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Geochemical and Geochronological Constraints on the Nature of the Immediate Basement next to the Mesoarchaean Auriferous Witwatersrand Basin, South Africa

Geochemical and Geochronological Constraints on the Nature of the Immediate Basement next to the... Abstract A combined petrological, geochemical, and geochronological (Rb–Sr and Sm–Nd whole-rock, U–Pb and Lu–Hf zircon, and Ar–Ar hornblende) study on a section of pre-Witwatersrand basement drilled at the northwestern margin of the Witwatersrand Basin has revealed new insights into the nature and tectonic setting of the likely source area for some of the Mesoarchaean auriferous Witwatersrand sediments. The protoliths of intersected altered granite and hornblende metagabbro are of indistinguishable age (3062 ± 5 Ma) and have very similar geochemical signatures. Trace element characteristics typical of calc-alkaline magmatism and evidence of variable contamination with older crust (subchondritic ϵNd and ϵHf in zircon) point to an active continental margin setting. The Ar–Ar hornblende ages are within error of the magmatic crystallization age or slightly older. Alteration of presumably primary magmatic hornblende to magnesio-hornblende immediately after gabbro emplacement during late magmatic autometasomatism is suggested. The presence of hydrous melts (>4 wt % H2O), comparable with fertile Au-bearing magmatic–hydrothermal mineralizing systems in Phanerozoic volcanic arcs, is inferred. Thus, a kind of hinterland is proposed for the Witwatersrand that compares favourably with the tectonic domains that are known to host the majority of post-Archaean gold deposits. Later retrograde hydrothermal alteration at c. 2720 and 2630 Ma led to variable Pb loss in zircon and the resetting of the whole-rock Rb–Sr isotope system whereas the Ar–Ar and Lu–Hf isotope systems in the hornblende and zircon grains, respectively, were not significantly affected. Comparison with published data suggests that these alteration events are the same as those that affected the Witwatersrand Basin fill associated with major early Ventersdorp flood basalt volcanism and possibly a pre-Transvaal thrusting event in response to contractional deformation in the Limpopo Belt. INTRODUCTION The Mesoarchaean Witwatersrand Basin in South Africa accounts for about 40% of all known gold (Frimmel, 2008) and thus represents by far the single most important gold depository known in the Earth's crust. The genesis of the ore bodies in the Witwatersrand goldfields has been a matter of intense debate (for a relatively recent summary of the arguments see Muntean et al., 2005); however, consensus is emerging that the gold entered the conglomeratic host-rocks essentially in the form of detrital particles that were subsequently mobilized on a local scale by various post-depositional fluids (for a review and summary of arguments see Frimmel et al., 2005). A palaeoplacer model invariably raises the question of the source of all that gold and thus of the nature of the hinterland at the time of detrital gold transport and sedimentation. The term ‘Witwatersrand Basin’, although widely accepted in the literature, is misleading because it refers to tectonically different basins whose fills were stacked on top of each other. Most of the Witwatersrand gold (>80%) is concentrated in quartz-pebble conglomerates (reefs) of the <2·90 to >2·78 Ga Central Rand Group. In contrast to the underlying <2·98 to 2·91 Ga West Rand Group (contributing <5% of Witwatersrand gold production) for which a passive margin setting is indicated, the predominantly fluvial sediments of the Central Rand Group were deposited in a foreland basin whose shape was similar to the current subcrop outline, except for the later up-doming of the central part as a result of the 2023 Ma Vredefort impact event (Fig. 1; Coward et al., 1995; Kositcin & Krapez, 2004; Frimmel et al., 2005). Fig. 1. Open in new tabDownload slide Main Archaean stratigraphic units of the Kaapvaal Craton. The West Rand and Central Rand Groups constitute the Witwatersrand Supergroup. Bold dashed line outlines the boundary of the Kaapvaal Craton as inferred from aeromagnetic data. The crustal blocks (amalgamated by 2·8 Ga) are separated by major lineaments; modified from Eglington & Armstrong (2004) and Frimmel et al. (2005). Information on the nature of the source rocks may be obtained from the study of the clastic sediments. Derivation of the various allogenic components of the Witwatersrand sediments, including gold, from older granite–greenstone terranes, such as the Barberton Greenstone Belt, has been suggested repeatedly by several workers (e.g. Viljoen et al., 1970; Robb & Meyer, 1990). A number of undoubtedly allogenic minerals indicative of both felsic and mafic sources, most importantly zircon and chromite, respectively, are present in the siliciclastic Witwatersrand sediments (for a detailed list of minerals see Feather & Koen, 1975). From the distribution of relatively immobile elements in Witwatersrand shales, Wronkiewicz & Condie (1987) concluded that the proportion of granite, basalt and komatiite in the source areas increased with time at the expense of tonalite. Vennemann et al. (1992, 1995) showed that the oxygen isotopic composition of the Witwatersrand quartz pebbles conforms to that of mesothermal vein quartz as found in greenstone-hosted orogenic gold deposits. Detrital zircon age spectra from the Witwatersrand (Kositcin & Krapez, 2004) agree well with available ages from the various granitoid–greenstone belts to the east, north and west (Poujol et al., 2003; for a comparison see Frimmel et al., 2005). The basal contacts of the numerous auriferous and uraniferous conglomerate beds typically represent an erosional surface above which older intrabasinal sedimentary units have been partially reworked. The sedimentary units higher in the succession, for example the 2·71 Ga Ventersdorp Contact Reef (contributing c. 15% of the total Witwatersrand gold production) above the Witwatersrand Supergroup, reflect considerable reworking of the underlying sediments. This reworking of intrabasinal sediments forms a major obstacle in the reconstruction of the primary source rocks and our attention is, therefore, directed towards the potential source rocks directly. Interestingly, most zircon grains in the arenitic units below and above single reefs of the Central Rand Group have ages between 2·96 and 2·83 Ga, whereas those from the auriferous conglomeratic reefs indicate a dominance of 3·05–3·09 Ga sources (Ruiz et al., 2006; N. Koglin, unpublished data, 2009). An imprecise Re–Os age of 3016 ± 110 Ma obtained by Kirk et al. (2002) for the gold itself (3033 ± 21 Ma obtained for rounded pyrite and gold combined) is clearly older than the age of sediment deposition and thus provides strong support for the palaeoplacer model. More importantly, these ages conform to the detrital zircon age peaks mentioned above for the reef units. These data indicate that 3·05–3·09 Ga pre-Witwatersrand units are by far more important as a potential source of the gold than the various younger, syn-depositional granitoids in the hinterland and thus they will be at the centre of this study. Numerous studies have been conducted on the main granite–greenstone terranes of the Kaapvaal Craton, especially on the Barberton Belt (for recent reviews and further references see Brandl et al., 2006; Robb et al., 2006; Zeh et al., 2009). Today's exposed granite–greenstone belts on the craton (Fig. 1) cannot, however, be the source regions of the Witwatersrand sediments and the gold therein. They are the deeply eroded remnants of what might have been a potential 3·5–2·8 Ga hinterland. Although the finer grained metasedimentary rocks of the Witwatersrand Supergroup and its stratigraphic equivalent, the Pongola Supergroup (Fig. 1), may have been sourced in distal hinterlands, the allogenic components of the coarse-grained, auriferous and uraniniferous conglomerate beds are likely to be derived from proximal sources. Within a given reef, both Au and U concentrations decrease from the basin margin towards its centre, with a systematic increase in the U/Au ratio down the palaeoslope. This observation has been explained by hydraulic sorting of allogenic gold and uraninite particles (Minter et al., 1986; Frimmel et al., 2005). Thus the source rocks for the high-energy detritus, including the gold, have to be sought in proximity to the former basin margin. Unfortunately, data on such pre-Witwatersrand units in the immediate vicinity of the Witwatersrand Basin are sparse because of a lack of outcrop. So far, two types of basement have been recognized in the vicinity of the Witwatersrand Basin: bimodal volcanic rocks with minor siliciclastic sedimentary material of the Dominion Group as well as various granitoid domes (with minor greenstones). From the spatial distribution of both basement types (Fig. 2) it is apparent that both could have supplied material, particularly into the Central Rand Basin. Fig. 2. Open in new tabDownload slide Surface and subsurface distribution of the Witwatersrand Supergroup, Dominion Group and Mesoarchaean granite–greenstone basement domes. Also shown are the palaeocurrent directions during Central Rand Group times (small arrows) as well as the position of the producing goldfields (from Frimmel et al., 2005). Larger open arrow indicates approximate position of the studied drill site (see Fig. 3). The precisely dated 3074 ± 6 Ma (Armstrong et al., 1991) Dominion Group represents the first supracrustal volcano-sedimentary succession above the Palaeo- to Mesoarchaean granitoid–greenstone basement. Its tectonic setting has been a matter of controversy. Whereas some workers have argued for a continental margin setting because of the calc-alkaline andesitic character of the mafic rocks and depletion in Nb, Ti and Zr (Burke et al., 1986; Crow & Condie, 1987), others have considered this geochemical signature not conclusive, inherited from the mantle source rocks, and proposed deposition in a continental rift setting because of the bimodal nature of the volcanism and the tholeiitic affinity of the mafic rocks (Bowen et al., 1986; Marsh et al., 1989; Jackson, 1992). Fragments of greenstone belts in the vicinity of the Witwatersrand Basin have been known so far only from the Johannesburg Dome to the north (Anhaeusser, 1973) and from the eastern margin of the Vredefort Dome (Minnitt et al., 1994; Fig. 2). Ultramafic–mafic complexes dominate in the former area. No precise age data exist but these complexes, which have been interpreted as reflecting a former suture zone (Anhaeusser, 2006), are considered to be older than the 3200–3340 Ma intrusive tonalitic to trondhjemitic gneisses in the Johannesburg Dome (Poujol & Anhaeusser, 2001). Ultramafic–mafic intrusions in the core of the Vredefort Dome are metamorphosed at high grade and a xenocryst age of 3425 Ma has been reported from there (Hart et al., 1990). Some petrographic, geochronological and geochemical information is available from a variety of granitoid bodies that occur in several basement domes to the north and NW of the Witwatersrand Basin as well as in the Vredefort Dome (Anhaeusser, 1973, 1999; Klemd & Hallbauer, 1987; Robb & Meyer, 1987; Barton et al., 1999; Poujol & Anhaeusser, 2001; Armstrong et al., 2006; for a summary and further references see Robb et al., 2006). Their ages range from 3340 to 2777 Ma. Most of them, notably the majority of the granitoid bodies in the Johannesburg, Vredefort and Westerdam–Coligny Domes, have ages that cluster around 3·1 Ga, whereas the youngest of these intrusions post-dates the onset of sediment deposition in the Witwatersrand Basin. Whereas the focus of the previous studies has been on the granitoids along the perimeter of the Witwatersrand Basin, mafic rocks are at the centre of this study. Here we report the first geochemical and isotope data on mafic and associated felsic intrusive rocks that were intersected in exploration drill holes through a basement horst at the northwestern margin of the Witwatersrand Basin. The significance of this site lies not only in the abundance of mafic rocks but also in the fact that the pre-Witwatersrand basement is not covered by Witwatersrand sediments but is directly overlain by rocks of the lowermost Transvaal Supergroup, as well as in its proximity to the margin of the Central Rand Basin. Consequently, the drill core can provide insights into the nature and make-up of at least a section of what has been a potential source of proximal Central Rand Group sediments. Specifically, we provide petrographic, mineral-chemical and whole-rock geochemical data, U–Pb and Lu–Hf isotope data on zircon grains, Rb–Sr and Sm–Nd whole-rock isotope data, as well as Ar–Ar hornblende data, all of which are used to constrain the age and to formulate a petrogenetic model for this section of proximal source rocks. DRILL CORE SETTING AND PETROGRAPHY The investigated core comes from a borehole (BH1) that was drilled by the Chamber of Mines Research Organization (supervision: D. K. Hallbauer) on the farm Rooidraai 85 IQ at 26°21′47′′S, 27°06′54′′E, 30 km west of Carletonville (Fig. 2). The local geology of the site is dominated by a ridge that consists mainly of strongly weathered granodiorite with minor pegmatite and quartz veins, overlain by a 2–5 m thick cover of Black Reef Formation quartzite and conglomerate (the basal lithostratigraphic unit of the Transvaal Supergroup), and soil (Fig. 3). The ridge is part of an approximately north–south-trending horst structure within the northwestern Witwatersrand Basin. The structure was shaped during syn-Ventersdorp (2·71 Ga) extension. Block faulting at that time made it possible that locally the pre-Witwatersrand basement became elevated to shallow levels and was eventually exposed at or near the surface. The block-bounding faults have, however, a protracted history of repeated reactivation from syn-Central Rand Group compression to modification by the 2024 Ma Vredefort impact (Brink et al., 2000), leaving behind cataclasites and locally pseudotachylyte. Fig. 3. Open in new tabDownload slide Local geological map of the investigated borehole (BH1) site on the farm Rooidraai 85 IQ. The borehole intersected 35 m of pre-Witwatersrand basement rocks beneath 4 m of quartzite of the Black Reef Formation and overlying saprolite and lateritic soil. The top 10 m of the basement consists of coarse-grained altered granite underlain by massive, coarse-grained hornblende metagabbro (or metadiorite). The contact between the two rock types in the core is characterized by a transition zone with pink alkali feldspar disseminated in the otherwise mafic to intermediate rocks, and by a strong enrichment in largely chloritized biotite over a 1 m distance from the granite into the metagabbro (or metadiorite). Macroscopically visible hydrothermal alteration is restricted to quartz–calcite–sulphide veinlets that cross-cut particularly the mafic rocks in variable orientations. The altered granite is macroscopically highly heterogeneous with equigranular, medium-grained and fine-grained domains. In places, centimetre-size pinkish alkali feldspar grains give the rock an overall coarse-grained, inequigranular appearance. Under the microscope, the main phases are anhedral microcline (c. 36 vol. %), plagioclase (c. 29 vol. %), and quartz (29 vol. %) with irregular to serrated grain boundaries. Microcline displays typical tartan twinning and is, in places, altered to muscovite (2 vol. %). Plagioclase shows polysynthetic albite twinning and is partly saussuritized to epidote and white mica. Quartz is partly recrystallized. Variably chloritized biotite occurs in minor amounts (2 vol. %). Apatite, titanite, zircon, and magnetite are present as accessory phases (<1 vol. %). The metagabbro (or metadiorite) is, except for the contact with the overlying granite, mineralogically fairly uniform, but texturally variable. The overall dark green, coarse- to fine-grained rock consists mainly of amphiboles (56 vol. %) and plagioclase (20 vol. %). The generally subhedral amphibole grains are mainly hornblende and they are typically larger (up to several millimetres) than the interstitial, anhedral plagioclase. Variably chloritized biotite makes up as much as 13 vol. %. Irregular patches of the hornblende grains are replaced by actinolite. Epidote and sericite, derived from the saussuritization of plagioclase, occur in addition to actinolite and chlorite, and quartz (1 vol. %) as secondary, metamorphic–hydrothermal phases in minor amounts. Microcline is present in minor amounts (5 vol. %), irregularly distributed throughout the metagabbro (metadiorite) and especially along the contacts to veins. Zircon is relatively abundant as is apatite (1 vol. %). Accessory titanite is secondary and formed during the chloritization of biotite and the oxidation of presumably primary ilmenite. The biotite-rich zone in the mafic portion immediately below the granite is 1·15 m in thickness and occurs as a dark greenish-grey, fine-grained rock that contains irregularly distributed, anhedral, pink microcline grains, several centimetres in size. Largely chloritized biotite constitutes on average 71 vol. % of the rock, with the remainder being predominantly saussuritized plagioclase (15 vol. %). Apatite is present in unusually high amounts (9 vol. %), mainly as euhedral inclusions in plagioclase. Other minor phases include quartz (1 vol. %), zircon, secondary titanite and relics of a probably primary opaque phase (2 vol. %). Of special interest is a strong enrichment in allanite that displays oscillatory zonation and occurs concentrated as euhedral, several millimeter long grains at the bottom of this biotite-rich zone. All of the above lithotypes are cross-cut by various millimetre- to centimetre-thick veinlets that contain variable amounts of quartz, calcite, chlorite, microcline, pyrite and chalcopyrite. Depending on the mineralogy, their colour ranges from white, grey, green to pink. Where they cut across metabasite, the latter is typically affected by hydrothermal growth of microcline and pyrite. The spatial distribution of the microcline in the metagabbro (metadiorite) is suggestive of post-magmatic K-metasomatism that is probably related to the emplacement of the granite or that of pegmatite pods in the vicinity of the borehole. Thus, some of the microcline observed in the granite may be secondary, in which case the original composition of the granite would be granodioritic rather than granitic, analogous to the granodiorite that occurs in outcrops near the borehole locality (Fig. 3). MINERAL CHEMISTRY Mineral chemical data were obtained by electron microprobe analysis (EMPA) on 160 spots of amphiboles, feldspars, biotite and chlorite, using a CAMECA S50 instrument at the University of Würzburg (acceleration voltage 15 kV, beam current 15 nA). The analytical errors are less than 1%, except for Na (<2%). The amphibole compositions cover a wide range from magnesio-hornblende to actinolite, irrespective of the assumed Fe2+/Fe3+ ratio (Table 1), with 7·10–7·72 Si p.f.u. As expected, the Al content (Al2O3 6·56–2·19 wt %) is negatively correlated with Si and positively correlated with Ti (TiO2 0·74–0·08 wt %). Table 1: Representative analyses of amphibole grains in the metagabbro Core-metres . 14·10–14·35 . 14·10–14·35 . 22·70–22·80 . 24·20–24·40 . 24·20–24·40 . 32·00–32·35 . SiO2 54·35 49·08 53·13 50·32 52·52 52·60 TiO2 0·08 0·74 0·11 0·38 0·29 0·26 Al2O3 2·19 6·56 3·17 5·37 4·19 3·83 Cr2O3 0·14 0·13 0·19 0·14 0·17 0·12 FeO* 9·24 12·08 8·59 10·35 9·74 9·00 MgO 18·25 15·53 18·58 16·56 17·39 17·98 MnO 0·20 0·30 0·17 0·21 0·22 0·18 CaO 12·75 12·14 12·36 12·39 12·49 12·45 Na2O 0·32 1·04 0·49 0·92 0·58 0·75 K2O 0·08 0·61 0·19 0·35 0·23 0·22 Total 97·60 98·21 96·98 96·99 97·82 97·39 Core-metres . 14·10–14·35 . 14·10–14·35 . 22·70–22·80 . 24·20–24·40 . 24·20–24·40 . 32·00–32·35 . SiO2 54·35 49·08 53·13 50·32 52·52 52·60 TiO2 0·08 0·74 0·11 0·38 0·29 0·26 Al2O3 2·19 6·56 3·17 5·37 4·19 3·83 Cr2O3 0·14 0·13 0·19 0·14 0·17 0·12 FeO* 9·24 12·08 8·59 10·35 9·74 9·00 MgO 18·25 15·53 18·58 16·56 17·39 17·98 MnO 0·20 0·30 0·17 0·21 0·22 0·18 CaO 12·75 12·14 12·36 12·39 12·49 12·45 Na2O 0·32 1·04 0·49 0·92 0·58 0·75 K2O 0·08 0·61 0·19 0·35 0·23 0·22 Total 97·60 98·21 96·98 96·99 97·82 97·39 (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) Si 7·72 7·70 7·66 7·10 7·08 7·01 7·59 7·56 7·50 7·29 7·27 7·22 7·48 7·47 7·41 7·50 7·49 7·44 Al(IV) 0·28 0·30 0·34 0·90 0·92 0·99 0·41 0·44 0·50 0·71 0·73 0·78 0·52 0·53 0·59 0·50 0·51 0·56 Sum T 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 Al(VI) 0·09 0·07 0·03 0·22 0·19 0·12 0·13 0·10 0·02 0·21 0·18 0·13 0·19 0·17 0·11 0·14 0·13 0·08 Fe3+ — 0·09 0·32 — 0·15 0·58 — 0·17 0·53 — 0·13 0·42 — 0·08 0·40 — 0·06 0·39 Ti 0·01 0·01 0·01 0·08 0·08 0·08 0·01 0·01 0·01 0·04 0·04 0·04 0·04 0·04 0·04 0·03 0·03 0·03 Cr 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·01 0·01 Mg 3·86 3·85 3·84 3·35 3·34 3·31 3·95 3·94 3·91 3·58 3·57 3·54 3·69 3·68 3·66 3·83 3·82 3·79 Fe2+ 1·02 0·96 0·76 1·33 1·22 0·85 0·89 0·76 0·49 1·15 1·06 0·82 1·06 1·01 0·74 0·99 0·95 0·68 Mn — — 0·02 — — 0·04 — — 0·02 — — 0·03 — — 0·03 — — 0·02 Sum C 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 Fe2+ 0·08 0·04 — 0·13 0·08 — 0·13 0·10 — 0·10 0·05 — 0·10 0·07 — 0·08 0·08 — Mn 0·02 0·02 — 0·04 0·04 — 0·02 0·02 — 0·03 0·03 — 0·03 0·03 — 0·02 0·02 — Ca 1·94 1·94 1·93 1·88 1·88 1·86 1·89 1·88 1·87 1·92 1·92 1·91 1·91 1·90 1·89 1·90 1·90 1·89 Na 0·07 0·14 0·13 0·09 0·11 0·11 Sum B 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·01 2·00 2·00 Na 0·09 0·09 0·02 0·29 0·29 0·15 0·13 0·13 0·00 0·26 0·26 0·16 0·16 0·16 0·05 0·21 0·21 0·10 K 0·01 0·01 0·01 0·11 0·11 0·11 0·03 0·03 0·03 0·06 0·06 0·06 0·04 0·04 0·04 0·04 0·04 0·04 Sum A 0·10 0·10 0·03 0·40 0·40 0·26 0·16 0·16 0·03 0·32 0·32 0·22 0·20 0·20 0·09 0·25 0·25 0·14 Total 15·14 15·10 15·03 15·45 15·40 15·26 15·20 15·16 15·03 15·37 15·32 15·22 15·24 15·20 15·09 15·25 15·25 15·14 (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) Si 7·72 7·70 7·66 7·10 7·08 7·01 7·59 7·56 7·50 7·29 7·27 7·22 7·48 7·47 7·41 7·50 7·49 7·44 Al(IV) 0·28 0·30 0·34 0·90 0·92 0·99 0·41 0·44 0·50 0·71 0·73 0·78 0·52 0·53 0·59 0·50 0·51 0·56 Sum T 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 Al(VI) 0·09 0·07 0·03 0·22 0·19 0·12 0·13 0·10 0·02 0·21 0·18 0·13 0·19 0·17 0·11 0·14 0·13 0·08 Fe3+ — 0·09 0·32 — 0·15 0·58 — 0·17 0·53 — 0·13 0·42 — 0·08 0·40 — 0·06 0·39 Ti 0·01 0·01 0·01 0·08 0·08 0·08 0·01 0·01 0·01 0·04 0·04 0·04 0·04 0·04 0·04 0·03 0·03 0·03 Cr 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·01 0·01 Mg 3·86 3·85 3·84 3·35 3·34 3·31 3·95 3·94 3·91 3·58 3·57 3·54 3·69 3·68 3·66 3·83 3·82 3·79 Fe2+ 1·02 0·96 0·76 1·33 1·22 0·85 0·89 0·76 0·49 1·15 1·06 0·82 1·06 1·01 0·74 0·99 0·95 0·68 Mn — — 0·02 — — 0·04 — — 0·02 — — 0·03 — — 0·03 — — 0·02 Sum C 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 Fe2+ 0·08 0·04 — 0·13 0·08 — 0·13 0·10 — 0·10 0·05 — 0·10 0·07 — 0·08 0·08 — Mn 0·02 0·02 — 0·04 0·04 — 0·02 0·02 — 0·03 0·03 — 0·03 0·03 — 0·02 0·02 — Ca 1·94 1·94 1·93 1·88 1·88 1·86 1·89 1·88 1·87 1·92 1·92 1·91 1·91 1·90 1·89 1·90 1·90 1·89 Na 0·07 0·14 0·13 0·09 0·11 0·11 Sum B 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·01 2·00 2·00 Na 0·09 0·09 0·02 0·29 0·29 0·15 0·13 0·13 0·00 0·26 0·26 0·16 0·16 0·16 0·05 0·21 0·21 0·10 K 0·01 0·01 0·01 0·11 0·11 0·11 0·03 0·03 0·03 0·06 0·06 0·06 0·04 0·04 0·04 0·04 0·04 0·04 Sum A 0·10 0·10 0·03 0·40 0·40 0·26 0·16 0·16 0·03 0·32 0·32 0·22 0·20 0·20 0·09 0·25 0·25 0·14 Total 15·14 15·10 15·03 15·45 15·40 15·26 15·20 15·16 15·03 15·37 15·32 15·22 15·24 15·20 15·09 15·25 15·25 15·14 *All Fe expressed as FeO. (a) Normalization assuming all Fe to be FeO on the basis of 24 (O,OH); (b) normalization on the basis of 15eNK; (c) normalization on the basis of 13eCNK. Open in new tab Table 1: Representative analyses of amphibole grains in the metagabbro Core-metres . 14·10–14·35 . 14·10–14·35 . 22·70–22·80 . 24·20–24·40 . 24·20–24·40 . 32·00–32·35 . SiO2 54·35 49·08 53·13 50·32 52·52 52·60 TiO2 0·08 0·74 0·11 0·38 0·29 0·26 Al2O3 2·19 6·56 3·17 5·37 4·19 3·83 Cr2O3 0·14 0·13 0·19 0·14 0·17 0·12 FeO* 9·24 12·08 8·59 10·35 9·74 9·00 MgO 18·25 15·53 18·58 16·56 17·39 17·98 MnO 0·20 0·30 0·17 0·21 0·22 0·18 CaO 12·75 12·14 12·36 12·39 12·49 12·45 Na2O 0·32 1·04 0·49 0·92 0·58 0·75 K2O 0·08 0·61 0·19 0·35 0·23 0·22 Total 97·60 98·21 96·98 96·99 97·82 97·39 Core-metres . 14·10–14·35 . 14·10–14·35 . 22·70–22·80 . 24·20–24·40 . 24·20–24·40 . 32·00–32·35 . SiO2 54·35 49·08 53·13 50·32 52·52 52·60 TiO2 0·08 0·74 0·11 0·38 0·29 0·26 Al2O3 2·19 6·56 3·17 5·37 4·19 3·83 Cr2O3 0·14 0·13 0·19 0·14 0·17 0·12 FeO* 9·24 12·08 8·59 10·35 9·74 9·00 MgO 18·25 15·53 18·58 16·56 17·39 17·98 MnO 0·20 0·30 0·17 0·21 0·22 0·18 CaO 12·75 12·14 12·36 12·39 12·49 12·45 Na2O 0·32 1·04 0·49 0·92 0·58 0·75 K2O 0·08 0·61 0·19 0·35 0·23 0·22 Total 97·60 98·21 96·98 96·99 97·82 97·39 (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) Si 7·72 7·70 7·66 7·10 7·08 7·01 7·59 7·56 7·50 7·29 7·27 7·22 7·48 7·47 7·41 7·50 7·49 7·44 Al(IV) 0·28 0·30 0·34 0·90 0·92 0·99 0·41 0·44 0·50 0·71 0·73 0·78 0·52 0·53 0·59 0·50 0·51 0·56 Sum T 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 Al(VI) 0·09 0·07 0·03 0·22 0·19 0·12 0·13 0·10 0·02 0·21 0·18 0·13 0·19 0·17 0·11 0·14 0·13 0·08 Fe3+ — 0·09 0·32 — 0·15 0·58 — 0·17 0·53 — 0·13 0·42 — 0·08 0·40 — 0·06 0·39 Ti 0·01 0·01 0·01 0·08 0·08 0·08 0·01 0·01 0·01 0·04 0·04 0·04 0·04 0·04 0·04 0·03 0·03 0·03 Cr 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·01 0·01 Mg 3·86 3·85 3·84 3·35 3·34 3·31 3·95 3·94 3·91 3·58 3·57 3·54 3·69 3·68 3·66 3·83 3·82 3·79 Fe2+ 1·02 0·96 0·76 1·33 1·22 0·85 0·89 0·76 0·49 1·15 1·06 0·82 1·06 1·01 0·74 0·99 0·95 0·68 Mn — — 0·02 — — 0·04 — — 0·02 — — 0·03 — — 0·03 — — 0·02 Sum C 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 Fe2+ 0·08 0·04 — 0·13 0·08 — 0·13 0·10 — 0·10 0·05 — 0·10 0·07 — 0·08 0·08 — Mn 0·02 0·02 — 0·04 0·04 — 0·02 0·02 — 0·03 0·03 — 0·03 0·03 — 0·02 0·02 — Ca 1·94 1·94 1·93 1·88 1·88 1·86 1·89 1·88 1·87 1·92 1·92 1·91 1·91 1·90 1·89 1·90 1·90 1·89 Na 0·07 0·14 0·13 0·09 0·11 0·11 Sum B 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·01 2·00 2·00 Na 0·09 0·09 0·02 0·29 0·29 0·15 0·13 0·13 0·00 0·26 0·26 0·16 0·16 0·16 0·05 0·21 0·21 0·10 K 0·01 0·01 0·01 0·11 0·11 0·11 0·03 0·03 0·03 0·06 0·06 0·06 0·04 0·04 0·04 0·04 0·04 0·04 Sum A 0·10 0·10 0·03 0·40 0·40 0·26 0·16 0·16 0·03 0·32 0·32 0·22 0·20 0·20 0·09 0·25 0·25 0·14 Total 15·14 15·10 15·03 15·45 15·40 15·26 15·20 15·16 15·03 15·37 15·32 15·22 15·24 15·20 15·09 15·25 15·25 15·14 (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) Si 7·72 7·70 7·66 7·10 7·08 7·01 7·59 7·56 7·50 7·29 7·27 7·22 7·48 7·47 7·41 7·50 7·49 7·44 Al(IV) 0·28 0·30 0·34 0·90 0·92 0·99 0·41 0·44 0·50 0·71 0·73 0·78 0·52 0·53 0·59 0·50 0·51 0·56 Sum T 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 Al(VI) 0·09 0·07 0·03 0·22 0·19 0·12 0·13 0·10 0·02 0·21 0·18 0·13 0·19 0·17 0·11 0·14 0·13 0·08 Fe3+ — 0·09 0·32 — 0·15 0·58 — 0·17 0·53 — 0·13 0·42 — 0·08 0·40 — 0·06 0·39 Ti 0·01 0·01 0·01 0·08 0·08 0·08 0·01 0·01 0·01 0·04 0·04 0·04 0·04 0·04 0·04 0·03 0·03 0·03 Cr 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·01 0·01 Mg 3·86 3·85 3·84 3·35 3·34 3·31 3·95 3·94 3·91 3·58 3·57 3·54 3·69 3·68 3·66 3·83 3·82 3·79 Fe2+ 1·02 0·96 0·76 1·33 1·22 0·85 0·89 0·76 0·49 1·15 1·06 0·82 1·06 1·01 0·74 0·99 0·95 0·68 Mn — — 0·02 — — 0·04 — — 0·02 — — 0·03 — — 0·03 — — 0·02 Sum C 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 Fe2+ 0·08 0·04 — 0·13 0·08 — 0·13 0·10 — 0·10 0·05 — 0·10 0·07 — 0·08 0·08 — Mn 0·02 0·02 — 0·04 0·04 — 0·02 0·02 — 0·03 0·03 — 0·03 0·03 — 0·02 0·02 — Ca 1·94 1·94 1·93 1·88 1·88 1·86 1·89 1·88 1·87 1·92 1·92 1·91 1·91 1·90 1·89 1·90 1·90 1·89 Na 0·07 0·14 0·13 0·09 0·11 0·11 Sum B 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·01 2·00 2·00 Na 0·09 0·09 0·02 0·29 0·29 0·15 0·13 0·13 0·00 0·26 0·26 0·16 0·16 0·16 0·05 0·21 0·21 0·10 K 0·01 0·01 0·01 0·11 0·11 0·11 0·03 0·03 0·03 0·06 0·06 0·06 0·04 0·04 0·04 0·04 0·04 0·04 Sum A 0·10 0·10 0·03 0·40 0·40 0·26 0·16 0·16 0·03 0·32 0·32 0·22 0·20 0·20 0·09 0·25 0·25 0·14 Total 15·14 15·10 15·03 15·45 15·40 15·26 15·20 15·16 15·03 15·37 15·32 15·22 15·24 15·20 15·09 15·25 15·25 15·14 *All Fe expressed as FeO. (a) Normalization assuming all Fe to be FeO on the basis of 24 (O,OH); (b) normalization on the basis of 15eNK; (c) normalization on the basis of 13eCNK. Open in new tab Plagioclase in the mafic rocks is oligoclase with XAn of 0·13. Considering the extent of saussuritization, the original anorthite content must have been considerably higher but can no longer be constrained. The plagioclase grains in the altered granite are all essentially stoichiometric albite. Again, a certain anorthite component has to be assumed for the original plagioclase grains in this rock, taking into account the extent of saussuritization. Overall, bearing in mind that both the felsic and mafic–intermediate rocks experienced the same post-intrusive alteration history, an originally more calcic plagioclase composition for the latter rocks compared with the granite can be inferred from the results obtained, but the initial anorthite content of the magmatic plagioclase remains elusive. Thus, based on purely petrographic criteria, it cannot be stated whether the protolith of the mafic drill core portion was a gabbro or a diorite. The alkali feldspar in the altered granite, as well as in the more mafic rocks and the veinlets, is almost pure K-feldspar throughout. Biotite is compositionally uniform and identical in both the hornblende-rich mafic portion and the biotite-rich zone. Its XFe is consistently 0·37; the TiO2 content is elevated and varies between 2·0 and 2·8 wt % (Table 2). Table 2: Representative compositions of biotite and chlorite in metagabbro . Biotite . . Chlorite . Core-metre: . 32·00–32·35 . 12·00–12·10 . . 22·70–22·80 . 14·10–14·35 . 14·10–14·35 . 5·65–5·75 . Rock type: . Hbl-gabbro . Bio-gabbro . . Hbl-gabbro . Vein . Bio-gabbro . Granite . SiO2 37·37 38·09 27·80 27·29 27·08 24·44 TiO2 1·97 2·76 0·05 0·03 0·01 0·01 Al2O3 15·24 13·88 19·87 19·73 18·51 18·65 Cr2O3 0·40 0·18 0·15 0·00 0·01 0·00 MgO 14·87 14·49 21·71 21·13 17·01 8·83 CaO 0·02 0·67 0·01 0·01 0·04 0·03 MnO 0·11 0·28 0·21 0·33 0·50 0·74 FeO* 15·68 15·49 17·98 18·48 24·63 36·12 BaO 0·01 0·00 0·00 0·00 0·00 0·00 Na2O 0·05 0·09 0·00 0·03 0·03 0·00 K2O 8·65 8·94 0·00 0·01 0·02 0·03 Total 94·37 94·87 87·79 87·03 87·83 88·85 Si 5·63 5·73 Si 5·62 5·59 5·69 5·44 AlIV 2·37 2·28 AlIV 2·38 2·41 2·31 2·57 T-site 8·00 8·00 T-site 8·00 8·00 8·00 8·00 AlVI 0·33 0·18 AlVI 2·35 2·35 2·27 2·32 Cr 0·05 0·02 Ti 0·01 0·01 0·00 0·00 Ti 0·22 0·31 Fe2+ 3·04 3·16 4·32 6·72 Fe2+ 1·97 1·95 Mn 0·04 0·06 0·09 0·14 Mn 0·01 0·04 Mg 6·54 6·45 5·33 2·93 Mg 3·34 3·25 Na 0·00 0·01 0·01 0·00 O-site 5·93 5·75 K 0·00 0·00 0·01 0·01 Ba 0·00 0·00 Sum 11·97 12·03 12·02 12·12 Ca 0·00 0·11 Na 0·02 0·03 K 1·66 1·71 A-site 1·68 1·85 Total 19·97 20·03 20·02 20·12 Total 15·61 15·59 XFe 0·32 0·33 0·45 0·70 XFe 0·37 0·37 T (°C)† 294 298 297 342 . Biotite . . Chlorite . Core-metre: . 32·00–32·35 . 12·00–12·10 . . 22·70–22·80 . 14·10–14·35 . 14·10–14·35 . 5·65–5·75 . Rock type: . Hbl-gabbro . Bio-gabbro . . Hbl-gabbro . Vein . Bio-gabbro . Granite . SiO2 37·37 38·09 27·80 27·29 27·08 24·44 TiO2 1·97 2·76 0·05 0·03 0·01 0·01 Al2O3 15·24 13·88 19·87 19·73 18·51 18·65 Cr2O3 0·40 0·18 0·15 0·00 0·01 0·00 MgO 14·87 14·49 21·71 21·13 17·01 8·83 CaO 0·02 0·67 0·01 0·01 0·04 0·03 MnO 0·11 0·28 0·21 0·33 0·50 0·74 FeO* 15·68 15·49 17·98 18·48 24·63 36·12 BaO 0·01 0·00 0·00 0·00 0·00 0·00 Na2O 0·05 0·09 0·00 0·03 0·03 0·00 K2O 8·65 8·94 0·00 0·01 0·02 0·03 Total 94·37 94·87 87·79 87·03 87·83 88·85 Si 5·63 5·73 Si 5·62 5·59 5·69 5·44 AlIV 2·37 2·28 AlIV 2·38 2·41 2·31 2·57 T-site 8·00 8·00 T-site 8·00 8·00 8·00 8·00 AlVI 0·33 0·18 AlVI 2·35 2·35 2·27 2·32 Cr 0·05 0·02 Ti 0·01 0·01 0·00 0·00 Ti 0·22 0·31 Fe2+ 3·04 3·16 4·32 6·72 Fe2+ 1·97 1·95 Mn 0·04 0·06 0·09 0·14 Mn 0·01 0·04 Mg 6·54 6·45 5·33 2·93 Mg 3·34 3·25 Na 0·00 0·01 0·01 0·00 O-site 5·93 5·75 K 0·00 0·00 0·01 0·01 Ba 0·00 0·00 Sum 11·97 12·03 12·02 12·12 Ca 0·00 0·11 Na 0·02 0·03 K 1·66 1·71 A-site 1·68 1·85 Total 19·97 20·03 20·02 20·12 Total 15·61 15·59 XFe 0·32 0·33 0·45 0·70 XFe 0·37 0·37 T (°C)† 294 298 297 342 Biotite normalization on the basis of 22 O, chlorite normalization on the basis of 36 (O,OH). *All Fe as FeO. †Temperature calculated according to Kranidiotis & McLean (1987) for chlorite with XFe < 0·50. Open in new tab Table 2: Representative compositions of biotite and chlorite in metagabbro . Biotite . . Chlorite . Core-metre: . 32·00–32·35 . 12·00–12·10 . . 22·70–22·80 . 14·10–14·35 . 14·10–14·35 . 5·65–5·75 . Rock type: . Hbl-gabbro . Bio-gabbro . . Hbl-gabbro . Vein . Bio-gabbro . Granite . SiO2 37·37 38·09 27·80 27·29 27·08 24·44 TiO2 1·97 2·76 0·05 0·03 0·01 0·01 Al2O3 15·24 13·88 19·87 19·73 18·51 18·65 Cr2O3 0·40 0·18 0·15 0·00 0·01 0·00 MgO 14·87 14·49 21·71 21·13 17·01 8·83 CaO 0·02 0·67 0·01 0·01 0·04 0·03 MnO 0·11 0·28 0·21 0·33 0·50 0·74 FeO* 15·68 15·49 17·98 18·48 24·63 36·12 BaO 0·01 0·00 0·00 0·00 0·00 0·00 Na2O 0·05 0·09 0·00 0·03 0·03 0·00 K2O 8·65 8·94 0·00 0·01 0·02 0·03 Total 94·37 94·87 87·79 87·03 87·83 88·85 Si 5·63 5·73 Si 5·62 5·59 5·69 5·44 AlIV 2·37 2·28 AlIV 2·38 2·41 2·31 2·57 T-site 8·00 8·00 T-site 8·00 8·00 8·00 8·00 AlVI 0·33 0·18 AlVI 2·35 2·35 2·27 2·32 Cr 0·05 0·02 Ti 0·01 0·01 0·00 0·00 Ti 0·22 0·31 Fe2+ 3·04 3·16 4·32 6·72 Fe2+ 1·97 1·95 Mn 0·04 0·06 0·09 0·14 Mn 0·01 0·04 Mg 6·54 6·45 5·33 2·93 Mg 3·34 3·25 Na 0·00 0·01 0·01 0·00 O-site 5·93 5·75 K 0·00 0·00 0·01 0·01 Ba 0·00 0·00 Sum 11·97 12·03 12·02 12·12 Ca 0·00 0·11 Na 0·02 0·03 K 1·66 1·71 A-site 1·68 1·85 Total 19·97 20·03 20·02 20·12 Total 15·61 15·59 XFe 0·32 0·33 0·45 0·70 XFe 0·37 0·37 T (°C)† 294 298 297 342 . Biotite . . Chlorite . Core-metre: . 32·00–32·35 . 12·00–12·10 . . 22·70–22·80 . 14·10–14·35 . 14·10–14·35 . 5·65–5·75 . Rock type: . Hbl-gabbro . Bio-gabbro . . Hbl-gabbro . Vein . Bio-gabbro . Granite . SiO2 37·37 38·09 27·80 27·29 27·08 24·44 TiO2 1·97 2·76 0·05 0·03 0·01 0·01 Al2O3 15·24 13·88 19·87 19·73 18·51 18·65 Cr2O3 0·40 0·18 0·15 0·00 0·01 0·00 MgO 14·87 14·49 21·71 21·13 17·01 8·83 CaO 0·02 0·67 0·01 0·01 0·04 0·03 MnO 0·11 0·28 0·21 0·33 0·50 0·74 FeO* 15·68 15·49 17·98 18·48 24·63 36·12 BaO 0·01 0·00 0·00 0·00 0·00 0·00 Na2O 0·05 0·09 0·00 0·03 0·03 0·00 K2O 8·65 8·94 0·00 0·01 0·02 0·03 Total 94·37 94·87 87·79 87·03 87·83 88·85 Si 5·63 5·73 Si 5·62 5·59 5·69 5·44 AlIV 2·37 2·28 AlIV 2·38 2·41 2·31 2·57 T-site 8·00 8·00 T-site 8·00 8·00 8·00 8·00 AlVI 0·33 0·18 AlVI 2·35 2·35 2·27 2·32 Cr 0·05 0·02 Ti 0·01 0·01 0·00 0·00 Ti 0·22 0·31 Fe2+ 3·04 3·16 4·32 6·72 Fe2+ 1·97 1·95 Mn 0·04 0·06 0·09 0·14 Mn 0·01 0·04 Mg 6·54 6·45 5·33 2·93 Mg 3·34 3·25 Na 0·00 0·01 0·01 0·00 O-site 5·93 5·75 K 0·00 0·00 0·01 0·01 Ba 0·00 0·00 Sum 11·97 12·03 12·02 12·12 Ca 0·00 0·11 Na 0·02 0·03 K 1·66 1·71 A-site 1·68 1·85 Total 19·97 20·03 20·02 20·12 Total 15·61 15·59 XFe 0·32 0·33 0·45 0·70 XFe 0·37 0·37 T (°C)† 294 298 297 342 Biotite normalization on the basis of 22 O, chlorite normalization on the basis of 36 (O,OH). *All Fe as FeO. †Temperature calculated according to Kranidiotis & McLean (1987) for chlorite with XFe < 0·50. Open in new tab Secondary chlorite in the hornblende metagabbro (metadiorite), where it replaces biotite, as well as in the cross-cutting veinlets has an XFe of c. 0·33 (Table 2). It is slightly more enriched in Fe in the biotite-rich zone (XFe = 0·45) and even more so where it replaced biotite in the altered granite (XFe = 0·70). CONSTRAINTS ON METAMORPHIC–HYDROTHERMAL OVERPRINT The original magmatic mineral assemblage K-feldspar–plagioclase–quartz–biotite in the granite or granodiorite shows evidence of potassic alteration in the form of secondary microcline growth and a low-grade metamorphic overprint in the form of partial recrystallization of quartz and the sericitization and saussuritization of microcline and plagioclase, respectively, as well as the chloritization of biotite. Qualitatively, a retrograde overprint at lower greenschist-facies conditions can be inferred. The primary mineral assemblage in the gabbroic or dioritic rock is not easily reconstructed. The actinolitic domains are evidently metamorphic but the dominant magnesio-hornblende could be either a primary magmatic or an earlier metamorphic phase. A magmatic origin for the hornblende seems indicated by a lack of any pyroxene relics anywhere in the samples as well as the lack of any other evidence of medium-grade metamorphism having affected the area. However, in the presence of Ti-minerals, such as titanite, in the rock a higher Ti content than detected would be expected in the hornblende if it were indeed magmatic. Similarly, the original magmatic plagioclase composition remains unknown because of retrograde alteration to oligoclase. The biotite remnants are presumed to be magmatic because of their elevated Ti content, indicative of a relatively high formation temperature. A retrograde overprint is reflected by chloritization of the biotite, formation of actinolite and the saussuritization of plagioclase, all of which indicate lower greenschist-facies conditions, also supported by the limited extent of quartz recrystallization in the adjacent altered granite. Assuming a relatively low pressure of c. 2 kbar (estimated from Na contents in the crystallographic B-site of amphibole), application of the plagioclase–amphibole thermometer (Holland & Blundy, 1994), specifically the edenite–richterite thermometer, yielded a mean temperature of 439 ± 48°C for 23 actinolite–plagioclase pairs, largely dependent on the XAb (0·86–0·99). The temperature at which the hydrothermal alteration, evident in the various veinlets, took place appears to have been similar to that of the pervasive retrograde overprint. Some of the microcline, notably the porphyroblastic textural variety and those grains that occur in the mafic–intermediate rocks adjacent to the hydrothermal veins, must be hydrothermal. The infiltrating fluid, whose pH is constrained by the stability of K-feldspar, was carbonic and S-bearing, as is evident from the precipitation of calcite, pyrite and chalcopyrite in the veinlets. GEOCHEMISTRY Major and trace elements Notwithstanding the low-grade pervasive metamorphic and local hydrothermal overprint, the whole-rock geochemical compositions of the three principal rock types, hornblende metagabbro (metadiorite), altered granite and the biotite-rich zone, have been used in an attempt to assess the genetic relationship between the felsic and more mafic rocks in the drill core and to constrain the likely tectonic setting of the magmatism. Altogether, 17 hornblende metagabbro (metadiorite) samples, five samples of the biotite-rich zone and seven altered granite samples were analysed for their major and trace element concentrations (Table 3). In addition, four samples of intensely veined metagabbro (metadiorite) were analysed to assess the effect of hydrothermal alteration on the overall geochemistry of the mafic–intermediate rocks. The major elements and selected trace element contents were determined on fusion disks by conventional X-ray fluorescence spectroscopy (XRF) using a Philips PW1480 instrument, whereas most of the trace element (including the rare earth elements, REE) concentrations were obtained by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS), using a Merchantek 266 LUV laser connected to an Agilent 7500i mass spectrometer, both at the Geodynamics and Geomaterials Research Division of the University of Würzburg. External calibration was carried out with the aid of the NIST 612 glass standard (Pearce et al., 1997). Reproducibility was tested by repeated analyses of the NIST 612 and 614 glass standards and the in-house standard BE-N. The lower limit of detection for most trace elements is below 0·02 ppm, except for Cr (0·54 ppm), Cu (0·18 ppm), Gd (0·07 ppm) and Pb (0·12 ppm). The results are given in Table 3. An additional 13 hornblende metagabbro (metadiorite) and four altered granite samples were analysed for both major and selected trace element concentrations by XRF only. The results are very similar to those above and are used in the data analysis but are not included in Table 3. Table 3: Geochemical analyses of different rock types in borehole BH1 on the farm Rooidraai Rock type: . Granite . Biotite-rich zone . Core- . 0·15– . 1·65– . 2·20– . 3·10– . 4·25– . 5·10– . 6·10– . 7·00– . 7·00– . 7·40– . 8·00– . 9·30– . metre: . 0·35 . 1·75 . 2·45 . 3·30 . 4·40 . 5·30 . 6·20 . 7·30A . 7·30B . 7·70 . 8·10 . 9·55 . Depth: . 4·25 . 5·7 . 6·3 . 7·2 . 8·3 . 9·20 . 10·15 . 11·15 . 11·15 . 11·55 . 12·05 . 13·4 . SiO2 (%) 72·12 74·25 70·81 72·97 63·32 70·29 73·26 47·69 44·96 51·55 50·73 53·42 TiO2 0·16 0·07 0·18 0·11 0·50 0·22 0·06 1·67 2·23 1·48 1·12 0·57 Al2O3 13·96 13·48 14·60 13·80 15·81 13·96 13·79 17·65 14·30 12·39 12·08 13·10 Fe2O3* 1·33 0·76 2·19 0·83 4·38 2·42 0·70 12·71 15·06 11·9 11·60 6·56 MnO 0·02 b.d. 0·04 0·02 0·10 0·04 0·01 0·15 0·24 0·23 0·22 0·16 MgO 0·30 0·17 0·90 0·24 1·50 0·59 0·18 6·30 11·14 9·91 11·82 7·95 CaO 0·93 0·59 0·75 1·25 3·08 1·74 0·72 2·91 3·07 3·98 3·05 7·13 Na2O 3·90 3·68 4·86 4·13 5·41 4·07 3·26 4·82 1·17 1·55 1·08 3·46 K2O 5·38 5·44 3·48 4·86 2·25 4·54 6·46 1·43 1·63 1·46 3·20 2·56 P2O5 0·06 0·02 0·09 0·03 0·28 0·06 0·03 0·70 0·66 0·52 0·67 0·37 LOI 0·97 0·76 1·32 1·31 2·86 1·73 0·81 3·51 5·87 5·17 4·26 4·35 Total 99·13 99·22 99·22 99·55 99·49 99·66 99·28 99·54 100·33 100·14 99·83 99·63 Sc (ppm) 3·02 2·02 3·23 2·32 10·22 3·43 1·48 23·84 42·58 36·93 32·30 23·24 V 20·00 b.d. 23·00 b.d. 45·00 11·00 11·00 169·0 145·0 118·0 134·0 103·0 Cr 6·55 7·46 7·35 6·42 8·58 6·76 6·52 104·1 634·8 744·3 872·8 440·6 Co 35·0 43·0 27·0 39·0 40·0 46·0 49·0 35·0 49·0 39·0 59·0 41·0 Ni 3·98 3·45 4·39 2·92 6·38 4·49 2·69 52·00 125·60 121·75 162·80 103·60 Cu 7·45 6·54 8·58 5·62 12·52 8·50 5·37 23·92 27·16 14·33 12·86 40·20 Zn 31·0 17·0 51·0 21·0 94·0 37·0 29·0 208·0 320·0 265·0 284·0 117·0 Ga 21·0 18·0 28·0 17·0 24·0 19·0 19·0 38·0 50·0 26·0 30·0 20·0 Rb 212 213 210 202 140 186 232 110 65 167 471 147 Sr 211 185 124 182 264 236 203 284 105 177 185 320 Y 17·64 9·03 18·40 11·84 50·12 28·76 7·02 98·74 111·00 93·68 151·80 95·24 Zr 183·6 62·7 226·6 99·9 205·2 321·6 53·4 330·0 47·3 85·3 71·8 306·4 Nb 13·00 11·03 13·42 9·57 50·10 14·10 8·09 94·02 165·20 110·00 135·60 41·58 Mo 0·64 0·63 1·35 6·28 0·74 7·43 0·56 0·49 0·42 0·61 0·37 0·53 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. 16·00 18·00 21·00 18·00 b.d. Cs 2·88 2·19 9·95 3·24 2·45 2·09 2·19 4·59 6·16 8·95 19·74 2·51 Ba 638 615 357 562 247 622 757 179 270 192 216 452 La 47·6 17·5 7·1 28·4 71·8 95·2 8·5 103·7 88·5 55·8 439·8 49·7 Ce 81·9 30·6 11·5 46·7 132·6 169·8 16·5 205·4 190·2 115·8 895·6 84·5 Pr 8·86 3·56 1·49 5·04 15·26 18·88 1·90 22·26 21·44 13·50 88·74 10·50 Nd 31·54 11·86 6·31 18·22 56·80 67·56 6·51 85·84 78·64 55·13 290·20 43·88 Sm 5·63 2·32 1·87 3·30 11·72 10·98 1·60 18·18 19·16 15·00 48·64 11·84 Eu 0·91 0·68 0·51 0·80 1·25 1·26 0·73 1·25 1·16 1·20 2·09 1·31 Gd 4·52 2·16 2·42 2·71 10·17 8·09 1·38 19·02 20·60 14·90 35·04 12·34 Tb 0·61 0·29 0·45 0·38 1·54 1·06 0·24 2·85 3·26 2·53 5·01 2·20 Dy 3·66 2·02 3·18 2·21 9·31 5·83 1·40 18·00 20·60 16·30 28·24 15·48 Ho 0·71 0·37 0·61 0·47 1·78 1·10 0·27 3·47 3·87 2·99 5·18 3·22 Er 1·70 1·09 1·71 1·35 4·94 2·81 0·82 8·67 10·14 8·46 13·20 9·89 Tm 0·25 0·16 0·25 0·16 0·68 0·43 0·14 1·05 1·26 1·18 1·78 1·58 Yb 1·65 1·12 1·58 1·28 4·41 2·90 0·99 6·25 7·64 7·28 10·33 11·74 Lu 0·24 0·15 0·26 0·19 0·59 0·41 0·13 0·80 0·98 0·96 1·40 1·55 Hf 4·94 2·13 5·97 3·06 5·47 8·11 1·92 8·37 1·23 2·14 1·72 7·45 Ta 1·32 1·04 0·60 0·57 3·22 1·64 1·03 2·67 5·02 3·79 5·43 2·14 Pb 21·74 17·56 10·56 17·60 8·58 15·62 17·30 7·10 7·14 5·06 17·44 4·90 Th 14·06 5·58 7·04 12·76 10·86 17·38 3·04 17·00 20·06 15·10 72·40 2·60 U 5·40 4·71 5·90 7·50 5·10 4·38 2·07 2·47 1·97 4·01 4·04 4·36 Rock type: . Granite . Biotite-rich zone . Core- . 0·15– . 1·65– . 2·20– . 3·10– . 4·25– . 5·10– . 6·10– . 7·00– . 7·00– . 7·40– . 8·00– . 9·30– . metre: . 0·35 . 1·75 . 2·45 . 3·30 . 4·40 . 5·30 . 6·20 . 7·30A . 7·30B . 7·70 . 8·10 . 9·55 . Depth: . 4·25 . 5·7 . 6·3 . 7·2 . 8·3 . 9·20 . 10·15 . 11·15 . 11·15 . 11·55 . 12·05 . 13·4 . SiO2 (%) 72·12 74·25 70·81 72·97 63·32 70·29 73·26 47·69 44·96 51·55 50·73 53·42 TiO2 0·16 0·07 0·18 0·11 0·50 0·22 0·06 1·67 2·23 1·48 1·12 0·57 Al2O3 13·96 13·48 14·60 13·80 15·81 13·96 13·79 17·65 14·30 12·39 12·08 13·10 Fe2O3* 1·33 0·76 2·19 0·83 4·38 2·42 0·70 12·71 15·06 11·9 11·60 6·56 MnO 0·02 b.d. 0·04 0·02 0·10 0·04 0·01 0·15 0·24 0·23 0·22 0·16 MgO 0·30 0·17 0·90 0·24 1·50 0·59 0·18 6·30 11·14 9·91 11·82 7·95 CaO 0·93 0·59 0·75 1·25 3·08 1·74 0·72 2·91 3·07 3·98 3·05 7·13 Na2O 3·90 3·68 4·86 4·13 5·41 4·07 3·26 4·82 1·17 1·55 1·08 3·46 K2O 5·38 5·44 3·48 4·86 2·25 4·54 6·46 1·43 1·63 1·46 3·20 2·56 P2O5 0·06 0·02 0·09 0·03 0·28 0·06 0·03 0·70 0·66 0·52 0·67 0·37 LOI 0·97 0·76 1·32 1·31 2·86 1·73 0·81 3·51 5·87 5·17 4·26 4·35 Total 99·13 99·22 99·22 99·55 99·49 99·66 99·28 99·54 100·33 100·14 99·83 99·63 Sc (ppm) 3·02 2·02 3·23 2·32 10·22 3·43 1·48 23·84 42·58 36·93 32·30 23·24 V 20·00 b.d. 23·00 b.d. 45·00 11·00 11·00 169·0 145·0 118·0 134·0 103·0 Cr 6·55 7·46 7·35 6·42 8·58 6·76 6·52 104·1 634·8 744·3 872·8 440·6 Co 35·0 43·0 27·0 39·0 40·0 46·0 49·0 35·0 49·0 39·0 59·0 41·0 Ni 3·98 3·45 4·39 2·92 6·38 4·49 2·69 52·00 125·60 121·75 162·80 103·60 Cu 7·45 6·54 8·58 5·62 12·52 8·50 5·37 23·92 27·16 14·33 12·86 40·20 Zn 31·0 17·0 51·0 21·0 94·0 37·0 29·0 208·0 320·0 265·0 284·0 117·0 Ga 21·0 18·0 28·0 17·0 24·0 19·0 19·0 38·0 50·0 26·0 30·0 20·0 Rb 212 213 210 202 140 186 232 110 65 167 471 147 Sr 211 185 124 182 264 236 203 284 105 177 185 320 Y 17·64 9·03 18·40 11·84 50·12 28·76 7·02 98·74 111·00 93·68 151·80 95·24 Zr 183·6 62·7 226·6 99·9 205·2 321·6 53·4 330·0 47·3 85·3 71·8 306·4 Nb 13·00 11·03 13·42 9·57 50·10 14·10 8·09 94·02 165·20 110·00 135·60 41·58 Mo 0·64 0·63 1·35 6·28 0·74 7·43 0·56 0·49 0·42 0·61 0·37 0·53 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. 16·00 18·00 21·00 18·00 b.d. Cs 2·88 2·19 9·95 3·24 2·45 2·09 2·19 4·59 6·16 8·95 19·74 2·51 Ba 638 615 357 562 247 622 757 179 270 192 216 452 La 47·6 17·5 7·1 28·4 71·8 95·2 8·5 103·7 88·5 55·8 439·8 49·7 Ce 81·9 30·6 11·5 46·7 132·6 169·8 16·5 205·4 190·2 115·8 895·6 84·5 Pr 8·86 3·56 1·49 5·04 15·26 18·88 1·90 22·26 21·44 13·50 88·74 10·50 Nd 31·54 11·86 6·31 18·22 56·80 67·56 6·51 85·84 78·64 55·13 290·20 43·88 Sm 5·63 2·32 1·87 3·30 11·72 10·98 1·60 18·18 19·16 15·00 48·64 11·84 Eu 0·91 0·68 0·51 0·80 1·25 1·26 0·73 1·25 1·16 1·20 2·09 1·31 Gd 4·52 2·16 2·42 2·71 10·17 8·09 1·38 19·02 20·60 14·90 35·04 12·34 Tb 0·61 0·29 0·45 0·38 1·54 1·06 0·24 2·85 3·26 2·53 5·01 2·20 Dy 3·66 2·02 3·18 2·21 9·31 5·83 1·40 18·00 20·60 16·30 28·24 15·48 Ho 0·71 0·37 0·61 0·47 1·78 1·10 0·27 3·47 3·87 2·99 5·18 3·22 Er 1·70 1·09 1·71 1·35 4·94 2·81 0·82 8·67 10·14 8·46 13·20 9·89 Tm 0·25 0·16 0·25 0·16 0·68 0·43 0·14 1·05 1·26 1·18 1·78 1·58 Yb 1·65 1·12 1·58 1·28 4·41 2·90 0·99 6·25 7·64 7·28 10·33 11·74 Lu 0·24 0·15 0·26 0·19 0·59 0·41 0·13 0·80 0·98 0·96 1·40 1·55 Hf 4·94 2·13 5·97 3·06 5·47 8·11 1·92 8·37 1·23 2·14 1·72 7·45 Ta 1·32 1·04 0·60 0·57 3·22 1·64 1·03 2·67 5·02 3·79 5·43 2·14 Pb 21·74 17·56 10·56 17·60 8·58 15·62 17·30 7·10 7·14 5·06 17·44 4·90 Th 14·06 5·58 7·04 12·76 10·86 17·38 3·04 17·00 20·06 15·10 72·40 2·60 U 5·40 4·71 5·90 7·50 5·10 4·38 2·07 2·47 1·97 4·01 4·04 4·36 Rock type: . Hbl-gabbro with veins . Hornblende gabbro . . . . . . . . . . dyke . . . . Core- . 10·10– . 11·00– . 12·50– . 28·00– . 14·70– . 15·60– . 18·70– . 18·80– . 19·50– . 19·60– . 20·40– . 21·10– . metre: . 10·35 . 11·25 . 12·70 . 28·35 . 15·00 . 15·80 . 18·80 . 19·00 . 19·60 . 20·00 . 20·60 . 21·40 . Depth: . 14·25 . 15·15 . 16·6 . 32·2 . 18·85 . 19·7 . 22·75 . 22·9 . 23·55 . 23·8 . 24·5 . 25·25 . SiO2 (%) 52·83 51·83 53·00 52·75 51·98 52·72 53·05 53·21 51·61 52·70 51·33 51·23 TiO2 0·53 0·52 0·49 0·43 0·47 0·41 0·33 0·32 0·29 0·34 0·42 0·50 Al2O3 10·02 9·61 8·84 8·45 8·64 9·02 8·78 8·69 7·64 8·49 8·40 8·91 Fe2O3* 9·06 9·32 9·07 8·49 9·05 8·47 8·25 8·42 8·93 8·70 8·94 9·42 MnO 0·17 0·18 0·17 0·16 0·16 0·15 0·15 0·15 0·20 0·17 0·17 0·17 MgO 12·60 13·18 13·32 13·70 13·83 13·82 13·35 13·68 13·73 13·85 14·71 13·57 CaO 8·10 9·00 9·18 9·47 9·23 9·02 8·81 8·99 9·31 8·95 9·04 9·27 Na2O 1·79 1·79 1·62 1·37 1·43 1·57 1·51 1·64 0·75 1·52 1·25 1·50 K2O 2·93 2·42 2·21 2·35 2·22 2·50 2·68 2·37 0·85 2·12 2·05 2·09 P2O5 0·40 0·40 0·39 0·32 0·35 0·32 0·20 0·23 0·18 0·23 0·30 0·38 LOI 2·08 2·67 2·52 2·88 2·20 2·04 2·13 2·04 6·09 2·10 2·49 2·36 Total 100·51 100·92 100·81 100·37 99·56 100·04 99·24 99·74 99·58 99·17 99·10 99·40 Sc (ppm) 29·66 37·72 29·90 32·00 31·16 29·54 28·58 27·90 36·68 29·18 31·84 32·82 V 143·00 144·00 142·00 128·00 122·00 115·00 96·00 98·00 105·00 108·00 116·00 142·00 Cr 891·80 1174·80 914·40 1382·60 1286·20 1204·40 1165·20 1230·00 1509·00 1231·80 1325·00 1059·80 Co 58·00 53·00 64·00 59·00 62·00 59·00 67·00 60·00 65·00 65·00 60·00 69·00 Ni 179·20 236·40 184·60 246·20 221·00 206·60 211·80 220·00 249·40 232·00 246·60 200·60 Cu 49·98 51·94 63·64 69·80 45·04 48·26 50·70 51·56 28·14 56·26 58·18 43·92 Zn 110·00 119·00 112·00 91·00 84·00 77·00 72·00 74·00 137·00 89·00 94·00 88·00 Ga 22·00 17·00 21·00 24·00 12·00 15·00 15·00 8·00 23·00 16·00 9·00 9·00 Rb 169·00 131·60 120·40 135·40 89·00 107·00 97·00 121·40 65·46 116·00 90·00 127·60 Sr 245·00 251·00 244·00 244·00 248·00 311·00 261·00 268·00 128·00 228·00 212·00 277·00 Y 30·22 25·54 18·82 18·52 18·24 16·00 15·90 15·76 48·68 23·04 15·48 17·16 Zr 111·80 120·00 95·64 63·04 99·80 97·84 118·80 112·80 150·80 86·76 70·86 105·00 Nb 28·22 21·78 13·80 14·44 12·58 10·78 10·27 9·51 20·38 15·14 10·51 8·30 Mo 0·47 0·96 9·27 0·68 0·57 0·46 0·43 0·73 0·48 0·55 0·76 0·59 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·45 1·53 2·04 2·05 1·43 1·53 1·13 1·37 3·51 2·09 2·10 2·16 Ba 687·00 591·00 513·00 624·00 482·00 531·00 592·00 533·00 171·00 538·00 553·00 557·00 La 26·54 31·20 32·16 23·80 23·74 23·12 18·08 21·04 14·26 26·38 21·56 26·14 Ce 67·42 54·76 57·68 47·42 52·62 49·52 43·92 46·26 39·24 45·72 48·86 52·22 Pr 8·33 6·40 6·53 5·52 6·12 5·72 5·30 5·49 5·64 5·77 5·56 6·30 Nd 31·48 25·50 26·14 22·48 23·36 22·00 20·92 21·68 24·90 23·74 21·26 25·68 Sm 6·40 5·48 5·58 5·06 4·77 4·56 4·38 4·56 6·48 5·49 3·64 5·84 Eu 1·54 1·26 1·37 1·19 1·05 1·05 1·05 1·13 0·99 1·14 1·05 1·38 Gd 5·28 5·05 4·99 4·41 4·28 3·63 3·43 3·94 6·32 4·76 3·45 4·66 Tb 0·76 0·69 0·66 0·53 0·57 0·50 0·48 0·54 1·13 0·72 0·47 0·65 Dy 4·75 3·50 3·90 3·33 3·58 2·95 2·67 3·19 8·33 4·11 2·82 3·74 Ho 0·92 0·64 0·73 0·60 0·66 0·60 0·53 0·55 1·71 0·80 0·52 0·71 Er 2·81 1·57 2·00 1·60 1·81 1·47 1·47 1·57 5·25 2·25 1·49 1·82 Tm 0·49 0·25 0·29 0·20 0·24 0·20 0·21 0·25 0·96 0·37 0·22 0·27 Yb 4·10 1·66 2·10 1·73 1·93 1·57 1·57 1·69 7·54 2·86 1·74 1·72 Lu 0·62 0·23 0·31 0·24 0·28 0·25 0·24 0·23 1·12 0·42 0·23 0·23 Hf 2·67 2·18 2·67 1·71 2·71 2·48 2·83 3·10 3·70 2·55 1·92 2·98 Ta 1·70 1·41 1·48 1·78 1·96 1·48 1·46 1·14 1·30 1·43 1·07 1·30 Pb 5·97 4·51 5·82 5·77 6·33 6·07 6·29 7·11 3·85 6·71 4·86 5·89 Th 1·93 1·52 2·42 2·00 2·04 1·90 1·80 1·89 1·63 1·70 1·54 6·11 U 2·48 1·58 2·49 1·95 2·47 2·23 1·60 2·28 6·55 2·02 1·89 2·29 Rock type: . Hbl-gabbro with veins . Hornblende gabbro . . . . . . . . . . dyke . . . . Core- . 10·10– . 11·00– . 12·50– . 28·00– . 14·70– . 15·60– . 18·70– . 18·80– . 19·50– . 19·60– . 20·40– . 21·10– . metre: . 10·35 . 11·25 . 12·70 . 28·35 . 15·00 . 15·80 . 18·80 . 19·00 . 19·60 . 20·00 . 20·60 . 21·40 . Depth: . 14·25 . 15·15 . 16·6 . 32·2 . 18·85 . 19·7 . 22·75 . 22·9 . 23·55 . 23·8 . 24·5 . 25·25 . SiO2 (%) 52·83 51·83 53·00 52·75 51·98 52·72 53·05 53·21 51·61 52·70 51·33 51·23 TiO2 0·53 0·52 0·49 0·43 0·47 0·41 0·33 0·32 0·29 0·34 0·42 0·50 Al2O3 10·02 9·61 8·84 8·45 8·64 9·02 8·78 8·69 7·64 8·49 8·40 8·91 Fe2O3* 9·06 9·32 9·07 8·49 9·05 8·47 8·25 8·42 8·93 8·70 8·94 9·42 MnO 0·17 0·18 0·17 0·16 0·16 0·15 0·15 0·15 0·20 0·17 0·17 0·17 MgO 12·60 13·18 13·32 13·70 13·83 13·82 13·35 13·68 13·73 13·85 14·71 13·57 CaO 8·10 9·00 9·18 9·47 9·23 9·02 8·81 8·99 9·31 8·95 9·04 9·27 Na2O 1·79 1·79 1·62 1·37 1·43 1·57 1·51 1·64 0·75 1·52 1·25 1·50 K2O 2·93 2·42 2·21 2·35 2·22 2·50 2·68 2·37 0·85 2·12 2·05 2·09 P2O5 0·40 0·40 0·39 0·32 0·35 0·32 0·20 0·23 0·18 0·23 0·30 0·38 LOI 2·08 2·67 2·52 2·88 2·20 2·04 2·13 2·04 6·09 2·10 2·49 2·36 Total 100·51 100·92 100·81 100·37 99·56 100·04 99·24 99·74 99·58 99·17 99·10 99·40 Sc (ppm) 29·66 37·72 29·90 32·00 31·16 29·54 28·58 27·90 36·68 29·18 31·84 32·82 V 143·00 144·00 142·00 128·00 122·00 115·00 96·00 98·00 105·00 108·00 116·00 142·00 Cr 891·80 1174·80 914·40 1382·60 1286·20 1204·40 1165·20 1230·00 1509·00 1231·80 1325·00 1059·80 Co 58·00 53·00 64·00 59·00 62·00 59·00 67·00 60·00 65·00 65·00 60·00 69·00 Ni 179·20 236·40 184·60 246·20 221·00 206·60 211·80 220·00 249·40 232·00 246·60 200·60 Cu 49·98 51·94 63·64 69·80 45·04 48·26 50·70 51·56 28·14 56·26 58·18 43·92 Zn 110·00 119·00 112·00 91·00 84·00 77·00 72·00 74·00 137·00 89·00 94·00 88·00 Ga 22·00 17·00 21·00 24·00 12·00 15·00 15·00 8·00 23·00 16·00 9·00 9·00 Rb 169·00 131·60 120·40 135·40 89·00 107·00 97·00 121·40 65·46 116·00 90·00 127·60 Sr 245·00 251·00 244·00 244·00 248·00 311·00 261·00 268·00 128·00 228·00 212·00 277·00 Y 30·22 25·54 18·82 18·52 18·24 16·00 15·90 15·76 48·68 23·04 15·48 17·16 Zr 111·80 120·00 95·64 63·04 99·80 97·84 118·80 112·80 150·80 86·76 70·86 105·00 Nb 28·22 21·78 13·80 14·44 12·58 10·78 10·27 9·51 20·38 15·14 10·51 8·30 Mo 0·47 0·96 9·27 0·68 0·57 0·46 0·43 0·73 0·48 0·55 0·76 0·59 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·45 1·53 2·04 2·05 1·43 1·53 1·13 1·37 3·51 2·09 2·10 2·16 Ba 687·00 591·00 513·00 624·00 482·00 531·00 592·00 533·00 171·00 538·00 553·00 557·00 La 26·54 31·20 32·16 23·80 23·74 23·12 18·08 21·04 14·26 26·38 21·56 26·14 Ce 67·42 54·76 57·68 47·42 52·62 49·52 43·92 46·26 39·24 45·72 48·86 52·22 Pr 8·33 6·40 6·53 5·52 6·12 5·72 5·30 5·49 5·64 5·77 5·56 6·30 Nd 31·48 25·50 26·14 22·48 23·36 22·00 20·92 21·68 24·90 23·74 21·26 25·68 Sm 6·40 5·48 5·58 5·06 4·77 4·56 4·38 4·56 6·48 5·49 3·64 5·84 Eu 1·54 1·26 1·37 1·19 1·05 1·05 1·05 1·13 0·99 1·14 1·05 1·38 Gd 5·28 5·05 4·99 4·41 4·28 3·63 3·43 3·94 6·32 4·76 3·45 4·66 Tb 0·76 0·69 0·66 0·53 0·57 0·50 0·48 0·54 1·13 0·72 0·47 0·65 Dy 4·75 3·50 3·90 3·33 3·58 2·95 2·67 3·19 8·33 4·11 2·82 3·74 Ho 0·92 0·64 0·73 0·60 0·66 0·60 0·53 0·55 1·71 0·80 0·52 0·71 Er 2·81 1·57 2·00 1·60 1·81 1·47 1·47 1·57 5·25 2·25 1·49 1·82 Tm 0·49 0·25 0·29 0·20 0·24 0·20 0·21 0·25 0·96 0·37 0·22 0·27 Yb 4·10 1·66 2·10 1·73 1·93 1·57 1·57 1·69 7·54 2·86 1·74 1·72 Lu 0·62 0·23 0·31 0·24 0·28 0·25 0·24 0·23 1·12 0·42 0·23 0·23 Hf 2·67 2·18 2·67 1·71 2·71 2·48 2·83 3·10 3·70 2·55 1·92 2·98 Ta 1·70 1·41 1·48 1·78 1·96 1·48 1·46 1·14 1·30 1·43 1·07 1·30 Pb 5·97 4·51 5·82 5·77 6·33 6·07 6·29 7·11 3·85 6·71 4·86 5·89 Th 1·93 1·52 2·42 2·00 2·04 1·90 1·80 1·89 1·63 1·70 1·54 6·11 U 2·48 1·58 2·49 1·95 2·47 2·23 1·60 2·28 6·55 2·02 1·89 2·29 Rock type: . Hornblende gabbro . Core- . 22·20– . 24·70– . 25·45– . 26·10– . 26·55– . 29·00– . 31·80– . 34·65– . metre: . 22·40 . 25·00 . 25·70 . 26·30 . 26·80 . 29·30 . 32·00 . 35·00 . Depth: . 26·3 . 28·85 . 29·6 . 30·2 . 30·7 . 33·15 . 35·9 . 38·8 . SiO2 (%) 51·38 51·12 50·66 50·52 51·07 51·78 52·72 52·53 TiO2 0·50 0·46 0·46 0·44 0·41 0·50 0·47 0·43 Al2O3 8·71 8·59 8·83 8·84 8·75 8·76 9·25 9·35 Fe2O3* 9·49 9·31 9·27 9·27 9·10 9·07 8·64 8·78 MnO 0·17 0·17 0·16 0·16 0·16 0·16 0·16 0·16 MgO 13·82 13·95 14·10 14·45 14·23 13·56 13·68 14·01 CaO 9·26 9·36 9·07 9·11 8·98 8·86 8·65 8·85 Na2O 1·41 1·50 1·51 1·48 1·36 1·48 1·52 1·61 K2O 2·17 1·80 1·80 1·44 1·86 2·21 2·67 2·51 P2O5 0·40 0·37 0·34 0·35 0·34 0·35 0·31 0·34 LOI 2·23 2·44 2·75 2·80 2·65 2·24 2·06 2·20 Total 99·54 99·07 98·95 98·86 98·91 98·97 100·13 100·77 Sc (ppm) 34·08 33·04 30·34 30·92 29·08 30·44 29·80 30·02 V 145·00 136·00 134·00 127·00 112·00 130·00 127·00 122·00 Cr 1035·50 1122·60 1163·80 1206·20 1218·20 1296·60 1203·60 1229·40 Co 64·00 57·00 70·00 74·00 67·00 67·00 67·00 59·00 Ni 196·00 212·40 230·40 232·80 239·00 228·00 209·40 214·60 Cu 54·98 40·92 46·88 54·32 38·68 54·58 54·24 54·34 Zn 93·00 120·00 81·00 73·00 75·00 91·00 91·00 94·00 Ga 11·00 11·00 16·00 19·00 17·00 10·00 9·00 14·00 Rb 93·00 83·00 110·60 75·00 101·48 101·00 115·00 96·00 Sr 266·00 257·00 277·00 289·00 277·00 255·00 324·00 280·00 Y 16·45 15·76 15·02 16·12 16·24 19·16 18·06 21·50 Zr 89·00 91·04 84·68 57·90 72·80 84·48 78·72 81·86 Nb 7·81 7·38 7·47 7·87 8·01 14·32 11·80 11·70 Mo 0·46 0·49 0·75 3·45 0·85 0·43 2·24 0·62 Sn b.d. 16·00 b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·28 1·21 2·19 2·46 1·89 2·78 2·09 1·60 Ba 573·00 458·00 520·00 353·00 599·00 579·00 751·00 665·00 La 22·98 20·28 23·48 23·46 24·00 21·94 19·98 22·32 Ce 48·45 44·54 45·24 50·26 50·52 49·22 45·58 51·92 Pr 5·81 5·48 5·43 6·02 6·08 5·81 5·34 6·12 Nd 24·20 22·38 22·68 23·24 24·84 23·68 21·28 24·30 Sm 5·03 5·11 4·62 5·38 5·26 5·53 4·72 5·25 Eu 1·25 1·12 1·19 1·20 1·20 1·18 1·12 1·23 Gd 4·59 4·30 3·99 4·49 4·04 4·57 4·03 4·50 Tb 0·61 0·62 0·56 0·59 0·58 0·64 0·58 0·65 Dy 3·39 3·23 3·37 3·21 3·38 3·72 3·27 3·99 Ho 0·62 0·59 0·57 0·62 0·59 0·70 0·58 0·75 Er 1·65 1·47 1·60 1·39 1·59 1·97 1·80 2·15 Tm 0·22 0·21 0·20 0·20 0·23 0·31 0·26 0·34 Yb 1·54 1·59 1·45 1·56 1·46 2·19 1·97 2·20 Lu 0·23 0·20 0·21 0·18 0·21 0·31 0·28 0·29 Hf 2·36 2·28 2·30 1·82 2·28 2·40 2·27 2·46 Ta 1·16 1·17 0·98 0·76 0·63 1·71 1·13 1·64 Pb 4·69 4·09 5·10 4·51 5·79 5·70 6·04 7·03 Th 2·39 1·94 2·65 2·56 1·92 2·71 1·75 2·47 U 1·82 2·10 2·12 1·65 1·63 3·02 1·88 2·20 Rock type: . Hornblende gabbro . Core- . 22·20– . 24·70– . 25·45– . 26·10– . 26·55– . 29·00– . 31·80– . 34·65– . metre: . 22·40 . 25·00 . 25·70 . 26·30 . 26·80 . 29·30 . 32·00 . 35·00 . Depth: . 26·3 . 28·85 . 29·6 . 30·2 . 30·7 . 33·15 . 35·9 . 38·8 . SiO2 (%) 51·38 51·12 50·66 50·52 51·07 51·78 52·72 52·53 TiO2 0·50 0·46 0·46 0·44 0·41 0·50 0·47 0·43 Al2O3 8·71 8·59 8·83 8·84 8·75 8·76 9·25 9·35 Fe2O3* 9·49 9·31 9·27 9·27 9·10 9·07 8·64 8·78 MnO 0·17 0·17 0·16 0·16 0·16 0·16 0·16 0·16 MgO 13·82 13·95 14·10 14·45 14·23 13·56 13·68 14·01 CaO 9·26 9·36 9·07 9·11 8·98 8·86 8·65 8·85 Na2O 1·41 1·50 1·51 1·48 1·36 1·48 1·52 1·61 K2O 2·17 1·80 1·80 1·44 1·86 2·21 2·67 2·51 P2O5 0·40 0·37 0·34 0·35 0·34 0·35 0·31 0·34 LOI 2·23 2·44 2·75 2·80 2·65 2·24 2·06 2·20 Total 99·54 99·07 98·95 98·86 98·91 98·97 100·13 100·77 Sc (ppm) 34·08 33·04 30·34 30·92 29·08 30·44 29·80 30·02 V 145·00 136·00 134·00 127·00 112·00 130·00 127·00 122·00 Cr 1035·50 1122·60 1163·80 1206·20 1218·20 1296·60 1203·60 1229·40 Co 64·00 57·00 70·00 74·00 67·00 67·00 67·00 59·00 Ni 196·00 212·40 230·40 232·80 239·00 228·00 209·40 214·60 Cu 54·98 40·92 46·88 54·32 38·68 54·58 54·24 54·34 Zn 93·00 120·00 81·00 73·00 75·00 91·00 91·00 94·00 Ga 11·00 11·00 16·00 19·00 17·00 10·00 9·00 14·00 Rb 93·00 83·00 110·60 75·00 101·48 101·00 115·00 96·00 Sr 266·00 257·00 277·00 289·00 277·00 255·00 324·00 280·00 Y 16·45 15·76 15·02 16·12 16·24 19·16 18·06 21·50 Zr 89·00 91·04 84·68 57·90 72·80 84·48 78·72 81·86 Nb 7·81 7·38 7·47 7·87 8·01 14·32 11·80 11·70 Mo 0·46 0·49 0·75 3·45 0·85 0·43 2·24 0·62 Sn b.d. 16·00 b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·28 1·21 2·19 2·46 1·89 2·78 2·09 1·60 Ba 573·00 458·00 520·00 353·00 599·00 579·00 751·00 665·00 La 22·98 20·28 23·48 23·46 24·00 21·94 19·98 22·32 Ce 48·45 44·54 45·24 50·26 50·52 49·22 45·58 51·92 Pr 5·81 5·48 5·43 6·02 6·08 5·81 5·34 6·12 Nd 24·20 22·38 22·68 23·24 24·84 23·68 21·28 24·30 Sm 5·03 5·11 4·62 5·38 5·26 5·53 4·72 5·25 Eu 1·25 1·12 1·19 1·20 1·20 1·18 1·12 1·23 Gd 4·59 4·30 3·99 4·49 4·04 4·57 4·03 4·50 Tb 0·61 0·62 0·56 0·59 0·58 0·64 0·58 0·65 Dy 3·39 3·23 3·37 3·21 3·38 3·72 3·27 3·99 Ho 0·62 0·59 0·57 0·62 0·59 0·70 0·58 0·75 Er 1·65 1·47 1·60 1·39 1·59 1·97 1·80 2·15 Tm 0·22 0·21 0·20 0·20 0·23 0·31 0·26 0·34 Yb 1·54 1·59 1·45 1·56 1·46 2·19 1·97 2·20 Lu 0·23 0·20 0·21 0·18 0·21 0·31 0·28 0·29 Hf 2·36 2·28 2·30 1·82 2·28 2·40 2·27 2·46 Ta 1·16 1·17 0·98 0·76 0·63 1·71 1·13 1·64 Pb 4·69 4·09 5·10 4·51 5·79 5·70 6·04 7·03 Th 2·39 1·94 2·65 2·56 1·92 2·71 1·75 2·47 U 1·82 2·10 2·12 1·65 1·63 3·02 1·88 2·20 *All Fe as Fe2O3. Open in new tab Table 3: Geochemical analyses of different rock types in borehole BH1 on the farm Rooidraai Rock type: . Granite . Biotite-rich zone . Core- . 0·15– . 1·65– . 2·20– . 3·10– . 4·25– . 5·10– . 6·10– . 7·00– . 7·00– . 7·40– . 8·00– . 9·30– . metre: . 0·35 . 1·75 . 2·45 . 3·30 . 4·40 . 5·30 . 6·20 . 7·30A . 7·30B . 7·70 . 8·10 . 9·55 . Depth: . 4·25 . 5·7 . 6·3 . 7·2 . 8·3 . 9·20 . 10·15 . 11·15 . 11·15 . 11·55 . 12·05 . 13·4 . SiO2 (%) 72·12 74·25 70·81 72·97 63·32 70·29 73·26 47·69 44·96 51·55 50·73 53·42 TiO2 0·16 0·07 0·18 0·11 0·50 0·22 0·06 1·67 2·23 1·48 1·12 0·57 Al2O3 13·96 13·48 14·60 13·80 15·81 13·96 13·79 17·65 14·30 12·39 12·08 13·10 Fe2O3* 1·33 0·76 2·19 0·83 4·38 2·42 0·70 12·71 15·06 11·9 11·60 6·56 MnO 0·02 b.d. 0·04 0·02 0·10 0·04 0·01 0·15 0·24 0·23 0·22 0·16 MgO 0·30 0·17 0·90 0·24 1·50 0·59 0·18 6·30 11·14 9·91 11·82 7·95 CaO 0·93 0·59 0·75 1·25 3·08 1·74 0·72 2·91 3·07 3·98 3·05 7·13 Na2O 3·90 3·68 4·86 4·13 5·41 4·07 3·26 4·82 1·17 1·55 1·08 3·46 K2O 5·38 5·44 3·48 4·86 2·25 4·54 6·46 1·43 1·63 1·46 3·20 2·56 P2O5 0·06 0·02 0·09 0·03 0·28 0·06 0·03 0·70 0·66 0·52 0·67 0·37 LOI 0·97 0·76 1·32 1·31 2·86 1·73 0·81 3·51 5·87 5·17 4·26 4·35 Total 99·13 99·22 99·22 99·55 99·49 99·66 99·28 99·54 100·33 100·14 99·83 99·63 Sc (ppm) 3·02 2·02 3·23 2·32 10·22 3·43 1·48 23·84 42·58 36·93 32·30 23·24 V 20·00 b.d. 23·00 b.d. 45·00 11·00 11·00 169·0 145·0 118·0 134·0 103·0 Cr 6·55 7·46 7·35 6·42 8·58 6·76 6·52 104·1 634·8 744·3 872·8 440·6 Co 35·0 43·0 27·0 39·0 40·0 46·0 49·0 35·0 49·0 39·0 59·0 41·0 Ni 3·98 3·45 4·39 2·92 6·38 4·49 2·69 52·00 125·60 121·75 162·80 103·60 Cu 7·45 6·54 8·58 5·62 12·52 8·50 5·37 23·92 27·16 14·33 12·86 40·20 Zn 31·0 17·0 51·0 21·0 94·0 37·0 29·0 208·0 320·0 265·0 284·0 117·0 Ga 21·0 18·0 28·0 17·0 24·0 19·0 19·0 38·0 50·0 26·0 30·0 20·0 Rb 212 213 210 202 140 186 232 110 65 167 471 147 Sr 211 185 124 182 264 236 203 284 105 177 185 320 Y 17·64 9·03 18·40 11·84 50·12 28·76 7·02 98·74 111·00 93·68 151·80 95·24 Zr 183·6 62·7 226·6 99·9 205·2 321·6 53·4 330·0 47·3 85·3 71·8 306·4 Nb 13·00 11·03 13·42 9·57 50·10 14·10 8·09 94·02 165·20 110·00 135·60 41·58 Mo 0·64 0·63 1·35 6·28 0·74 7·43 0·56 0·49 0·42 0·61 0·37 0·53 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. 16·00 18·00 21·00 18·00 b.d. Cs 2·88 2·19 9·95 3·24 2·45 2·09 2·19 4·59 6·16 8·95 19·74 2·51 Ba 638 615 357 562 247 622 757 179 270 192 216 452 La 47·6 17·5 7·1 28·4 71·8 95·2 8·5 103·7 88·5 55·8 439·8 49·7 Ce 81·9 30·6 11·5 46·7 132·6 169·8 16·5 205·4 190·2 115·8 895·6 84·5 Pr 8·86 3·56 1·49 5·04 15·26 18·88 1·90 22·26 21·44 13·50 88·74 10·50 Nd 31·54 11·86 6·31 18·22 56·80 67·56 6·51 85·84 78·64 55·13 290·20 43·88 Sm 5·63 2·32 1·87 3·30 11·72 10·98 1·60 18·18 19·16 15·00 48·64 11·84 Eu 0·91 0·68 0·51 0·80 1·25 1·26 0·73 1·25 1·16 1·20 2·09 1·31 Gd 4·52 2·16 2·42 2·71 10·17 8·09 1·38 19·02 20·60 14·90 35·04 12·34 Tb 0·61 0·29 0·45 0·38 1·54 1·06 0·24 2·85 3·26 2·53 5·01 2·20 Dy 3·66 2·02 3·18 2·21 9·31 5·83 1·40 18·00 20·60 16·30 28·24 15·48 Ho 0·71 0·37 0·61 0·47 1·78 1·10 0·27 3·47 3·87 2·99 5·18 3·22 Er 1·70 1·09 1·71 1·35 4·94 2·81 0·82 8·67 10·14 8·46 13·20 9·89 Tm 0·25 0·16 0·25 0·16 0·68 0·43 0·14 1·05 1·26 1·18 1·78 1·58 Yb 1·65 1·12 1·58 1·28 4·41 2·90 0·99 6·25 7·64 7·28 10·33 11·74 Lu 0·24 0·15 0·26 0·19 0·59 0·41 0·13 0·80 0·98 0·96 1·40 1·55 Hf 4·94 2·13 5·97 3·06 5·47 8·11 1·92 8·37 1·23 2·14 1·72 7·45 Ta 1·32 1·04 0·60 0·57 3·22 1·64 1·03 2·67 5·02 3·79 5·43 2·14 Pb 21·74 17·56 10·56 17·60 8·58 15·62 17·30 7·10 7·14 5·06 17·44 4·90 Th 14·06 5·58 7·04 12·76 10·86 17·38 3·04 17·00 20·06 15·10 72·40 2·60 U 5·40 4·71 5·90 7·50 5·10 4·38 2·07 2·47 1·97 4·01 4·04 4·36 Rock type: . Granite . Biotite-rich zone . Core- . 0·15– . 1·65– . 2·20– . 3·10– . 4·25– . 5·10– . 6·10– . 7·00– . 7·00– . 7·40– . 8·00– . 9·30– . metre: . 0·35 . 1·75 . 2·45 . 3·30 . 4·40 . 5·30 . 6·20 . 7·30A . 7·30B . 7·70 . 8·10 . 9·55 . Depth: . 4·25 . 5·7 . 6·3 . 7·2 . 8·3 . 9·20 . 10·15 . 11·15 . 11·15 . 11·55 . 12·05 . 13·4 . SiO2 (%) 72·12 74·25 70·81 72·97 63·32 70·29 73·26 47·69 44·96 51·55 50·73 53·42 TiO2 0·16 0·07 0·18 0·11 0·50 0·22 0·06 1·67 2·23 1·48 1·12 0·57 Al2O3 13·96 13·48 14·60 13·80 15·81 13·96 13·79 17·65 14·30 12·39 12·08 13·10 Fe2O3* 1·33 0·76 2·19 0·83 4·38 2·42 0·70 12·71 15·06 11·9 11·60 6·56 MnO 0·02 b.d. 0·04 0·02 0·10 0·04 0·01 0·15 0·24 0·23 0·22 0·16 MgO 0·30 0·17 0·90 0·24 1·50 0·59 0·18 6·30 11·14 9·91 11·82 7·95 CaO 0·93 0·59 0·75 1·25 3·08 1·74 0·72 2·91 3·07 3·98 3·05 7·13 Na2O 3·90 3·68 4·86 4·13 5·41 4·07 3·26 4·82 1·17 1·55 1·08 3·46 K2O 5·38 5·44 3·48 4·86 2·25 4·54 6·46 1·43 1·63 1·46 3·20 2·56 P2O5 0·06 0·02 0·09 0·03 0·28 0·06 0·03 0·70 0·66 0·52 0·67 0·37 LOI 0·97 0·76 1·32 1·31 2·86 1·73 0·81 3·51 5·87 5·17 4·26 4·35 Total 99·13 99·22 99·22 99·55 99·49 99·66 99·28 99·54 100·33 100·14 99·83 99·63 Sc (ppm) 3·02 2·02 3·23 2·32 10·22 3·43 1·48 23·84 42·58 36·93 32·30 23·24 V 20·00 b.d. 23·00 b.d. 45·00 11·00 11·00 169·0 145·0 118·0 134·0 103·0 Cr 6·55 7·46 7·35 6·42 8·58 6·76 6·52 104·1 634·8 744·3 872·8 440·6 Co 35·0 43·0 27·0 39·0 40·0 46·0 49·0 35·0 49·0 39·0 59·0 41·0 Ni 3·98 3·45 4·39 2·92 6·38 4·49 2·69 52·00 125·60 121·75 162·80 103·60 Cu 7·45 6·54 8·58 5·62 12·52 8·50 5·37 23·92 27·16 14·33 12·86 40·20 Zn 31·0 17·0 51·0 21·0 94·0 37·0 29·0 208·0 320·0 265·0 284·0 117·0 Ga 21·0 18·0 28·0 17·0 24·0 19·0 19·0 38·0 50·0 26·0 30·0 20·0 Rb 212 213 210 202 140 186 232 110 65 167 471 147 Sr 211 185 124 182 264 236 203 284 105 177 185 320 Y 17·64 9·03 18·40 11·84 50·12 28·76 7·02 98·74 111·00 93·68 151·80 95·24 Zr 183·6 62·7 226·6 99·9 205·2 321·6 53·4 330·0 47·3 85·3 71·8 306·4 Nb 13·00 11·03 13·42 9·57 50·10 14·10 8·09 94·02 165·20 110·00 135·60 41·58 Mo 0·64 0·63 1·35 6·28 0·74 7·43 0·56 0·49 0·42 0·61 0·37 0·53 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. 16·00 18·00 21·00 18·00 b.d. Cs 2·88 2·19 9·95 3·24 2·45 2·09 2·19 4·59 6·16 8·95 19·74 2·51 Ba 638 615 357 562 247 622 757 179 270 192 216 452 La 47·6 17·5 7·1 28·4 71·8 95·2 8·5 103·7 88·5 55·8 439·8 49·7 Ce 81·9 30·6 11·5 46·7 132·6 169·8 16·5 205·4 190·2 115·8 895·6 84·5 Pr 8·86 3·56 1·49 5·04 15·26 18·88 1·90 22·26 21·44 13·50 88·74 10·50 Nd 31·54 11·86 6·31 18·22 56·80 67·56 6·51 85·84 78·64 55·13 290·20 43·88 Sm 5·63 2·32 1·87 3·30 11·72 10·98 1·60 18·18 19·16 15·00 48·64 11·84 Eu 0·91 0·68 0·51 0·80 1·25 1·26 0·73 1·25 1·16 1·20 2·09 1·31 Gd 4·52 2·16 2·42 2·71 10·17 8·09 1·38 19·02 20·60 14·90 35·04 12·34 Tb 0·61 0·29 0·45 0·38 1·54 1·06 0·24 2·85 3·26 2·53 5·01 2·20 Dy 3·66 2·02 3·18 2·21 9·31 5·83 1·40 18·00 20·60 16·30 28·24 15·48 Ho 0·71 0·37 0·61 0·47 1·78 1·10 0·27 3·47 3·87 2·99 5·18 3·22 Er 1·70 1·09 1·71 1·35 4·94 2·81 0·82 8·67 10·14 8·46 13·20 9·89 Tm 0·25 0·16 0·25 0·16 0·68 0·43 0·14 1·05 1·26 1·18 1·78 1·58 Yb 1·65 1·12 1·58 1·28 4·41 2·90 0·99 6·25 7·64 7·28 10·33 11·74 Lu 0·24 0·15 0·26 0·19 0·59 0·41 0·13 0·80 0·98 0·96 1·40 1·55 Hf 4·94 2·13 5·97 3·06 5·47 8·11 1·92 8·37 1·23 2·14 1·72 7·45 Ta 1·32 1·04 0·60 0·57 3·22 1·64 1·03 2·67 5·02 3·79 5·43 2·14 Pb 21·74 17·56 10·56 17·60 8·58 15·62 17·30 7·10 7·14 5·06 17·44 4·90 Th 14·06 5·58 7·04 12·76 10·86 17·38 3·04 17·00 20·06 15·10 72·40 2·60 U 5·40 4·71 5·90 7·50 5·10 4·38 2·07 2·47 1·97 4·01 4·04 4·36 Rock type: . Hbl-gabbro with veins . Hornblende gabbro . . . . . . . . . . dyke . . . . Core- . 10·10– . 11·00– . 12·50– . 28·00– . 14·70– . 15·60– . 18·70– . 18·80– . 19·50– . 19·60– . 20·40– . 21·10– . metre: . 10·35 . 11·25 . 12·70 . 28·35 . 15·00 . 15·80 . 18·80 . 19·00 . 19·60 . 20·00 . 20·60 . 21·40 . Depth: . 14·25 . 15·15 . 16·6 . 32·2 . 18·85 . 19·7 . 22·75 . 22·9 . 23·55 . 23·8 . 24·5 . 25·25 . SiO2 (%) 52·83 51·83 53·00 52·75 51·98 52·72 53·05 53·21 51·61 52·70 51·33 51·23 TiO2 0·53 0·52 0·49 0·43 0·47 0·41 0·33 0·32 0·29 0·34 0·42 0·50 Al2O3 10·02 9·61 8·84 8·45 8·64 9·02 8·78 8·69 7·64 8·49 8·40 8·91 Fe2O3* 9·06 9·32 9·07 8·49 9·05 8·47 8·25 8·42 8·93 8·70 8·94 9·42 MnO 0·17 0·18 0·17 0·16 0·16 0·15 0·15 0·15 0·20 0·17 0·17 0·17 MgO 12·60 13·18 13·32 13·70 13·83 13·82 13·35 13·68 13·73 13·85 14·71 13·57 CaO 8·10 9·00 9·18 9·47 9·23 9·02 8·81 8·99 9·31 8·95 9·04 9·27 Na2O 1·79 1·79 1·62 1·37 1·43 1·57 1·51 1·64 0·75 1·52 1·25 1·50 K2O 2·93 2·42 2·21 2·35 2·22 2·50 2·68 2·37 0·85 2·12 2·05 2·09 P2O5 0·40 0·40 0·39 0·32 0·35 0·32 0·20 0·23 0·18 0·23 0·30 0·38 LOI 2·08 2·67 2·52 2·88 2·20 2·04 2·13 2·04 6·09 2·10 2·49 2·36 Total 100·51 100·92 100·81 100·37 99·56 100·04 99·24 99·74 99·58 99·17 99·10 99·40 Sc (ppm) 29·66 37·72 29·90 32·00 31·16 29·54 28·58 27·90 36·68 29·18 31·84 32·82 V 143·00 144·00 142·00 128·00 122·00 115·00 96·00 98·00 105·00 108·00 116·00 142·00 Cr 891·80 1174·80 914·40 1382·60 1286·20 1204·40 1165·20 1230·00 1509·00 1231·80 1325·00 1059·80 Co 58·00 53·00 64·00 59·00 62·00 59·00 67·00 60·00 65·00 65·00 60·00 69·00 Ni 179·20 236·40 184·60 246·20 221·00 206·60 211·80 220·00 249·40 232·00 246·60 200·60 Cu 49·98 51·94 63·64 69·80 45·04 48·26 50·70 51·56 28·14 56·26 58·18 43·92 Zn 110·00 119·00 112·00 91·00 84·00 77·00 72·00 74·00 137·00 89·00 94·00 88·00 Ga 22·00 17·00 21·00 24·00 12·00 15·00 15·00 8·00 23·00 16·00 9·00 9·00 Rb 169·00 131·60 120·40 135·40 89·00 107·00 97·00 121·40 65·46 116·00 90·00 127·60 Sr 245·00 251·00 244·00 244·00 248·00 311·00 261·00 268·00 128·00 228·00 212·00 277·00 Y 30·22 25·54 18·82 18·52 18·24 16·00 15·90 15·76 48·68 23·04 15·48 17·16 Zr 111·80 120·00 95·64 63·04 99·80 97·84 118·80 112·80 150·80 86·76 70·86 105·00 Nb 28·22 21·78 13·80 14·44 12·58 10·78 10·27 9·51 20·38 15·14 10·51 8·30 Mo 0·47 0·96 9·27 0·68 0·57 0·46 0·43 0·73 0·48 0·55 0·76 0·59 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·45 1·53 2·04 2·05 1·43 1·53 1·13 1·37 3·51 2·09 2·10 2·16 Ba 687·00 591·00 513·00 624·00 482·00 531·00 592·00 533·00 171·00 538·00 553·00 557·00 La 26·54 31·20 32·16 23·80 23·74 23·12 18·08 21·04 14·26 26·38 21·56 26·14 Ce 67·42 54·76 57·68 47·42 52·62 49·52 43·92 46·26 39·24 45·72 48·86 52·22 Pr 8·33 6·40 6·53 5·52 6·12 5·72 5·30 5·49 5·64 5·77 5·56 6·30 Nd 31·48 25·50 26·14 22·48 23·36 22·00 20·92 21·68 24·90 23·74 21·26 25·68 Sm 6·40 5·48 5·58 5·06 4·77 4·56 4·38 4·56 6·48 5·49 3·64 5·84 Eu 1·54 1·26 1·37 1·19 1·05 1·05 1·05 1·13 0·99 1·14 1·05 1·38 Gd 5·28 5·05 4·99 4·41 4·28 3·63 3·43 3·94 6·32 4·76 3·45 4·66 Tb 0·76 0·69 0·66 0·53 0·57 0·50 0·48 0·54 1·13 0·72 0·47 0·65 Dy 4·75 3·50 3·90 3·33 3·58 2·95 2·67 3·19 8·33 4·11 2·82 3·74 Ho 0·92 0·64 0·73 0·60 0·66 0·60 0·53 0·55 1·71 0·80 0·52 0·71 Er 2·81 1·57 2·00 1·60 1·81 1·47 1·47 1·57 5·25 2·25 1·49 1·82 Tm 0·49 0·25 0·29 0·20 0·24 0·20 0·21 0·25 0·96 0·37 0·22 0·27 Yb 4·10 1·66 2·10 1·73 1·93 1·57 1·57 1·69 7·54 2·86 1·74 1·72 Lu 0·62 0·23 0·31 0·24 0·28 0·25 0·24 0·23 1·12 0·42 0·23 0·23 Hf 2·67 2·18 2·67 1·71 2·71 2·48 2·83 3·10 3·70 2·55 1·92 2·98 Ta 1·70 1·41 1·48 1·78 1·96 1·48 1·46 1·14 1·30 1·43 1·07 1·30 Pb 5·97 4·51 5·82 5·77 6·33 6·07 6·29 7·11 3·85 6·71 4·86 5·89 Th 1·93 1·52 2·42 2·00 2·04 1·90 1·80 1·89 1·63 1·70 1·54 6·11 U 2·48 1·58 2·49 1·95 2·47 2·23 1·60 2·28 6·55 2·02 1·89 2·29 Rock type: . Hbl-gabbro with veins . Hornblende gabbro . . . . . . . . . . dyke . . . . Core- . 10·10– . 11·00– . 12·50– . 28·00– . 14·70– . 15·60– . 18·70– . 18·80– . 19·50– . 19·60– . 20·40– . 21·10– . metre: . 10·35 . 11·25 . 12·70 . 28·35 . 15·00 . 15·80 . 18·80 . 19·00 . 19·60 . 20·00 . 20·60 . 21·40 . Depth: . 14·25 . 15·15 . 16·6 . 32·2 . 18·85 . 19·7 . 22·75 . 22·9 . 23·55 . 23·8 . 24·5 . 25·25 . SiO2 (%) 52·83 51·83 53·00 52·75 51·98 52·72 53·05 53·21 51·61 52·70 51·33 51·23 TiO2 0·53 0·52 0·49 0·43 0·47 0·41 0·33 0·32 0·29 0·34 0·42 0·50 Al2O3 10·02 9·61 8·84 8·45 8·64 9·02 8·78 8·69 7·64 8·49 8·40 8·91 Fe2O3* 9·06 9·32 9·07 8·49 9·05 8·47 8·25 8·42 8·93 8·70 8·94 9·42 MnO 0·17 0·18 0·17 0·16 0·16 0·15 0·15 0·15 0·20 0·17 0·17 0·17 MgO 12·60 13·18 13·32 13·70 13·83 13·82 13·35 13·68 13·73 13·85 14·71 13·57 CaO 8·10 9·00 9·18 9·47 9·23 9·02 8·81 8·99 9·31 8·95 9·04 9·27 Na2O 1·79 1·79 1·62 1·37 1·43 1·57 1·51 1·64 0·75 1·52 1·25 1·50 K2O 2·93 2·42 2·21 2·35 2·22 2·50 2·68 2·37 0·85 2·12 2·05 2·09 P2O5 0·40 0·40 0·39 0·32 0·35 0·32 0·20 0·23 0·18 0·23 0·30 0·38 LOI 2·08 2·67 2·52 2·88 2·20 2·04 2·13 2·04 6·09 2·10 2·49 2·36 Total 100·51 100·92 100·81 100·37 99·56 100·04 99·24 99·74 99·58 99·17 99·10 99·40 Sc (ppm) 29·66 37·72 29·90 32·00 31·16 29·54 28·58 27·90 36·68 29·18 31·84 32·82 V 143·00 144·00 142·00 128·00 122·00 115·00 96·00 98·00 105·00 108·00 116·00 142·00 Cr 891·80 1174·80 914·40 1382·60 1286·20 1204·40 1165·20 1230·00 1509·00 1231·80 1325·00 1059·80 Co 58·00 53·00 64·00 59·00 62·00 59·00 67·00 60·00 65·00 65·00 60·00 69·00 Ni 179·20 236·40 184·60 246·20 221·00 206·60 211·80 220·00 249·40 232·00 246·60 200·60 Cu 49·98 51·94 63·64 69·80 45·04 48·26 50·70 51·56 28·14 56·26 58·18 43·92 Zn 110·00 119·00 112·00 91·00 84·00 77·00 72·00 74·00 137·00 89·00 94·00 88·00 Ga 22·00 17·00 21·00 24·00 12·00 15·00 15·00 8·00 23·00 16·00 9·00 9·00 Rb 169·00 131·60 120·40 135·40 89·00 107·00 97·00 121·40 65·46 116·00 90·00 127·60 Sr 245·00 251·00 244·00 244·00 248·00 311·00 261·00 268·00 128·00 228·00 212·00 277·00 Y 30·22 25·54 18·82 18·52 18·24 16·00 15·90 15·76 48·68 23·04 15·48 17·16 Zr 111·80 120·00 95·64 63·04 99·80 97·84 118·80 112·80 150·80 86·76 70·86 105·00 Nb 28·22 21·78 13·80 14·44 12·58 10·78 10·27 9·51 20·38 15·14 10·51 8·30 Mo 0·47 0·96 9·27 0·68 0·57 0·46 0·43 0·73 0·48 0·55 0·76 0·59 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·45 1·53 2·04 2·05 1·43 1·53 1·13 1·37 3·51 2·09 2·10 2·16 Ba 687·00 591·00 513·00 624·00 482·00 531·00 592·00 533·00 171·00 538·00 553·00 557·00 La 26·54 31·20 32·16 23·80 23·74 23·12 18·08 21·04 14·26 26·38 21·56 26·14 Ce 67·42 54·76 57·68 47·42 52·62 49·52 43·92 46·26 39·24 45·72 48·86 52·22 Pr 8·33 6·40 6·53 5·52 6·12 5·72 5·30 5·49 5·64 5·77 5·56 6·30 Nd 31·48 25·50 26·14 22·48 23·36 22·00 20·92 21·68 24·90 23·74 21·26 25·68 Sm 6·40 5·48 5·58 5·06 4·77 4·56 4·38 4·56 6·48 5·49 3·64 5·84 Eu 1·54 1·26 1·37 1·19 1·05 1·05 1·05 1·13 0·99 1·14 1·05 1·38 Gd 5·28 5·05 4·99 4·41 4·28 3·63 3·43 3·94 6·32 4·76 3·45 4·66 Tb 0·76 0·69 0·66 0·53 0·57 0·50 0·48 0·54 1·13 0·72 0·47 0·65 Dy 4·75 3·50 3·90 3·33 3·58 2·95 2·67 3·19 8·33 4·11 2·82 3·74 Ho 0·92 0·64 0·73 0·60 0·66 0·60 0·53 0·55 1·71 0·80 0·52 0·71 Er 2·81 1·57 2·00 1·60 1·81 1·47 1·47 1·57 5·25 2·25 1·49 1·82 Tm 0·49 0·25 0·29 0·20 0·24 0·20 0·21 0·25 0·96 0·37 0·22 0·27 Yb 4·10 1·66 2·10 1·73 1·93 1·57 1·57 1·69 7·54 2·86 1·74 1·72 Lu 0·62 0·23 0·31 0·24 0·28 0·25 0·24 0·23 1·12 0·42 0·23 0·23 Hf 2·67 2·18 2·67 1·71 2·71 2·48 2·83 3·10 3·70 2·55 1·92 2·98 Ta 1·70 1·41 1·48 1·78 1·96 1·48 1·46 1·14 1·30 1·43 1·07 1·30 Pb 5·97 4·51 5·82 5·77 6·33 6·07 6·29 7·11 3·85 6·71 4·86 5·89 Th 1·93 1·52 2·42 2·00 2·04 1·90 1·80 1·89 1·63 1·70 1·54 6·11 U 2·48 1·58 2·49 1·95 2·47 2·23 1·60 2·28 6·55 2·02 1·89 2·29 Rock type: . Hornblende gabbro . Core- . 22·20– . 24·70– . 25·45– . 26·10– . 26·55– . 29·00– . 31·80– . 34·65– . metre: . 22·40 . 25·00 . 25·70 . 26·30 . 26·80 . 29·30 . 32·00 . 35·00 . Depth: . 26·3 . 28·85 . 29·6 . 30·2 . 30·7 . 33·15 . 35·9 . 38·8 . SiO2 (%) 51·38 51·12 50·66 50·52 51·07 51·78 52·72 52·53 TiO2 0·50 0·46 0·46 0·44 0·41 0·50 0·47 0·43 Al2O3 8·71 8·59 8·83 8·84 8·75 8·76 9·25 9·35 Fe2O3* 9·49 9·31 9·27 9·27 9·10 9·07 8·64 8·78 MnO 0·17 0·17 0·16 0·16 0·16 0·16 0·16 0·16 MgO 13·82 13·95 14·10 14·45 14·23 13·56 13·68 14·01 CaO 9·26 9·36 9·07 9·11 8·98 8·86 8·65 8·85 Na2O 1·41 1·50 1·51 1·48 1·36 1·48 1·52 1·61 K2O 2·17 1·80 1·80 1·44 1·86 2·21 2·67 2·51 P2O5 0·40 0·37 0·34 0·35 0·34 0·35 0·31 0·34 LOI 2·23 2·44 2·75 2·80 2·65 2·24 2·06 2·20 Total 99·54 99·07 98·95 98·86 98·91 98·97 100·13 100·77 Sc (ppm) 34·08 33·04 30·34 30·92 29·08 30·44 29·80 30·02 V 145·00 136·00 134·00 127·00 112·00 130·00 127·00 122·00 Cr 1035·50 1122·60 1163·80 1206·20 1218·20 1296·60 1203·60 1229·40 Co 64·00 57·00 70·00 74·00 67·00 67·00 67·00 59·00 Ni 196·00 212·40 230·40 232·80 239·00 228·00 209·40 214·60 Cu 54·98 40·92 46·88 54·32 38·68 54·58 54·24 54·34 Zn 93·00 120·00 81·00 73·00 75·00 91·00 91·00 94·00 Ga 11·00 11·00 16·00 19·00 17·00 10·00 9·00 14·00 Rb 93·00 83·00 110·60 75·00 101·48 101·00 115·00 96·00 Sr 266·00 257·00 277·00 289·00 277·00 255·00 324·00 280·00 Y 16·45 15·76 15·02 16·12 16·24 19·16 18·06 21·50 Zr 89·00 91·04 84·68 57·90 72·80 84·48 78·72 81·86 Nb 7·81 7·38 7·47 7·87 8·01 14·32 11·80 11·70 Mo 0·46 0·49 0·75 3·45 0·85 0·43 2·24 0·62 Sn b.d. 16·00 b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·28 1·21 2·19 2·46 1·89 2·78 2·09 1·60 Ba 573·00 458·00 520·00 353·00 599·00 579·00 751·00 665·00 La 22·98 20·28 23·48 23·46 24·00 21·94 19·98 22·32 Ce 48·45 44·54 45·24 50·26 50·52 49·22 45·58 51·92 Pr 5·81 5·48 5·43 6·02 6·08 5·81 5·34 6·12 Nd 24·20 22·38 22·68 23·24 24·84 23·68 21·28 24·30 Sm 5·03 5·11 4·62 5·38 5·26 5·53 4·72 5·25 Eu 1·25 1·12 1·19 1·20 1·20 1·18 1·12 1·23 Gd 4·59 4·30 3·99 4·49 4·04 4·57 4·03 4·50 Tb 0·61 0·62 0·56 0·59 0·58 0·64 0·58 0·65 Dy 3·39 3·23 3·37 3·21 3·38 3·72 3·27 3·99 Ho 0·62 0·59 0·57 0·62 0·59 0·70 0·58 0·75 Er 1·65 1·47 1·60 1·39 1·59 1·97 1·80 2·15 Tm 0·22 0·21 0·20 0·20 0·23 0·31 0·26 0·34 Yb 1·54 1·59 1·45 1·56 1·46 2·19 1·97 2·20 Lu 0·23 0·20 0·21 0·18 0·21 0·31 0·28 0·29 Hf 2·36 2·28 2·30 1·82 2·28 2·40 2·27 2·46 Ta 1·16 1·17 0·98 0·76 0·63 1·71 1·13 1·64 Pb 4·69 4·09 5·10 4·51 5·79 5·70 6·04 7·03 Th 2·39 1·94 2·65 2·56 1·92 2·71 1·75 2·47 U 1·82 2·10 2·12 1·65 1·63 3·02 1·88 2·20 Rock type: . Hornblende gabbro . Core- . 22·20– . 24·70– . 25·45– . 26·10– . 26·55– . 29·00– . 31·80– . 34·65– . metre: . 22·40 . 25·00 . 25·70 . 26·30 . 26·80 . 29·30 . 32·00 . 35·00 . Depth: . 26·3 . 28·85 . 29·6 . 30·2 . 30·7 . 33·15 . 35·9 . 38·8 . SiO2 (%) 51·38 51·12 50·66 50·52 51·07 51·78 52·72 52·53 TiO2 0·50 0·46 0·46 0·44 0·41 0·50 0·47 0·43 Al2O3 8·71 8·59 8·83 8·84 8·75 8·76 9·25 9·35 Fe2O3* 9·49 9·31 9·27 9·27 9·10 9·07 8·64 8·78 MnO 0·17 0·17 0·16 0·16 0·16 0·16 0·16 0·16 MgO 13·82 13·95 14·10 14·45 14·23 13·56 13·68 14·01 CaO 9·26 9·36 9·07 9·11 8·98 8·86 8·65 8·85 Na2O 1·41 1·50 1·51 1·48 1·36 1·48 1·52 1·61 K2O 2·17 1·80 1·80 1·44 1·86 2·21 2·67 2·51 P2O5 0·40 0·37 0·34 0·35 0·34 0·35 0·31 0·34 LOI 2·23 2·44 2·75 2·80 2·65 2·24 2·06 2·20 Total 99·54 99·07 98·95 98·86 98·91 98·97 100·13 100·77 Sc (ppm) 34·08 33·04 30·34 30·92 29·08 30·44 29·80 30·02 V 145·00 136·00 134·00 127·00 112·00 130·00 127·00 122·00 Cr 1035·50 1122·60 1163·80 1206·20 1218·20 1296·60 1203·60 1229·40 Co 64·00 57·00 70·00 74·00 67·00 67·00 67·00 59·00 Ni 196·00 212·40 230·40 232·80 239·00 228·00 209·40 214·60 Cu 54·98 40·92 46·88 54·32 38·68 54·58 54·24 54·34 Zn 93·00 120·00 81·00 73·00 75·00 91·00 91·00 94·00 Ga 11·00 11·00 16·00 19·00 17·00 10·00 9·00 14·00 Rb 93·00 83·00 110·60 75·00 101·48 101·00 115·00 96·00 Sr 266·00 257·00 277·00 289·00 277·00 255·00 324·00 280·00 Y 16·45 15·76 15·02 16·12 16·24 19·16 18·06 21·50 Zr 89·00 91·04 84·68 57·90 72·80 84·48 78·72 81·86 Nb 7·81 7·38 7·47 7·87 8·01 14·32 11·80 11·70 Mo 0·46 0·49 0·75 3·45 0·85 0·43 2·24 0·62 Sn b.d. 16·00 b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·28 1·21 2·19 2·46 1·89 2·78 2·09 1·60 Ba 573·00 458·00 520·00 353·00 599·00 579·00 751·00 665·00 La 22·98 20·28 23·48 23·46 24·00 21·94 19·98 22·32 Ce 48·45 44·54 45·24 50·26 50·52 49·22 45·58 51·92 Pr 5·81 5·48 5·43 6·02 6·08 5·81 5·34 6·12 Nd 24·20 22·38 22·68 23·24 24·84 23·68 21·28 24·30 Sm 5·03 5·11 4·62 5·38 5·26 5·53 4·72 5·25 Eu 1·25 1·12 1·19 1·20 1·20 1·18 1·12 1·23 Gd 4·59 4·30 3·99 4·49 4·04 4·57 4·03 4·50 Tb 0·61 0·62 0·56 0·59 0·58 0·64 0·58 0·65 Dy 3·39 3·23 3·37 3·21 3·38 3·72 3·27 3·99 Ho 0·62 0·59 0·57 0·62 0·59 0·70 0·58 0·75 Er 1·65 1·47 1·60 1·39 1·59 1·97 1·80 2·15 Tm 0·22 0·21 0·20 0·20 0·23 0·31 0·26 0·34 Yb 1·54 1·59 1·45 1·56 1·46 2·19 1·97 2·20 Lu 0·23 0·20 0·21 0·18 0·21 0·31 0·28 0·29 Hf 2·36 2·28 2·30 1·82 2·28 2·40 2·27 2·46 Ta 1·16 1·17 0·98 0·76 0·63 1·71 1·13 1·64 Pb 4·69 4·09 5·10 4·51 5·79 5·70 6·04 7·03 Th 2·39 1·94 2·65 2·56 1·92 2·71 1·75 2·47 U 1·82 2·10 2·12 1·65 1·63 3·02 1·88 2·20 *All Fe as Fe2O3. Open in new tab In a total alkalis–silica (TAS) diagram (Fig. 4a) the strong bimodal distribution of SiO2 contents reflects the polarity in felsic and mafic rock compositions in the drill core. Almost all of the felsic samples conform geochemically to metaluminous to peraluminous granitic compositions. According to the TAS classification as proposed by Middlemost (1994), specifically the SiO2 contents between 50·5 and 53·4 wt %, all of the mafic rocks would be classified as either gabbro or gabbroic diorite. For simplicity, they will be referred to in the following discussion as gabbro, with the understanding that a dioritic composition of the protolith cannot be excluded in all cases. In the TAS space all of the mafic samples, except for the biotite-rich zone, are subalkaline. In terms of K contents, both the mafic and felsic rocks could theoretically correspond to high-K calc-alkaline compositions. The measured K concentrations are unlikely to be representative of the protolith's composition, however, considering the evidence of hydrothermal overprinting and post-magmatic K-metasomatism. At least two stages of hydrothermal alteration can be distinguished: (1) a higher-temperature, more or less pervasive potassic alteration as evident from the distribution of microcline in the metagabbro and the formation of the biotite-rich zone near the contact between the metagabbro and the granite; (2) a lower temperature, fracture-controlled alteration along veins. Fig. 4. Open in new tabDownload slide Harker variation diagrams of Na2O + K2O (a), CaO + MgO (b), and FeO*/MgO (c) vs SiO2 for the various rock types in the investigated drill core. Distinction between calc-alkaline and tholeiitic compositions in (c) after Miyashiro (1974). The expected high concentration of K in what is referred to as the ‘biotite-rich zone’ is notably absent (Fig. 4a). This is due to almost complete chloritization of the biotite in that zone. Of interest is an enrichment in P and Ti in this previously biotite-rich zone (Fig. 5a and b), which reflects the relatively high proportion of apatite and titanite. Fig. 5. Open in new tabDownload slide TiO2 (a) and P2O5 vs SiO2 (b), and Cr vs Cu (c) for the various rock types in the investigated drill core. The mafic samples with the greatest extent of hydrothermal veining do not differ significantly in their major element concentrations from those samples that are not visibly veined (Fig. 4). Thus the lower temperature alteration did not pervasively alter the whole-rock composition. In contrast, the earlier K-metasomatism must have affected most, if not all, of the studied drill core and, consequently, any diagram relying on alkali element distribution for the characterization of the original melt composition cannot be reliable. A more reliable discriminant in this regard may be the relative proportion of Fe as shown in a FeO*/MgO vs SiO2 diagram (Miyashiro, 1974). All the hornblende metagabbro samples plot in a tight cluster in the field of calc-alkaline compositions (Fig. 4c), even those that contain hydrothermal veins. The granite samples show a wider spread but all of them, with the exception of two extreme outliers (highly altered granite samples), also plot in the calc-alkaline field. The Ni and Cr concentrations are higher by about two to three orders of magnitude in the relatively mafic rocks compared with the altered granite, with the biotite-rich zone samples plotting in an intermediate position (Fig. 5c). Elevated Cu contents in some samples are related to the hydrothermal alteration because the veined metagabbro samples contain an order of magnitude more Cu than the others. The hornblende metagabbro is enriched in Ni relative to normal mid-ocean ridge basalt (N-MORB) by a factor of 1·5, whereas the biotite-rich transition zone is depleted (Ni* = 0·75). All analysed mafic rocks (without visible veining) are depleted in Cu, with MORB-normalized Cu in the hornblende metagabbro being 0·73 and in the biotite-rich zone only 0·32. Considering the altered nature of the studied rocks, even the FeO–MgO–SiO2 distribution may no longer be representative of the original composition, and potentially more reliable information on the likely setting of the magmatic protoliths may be obtained from the relationships between the least mobile elements; that is, the REE and high field strength elements (HFSE). The relationships between Nb, Y, Ta, and Yb in all the granite samples with a narrow range in SiO2 that is within the range used by Pearce et al. (1984) for the construction of their discrimination diagrams conform to those of volcanic arc granites (Fig. 6). The least hydrothermally altered mafic compositions were normalized against mid-oceanic ridge basalt (N-MORB). The resulting diagram (Fig. 7) shows an overall enrichment in the less compatible elements, negative Nb, Zr and Hf, as well as a strong negative Ti anomalies and a positive Ho anomaly in all mafic samples. The trace element concentrations in the biotite-rich transition zone follow a similar pattern but this zone is markedly enriched in Th and the REE, which can be explained by the high concentration of allanite in this zone. The same zone is particularly strongly depleted in Zr and Hf, reflecting a low zircon content. Fig. 6. Open in new tabDownload slide Nb vs Y (a) and Ta vs Yb diagrams (b) for the granitic rocks; discrimination of genetic types according to tectonic setting from Pearce et al. (1984). For comparison, the composition of quartz–feldspar porphyries of the Dominion Group is also shown (from Marsh et al., 1989) as a grey shaded field in (a). Fig. 7. Open in new tabDownload slide N-MORB-normalized trace element patterns for mafic rocks in the investigated drill core. Elements are ordered according to decreasing compatibility. N-MORB composition from Hofmann (1988). All analysed samples are enriched in the light REE (LREE) relative to chondrite (Fig. 8) but also relative to N-MORB (not shown). The strongest enrichment is noted in the biotite-rich zone and some of the granite samples. The hornblende metagabbro samples display a very uniform REE distribution that is very similar to that of those metagabbro samples with hydrothermal vein-type alteration. Thus, the low-temperature hydrothermal alteration is regarded as not having significantly affected the overall REE patterns. Apart from the overall LREE enrichment, the gabbroic samples show no elemental anomalies. Notably, they lack any significant Eu anomaly. In contrast, the (chloritized) biotite-rich transition zone is characterized by a marked negative Eu anomaly, whereas most granite samples yielded a less pronounced negative Eu anomaly (Fig. 8). Fig. 8. Open in new tabDownload slide Chondrite-normalized rare earth element patterns in the principal rock types of the investigated drill core; normalization values from Sun & McDonough (1989). Rb–Sr and Sm–Nd isotopes Rb–Sr isotope analyses of five hornblende metagabbro and five metagranite samples (Table 4) were carried out at the Institute of Mineralogy, University of Münster. Whole-rock powders (c. 100 mg) were mixed with a 87Rb–84Sr spike in Teflon screw-top vials and dissolved in a HF–HNO3 (5:1) mixture on a hot plate overnight. After evaporation and drying, 6N HCl was added to the residue and mixed to homogenization. After a second evaporation to dryness, Rb and Sr were separated by standard ion-exchange procedures (AG 50W-X8 resin) on quartz glass columns using 2·5N and 6N HCl as eluents. Rb was loaded with H2O on Ta double filaments and Sr was loaded with TaF5 on W single filaments. The Rb and Sr isotope ratios were measured with a VG Sector 54 and a Finnigan Triton multicollector thermal ionization mass spectrometer, respectively. Correction for mass fractionation is based on a 86Sr/88Sr ratio of 0·1194. Rb ratios were corrected for mass fractionation using a factor deduced from multiple measurements of the Rb standard NBS 607. Total procedural blanks were less than 15 pg for Rb and less than 30 pg for Sr. Based on repeated measurements the 87Rb/86Sr ratios were assigned an uncertainty of 1% (2σ). In the course of this study, repeated runs of NBS standard 987 gave an average 87Sr/86Sr ratio of 0·710223 ± 0·000018 (2σ, n = 16). Table 4: Rb–Sr and Sm–Nd isotope data for hornblende metagabbro and metagranite samples Core length . Rock . Rb . Sr . 87Rb/86Sr . 87Sr/86Sr . Error (2σ) . Sm . Nd . 147Sm/144Nd . 143Nd/144Nd . Error (2σ) . TDM . ϵNd . . . (ppm) . (ppm) . . . . (ppm) . (ppm) . . . . (Ma) . (3062 Ma) . 22·20–22·40 Hbl-metagabbro 97·6 321 0·8842 0·745716 0·000013 5·03 24·20 0·12551 0·511292 0·000009 3106·2 1·85 18·70–18·80 Hbl-gabbro 98·1 293 0·9733 0·746338 0·000013 4·38 20·90 0·12967 0·511192 0·000009 3437·7 −1·78 15·60–15·80 Hbl-gabbro 102 322 0·9211 0·743865 0·000016 4·56 22·00 0·12929 0·511030 0·000008 3708 −4·80 31·80–32·00 Hbl-gabbro 110 342 0·9384 0·746976 0·000016 4·72 21·30 0·13381 0·511336 0·000010 3343·7 −0·59 19·60–20·00 Hbl-gabbro 85·8 257 0·9704 0·746410 0·000015 5·10–5·20 granite 187 231 2·3642 0·798783 0·000013 11·00 67·60 0·098243 0·510640 0·000011 3226·5 −0·13 3·10–3·30 granite 203 177 3·3529 0·837933 0·000018 3·30 18·20 0·109476 0·510859 0·000008 3256·2 −0·29 1·65–1·75 granite 203 179 3·3203 0·836689 0·000027 6·10–6·20 granite 235 203 3·4030 0·839558 0·000015 1·60 6·51 0·148403 0·511133 0·000009 4545 −10·38 0·15–0·35 granite 214 205 3·0502 0·826980 0·000013 5·63 31·50 0·10791 0·510753 0·000008 3358 −1·74 Core length . Rock . Rb . Sr . 87Rb/86Sr . 87Sr/86Sr . Error (2σ) . Sm . Nd . 147Sm/144Nd . 143Nd/144Nd . Error (2σ) . TDM . ϵNd . . . (ppm) . (ppm) . . . . (ppm) . (ppm) . . . . (Ma) . (3062 Ma) . 22·20–22·40 Hbl-metagabbro 97·6 321 0·8842 0·745716 0·000013 5·03 24·20 0·12551 0·511292 0·000009 3106·2 1·85 18·70–18·80 Hbl-gabbro 98·1 293 0·9733 0·746338 0·000013 4·38 20·90 0·12967 0·511192 0·000009 3437·7 −1·78 15·60–15·80 Hbl-gabbro 102 322 0·9211 0·743865 0·000016 4·56 22·00 0·12929 0·511030 0·000008 3708 −4·80 31·80–32·00 Hbl-gabbro 110 342 0·9384 0·746976 0·000016 4·72 21·30 0·13381 0·511336 0·000010 3343·7 −0·59 19·60–20·00 Hbl-gabbro 85·8 257 0·9704 0·746410 0·000015 5·10–5·20 granite 187 231 2·3642 0·798783 0·000013 11·00 67·60 0·098243 0·510640 0·000011 3226·5 −0·13 3·10–3·30 granite 203 177 3·3529 0·837933 0·000018 3·30 18·20 0·109476 0·510859 0·000008 3256·2 −0·29 1·65–1·75 granite 203 179 3·3203 0·836689 0·000027 6·10–6·20 granite 235 203 3·4030 0·839558 0·000015 1·60 6·51 0·148403 0·511133 0·000009 4545 −10·38 0·15–0·35 granite 214 205 3·0502 0·826980 0·000013 5·63 31·50 0·10791 0·510753 0·000008 3358 −1·74 Open in new tab Table 4: Rb–Sr and Sm–Nd isotope data for hornblende metagabbro and metagranite samples Core length . Rock . Rb . Sr . 87Rb/86Sr . 87Sr/86Sr . Error (2σ) . Sm . Nd . 147Sm/144Nd . 143Nd/144Nd . Error (2σ) . TDM . ϵNd . . . (ppm) . (ppm) . . . . (ppm) . (ppm) . . . . (Ma) . (3062 Ma) . 22·20–22·40 Hbl-metagabbro 97·6 321 0·8842 0·745716 0·000013 5·03 24·20 0·12551 0·511292 0·000009 3106·2 1·85 18·70–18·80 Hbl-gabbro 98·1 293 0·9733 0·746338 0·000013 4·38 20·90 0·12967 0·511192 0·000009 3437·7 −1·78 15·60–15·80 Hbl-gabbro 102 322 0·9211 0·743865 0·000016 4·56 22·00 0·12929 0·511030 0·000008 3708 −4·80 31·80–32·00 Hbl-gabbro 110 342 0·9384 0·746976 0·000016 4·72 21·30 0·13381 0·511336 0·000010 3343·7 −0·59 19·60–20·00 Hbl-gabbro 85·8 257 0·9704 0·746410 0·000015 5·10–5·20 granite 187 231 2·3642 0·798783 0·000013 11·00 67·60 0·098243 0·510640 0·000011 3226·5 −0·13 3·10–3·30 granite 203 177 3·3529 0·837933 0·000018 3·30 18·20 0·109476 0·510859 0·000008 3256·2 −0·29 1·65–1·75 granite 203 179 3·3203 0·836689 0·000027 6·10–6·20 granite 235 203 3·4030 0·839558 0·000015 1·60 6·51 0·148403 0·511133 0·000009 4545 −10·38 0·15–0·35 granite 214 205 3·0502 0·826980 0·000013 5·63 31·50 0·10791 0·510753 0·000008 3358 −1·74 Core length . Rock . Rb . Sr . 87Rb/86Sr . 87Sr/86Sr . Error (2σ) . Sm . Nd . 147Sm/144Nd . 143Nd/144Nd . Error (2σ) . TDM . ϵNd . . . (ppm) . (ppm) . . . . (ppm) . (ppm) . . . . (Ma) . (3062 Ma) . 22·20–22·40 Hbl-metagabbro 97·6 321 0·8842 0·745716 0·000013 5·03 24·20 0·12551 0·511292 0·000009 3106·2 1·85 18·70–18·80 Hbl-gabbro 98·1 293 0·9733 0·746338 0·000013 4·38 20·90 0·12967 0·511192 0·000009 3437·7 −1·78 15·60–15·80 Hbl-gabbro 102 322 0·9211 0·743865 0·000016 4·56 22·00 0·12929 0·511030 0·000008 3708 −4·80 31·80–32·00 Hbl-gabbro 110 342 0·9384 0·746976 0·000016 4·72 21·30 0·13381 0·511336 0·000010 3343·7 −0·59 19·60–20·00 Hbl-gabbro 85·8 257 0·9704 0·746410 0·000015 5·10–5·20 granite 187 231 2·3642 0·798783 0·000013 11·00 67·60 0·098243 0·510640 0·000011 3226·5 −0·13 3·10–3·30 granite 203 177 3·3529 0·837933 0·000018 3·30 18·20 0·109476 0·510859 0·000008 3256·2 −0·29 1·65–1·75 granite 203 179 3·3203 0·836689 0·000027 6·10–6·20 granite 235 203 3·4030 0·839558 0·000015 1·60 6·51 0·148403 0·511133 0·000009 4545 −10·38 0·15–0·35 granite 214 205 3·0502 0·826980 0·000013 5·63 31·50 0·10791 0·510753 0·000008 3358 −1·74 Open in new tab Both the Rb and Sr concentrations and respective isotope ratios are very similar for the hornblende metagabbro samples, whereas some spread and overall higher Rb concentrations, and thus more radiogenic Sr is noted for the granitic samples. Combining all 10 analyses yields an errorchron, the slope of which corresponds to an imprecise age of 2633 ± 50 Ma (Fig. 9) using a Rb decay constant of 1·42 × 10−11 (Steiger & Jäger, 1977). The calculated initial 87Sr/86Sr ratio is 0·7100 ± 0·0017. Using only the five granite data points would result in an errorchron ‘age’ of 2721 ± 150 Ma and an initial 87Sr/86Sr ratio of 0·7059 ± 0·0069 (MSWD = 226). Fig. 9. Open in new tabDownload slide Rb–Sr isochron plot for the granite and hornblende metagabbro. The Nd isotope ratios (Table 4) were measured on a VG Sector 7-collector mass spectrometer in multi-dynamic mode at the Department of Geological Sciences, University of Cape Town, following the standard chemical separation techniques described by le Roex & Lanyon (1998). A depleted mantle isotopic composition of 143Nd/144Nd = 0·5131 and 147Sm/144Nd = 0·2136 (Henry et al., 2000), and a 147Sm decay constant of 6·54 × 10−12 (Lugmair & Marti, 1978) were used for the calculation of the Nd model ages and the ϵNd values. The ϵNd values at the time of likely formation (3·07 Ga: see Fig. 10 and the subsequent section on geochronology) obtained for most of the granite and the hornblende metagabbro samples overlap and cluster between −1·8 and +1·9, but for one felsic and one mafic sample they are much lower, at −10·4 and −4·8, respectively. The subchondritic ϵNd values, corresponding to calculated TDM model ages that are higher than the age of magmatism (see below, Table 4), might be an indication of contamination by older crust. This argument is, however, not conclusive because the model age strongly depends on the preferred model for the evolution of the mantle composition in the Archaean—a controversial topic that is beyond the scope of this paper. Fig. 10. Open in new tabDownload slide ϵNd evolution diagram for the granite and hornblende metagabbro; for comparison the data field for the Dominion Group volcanic rocks (recalculated data from Marsh et al., 1989) is also shown. U–PB AND HF ISOTOPE DATA ON ZIRCON To constrain the age and to further characterize the source of the two principal magmatic rock types in the drill core, single zircon grains were separated from both the variably altered granite and the hornblende metagabbro by standard crushing and heavy mineral separation techniques. Polished zircon grain mounts were imaged by scanning electron microscope cathodoluminescence (CL) using a JEOL JSM-6400 electron microprobe at the Institute of Geosciences, University of Frankfurt. Selected zircon domains were analysed for their U–Pb isotopic composition by LA-ICPMS at the same institution, using a Thermo-Finnigan element II sector field ICPMS system coupled to a New Wave UP213 UV laser system. The results are listed in Table 5. Analytical details, data processing, and error calculations have been given by Gerdes & Zeh (2006, 2008). Concordia and upper intercept ages on concordia diagrams were calculated using the Isoplot/Ex 2.49 software (Ludwig, 2000). Table 5: U–Pb isotope data of single zircon domains Sample . U1 . Pb1 . Th/U1 . 206Pb/204Pb . 206Pb/238U2 . ±2σ . 207Pb/235U3 . ±2σ . 207Pb/206Pb2 . ±2σ . rho4 . Age (Ma) . no. . (ppm) . (ppm) . . . . (%) . . (%) . . (%) . . 206Pb/238U . ±2σ (%) . 207Pb/235U . ±2σ (%) . 207Pb/206Pb . ±2σ (%) . Con.5 (%) . BH1-metagranite A1 500 120 0·87 895·52 0·18910 3·2 4·888 3·6 0·18747 1·6 0·89 1116 33 1800 30 2720 27 41 A2 677 77 0·63 663·17 0·07754 10·5 1·721 11·4 0·16095 4·5 0·92 481 49 1016 76 2466 76 20 A3 311 147 0·78 2492·52 0·38819 9·5 11·548 9·8 0·21577 2·5 0·97 2114 173 2569 96 2949 40 72 A4 1088 186 0·46 541·40 0·12430 8·9 2·915 9·6 0·17011 3·4 0·93 755 64 1386 75 2559 57 30 A5 269 186 0·65 867·89 0·51473 8·6 13·515 9·4 0·19043 3·8 0·92 2677 192 2716 93 2746 62 97 A6 312 107 0·57 1294·63 0·23743 10·4 6·166 10·8 0·18835 2·9 0·96 1373 130 2000 99 2728 47 50 A7 303 93 0·84 4960·32 0·27662 5·6 7·148 6·5 0·18742 3·3 0·86 1574 79 2130 59 2720 54 58 A8 408 94 0·83 5374·14 0·20430 5·3 4·790 5·9 0·17006 2·6 0·90 1198 58 1783 51 2558 44 47 A9 426 67 0·85 6272·33 0·13968 5·5 2·776 6·2 0·14415 2·7 0·90 843 44 1349 47 2278 47 37 A10 341 116 0·92 906·62 0·22827 7·2 5·842 7·9 0·18561 3·1 0·92 1325 87 1953 71 2704 51 49 A11 198 114 0·60 1132·41 0·38801 3·3 11·750 3·7 0·21964 1·8 0·88 2114 59 2585 36 2978 29 71 A13 202 115 0·53 6420·44 0·48639 5·9 13·734 6·3 0·20479 2·4 0·93 2555 125 2732 62 2865 39 89 A14 67 47 0·49 21055·69 0·59171 4·0 19·133 4·3 0·23451 1·6 0·93 2996 96 3049 42 3083 25 97 A15 64 45 0·92 5691·13 0·60549 4·5 19·117 4·9 0·22898 1·9 0·92 3052 111 3048 48 3045 31 100 A16 163 92 1·03 30326·75 0·48851 4·2 14·399 5·3 0·21378 3·2 0·80 2564 89 2776 51 2934 51 87 A17 1316 80 0·53 407·67 0·04230 13·3 0·631 13·5 0·10811 2·1 0·99 267 35 496 54 1768 39 15 A18 183 47 1·58 5614·70 0·19251 7·1 5·283 7·3 0·19904 1·9 0·97 1135 74 1866 65 2818 30 40 BH1-hornblende metagabbro A1 331 66 0·71 5274·18 0·16279 6·4 4·624 6·6 0·20600 1·6 0·97 972 58 1754 56 2874 25 34 A2 335 81 1·82 1301·30 0·17600 3·9 4·430 4·3 0·18255 1·8 0·91 1045 38 1718 37 2676 30 39 A3 510 61 1·41 823·85 0·07949 2·4 1·650 3·4 0·15056 2·4 0·71 493 11 990 21 2352 40 21 A4 163 81 0·68 30538·03 0·44797 2·0 12·878 2·4 0·20849 1·4 0·81 2386 40 2671 23 2894 23 82 A5 358 82 1·11 17357·28 0·19269 2·4 5·836 2·6 0·21965 1·0 0·92 1136 25 1952 23 2978 16 38 A6 99 70 0·49 10869·06 0·60732 1·8 19·290 2·0 0·23036 1·0 0·87 3059 43 3056 20 3055 16 100 A7 92 68 0·64 7895·48 0·60824 1·3 19·335 1·6 0·23055 0·9 0·82 3063 32 3059 16 3056 15 100 A8 106 74 0·34 20377·72 0·60554 2·3 19·376 2·4 0·23207 0·8 0·94 3052 55 3061 23 3066 13 100 A9 128 89 0·36 34568·41 0·60691 2·4 19·397 2·6 0·23180 0·9 0·94 3058 60 3062 25 3065 14 100 A10 104 70 0·21 13214·50 0·60098 2·2 19·169 2·3 0·23133 0·8 0·94 3034 53 3050 23 3061 13 99 A11 116 84 0·35 2943·58 0·60482 2·3 19·320 2·6 0·23167 1·1 0·90 3049 56 3058 25 3064 18 100 A12 216 193 1·52 1579·22 0·61810 1·5 19·752 2·6 0·23177 2·1 0·59 3102 37 3079 25 3064 33 101 A13 302 45 0·66 4501·20 0·11856 3·3 2·994 3·5 0·18316 1·2 0·94 722 22 1406 27 2682 19 27 A14 407 53 0·51 1163·51 0·1071 3·4 2·741 4·1 0·1855 2·2 0·84 656 21 1340 31 2703 37 24 A15 374 93 0·87 1010·09 0·20086 1·9 5·0723 2·1 0·18315 1·0 0·89 1180 20 1831 18 2682 16 44 A16 220 118 0·71 1280·60 0·4381 2·9 13·393 3·2 0·2217 1·3 0·91 2342 57 2708 31 2993 21 78 A17 93 76 0·71 798·30 0·6289 2·3 20·089 2·7 0·2317 1·5 0·84 3145 57 3096 27 3064 24 103 A19 526 351 4·20 66·11 0·0187 19·9 0·53 20·0 0·2053 1·7 1·00 119 24 432 73 2869 28 4 A20 499 313 3·03 85·50 0·4148 3·5 10·94 5·1 0·1913 3·7 0·69 2237 67 2518 49 2754 60 81 A21 542 287 0·77 97·69 0·3263 2·7 7·416 4·3 0·1648 3·3 0·64 1820 44 2163 39 2506 56 73 A22 198 96 0·22 598·54 0·3938 4·9 12·0728 5·1 0·2223 1·6 0·95 2140 89 2610 49 2998 26 71 A23 325 237 0·90 155·36 0·5400 2·5 14·9350 2·9 0·2006 1·4 0·88 2783 57 2811 28 2831 23 98 A24 571 105 0·88 539·58 0·1434 2·6 3·8585 2·8 0·1951 0·9 0·94 864 21 1605 23 2786 15 31 A25 166 98 0·29 7837·33 0·5462 2·7 16·0264 2·8 0·2128 1·0 0·94 2810 61 2878 27 2927 15 96 A26 375 70 1·83 548·62 0·1385 2·1 3·2441 3·4 0·1699 2·6 0·64 836 17 1468 27 2556 44 33 A27 55 41 0·55 3577·39 0·6136 4·0 19·8287 4·2 0·2344 1·1 0·96 3085 99 3083 41 3082 18 100 A28 143 87 0·40 1039·91 0·5287 2·0 16·8013 2·1 0·2305 0·8 0·93 2736 44 2924 21 3055 12 90 A29 82 42 0·65 1804·93 0·3911 3·1 12·49 3·4 0·2315 1·4 0·91 2128 57 2642 33 3063 22 69 Zircon standard GJ1-4 178 82 1·09 12827·13 0·37576 3·6 11·364 4·2 0·21933 2·1 0·87 2056 64 2553 40 2976 34 69 Sample . U1 . Pb1 . Th/U1 . 206Pb/204Pb . 206Pb/238U2 . ±2σ . 207Pb/235U3 . ±2σ . 207Pb/206Pb2 . ±2σ . rho4 . Age (Ma) . no. . (ppm) . (ppm) . . . . (%) . . (%) . . (%) . . 206Pb/238U . ±2σ (%) . 207Pb/235U . ±2σ (%) . 207Pb/206Pb . ±2σ (%) . Con.5 (%) . BH1-metagranite A1 500 120 0·87 895·52 0·18910 3·2 4·888 3·6 0·18747 1·6 0·89 1116 33 1800 30 2720 27 41 A2 677 77 0·63 663·17 0·07754 10·5 1·721 11·4 0·16095 4·5 0·92 481 49 1016 76 2466 76 20 A3 311 147 0·78 2492·52 0·38819 9·5 11·548 9·8 0·21577 2·5 0·97 2114 173 2569 96 2949 40 72 A4 1088 186 0·46 541·40 0·12430 8·9 2·915 9·6 0·17011 3·4 0·93 755 64 1386 75 2559 57 30 A5 269 186 0·65 867·89 0·51473 8·6 13·515 9·4 0·19043 3·8 0·92 2677 192 2716 93 2746 62 97 A6 312 107 0·57 1294·63 0·23743 10·4 6·166 10·8 0·18835 2·9 0·96 1373 130 2000 99 2728 47 50 A7 303 93 0·84 4960·32 0·27662 5·6 7·148 6·5 0·18742 3·3 0·86 1574 79 2130 59 2720 54 58 A8 408 94 0·83 5374·14 0·20430 5·3 4·790 5·9 0·17006 2·6 0·90 1198 58 1783 51 2558 44 47 A9 426 67 0·85 6272·33 0·13968 5·5 2·776 6·2 0·14415 2·7 0·90 843 44 1349 47 2278 47 37 A10 341 116 0·92 906·62 0·22827 7·2 5·842 7·9 0·18561 3·1 0·92 1325 87 1953 71 2704 51 49 A11 198 114 0·60 1132·41 0·38801 3·3 11·750 3·7 0·21964 1·8 0·88 2114 59 2585 36 2978 29 71 A13 202 115 0·53 6420·44 0·48639 5·9 13·734 6·3 0·20479 2·4 0·93 2555 125 2732 62 2865 39 89 A14 67 47 0·49 21055·69 0·59171 4·0 19·133 4·3 0·23451 1·6 0·93 2996 96 3049 42 3083 25 97 A15 64 45 0·92 5691·13 0·60549 4·5 19·117 4·9 0·22898 1·9 0·92 3052 111 3048 48 3045 31 100 A16 163 92 1·03 30326·75 0·48851 4·2 14·399 5·3 0·21378 3·2 0·80 2564 89 2776 51 2934 51 87 A17 1316 80 0·53 407·67 0·04230 13·3 0·631 13·5 0·10811 2·1 0·99 267 35 496 54 1768 39 15 A18 183 47 1·58 5614·70 0·19251 7·1 5·283 7·3 0·19904 1·9 0·97 1135 74 1866 65 2818 30 40 BH1-hornblende metagabbro A1 331 66 0·71 5274·18 0·16279 6·4 4·624 6·6 0·20600 1·6 0·97 972 58 1754 56 2874 25 34 A2 335 81 1·82 1301·30 0·17600 3·9 4·430 4·3 0·18255 1·8 0·91 1045 38 1718 37 2676 30 39 A3 510 61 1·41 823·85 0·07949 2·4 1·650 3·4 0·15056 2·4 0·71 493 11 990 21 2352 40 21 A4 163 81 0·68 30538·03 0·44797 2·0 12·878 2·4 0·20849 1·4 0·81 2386 40 2671 23 2894 23 82 A5 358 82 1·11 17357·28 0·19269 2·4 5·836 2·6 0·21965 1·0 0·92 1136 25 1952 23 2978 16 38 A6 99 70 0·49 10869·06 0·60732 1·8 19·290 2·0 0·23036 1·0 0·87 3059 43 3056 20 3055 16 100 A7 92 68 0·64 7895·48 0·60824 1·3 19·335 1·6 0·23055 0·9 0·82 3063 32 3059 16 3056 15 100 A8 106 74 0·34 20377·72 0·60554 2·3 19·376 2·4 0·23207 0·8 0·94 3052 55 3061 23 3066 13 100 A9 128 89 0·36 34568·41 0·60691 2·4 19·397 2·6 0·23180 0·9 0·94 3058 60 3062 25 3065 14 100 A10 104 70 0·21 13214·50 0·60098 2·2 19·169 2·3 0·23133 0·8 0·94 3034 53 3050 23 3061 13 99 A11 116 84 0·35 2943·58 0·60482 2·3 19·320 2·6 0·23167 1·1 0·90 3049 56 3058 25 3064 18 100 A12 216 193 1·52 1579·22 0·61810 1·5 19·752 2·6 0·23177 2·1 0·59 3102 37 3079 25 3064 33 101 A13 302 45 0·66 4501·20 0·11856 3·3 2·994 3·5 0·18316 1·2 0·94 722 22 1406 27 2682 19 27 A14 407 53 0·51 1163·51 0·1071 3·4 2·741 4·1 0·1855 2·2 0·84 656 21 1340 31 2703 37 24 A15 374 93 0·87 1010·09 0·20086 1·9 5·0723 2·1 0·18315 1·0 0·89 1180 20 1831 18 2682 16 44 A16 220 118 0·71 1280·60 0·4381 2·9 13·393 3·2 0·2217 1·3 0·91 2342 57 2708 31 2993 21 78 A17 93 76 0·71 798·30 0·6289 2·3 20·089 2·7 0·2317 1·5 0·84 3145 57 3096 27 3064 24 103 A19 526 351 4·20 66·11 0·0187 19·9 0·53 20·0 0·2053 1·7 1·00 119 24 432 73 2869 28 4 A20 499 313 3·03 85·50 0·4148 3·5 10·94 5·1 0·1913 3·7 0·69 2237 67 2518 49 2754 60 81 A21 542 287 0·77 97·69 0·3263 2·7 7·416 4·3 0·1648 3·3 0·64 1820 44 2163 39 2506 56 73 A22 198 96 0·22 598·54 0·3938 4·9 12·0728 5·1 0·2223 1·6 0·95 2140 89 2610 49 2998 26 71 A23 325 237 0·90 155·36 0·5400 2·5 14·9350 2·9 0·2006 1·4 0·88 2783 57 2811 28 2831 23 98 A24 571 105 0·88 539·58 0·1434 2·6 3·8585 2·8 0·1951 0·9 0·94 864 21 1605 23 2786 15 31 A25 166 98 0·29 7837·33 0·5462 2·7 16·0264 2·8 0·2128 1·0 0·94 2810 61 2878 27 2927 15 96 A26 375 70 1·83 548·62 0·1385 2·1 3·2441 3·4 0·1699 2·6 0·64 836 17 1468 27 2556 44 33 A27 55 41 0·55 3577·39 0·6136 4·0 19·8287 4·2 0·2344 1·1 0·96 3085 99 3083 41 3082 18 100 A28 143 87 0·40 1039·91 0·5287 2·0 16·8013 2·1 0·2305 0·8 0·93 2736 44 2924 21 3055 12 90 A29 82 42 0·65 1804·93 0·3911 3·1 12·49 3·4 0·2315 1·4 0·91 2128 57 2642 33 3063 22 69 Zircon standard GJ1-4 178 82 1·09 12827·13 0·37576 3·6 11·364 4·2 0·21933 2·1 0·87 2056 64 2553 40 2976 34 69 1Calculated relative to GJ-1 reference zircon. 2Corrected for background, within-run Pb/U fractionation and common Pb using Stacey & Kramers (1975) model Pb composition and subsequently normalized to GJ-1 values. 3Calculated using 207Pb/206Pb/(238U/206Pb × 1/137·88). 4Rho is the error correlation; that is, error(206Pb/238U)/error(207Pb/235U). 5Degree of concordance. Open in new tab Table 5: U–Pb isotope data of single zircon domains Sample . U1 . Pb1 . Th/U1 . 206Pb/204Pb . 206Pb/238U2 . ±2σ . 207Pb/235U3 . ±2σ . 207Pb/206Pb2 . ±2σ . rho4 . Age (Ma) . no. . (ppm) . (ppm) . . . . (%) . . (%) . . (%) . . 206Pb/238U . ±2σ (%) . 207Pb/235U . ±2σ (%) . 207Pb/206Pb . ±2σ (%) . Con.5 (%) . BH1-metagranite A1 500 120 0·87 895·52 0·18910 3·2 4·888 3·6 0·18747 1·6 0·89 1116 33 1800 30 2720 27 41 A2 677 77 0·63 663·17 0·07754 10·5 1·721 11·4 0·16095 4·5 0·92 481 49 1016 76 2466 76 20 A3 311 147 0·78 2492·52 0·38819 9·5 11·548 9·8 0·21577 2·5 0·97 2114 173 2569 96 2949 40 72 A4 1088 186 0·46 541·40 0·12430 8·9 2·915 9·6 0·17011 3·4 0·93 755 64 1386 75 2559 57 30 A5 269 186 0·65 867·89 0·51473 8·6 13·515 9·4 0·19043 3·8 0·92 2677 192 2716 93 2746 62 97 A6 312 107 0·57 1294·63 0·23743 10·4 6·166 10·8 0·18835 2·9 0·96 1373 130 2000 99 2728 47 50 A7 303 93 0·84 4960·32 0·27662 5·6 7·148 6·5 0·18742 3·3 0·86 1574 79 2130 59 2720 54 58 A8 408 94 0·83 5374·14 0·20430 5·3 4·790 5·9 0·17006 2·6 0·90 1198 58 1783 51 2558 44 47 A9 426 67 0·85 6272·33 0·13968 5·5 2·776 6·2 0·14415 2·7 0·90 843 44 1349 47 2278 47 37 A10 341 116 0·92 906·62 0·22827 7·2 5·842 7·9 0·18561 3·1 0·92 1325 87 1953 71 2704 51 49 A11 198 114 0·60 1132·41 0·38801 3·3 11·750 3·7 0·21964 1·8 0·88 2114 59 2585 36 2978 29 71 A13 202 115 0·53 6420·44 0·48639 5·9 13·734 6·3 0·20479 2·4 0·93 2555 125 2732 62 2865 39 89 A14 67 47 0·49 21055·69 0·59171 4·0 19·133 4·3 0·23451 1·6 0·93 2996 96 3049 42 3083 25 97 A15 64 45 0·92 5691·13 0·60549 4·5 19·117 4·9 0·22898 1·9 0·92 3052 111 3048 48 3045 31 100 A16 163 92 1·03 30326·75 0·48851 4·2 14·399 5·3 0·21378 3·2 0·80 2564 89 2776 51 2934 51 87 A17 1316 80 0·53 407·67 0·04230 13·3 0·631 13·5 0·10811 2·1 0·99 267 35 496 54 1768 39 15 A18 183 47 1·58 5614·70 0·19251 7·1 5·283 7·3 0·19904 1·9 0·97 1135 74 1866 65 2818 30 40 BH1-hornblende metagabbro A1 331 66 0·71 5274·18 0·16279 6·4 4·624 6·6 0·20600 1·6 0·97 972 58 1754 56 2874 25 34 A2 335 81 1·82 1301·30 0·17600 3·9 4·430 4·3 0·18255 1·8 0·91 1045 38 1718 37 2676 30 39 A3 510 61 1·41 823·85 0·07949 2·4 1·650 3·4 0·15056 2·4 0·71 493 11 990 21 2352 40 21 A4 163 81 0·68 30538·03 0·44797 2·0 12·878 2·4 0·20849 1·4 0·81 2386 40 2671 23 2894 23 82 A5 358 82 1·11 17357·28 0·19269 2·4 5·836 2·6 0·21965 1·0 0·92 1136 25 1952 23 2978 16 38 A6 99 70 0·49 10869·06 0·60732 1·8 19·290 2·0 0·23036 1·0 0·87 3059 43 3056 20 3055 16 100 A7 92 68 0·64 7895·48 0·60824 1·3 19·335 1·6 0·23055 0·9 0·82 3063 32 3059 16 3056 15 100 A8 106 74 0·34 20377·72 0·60554 2·3 19·376 2·4 0·23207 0·8 0·94 3052 55 3061 23 3066 13 100 A9 128 89 0·36 34568·41 0·60691 2·4 19·397 2·6 0·23180 0·9 0·94 3058 60 3062 25 3065 14 100 A10 104 70 0·21 13214·50 0·60098 2·2 19·169 2·3 0·23133 0·8 0·94 3034 53 3050 23 3061 13 99 A11 116 84 0·35 2943·58 0·60482 2·3 19·320 2·6 0·23167 1·1 0·90 3049 56 3058 25 3064 18 100 A12 216 193 1·52 1579·22 0·61810 1·5 19·752 2·6 0·23177 2·1 0·59 3102 37 3079 25 3064 33 101 A13 302 45 0·66 4501·20 0·11856 3·3 2·994 3·5 0·18316 1·2 0·94 722 22 1406 27 2682 19 27 A14 407 53 0·51 1163·51 0·1071 3·4 2·741 4·1 0·1855 2·2 0·84 656 21 1340 31 2703 37 24 A15 374 93 0·87 1010·09 0·20086 1·9 5·0723 2·1 0·18315 1·0 0·89 1180 20 1831 18 2682 16 44 A16 220 118 0·71 1280·60 0·4381 2·9 13·393 3·2 0·2217 1·3 0·91 2342 57 2708 31 2993 21 78 A17 93 76 0·71 798·30 0·6289 2·3 20·089 2·7 0·2317 1·5 0·84 3145 57 3096 27 3064 24 103 A19 526 351 4·20 66·11 0·0187 19·9 0·53 20·0 0·2053 1·7 1·00 119 24 432 73 2869 28 4 A20 499 313 3·03 85·50 0·4148 3·5 10·94 5·1 0·1913 3·7 0·69 2237 67 2518 49 2754 60 81 A21 542 287 0·77 97·69 0·3263 2·7 7·416 4·3 0·1648 3·3 0·64 1820 44 2163 39 2506 56 73 A22 198 96 0·22 598·54 0·3938 4·9 12·0728 5·1 0·2223 1·6 0·95 2140 89 2610 49 2998 26 71 A23 325 237 0·90 155·36 0·5400 2·5 14·9350 2·9 0·2006 1·4 0·88 2783 57 2811 28 2831 23 98 A24 571 105 0·88 539·58 0·1434 2·6 3·8585 2·8 0·1951 0·9 0·94 864 21 1605 23 2786 15 31 A25 166 98 0·29 7837·33 0·5462 2·7 16·0264 2·8 0·2128 1·0 0·94 2810 61 2878 27 2927 15 96 A26 375 70 1·83 548·62 0·1385 2·1 3·2441 3·4 0·1699 2·6 0·64 836 17 1468 27 2556 44 33 A27 55 41 0·55 3577·39 0·6136 4·0 19·8287 4·2 0·2344 1·1 0·96 3085 99 3083 41 3082 18 100 A28 143 87 0·40 1039·91 0·5287 2·0 16·8013 2·1 0·2305 0·8 0·93 2736 44 2924 21 3055 12 90 A29 82 42 0·65 1804·93 0·3911 3·1 12·49 3·4 0·2315 1·4 0·91 2128 57 2642 33 3063 22 69 Zircon standard GJ1-4 178 82 1·09 12827·13 0·37576 3·6 11·364 4·2 0·21933 2·1 0·87 2056 64 2553 40 2976 34 69 Sample . U1 . Pb1 . Th/U1 . 206Pb/204Pb . 206Pb/238U2 . ±2σ . 207Pb/235U3 . ±2σ . 207Pb/206Pb2 . ±2σ . rho4 . Age (Ma) . no. . (ppm) . (ppm) . . . . (%) . . (%) . . (%) . . 206Pb/238U . ±2σ (%) . 207Pb/235U . ±2σ (%) . 207Pb/206Pb . ±2σ (%) . Con.5 (%) . BH1-metagranite A1 500 120 0·87 895·52 0·18910 3·2 4·888 3·6 0·18747 1·6 0·89 1116 33 1800 30 2720 27 41 A2 677 77 0·63 663·17 0·07754 10·5 1·721 11·4 0·16095 4·5 0·92 481 49 1016 76 2466 76 20 A3 311 147 0·78 2492·52 0·38819 9·5 11·548 9·8 0·21577 2·5 0·97 2114 173 2569 96 2949 40 72 A4 1088 186 0·46 541·40 0·12430 8·9 2·915 9·6 0·17011 3·4 0·93 755 64 1386 75 2559 57 30 A5 269 186 0·65 867·89 0·51473 8·6 13·515 9·4 0·19043 3·8 0·92 2677 192 2716 93 2746 62 97 A6 312 107 0·57 1294·63 0·23743 10·4 6·166 10·8 0·18835 2·9 0·96 1373 130 2000 99 2728 47 50 A7 303 93 0·84 4960·32 0·27662 5·6 7·148 6·5 0·18742 3·3 0·86 1574 79 2130 59 2720 54 58 A8 408 94 0·83 5374·14 0·20430 5·3 4·790 5·9 0·17006 2·6 0·90 1198 58 1783 51 2558 44 47 A9 426 67 0·85 6272·33 0·13968 5·5 2·776 6·2 0·14415 2·7 0·90 843 44 1349 47 2278 47 37 A10 341 116 0·92 906·62 0·22827 7·2 5·842 7·9 0·18561 3·1 0·92 1325 87 1953 71 2704 51 49 A11 198 114 0·60 1132·41 0·38801 3·3 11·750 3·7 0·21964 1·8 0·88 2114 59 2585 36 2978 29 71 A13 202 115 0·53 6420·44 0·48639 5·9 13·734 6·3 0·20479 2·4 0·93 2555 125 2732 62 2865 39 89 A14 67 47 0·49 21055·69 0·59171 4·0 19·133 4·3 0·23451 1·6 0·93 2996 96 3049 42 3083 25 97 A15 64 45 0·92 5691·13 0·60549 4·5 19·117 4·9 0·22898 1·9 0·92 3052 111 3048 48 3045 31 100 A16 163 92 1·03 30326·75 0·48851 4·2 14·399 5·3 0·21378 3·2 0·80 2564 89 2776 51 2934 51 87 A17 1316 80 0·53 407·67 0·04230 13·3 0·631 13·5 0·10811 2·1 0·99 267 35 496 54 1768 39 15 A18 183 47 1·58 5614·70 0·19251 7·1 5·283 7·3 0·19904 1·9 0·97 1135 74 1866 65 2818 30 40 BH1-hornblende metagabbro A1 331 66 0·71 5274·18 0·16279 6·4 4·624 6·6 0·20600 1·6 0·97 972 58 1754 56 2874 25 34 A2 335 81 1·82 1301·30 0·17600 3·9 4·430 4·3 0·18255 1·8 0·91 1045 38 1718 37 2676 30 39 A3 510 61 1·41 823·85 0·07949 2·4 1·650 3·4 0·15056 2·4 0·71 493 11 990 21 2352 40 21 A4 163 81 0·68 30538·03 0·44797 2·0 12·878 2·4 0·20849 1·4 0·81 2386 40 2671 23 2894 23 82 A5 358 82 1·11 17357·28 0·19269 2·4 5·836 2·6 0·21965 1·0 0·92 1136 25 1952 23 2978 16 38 A6 99 70 0·49 10869·06 0·60732 1·8 19·290 2·0 0·23036 1·0 0·87 3059 43 3056 20 3055 16 100 A7 92 68 0·64 7895·48 0·60824 1·3 19·335 1·6 0·23055 0·9 0·82 3063 32 3059 16 3056 15 100 A8 106 74 0·34 20377·72 0·60554 2·3 19·376 2·4 0·23207 0·8 0·94 3052 55 3061 23 3066 13 100 A9 128 89 0·36 34568·41 0·60691 2·4 19·397 2·6 0·23180 0·9 0·94 3058 60 3062 25 3065 14 100 A10 104 70 0·21 13214·50 0·60098 2·2 19·169 2·3 0·23133 0·8 0·94 3034 53 3050 23 3061 13 99 A11 116 84 0·35 2943·58 0·60482 2·3 19·320 2·6 0·23167 1·1 0·90 3049 56 3058 25 3064 18 100 A12 216 193 1·52 1579·22 0·61810 1·5 19·752 2·6 0·23177 2·1 0·59 3102 37 3079 25 3064 33 101 A13 302 45 0·66 4501·20 0·11856 3·3 2·994 3·5 0·18316 1·2 0·94 722 22 1406 27 2682 19 27 A14 407 53 0·51 1163·51 0·1071 3·4 2·741 4·1 0·1855 2·2 0·84 656 21 1340 31 2703 37 24 A15 374 93 0·87 1010·09 0·20086 1·9 5·0723 2·1 0·18315 1·0 0·89 1180 20 1831 18 2682 16 44 A16 220 118 0·71 1280·60 0·4381 2·9 13·393 3·2 0·2217 1·3 0·91 2342 57 2708 31 2993 21 78 A17 93 76 0·71 798·30 0·6289 2·3 20·089 2·7 0·2317 1·5 0·84 3145 57 3096 27 3064 24 103 A19 526 351 4·20 66·11 0·0187 19·9 0·53 20·0 0·2053 1·7 1·00 119 24 432 73 2869 28 4 A20 499 313 3·03 85·50 0·4148 3·5 10·94 5·1 0·1913 3·7 0·69 2237 67 2518 49 2754 60 81 A21 542 287 0·77 97·69 0·3263 2·7 7·416 4·3 0·1648 3·3 0·64 1820 44 2163 39 2506 56 73 A22 198 96 0·22 598·54 0·3938 4·9 12·0728 5·1 0·2223 1·6 0·95 2140 89 2610 49 2998 26 71 A23 325 237 0·90 155·36 0·5400 2·5 14·9350 2·9 0·2006 1·4 0·88 2783 57 2811 28 2831 23 98 A24 571 105 0·88 539·58 0·1434 2·6 3·8585 2·8 0·1951 0·9 0·94 864 21 1605 23 2786 15 31 A25 166 98 0·29 7837·33 0·5462 2·7 16·0264 2·8 0·2128 1·0 0·94 2810 61 2878 27 2927 15 96 A26 375 70 1·83 548·62 0·1385 2·1 3·2441 3·4 0·1699 2·6 0·64 836 17 1468 27 2556 44 33 A27 55 41 0·55 3577·39 0·6136 4·0 19·8287 4·2 0·2344 1·1 0·96 3085 99 3083 41 3082 18 100 A28 143 87 0·40 1039·91 0·5287 2·0 16·8013 2·1 0·2305 0·8 0·93 2736 44 2924 21 3055 12 90 A29 82 42 0·65 1804·93 0·3911 3·1 12·49 3·4 0·2315 1·4 0·91 2128 57 2642 33 3063 22 69 Zircon standard GJ1-4 178 82 1·09 12827·13 0·37576 3·6 11·364 4·2 0·21933 2·1 0·87 2056 64 2553 40 2976 34 69 1Calculated relative to GJ-1 reference zircon. 2Corrected for background, within-run Pb/U fractionation and common Pb using Stacey & Kramers (1975) model Pb composition and subsequently normalized to GJ-1 values. 3Calculated using 207Pb/206Pb/(238U/206Pb × 1/137·88). 4Rho is the error correlation; that is, error(206Pb/238U)/error(207Pb/235U). 5Degree of concordance. Open in new tab Subsequently, the same zircon domains were analysed for their Lu, Hf, and Yb isotopic composition (Table 6) using the same laser system, with a 40 μm spot size, and a Thermo-Finnigan Neptune multicollector (MC)-ICPMS system. The procedures for correction of isobaric interferences between Lu and Yb, instrumental mass fractionation, and comparison with standards have been detailed by Gerdes & Zeh (2006, 2008). Multiple analyses by LA-MC-ICPMS of the GJ1 zircon standard during the period of this study yielded 176Hf/177Hf of 0·281998 ± 0·000015 (n = 10). All uncertainties are reported at the 2σ level. Table 6: Lu–Hf isotope data for single zircon domains Sample . 176Yb/177Hf1 . ±2σ . 176Lu/177Hf1 . ±2σ . 178Hf/177Hf . 180Hf/177Hf . SigHf2 . 176Hf/177Hf . ±2σ3 . 176Hf/177Hf(t)4 . ϵHf(t)5 . ±2σ . TDM5 . Age5 . ±2σ . no. . . . . . . . (V) . . . . 3062 Ma . . (Ga) . (Ma) . . BH1-granite A1 0·1010 28 0·00241 4 1·46740 1·88685 7 0·281009 23 0·280883 1·7 0·8 3·31 2720 27 A2 0·0565 40 0·00137 7 1·46731 1·88673 8 0·280916 22 0·280851 0·6 0·8 3·37 2466 76 A3 0·0947 29 0·00223 7 1·46709 1·88610 8 0·280951 24 0·280825 0·1 0·9 3·40 2949 40 A4 0·1068 73 0·00257 15 1·46734 1·88681 7 0·280986 27 0·280861 0·6 0·9 3·37 2559 57 A5 0·1037 31 0·00247 6 1·46719 1·88654 7 0·280951 24 0·280821 −0·5 0·9 3·43 2746 62 A6 0·1078 36 0·00260 11 1·46723 1·88632 7 0·280958 25 0·280822 −0·5 0·9 3·43 2728 47 A7 0·0638 52 0·00160 12 1·46715 1·88631 9 0·280919 26 0·280836 0·2 0·9 3·39 2720 54 A8 0·0846 25 0·00208 8 1·46732 1·88638 9 0·280934 28 0·280833 −0·2 1·0 3·41 2558 44 A9 0·1225 40 0·00302 12 1·46704 1·88593 9 0·280974 26 0·280842 −0·8 0·9 3·45 2278 47 A10 0·09269 74 0·00230 19 1·46724 1·88647 8 0·280969 27 0·280837 0·5 1·0 3·37 2704 51 A11 0·0786 59 0·00200 15 1·46709 1·88607 8 0·280981 23 0·280866 1·6 0·8 3·31 2978 29 A13 0·0835 34 0·00209 9 1·46721 1·88631 9 0·280891 20 0·280776 −1·8 0·7 3·50 2865 39 A14* 0·0409 18 0·00108 4 1·46727 1·88693 7 0·280878 23 0·280814 −0·1 0·8 3·41 3083 25 A15* 0·0337 8 0·00090 2 1·46725 1·88671 7 0·280886 25 0·280833 0·5 0·9 3·37 3045 31 A18 0·0893 42 0·00215 11 1·46735 1·88680 8 0·280959 25 0·280843 0·5 0·9 3·37 2818 30 BH1-gabbro A1 0·0364 69 0·00114 21 1·46745 1·88674 8 0·280839 21 0·280777 −1·7 0·8 3·49 2874 25 A2 0·0615 49 0·00187 12 1·46711 1·88655 7 0·280865 23 0·280769 −2·3 0·8 3·53 2676 30 A3 0·0255 75 0·00072 18 1·46690 1·88588 8 0·280827 21 0·280795 −1·2 0·7 3·47 2352 40 A4 0·0159 12 0·00059 5 1·46720 1·88657 13 0·280810 19 0·280777 −1·6 0·7 3·49 2894 23 A5 0·0183 4 0·00066 1 1·46720 1·88649 13 0·280823 15 0·280785 −1·2 0·5 3·47 2978 16 A6* 0·0132 10 0·00042 3 1·46721 1·88628 8 0·280833 24 0·280809 −0·4 0·9 3·42 3055 16 A7* 0·0156 12 0·00049 2 1·46717 1·88625 8 0·280803 18 0·280775 −1·6 0·6 3·49 3056 15 A8* 0·0280 37 0·00090 12 1·46720 1·88669 7 0·280859 26 0·280806 −0·5 0·9 3·43 3066 13 A9* 0·0292 35 0·00094 12 1·46711 1·88631 8 0·280846 23 0·280791 −1·0 0·8 3·46 3065 14 A10* 0·0121 15 0·00040 5 1·46713 1·88650 8 0·280815 18 0·280791 −1·0 0·7 3·45 3061 13 A11* 0·0317 107 0·00091 31 1·46717 1·88649 10 0·280855 23 0·280801 −0·6 0·8 3·43 3064 18 A12* 0·0423 111 0·00129 34 1·46733 1·88661 7 0·280868 23 0·280792 −0·9 0·8 3·45 3064 33 A13 0·0415 54 0·00126 14 1·46732 1·88662 8 0·280855 20 0·280790 −1·3 0·7 3·48 2682 19 A14 0·0108 13 0·00035 3 1·46710 1·88654 9 0·280807 25 0·280789 −1·2 0·9 3·46 2703 37 A16 0·0296 23 0·00088 7 1·46704 1·88603 6 0·280826 23 0·280775 −1·6 0·8 3·49 2993 21 A17* 0·0311 108 0·00097 35 1·46738 1·88678 8 0·280826 22 0·280769 −1·8 0·8 3·50 3064 24 A19 0·1386 30 0·00389 6 1·46727 1·88667 8 0·281003 24 0·280789 −1·6 0·8 3·49 2869 28 A21 0·0659 24 0·00200 4 1·46739 1·88695 6 0·280899 31 0·280803 −1·3 1·1 3·48 2506 56 A22 0·0049 3 0·00015 1 1·46723 1·88660 12 0·280786 16 0·280777 −1·5 0·6 3·48 2998 26 A23 0·0655 47 0·00181 14 1·46710 1·88640 7 0·280910 25 0·280812 −0·5 0·9 3·43 2831 23 A24 0·0662 16 0·00181 6 1·46721 1·88664 6 0·280880 21 0·280784 −1·6 0·7 3·49 2786 15 A25 0·0106 4 0·00035 2 1·46714 1·88657 8 0·280812 15 0·280793 −1·0 0·5 3·45 2927 15 A26 0·0762 29 0·00227 5 1·46719 1·88676 7 0·280921 20 0·280810 −1·1 0·7 3·46 2556 44 A27* 0·0216 9 0·00067 4 1·46717 1·88657 7 0·280834 23 0·280794 −0·9 0·8 3·45 3082 18 A28* 0·0371 56 0·00115 19 1·46715 1·88680 7 0·280847 22 0·280780 −1·4 0·8 3·48 3055 12 A29* 0·0926 83 0·00277 24 1·46720 1·88638 7 0·280966 24 0·280803 −0·6 0·9 3·43 3063 22 JMC (50 ppb) (n = 7) 1·46718 1·88643 13 0·282162 14 GJ-1 0·0094 13 0·00031 6 1·46722 1·88656 9 0·282004 18 0·282000 −13·9 0·6 609 6 (n = 15) Sample . 176Yb/177Hf1 . ±2σ . 176Lu/177Hf1 . ±2σ . 178Hf/177Hf . 180Hf/177Hf . SigHf2 . 176Hf/177Hf . ±2σ3 . 176Hf/177Hf(t)4 . ϵHf(t)5 . ±2σ . TDM5 . Age5 . ±2σ . no. . . . . . . . (V) . . . . 3062 Ma . . (Ga) . (Ma) . . BH1-granite A1 0·1010 28 0·00241 4 1·46740 1·88685 7 0·281009 23 0·280883 1·7 0·8 3·31 2720 27 A2 0·0565 40 0·00137 7 1·46731 1·88673 8 0·280916 22 0·280851 0·6 0·8 3·37 2466 76 A3 0·0947 29 0·00223 7 1·46709 1·88610 8 0·280951 24 0·280825 0·1 0·9 3·40 2949 40 A4 0·1068 73 0·00257 15 1·46734 1·88681 7 0·280986 27 0·280861 0·6 0·9 3·37 2559 57 A5 0·1037 31 0·00247 6 1·46719 1·88654 7 0·280951 24 0·280821 −0·5 0·9 3·43 2746 62 A6 0·1078 36 0·00260 11 1·46723 1·88632 7 0·280958 25 0·280822 −0·5 0·9 3·43 2728 47 A7 0·0638 52 0·00160 12 1·46715 1·88631 9 0·280919 26 0·280836 0·2 0·9 3·39 2720 54 A8 0·0846 25 0·00208 8 1·46732 1·88638 9 0·280934 28 0·280833 −0·2 1·0 3·41 2558 44 A9 0·1225 40 0·00302 12 1·46704 1·88593 9 0·280974 26 0·280842 −0·8 0·9 3·45 2278 47 A10 0·09269 74 0·00230 19 1·46724 1·88647 8 0·280969 27 0·280837 0·5 1·0 3·37 2704 51 A11 0·0786 59 0·00200 15 1·46709 1·88607 8 0·280981 23 0·280866 1·6 0·8 3·31 2978 29 A13 0·0835 34 0·00209 9 1·46721 1·88631 9 0·280891 20 0·280776 −1·8 0·7 3·50 2865 39 A14* 0·0409 18 0·00108 4 1·46727 1·88693 7 0·280878 23 0·280814 −0·1 0·8 3·41 3083 25 A15* 0·0337 8 0·00090 2 1·46725 1·88671 7 0·280886 25 0·280833 0·5 0·9 3·37 3045 31 A18 0·0893 42 0·00215 11 1·46735 1·88680 8 0·280959 25 0·280843 0·5 0·9 3·37 2818 30 BH1-gabbro A1 0·0364 69 0·00114 21 1·46745 1·88674 8 0·280839 21 0·280777 −1·7 0·8 3·49 2874 25 A2 0·0615 49 0·00187 12 1·46711 1·88655 7 0·280865 23 0·280769 −2·3 0·8 3·53 2676 30 A3 0·0255 75 0·00072 18 1·46690 1·88588 8 0·280827 21 0·280795 −1·2 0·7 3·47 2352 40 A4 0·0159 12 0·00059 5 1·46720 1·88657 13 0·280810 19 0·280777 −1·6 0·7 3·49 2894 23 A5 0·0183 4 0·00066 1 1·46720 1·88649 13 0·280823 15 0·280785 −1·2 0·5 3·47 2978 16 A6* 0·0132 10 0·00042 3 1·46721 1·88628 8 0·280833 24 0·280809 −0·4 0·9 3·42 3055 16 A7* 0·0156 12 0·00049 2 1·46717 1·88625 8 0·280803 18 0·280775 −1·6 0·6 3·49 3056 15 A8* 0·0280 37 0·00090 12 1·46720 1·88669 7 0·280859 26 0·280806 −0·5 0·9 3·43 3066 13 A9* 0·0292 35 0·00094 12 1·46711 1·88631 8 0·280846 23 0·280791 −1·0 0·8 3·46 3065 14 A10* 0·0121 15 0·00040 5 1·46713 1·88650 8 0·280815 18 0·280791 −1·0 0·7 3·45 3061 13 A11* 0·0317 107 0·00091 31 1·46717 1·88649 10 0·280855 23 0·280801 −0·6 0·8 3·43 3064 18 A12* 0·0423 111 0·00129 34 1·46733 1·88661 7 0·280868 23 0·280792 −0·9 0·8 3·45 3064 33 A13 0·0415 54 0·00126 14 1·46732 1·88662 8 0·280855 20 0·280790 −1·3 0·7 3·48 2682 19 A14 0·0108 13 0·00035 3 1·46710 1·88654 9 0·280807 25 0·280789 −1·2 0·9 3·46 2703 37 A16 0·0296 23 0·00088 7 1·46704 1·88603 6 0·280826 23 0·280775 −1·6 0·8 3·49 2993 21 A17* 0·0311 108 0·00097 35 1·46738 1·88678 8 0·280826 22 0·280769 −1·8 0·8 3·50 3064 24 A19 0·1386 30 0·00389 6 1·46727 1·88667 8 0·281003 24 0·280789 −1·6 0·8 3·49 2869 28 A21 0·0659 24 0·00200 4 1·46739 1·88695 6 0·280899 31 0·280803 −1·3 1·1 3·48 2506 56 A22 0·0049 3 0·00015 1 1·46723 1·88660 12 0·280786 16 0·280777 −1·5 0·6 3·48 2998 26 A23 0·0655 47 0·00181 14 1·46710 1·88640 7 0·280910 25 0·280812 −0·5 0·9 3·43 2831 23 A24 0·0662 16 0·00181 6 1·46721 1·88664 6 0·280880 21 0·280784 −1·6 0·7 3·49 2786 15 A25 0·0106 4 0·00035 2 1·46714 1·88657 8 0·280812 15 0·280793 −1·0 0·5 3·45 2927 15 A26 0·0762 29 0·00227 5 1·46719 1·88676 7 0·280921 20 0·280810 −1·1 0·7 3·46 2556 44 A27* 0·0216 9 0·00067 4 1·46717 1·88657 7 0·280834 23 0·280794 −0·9 0·8 3·45 3082 18 A28* 0·0371 56 0·00115 19 1·46715 1·88680 7 0·280847 22 0·280780 −1·4 0·8 3·48 3055 12 A29* 0·0926 83 0·00277 24 1·46720 1·88638 7 0·280966 24 0·280803 −0·6 0·9 3·43 3063 22 JMC (50 ppb) (n = 7) 1·46718 1·88643 13 0·282162 14 GJ-1 0·0094 13 0·00031 6 1·46722 1·88656 9 0·282004 18 0·282000 −13·9 0·6 609 6 (n = 15) 1176Yb/177Hf = (176Yb/173Yb)true × 173Yb/177Hf)meas × [M173(Yb)/M177(Hf))]β(Hf). 176Lu/177Hf calculated in a similar way by using the 175Lu/177Hf. Quoted uncertainties (absolute) relate to the last quoted figure; Effect of inter-element fractionation on Lu/Hf is estimated to be about 6% or less based on analyses of the GJ-1 and Plesovice zircons. 2Mean Hf signal in volts. 3Uncertainties are quadratic additions of the within-run precision and the daily reproducibility of the 50 ppb JMC475 solution. Uncertainties for the JMC475 and GJ-1 are 2SD (2 standard deviations). 4Initial 176Hf/177Hf are calculated using the apparent Pb–Pb age determined by LA-ICPMS dating (see last two rows). 5All ϵHf and TDM are calculated for the emplacement age of 3062 Ma, TDM is the two-stage model age calculated by using the measured 176Hf/177Hf of each spot (first stage = emplacement age), a value of 0·0113 for the average continental crust (second stage), and a depleted mantle 176Lu/177Hf and 176Lu/177Hf of 0·0384 and 0·28325, respectively. *Most concordant analyses. Open in new tab Table 6: Lu–Hf isotope data for single zircon domains Sample . 176Yb/177Hf1 . ±2σ . 176Lu/177Hf1 . ±2σ . 178Hf/177Hf . 180Hf/177Hf . SigHf2 . 176Hf/177Hf . ±2σ3 . 176Hf/177Hf(t)4 . ϵHf(t)5 . ±2σ . TDM5 . Age5 . ±2σ . no. . . . . . . . (V) . . . . 3062 Ma . . (Ga) . (Ma) . . BH1-granite A1 0·1010 28 0·00241 4 1·46740 1·88685 7 0·281009 23 0·280883 1·7 0·8 3·31 2720 27 A2 0·0565 40 0·00137 7 1·46731 1·88673 8 0·280916 22 0·280851 0·6 0·8 3·37 2466 76 A3 0·0947 29 0·00223 7 1·46709 1·88610 8 0·280951 24 0·280825 0·1 0·9 3·40 2949 40 A4 0·1068 73 0·00257 15 1·46734 1·88681 7 0·280986 27 0·280861 0·6 0·9 3·37 2559 57 A5 0·1037 31 0·00247 6 1·46719 1·88654 7 0·280951 24 0·280821 −0·5 0·9 3·43 2746 62 A6 0·1078 36 0·00260 11 1·46723 1·88632 7 0·280958 25 0·280822 −0·5 0·9 3·43 2728 47 A7 0·0638 52 0·00160 12 1·46715 1·88631 9 0·280919 26 0·280836 0·2 0·9 3·39 2720 54 A8 0·0846 25 0·00208 8 1·46732 1·88638 9 0·280934 28 0·280833 −0·2 1·0 3·41 2558 44 A9 0·1225 40 0·00302 12 1·46704 1·88593 9 0·280974 26 0·280842 −0·8 0·9 3·45 2278 47 A10 0·09269 74 0·00230 19 1·46724 1·88647 8 0·280969 27 0·280837 0·5 1·0 3·37 2704 51 A11 0·0786 59 0·00200 15 1·46709 1·88607 8 0·280981 23 0·280866 1·6 0·8 3·31 2978 29 A13 0·0835 34 0·00209 9 1·46721 1·88631 9 0·280891 20 0·280776 −1·8 0·7 3·50 2865 39 A14* 0·0409 18 0·00108 4 1·46727 1·88693 7 0·280878 23 0·280814 −0·1 0·8 3·41 3083 25 A15* 0·0337 8 0·00090 2 1·46725 1·88671 7 0·280886 25 0·280833 0·5 0·9 3·37 3045 31 A18 0·0893 42 0·00215 11 1·46735 1·88680 8 0·280959 25 0·280843 0·5 0·9 3·37 2818 30 BH1-gabbro A1 0·0364 69 0·00114 21 1·46745 1·88674 8 0·280839 21 0·280777 −1·7 0·8 3·49 2874 25 A2 0·0615 49 0·00187 12 1·46711 1·88655 7 0·280865 23 0·280769 −2·3 0·8 3·53 2676 30 A3 0·0255 75 0·00072 18 1·46690 1·88588 8 0·280827 21 0·280795 −1·2 0·7 3·47 2352 40 A4 0·0159 12 0·00059 5 1·46720 1·88657 13 0·280810 19 0·280777 −1·6 0·7 3·49 2894 23 A5 0·0183 4 0·00066 1 1·46720 1·88649 13 0·280823 15 0·280785 −1·2 0·5 3·47 2978 16 A6* 0·0132 10 0·00042 3 1·46721 1·88628 8 0·280833 24 0·280809 −0·4 0·9 3·42 3055 16 A7* 0·0156 12 0·00049 2 1·46717 1·88625 8 0·280803 18 0·280775 −1·6 0·6 3·49 3056 15 A8* 0·0280 37 0·00090 12 1·46720 1·88669 7 0·280859 26 0·280806 −0·5 0·9 3·43 3066 13 A9* 0·0292 35 0·00094 12 1·46711 1·88631 8 0·280846 23 0·280791 −1·0 0·8 3·46 3065 14 A10* 0·0121 15 0·00040 5 1·46713 1·88650 8 0·280815 18 0·280791 −1·0 0·7 3·45 3061 13 A11* 0·0317 107 0·00091 31 1·46717 1·88649 10 0·280855 23 0·280801 −0·6 0·8 3·43 3064 18 A12* 0·0423 111 0·00129 34 1·46733 1·88661 7 0·280868 23 0·280792 −0·9 0·8 3·45 3064 33 A13 0·0415 54 0·00126 14 1·46732 1·88662 8 0·280855 20 0·280790 −1·3 0·7 3·48 2682 19 A14 0·0108 13 0·00035 3 1·46710 1·88654 9 0·280807 25 0·280789 −1·2 0·9 3·46 2703 37 A16 0·0296 23 0·00088 7 1·46704 1·88603 6 0·280826 23 0·280775 −1·6 0·8 3·49 2993 21 A17* 0·0311 108 0·00097 35 1·46738 1·88678 8 0·280826 22 0·280769 −1·8 0·8 3·50 3064 24 A19 0·1386 30 0·00389 6 1·46727 1·88667 8 0·281003 24 0·280789 −1·6 0·8 3·49 2869 28 A21 0·0659 24 0·00200 4 1·46739 1·88695 6 0·280899 31 0·280803 −1·3 1·1 3·48 2506 56 A22 0·0049 3 0·00015 1 1·46723 1·88660 12 0·280786 16 0·280777 −1·5 0·6 3·48 2998 26 A23 0·0655 47 0·00181 14 1·46710 1·88640 7 0·280910 25 0·280812 −0·5 0·9 3·43 2831 23 A24 0·0662 16 0·00181 6 1·46721 1·88664 6 0·280880 21 0·280784 −1·6 0·7 3·49 2786 15 A25 0·0106 4 0·00035 2 1·46714 1·88657 8 0·280812 15 0·280793 −1·0 0·5 3·45 2927 15 A26 0·0762 29 0·00227 5 1·46719 1·88676 7 0·280921 20 0·280810 −1·1 0·7 3·46 2556 44 A27* 0·0216 9 0·00067 4 1·46717 1·88657 7 0·280834 23 0·280794 −0·9 0·8 3·45 3082 18 A28* 0·0371 56 0·00115 19 1·46715 1·88680 7 0·280847 22 0·280780 −1·4 0·8 3·48 3055 12 A29* 0·0926 83 0·00277 24 1·46720 1·88638 7 0·280966 24 0·280803 −0·6 0·9 3·43 3063 22 JMC (50 ppb) (n = 7) 1·46718 1·88643 13 0·282162 14 GJ-1 0·0094 13 0·00031 6 1·46722 1·88656 9 0·282004 18 0·282000 −13·9 0·6 609 6 (n = 15) Sample . 176Yb/177Hf1 . ±2σ . 176Lu/177Hf1 . ±2σ . 178Hf/177Hf . 180Hf/177Hf . SigHf2 . 176Hf/177Hf . ±2σ3 . 176Hf/177Hf(t)4 . ϵHf(t)5 . ±2σ . TDM5 . Age5 . ±2σ . no. . . . . . . . (V) . . . . 3062 Ma . . (Ga) . (Ma) . . BH1-granite A1 0·1010 28 0·00241 4 1·46740 1·88685 7 0·281009 23 0·280883 1·7 0·8 3·31 2720 27 A2 0·0565 40 0·00137 7 1·46731 1·88673 8 0·280916 22 0·280851 0·6 0·8 3·37 2466 76 A3 0·0947 29 0·00223 7 1·46709 1·88610 8 0·280951 24 0·280825 0·1 0·9 3·40 2949 40 A4 0·1068 73 0·00257 15 1·46734 1·88681 7 0·280986 27 0·280861 0·6 0·9 3·37 2559 57 A5 0·1037 31 0·00247 6 1·46719 1·88654 7 0·280951 24 0·280821 −0·5 0·9 3·43 2746 62 A6 0·1078 36 0·00260 11 1·46723 1·88632 7 0·280958 25 0·280822 −0·5 0·9 3·43 2728 47 A7 0·0638 52 0·00160 12 1·46715 1·88631 9 0·280919 26 0·280836 0·2 0·9 3·39 2720 54 A8 0·0846 25 0·00208 8 1·46732 1·88638 9 0·280934 28 0·280833 −0·2 1·0 3·41 2558 44 A9 0·1225 40 0·00302 12 1·46704 1·88593 9 0·280974 26 0·280842 −0·8 0·9 3·45 2278 47 A10 0·09269 74 0·00230 19 1·46724 1·88647 8 0·280969 27 0·280837 0·5 1·0 3·37 2704 51 A11 0·0786 59 0·00200 15 1·46709 1·88607 8 0·280981 23 0·280866 1·6 0·8 3·31 2978 29 A13 0·0835 34 0·00209 9 1·46721 1·88631 9 0·280891 20 0·280776 −1·8 0·7 3·50 2865 39 A14* 0·0409 18 0·00108 4 1·46727 1·88693 7 0·280878 23 0·280814 −0·1 0·8 3·41 3083 25 A15* 0·0337 8 0·00090 2 1·46725 1·88671 7 0·280886 25 0·280833 0·5 0·9 3·37 3045 31 A18 0·0893 42 0·00215 11 1·46735 1·88680 8 0·280959 25 0·280843 0·5 0·9 3·37 2818 30 BH1-gabbro A1 0·0364 69 0·00114 21 1·46745 1·88674 8 0·280839 21 0·280777 −1·7 0·8 3·49 2874 25 A2 0·0615 49 0·00187 12 1·46711 1·88655 7 0·280865 23 0·280769 −2·3 0·8 3·53 2676 30 A3 0·0255 75 0·00072 18 1·46690 1·88588 8 0·280827 21 0·280795 −1·2 0·7 3·47 2352 40 A4 0·0159 12 0·00059 5 1·46720 1·88657 13 0·280810 19 0·280777 −1·6 0·7 3·49 2894 23 A5 0·0183 4 0·00066 1 1·46720 1·88649 13 0·280823 15 0·280785 −1·2 0·5 3·47 2978 16 A6* 0·0132 10 0·00042 3 1·46721 1·88628 8 0·280833 24 0·280809 −0·4 0·9 3·42 3055 16 A7* 0·0156 12 0·00049 2 1·46717 1·88625 8 0·280803 18 0·280775 −1·6 0·6 3·49 3056 15 A8* 0·0280 37 0·00090 12 1·46720 1·88669 7 0·280859 26 0·280806 −0·5 0·9 3·43 3066 13 A9* 0·0292 35 0·00094 12 1·46711 1·88631 8 0·280846 23 0·280791 −1·0 0·8 3·46 3065 14 A10* 0·0121 15 0·00040 5 1·46713 1·88650 8 0·280815 18 0·280791 −1·0 0·7 3·45 3061 13 A11* 0·0317 107 0·00091 31 1·46717 1·88649 10 0·280855 23 0·280801 −0·6 0·8 3·43 3064 18 A12* 0·0423 111 0·00129 34 1·46733 1·88661 7 0·280868 23 0·280792 −0·9 0·8 3·45 3064 33 A13 0·0415 54 0·00126 14 1·46732 1·88662 8 0·280855 20 0·280790 −1·3 0·7 3·48 2682 19 A14 0·0108 13 0·00035 3 1·46710 1·88654 9 0·280807 25 0·280789 −1·2 0·9 3·46 2703 37 A16 0·0296 23 0·00088 7 1·46704 1·88603 6 0·280826 23 0·280775 −1·6 0·8 3·49 2993 21 A17* 0·0311 108 0·00097 35 1·46738 1·88678 8 0·280826 22 0·280769 −1·8 0·8 3·50 3064 24 A19 0·1386 30 0·00389 6 1·46727 1·88667 8 0·281003 24 0·280789 −1·6 0·8 3·49 2869 28 A21 0·0659 24 0·00200 4 1·46739 1·88695 6 0·280899 31 0·280803 −1·3 1·1 3·48 2506 56 A22 0·0049 3 0·00015 1 1·46723 1·88660 12 0·280786 16 0·280777 −1·5 0·6 3·48 2998 26 A23 0·0655 47 0·00181 14 1·46710 1·88640 7 0·280910 25 0·280812 −0·5 0·9 3·43 2831 23 A24 0·0662 16 0·00181 6 1·46721 1·88664 6 0·280880 21 0·280784 −1·6 0·7 3·49 2786 15 A25 0·0106 4 0·00035 2 1·46714 1·88657 8 0·280812 15 0·280793 −1·0 0·5 3·45 2927 15 A26 0·0762 29 0·00227 5 1·46719 1·88676 7 0·280921 20 0·280810 −1·1 0·7 3·46 2556 44 A27* 0·0216 9 0·00067 4 1·46717 1·88657 7 0·280834 23 0·280794 −0·9 0·8 3·45 3082 18 A28* 0·0371 56 0·00115 19 1·46715 1·88680 7 0·280847 22 0·280780 −1·4 0·8 3·48 3055 12 A29* 0·0926 83 0·00277 24 1·46720 1·88638 7 0·280966 24 0·280803 −0·6 0·9 3·43 3063 22 JMC (50 ppb) (n = 7) 1·46718 1·88643 13 0·282162 14 GJ-1 0·0094 13 0·00031 6 1·46722 1·88656 9 0·282004 18 0·282000 −13·9 0·6 609 6 (n = 15) 1176Yb/177Hf = (176Yb/173Yb)true × 173Yb/177Hf)meas × [M173(Yb)/M177(Hf))]β(Hf). 176Lu/177Hf calculated in a similar way by using the 175Lu/177Hf. Quoted uncertainties (absolute) relate to the last quoted figure; Effect of inter-element fractionation on Lu/Hf is estimated to be about 6% or less based on analyses of the GJ-1 and Plesovice zircons. 2Mean Hf signal in volts. 3Uncertainties are quadratic additions of the within-run precision and the daily reproducibility of the 50 ppb JMC475 solution. Uncertainties for the JMC475 and GJ-1 are 2SD (2 standard deviations). 4Initial 176Hf/177Hf are calculated using the apparent Pb–Pb age determined by LA-ICPMS dating (see last two rows). 5All ϵHf and TDM are calculated for the emplacement age of 3062 Ma, TDM is the two-stage model age calculated by using the measured 176Hf/177Hf of each spot (first stage = emplacement age), a value of 0·0113 for the average continental crust (second stage), and a depleted mantle 176Lu/177Hf and 176Lu/177Hf of 0·0384 and 0·28325, respectively. *Most concordant analyses. Open in new tab The zircon grains in the granite are typically 0·1–0·2 mm in length and a number of them display complex internal structures. Some grains show a relic oscillatory zonation, overprinted by several patchy domains that point to recrystallization of the original magmatic zoning (Fig. 11a, insert). In some zircon grains, however, only compositional banding can be distinguished by bright and dull CL domains. Fig. 11. Open in new tabDownload slide Concordia diagrams for zircon grains from the granite (a) and the hornblende metagabbro (b). Data point error ellipses are 2σ. Also shown are cathodoluminescence images of typical zircon grains from each lithotype; scale bars represent 0·1 mm. Lines I represent discordia lines from the concordant crystallization age forced through the origin, lines II reflect a later thermal overprint (for further details see text). Many of the analysed zircon domains are rich in U, with contents up to 1316 ppm. These domains yielded the lowest 206Pb/204Pb ratios and the most discordant results. In contrast, those domains with the lowest U contents yielded perfectly concordant age data (Table 5). Two concordant domains (97 and 100% concordance) gave 207Pb–206Pb ages of 3083 and 3045 Ma, respectively. A third concordant analysis corresponds to a significantly younger age of 2746 ± 62 Ma. The rest of the analyses are variably discordant. In spite of this wide scatter of U–Pb age data, almost all (except for two) analysed domains yielded within error identical 176Hf/177Hf ratios (0·28084 ± 0·00003) calculated for the apparent 207Pb–206Pb age (Fig. 12a, Table 6). This indicates that all these zircon domains crystallized from the same magmatic source but underwent variable Pb loss as a result of later alteration (Gerdes & Zeh, 2008; Zeh et al., 2009). Two analyses deviate from the mean 176Hf/177Hf ratio. One of them has a lower 176Hf/177Hf and could reflect inheritance, whereas the other has a higher ratio, which might point to later incorporation of radiogenic Hf. Zircon formation during a later magmatic–metamorphic–hydrothermal event can be excluded on the basis of the combined U–Pb and Hf datasets shown in Fig. 12a. If such later zircon crystallization had taken place, the resulting domains should plot on, or above, the 176Lu/177Hf whole-rock evolution line. From the combined U–Pb and Lu–Hf isotope data it can be concluded that the oldest concordant ages best reflect the time of granite emplacement; that is, 3064 ± 20 Ma as calculated from the two most concordant analyses, A14 and A15 in Table 5 (Fig. 11a). Fig. 12. Open in new tabDownload slide Calculated 176Hf/177Hf ratio at the time of the apparent 207Pb–206Pb age vs apparent 207Pb–206Pb age for single zircon domains from the granite (a) and the hornblende metagabbro (b). The concordant analyses (97–103% concordance level) are shown as filled symbols. It should be noted that the corresponding ϵHf at the time of emplacement are all within a very narrow range close to zero, but slightly lower in the hornblende metagabbro. The zircon grains in the hornblende metagabbro are on average larger than (up to 0·8 mm in length) but of similar habit to those in the granite and also show complex internal structures with patchy bright and dull CL domains (Fig. 11b, insert). A large number of zircon domains, typically with U contents of much less than 200 ppm, yielded concordant results (Fig. 11b). Eight domains gave concordant (99–101%) age data that correspond to a concordia age of 3064 ± 7 Ma, including the error on the decay constant. This age is identical to an upper intercept age of 3063 ± 5 Ma obtained on 11 spot analyses (Fig. 11b), which is regarded as the best constraint on the time of emplacement. There are, however, also a number of other discordant zircon analyses that plot to the left of the discordia line as shown in Fig. 11b. All analyses, irrespective of the level of discordance, yielded within error identical 176Hf/177Hf ratios (0·28079 ± 0·000026) calculated for the apparent 207Pb–206Pb age (Fig. 12b, Table 6). This relationship is well reflected even by spot analyses obtained from different zircon domains that have different internal structure, U contents and 176Yb/177Hf ratios within the same grain (Fig. 12b, Tables 5 and 6). This indicates that all of these zircon grains were formed from an isotopically homogeneous magma. As with the zircon in the granite, multiple zircon growth can be excluded and the variable discordance, and 207Pb–206Pb ages, can be explained by multiple alteration events after zircon growth. Significantly, the U–Pb and Hf isotope data obtained on zircon domains from both the altered granite and the hornblende metagabbro appear very similar. Their respective emplacement ages and Hf isotope characteristics are identical within error. The ϵHf calculated for the time of emplacement (3063 Ma) ranges from −1·8 to +1·7 in the case of the granite and from −2·3 to −0·4 in the hornblende metagabbro. The mean ϵHf(t) values are slightly higher for the metagranite (+0·2) than for the hornblende metagabbro (–1·2). These values correspond to Hf model ages of 3·32 Ga for the metagranite and 3·47 Ga for the hornblende metagabbro, calculated by using the parameters as outlined in the legend of Table 6 and discussed by Zeh et al. (2007). The U–Pb isotope data indicate that the zircon grains in both rock types experienced multiple Pb loss. A major Pb loss event seemingly occurred at 2720 Ma, as reflected by one concordant data point obtained from the altered granite (spot A1, Table 5) and by several discordant analyses that yielded 207Pb–206Pb ages around 2700 Ma and plot on a reference line between the concordant datum and the origin (dashed line II in Fig. 11a). Subsequent Pb loss is further indicated by a few zircon analyses that yielded younger 207Pb–206Pb ages (Table 5). AR–AR AGE DATA FOR HORNBLENDE The above petrological and U–Pb zircon age data indicate that both the granitic and gabbroic protoliths intruded at about 3·07 Ga, and were altered afterwards, most probably at 2·7 Ga. At present, it is unclear whether the low-Ti composition of the magnesio-hornblende in the metagabbro is a result of a Neoarchaean alteration process or a primary feature related to the emplacement of the mafic intrusion. To assess the origin of the magnesio-hornblende in the metagabbro, we conducted 40Ar–39Ar analyses on hornblende separates from four positions within the mafic portion of the drill core. Between 60 and 120 mg of hornblende concentrates were enclosed in high-purity quartz vials and irradiated at the nuclear research reactor VR-1 in Prague, Czech Republic. Once cooled the samples were filled into annealed Ta capsules and subsequently analysed by stepwise heating experiments at the CEAL Laboratory of the Slovak Academy of Sciences in Bratislava. The analytical details for the fully automatic Ar-extraction and purification line are as described by Frimmel & Frank (1998; at that time the line was still housed at the former Institute of Geology, University of Vienna). The paper by Frimmel & Frank also includes details regarding corrections for mass discrimination and radioactive decay, as well as for the determination of the J-value (0·013552 ± 0·4%) and the definition of a plateau age. The K/Ca ratio was determined from the 39Ar/37Ar ratio (calculated for the end of irradiation) using a conversion factor of 0·247. The 40Ar/36Ar ratio of the line blank was close to air composition throughout the study (299 ± 1·0%). The errors of the calculated ages for single steps are given as 1σ. The errors of the plateau and total gas ages include an additional error of ±0·4% on the J-value. The hornblende separates from all four positions in the drill core yielded similar but not identical results with some significant subtle differences (Fig. 13, Table 7). The hornblende from position 15·1–15·5 m, nearest the overlying granite, gave a total gas age of 3064 ± 19 Ma. No distinct plateau can be recognized (Fig. 13a). Lower ages at the lowest temperature steps are most probably caused by secondary overgrowth of actinolite and/or chlorite. The highest ages were obtained on relatively low-temperature steps and this is regarded as reflecting the incorporation of excess Ar into the hornblende lattice as a consequence of partial chloritization. Hornblende in drill core 19·3–19·4 m yielded an identical total gas age of 3074 ± 21 Ma. Some Ar loss is indicated by lower apparent ages obtained for the lowest temperature steps, which are also characterized by elevated K/Ca (Fig. 13b). Most of the 39Ar released (65%) defines a very good plateau that corresponds to an age of 3078 ± 20 Ma. The analytical data were also used to construct 40Ar/36Ar vs 39Ar/36Ar as well as 39Ar/40Ar vs 36Ar/40Ar isotope correlation diagrams. The ages obtained from these diagrams (not shown) are all indistinguishable (3072 ± 8·5 and 3071 ± 11 Ma, respectively) from the plateau age and the total gas age, which is, therefore, regarded as dating the time of hornblende crystallization. Fig. 13. Open in new tabDownload slide 40Ar–39Ar incremental release spectra obtained for hornblende separates from different positions within the intersected hornblende metagabbro. Table 7: 40Ar–39Ar analytical data for incremental heating experiments on hornblende Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 15·1–15·5 m 1 800 4·3 478·71 97·9 0·631 1·56 302·58 0·9 2939·3 12·6 2 880 1·4 143·79 97·6 0·147 6·43 272·16 2·8 2787·3 40·1 3 950 9·6 1318·70 99·5 0·055 38·07 376·53 1·0 3263·0 15·2 4 980 34·8 4302·12 99·7 0·074 47·99 338·86 0·6 3105·4 9·4 5 1000 9·8 1116·66 99·7 0·079 51·34 313·06 1·0 2988·9 14·4 6 1015 4·9 556·58 99·8 0·083 52·70 308·58 0·7 2967·9 10·8 7 1045 6·5 730·72 99·9 0·097 64·59 306·97 0·7 2960·3 10·6 8 1095 3·3 374·96 99·8 0·090 55·77 311·53 0·6 2981·8 9·1 9 1160 9·6 1137·33 99·7 0·088 48·24 323·11 0·6 3035·2 8·5 10 1300 15·6 1869·61 99·6 0·079 41·29 329·39 0·6 3063·5 8·7 Total gas age 3063·7 18·9 1Measured; correcting factors: Daly/HF = 9·17 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 15·1–15·5 m 1 800 4·3 478·71 97·9 0·631 1·56 302·58 0·9 2939·3 12·6 2 880 1·4 143·79 97·6 0·147 6·43 272·16 2·8 2787·3 40·1 3 950 9·6 1318·70 99·5 0·055 38·07 376·53 1·0 3263·0 15·2 4 980 34·8 4302·12 99·7 0·074 47·99 338·86 0·6 3105·4 9·4 5 1000 9·8 1116·66 99·7 0·079 51·34 313·06 1·0 2988·9 14·4 6 1015 4·9 556·58 99·8 0·083 52·70 308·58 0·7 2967·9 10·8 7 1045 6·5 730·72 99·9 0·097 64·59 306·97 0·7 2960·3 10·6 8 1095 3·3 374·96 99·8 0·090 55·77 311·53 0·6 2981·8 9·1 9 1160 9·6 1137·33 99·7 0·088 48·24 323·11 0·6 3035·2 8·5 10 1300 15·6 1869·61 99·6 0·079 41·29 329·39 0·6 3063·5 8·7 Total gas age 3063·7 18·9 1Measured; correcting factors: Daly/HF = 9·17 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 19·3, 19·4 m 1 720 2·0 236·88 97·5 0·689 1·27 284·63 1·0 2851·2 13·9 2 760 1·3 142·03 97·7 1·195 0·88 264·65 1·7 2747·7 24·1 3 820 5·7 645·60 99·3 1·420 2·52 266·52 0·6 2757·7 8·9 4 850 1·2 142·48 98·0 0·366 3·09 270·54 1·5 2778·8 20·6 5 880 7·7 1081·27 99·2 0·093 21·20 332·69 0·7 3078·2 10·1 6 920 5·6 779·96 99·5 0·073 35·60 329·46 1·0 3063·8 14·4 7 950 4·7 654·01 99·1 0·071 23·12 330·10 1·2 3066·7 17·5 8 1000 35·9 5055·51 99·8 0·069 59·57 333·53 0·7 3081·9 10·9 9 1050 5·6 782·30 99·9 0·061 70·42 332·09 0·5 3075·5 7·6 10 1095 3·3 465·94 99·9 0·063 74·77 333·48 0·6 3081·7 8·4 11 1200 2·2 310·11 99·2 0·066 28·42 332·71 1·5 3078·3 22·6 12 1300 24·8 3742·31 99·7 0·057 55·23 357·17 1·2 3183·7 17·3 Total gas age 3073·7 21·3 65% plateau age 3078·1 19·6 1Measured; correcting factors: Daly/HF = 9·10 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 19·3, 19·4 m 1 720 2·0 236·88 97·5 0·689 1·27 284·63 1·0 2851·2 13·9 2 760 1·3 142·03 97·7 1·195 0·88 264·65 1·7 2747·7 24·1 3 820 5·7 645·60 99·3 1·420 2·52 266·52 0·6 2757·7 8·9 4 850 1·2 142·48 98·0 0·366 3·09 270·54 1·5 2778·8 20·6 5 880 7·7 1081·27 99·2 0·093 21·20 332·69 0·7 3078·2 10·1 6 920 5·6 779·96 99·5 0·073 35·60 329·46 1·0 3063·8 14·4 7 950 4·7 654·01 99·1 0·071 23·12 330·10 1·2 3066·7 17·5 8 1000 35·9 5055·51 99·8 0·069 59·57 333·53 0·7 3081·9 10·9 9 1050 5·6 782·30 99·9 0·061 70·42 332·09 0·5 3075·5 7·6 10 1095 3·3 465·94 99·9 0·063 74·77 333·48 0·6 3081·7 8·4 11 1200 2·2 310·11 99·2 0·066 28·42 332·71 1·5 3078·3 22·6 12 1300 24·8 3742·31 99·7 0·057 55·23 357·17 1·2 3183·7 17·3 Total gas age 3073·7 21·3 65% plateau age 3078·1 19·6 1Measured; correcting factors: Daly/HF = 9·10 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 26·8–26·9 m 1 800 5·5 281·94 97·3 0·262 2·98 295·721 1·0 2906·2 15·1 2 880 9·5 414·35 99·1 0·778 3·47 249·922 1·3 2667·5 18·1 3 950 10·9 598·29 99·0 0·073 21·79 316·749 0·9 3006·0 13·5 4 980 18·4 1194·83 99·7 0·057 50·42 376·066 1·3 3261·1 19·1 5 1000 19·6 1164·44 99·8 0·060 59·96 342·593 1·3 3121·6 19·4 6 1015 9·9 508·26 99·7 0·072 53·80 296·435 1·4 2909·7 20·9 7 1045 10·3 656·67 99·8 0·060 60·77 369·105 0·8 3233·0 12·5 8 1095 4·5 343·18 99·1 0·050 25·70 444·576 2·5 3516·8 38·6 9 1300 11·5 764·91 99·9 0·058 75·45 384·311 1·7 3293·9 26·2 Total gas age 3107·5 28·8 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 26·8–26·9 m 1 800 5·5 281·94 97·3 0·262 2·98 295·721 1·0 2906·2 15·1 2 880 9·5 414·35 99·1 0·778 3·47 249·922 1·3 2667·5 18·1 3 950 10·9 598·29 99·0 0·073 21·79 316·749 0·9 3006·0 13·5 4 980 18·4 1194·83 99·7 0·057 50·42 376·066 1·3 3261·1 19·1 5 1000 19·6 1164·44 99·8 0·060 59·96 342·593 1·3 3121·6 19·4 6 1015 9·9 508·26 99·7 0·072 53·80 296·435 1·4 2909·7 20·9 7 1045 10·3 656·67 99·8 0·060 60·77 369·105 0·8 3233·0 12·5 8 1095 4·5 343·18 99·1 0·050 25·70 444·576 2·5 3516·8 38·6 9 1300 11·5 764·91 99·9 0·058 75·45 384·311 1·7 3293·9 26·2 Total gas age 3107·5 28·8 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 30·1–30·2 m 1 800 7·1 831·15 98·63477 0·734 1·819598 319·415 0·7 3018·3 10·7 2 880 1·7 207·82 98·46037 0·144 7·334663 336·103 1·2 3093·3 18·4 3 950 14·6 1968·35 99·4128 0·066 29·60979 370·579 1·3 3239·0 19·6 4 980 16·3 1953·70 99·60721 0·077 37·92846 329·466 1·7 3063·8 24·3 5 1000 9·7 1086·63 99·49839 0·080 33·00942 306·726 1·3 2959·1 19·3 6 1015 4·1 468·07 99·20482 0·081 22·85102 313·544 1·5 2991·2 21·6 7 1045 3·7 412·32 99·13203 0·092 19·73829 304·459 2·6 2948·3 38·4 8 1095 4·6 533·32 98·68499 0·092 13·46074 316·918 1·3 3006·8 19·3 9 1160 13·7 1805·51 99·77105 0·072 50·79623 361·611 0·8 3202·2 12·4 10 1300 24·6 3087·86 99·70786 0·071 46·17452 344·218 0·8 3128·6 11·5 Total gas age 3101·4 26·1 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00020 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 30·1–30·2 m 1 800 7·1 831·15 98·63477 0·734 1·819598 319·415 0·7 3018·3 10·7 2 880 1·7 207·82 98·46037 0·144 7·334663 336·103 1·2 3093·3 18·4 3 950 14·6 1968·35 99·4128 0·066 29·60979 370·579 1·3 3239·0 19·6 4 980 16·3 1953·70 99·60721 0·077 37·92846 329·466 1·7 3063·8 24·3 5 1000 9·7 1086·63 99·49839 0·080 33·00942 306·726 1·3 2959·1 19·3 6 1015 4·1 468·07 99·20482 0·081 22·85102 313·544 1·5 2991·2 21·6 7 1045 3·7 412·32 99·13203 0·092 19·73829 304·459 2·6 2948·3 38·4 8 1095 4·6 533·32 98·68499 0·092 13·46074 316·918 1·3 3006·8 19·3 9 1160 13·7 1805·51 99·77105 0·072 50·79623 361·611 0·8 3202·2 12·4 10 1300 24·6 3087·86 99·70786 0·071 46·17452 344·218 0·8 3128·6 11·5 Total gas age 3101·4 26·1 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00020 Open in new tab Table 7: 40Ar–39Ar analytical data for incremental heating experiments on hornblende Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 15·1–15·5 m 1 800 4·3 478·71 97·9 0·631 1·56 302·58 0·9 2939·3 12·6 2 880 1·4 143·79 97·6 0·147 6·43 272·16 2·8 2787·3 40·1 3 950 9·6 1318·70 99·5 0·055 38·07 376·53 1·0 3263·0 15·2 4 980 34·8 4302·12 99·7 0·074 47·99 338·86 0·6 3105·4 9·4 5 1000 9·8 1116·66 99·7 0·079 51·34 313·06 1·0 2988·9 14·4 6 1015 4·9 556·58 99·8 0·083 52·70 308·58 0·7 2967·9 10·8 7 1045 6·5 730·72 99·9 0·097 64·59 306·97 0·7 2960·3 10·6 8 1095 3·3 374·96 99·8 0·090 55·77 311·53 0·6 2981·8 9·1 9 1160 9·6 1137·33 99·7 0·088 48·24 323·11 0·6 3035·2 8·5 10 1300 15·6 1869·61 99·6 0·079 41·29 329·39 0·6 3063·5 8·7 Total gas age 3063·7 18·9 1Measured; correcting factors: Daly/HF = 9·17 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 15·1–15·5 m 1 800 4·3 478·71 97·9 0·631 1·56 302·58 0·9 2939·3 12·6 2 880 1·4 143·79 97·6 0·147 6·43 272·16 2·8 2787·3 40·1 3 950 9·6 1318·70 99·5 0·055 38·07 376·53 1·0 3263·0 15·2 4 980 34·8 4302·12 99·7 0·074 47·99 338·86 0·6 3105·4 9·4 5 1000 9·8 1116·66 99·7 0·079 51·34 313·06 1·0 2988·9 14·4 6 1015 4·9 556·58 99·8 0·083 52·70 308·58 0·7 2967·9 10·8 7 1045 6·5 730·72 99·9 0·097 64·59 306·97 0·7 2960·3 10·6 8 1095 3·3 374·96 99·8 0·090 55·77 311·53 0·6 2981·8 9·1 9 1160 9·6 1137·33 99·7 0·088 48·24 323·11 0·6 3035·2 8·5 10 1300 15·6 1869·61 99·6 0·079 41·29 329·39 0·6 3063·5 8·7 Total gas age 3063·7 18·9 1Measured; correcting factors: Daly/HF = 9·17 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 19·3, 19·4 m 1 720 2·0 236·88 97·5 0·689 1·27 284·63 1·0 2851·2 13·9 2 760 1·3 142·03 97·7 1·195 0·88 264·65 1·7 2747·7 24·1 3 820 5·7 645·60 99·3 1·420 2·52 266·52 0·6 2757·7 8·9 4 850 1·2 142·48 98·0 0·366 3·09 270·54 1·5 2778·8 20·6 5 880 7·7 1081·27 99·2 0·093 21·20 332·69 0·7 3078·2 10·1 6 920 5·6 779·96 99·5 0·073 35·60 329·46 1·0 3063·8 14·4 7 950 4·7 654·01 99·1 0·071 23·12 330·10 1·2 3066·7 17·5 8 1000 35·9 5055·51 99·8 0·069 59·57 333·53 0·7 3081·9 10·9 9 1050 5·6 782·30 99·9 0·061 70·42 332·09 0·5 3075·5 7·6 10 1095 3·3 465·94 99·9 0·063 74·77 333·48 0·6 3081·7 8·4 11 1200 2·2 310·11 99·2 0·066 28·42 332·71 1·5 3078·3 22·6 12 1300 24·8 3742·31 99·7 0·057 55·23 357·17 1·2 3183·7 17·3 Total gas age 3073·7 21·3 65% plateau age 3078·1 19·6 1Measured; correcting factors: Daly/HF = 9·10 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 19·3, 19·4 m 1 720 2·0 236·88 97·5 0·689 1·27 284·63 1·0 2851·2 13·9 2 760 1·3 142·03 97·7 1·195 0·88 264·65 1·7 2747·7 24·1 3 820 5·7 645·60 99·3 1·420 2·52 266·52 0·6 2757·7 8·9 4 850 1·2 142·48 98·0 0·366 3·09 270·54 1·5 2778·8 20·6 5 880 7·7 1081·27 99·2 0·093 21·20 332·69 0·7 3078·2 10·1 6 920 5·6 779·96 99·5 0·073 35·60 329·46 1·0 3063·8 14·4 7 950 4·7 654·01 99·1 0·071 23·12 330·10 1·2 3066·7 17·5 8 1000 35·9 5055·51 99·8 0·069 59·57 333·53 0·7 3081·9 10·9 9 1050 5·6 782·30 99·9 0·061 70·42 332·09 0·5 3075·5 7·6 10 1095 3·3 465·94 99·9 0·063 74·77 333·48 0·6 3081·7 8·4 11 1200 2·2 310·11 99·2 0·066 28·42 332·71 1·5 3078·3 22·6 12 1300 24·8 3742·31 99·7 0·057 55·23 357·17 1·2 3183·7 17·3 Total gas age 3073·7 21·3 65% plateau age 3078·1 19·6 1Measured; correcting factors: Daly/HF = 9·10 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 26·8–26·9 m 1 800 5·5 281·94 97·3 0·262 2·98 295·721 1·0 2906·2 15·1 2 880 9·5 414·35 99·1 0·778 3·47 249·922 1·3 2667·5 18·1 3 950 10·9 598·29 99·0 0·073 21·79 316·749 0·9 3006·0 13·5 4 980 18·4 1194·83 99·7 0·057 50·42 376·066 1·3 3261·1 19·1 5 1000 19·6 1164·44 99·8 0·060 59·96 342·593 1·3 3121·6 19·4 6 1015 9·9 508·26 99·7 0·072 53·80 296·435 1·4 2909·7 20·9 7 1045 10·3 656·67 99·8 0·060 60·77 369·105 0·8 3233·0 12·5 8 1095 4·5 343·18 99·1 0·050 25·70 444·576 2·5 3516·8 38·6 9 1300 11·5 764·91 99·9 0·058 75·45 384·311 1·7 3293·9 26·2 Total gas age 3107·5 28·8 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 26·8–26·9 m 1 800 5·5 281·94 97·3 0·262 2·98 295·721 1·0 2906·2 15·1 2 880 9·5 414·35 99·1 0·778 3·47 249·922 1·3 2667·5 18·1 3 950 10·9 598·29 99·0 0·073 21·79 316·749 0·9 3006·0 13·5 4 980 18·4 1194·83 99·7 0·057 50·42 376·066 1·3 3261·1 19·1 5 1000 19·6 1164·44 99·8 0·060 59·96 342·593 1·3 3121·6 19·4 6 1015 9·9 508·26 99·7 0·072 53·80 296·435 1·4 2909·7 20·9 7 1045 10·3 656·67 99·8 0·060 60·77 369·105 0·8 3233·0 12·5 8 1095 4·5 343·18 99·1 0·050 25·70 444·576 2·5 3516·8 38·6 9 1300 11·5 764·91 99·9 0·058 75·45 384·311 1·7 3293·9 26·2 Total gas age 3107·5 28·8 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 30·1–30·2 m 1 800 7·1 831·15 98·63477 0·734 1·819598 319·415 0·7 3018·3 10·7 2 880 1·7 207·82 98·46037 0·144 7·334663 336·103 1·2 3093·3 18·4 3 950 14·6 1968·35 99·4128 0·066 29·60979 370·579 1·3 3239·0 19·6 4 980 16·3 1953·70 99·60721 0·077 37·92846 329·466 1·7 3063·8 24·3 5 1000 9·7 1086·63 99·49839 0·080 33·00942 306·726 1·3 2959·1 19·3 6 1015 4·1 468·07 99·20482 0·081 22·85102 313·544 1·5 2991·2 21·6 7 1045 3·7 412·32 99·13203 0·092 19·73829 304·459 2·6 2948·3 38·4 8 1095 4·6 533·32 98·68499 0·092 13·46074 316·918 1·3 3006·8 19·3 9 1160 13·7 1805·51 99·77105 0·072 50·79623 361·611 0·8 3202·2 12·4 10 1300 24·6 3087·86 99·70786 0·071 46·17452 344·218 0·8 3128·6 11·5 Total gas age 3101·4 26·1 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00020 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 30·1–30·2 m 1 800 7·1 831·15 98·63477 0·734 1·819598 319·415 0·7 3018·3 10·7 2 880 1·7 207·82 98·46037 0·144 7·334663 336·103 1·2 3093·3 18·4 3 950 14·6 1968·35 99·4128 0·066 29·60979 370·579 1·3 3239·0 19·6 4 980 16·3 1953·70 99·60721 0·077 37·92846 329·466 1·7 3063·8 24·3 5 1000 9·7 1086·63 99·49839 0·080 33·00942 306·726 1·3 2959·1 19·3 6 1015 4·1 468·07 99·20482 0·081 22·85102 313·544 1·5 2991·2 21·6 7 1045 3·7 412·32 99·13203 0·092 19·73829 304·459 2·6 2948·3 38·4 8 1095 4·6 533·32 98·68499 0·092 13·46074 316·918 1·3 3006·8 19·3 9 1160 13·7 1805·51 99·77105 0·072 50·79623 361·611 0·8 3202·2 12·4 10 1300 24·6 3087·86 99·70786 0·071 46·17452 344·218 0·8 3128·6 11·5 Total gas age 3101·4 26·1 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00020 Open in new tab The remaining two hornblende separates from the drill core sections 26·8–26·9 m and 30·1–30·2 m both yielded similar results with total gas ages of 3108 ± 29 and 3101 ± 26 Ma, respectively (Fig. 13c and d, Table 7). In both cases, a certain variability in the ages obtained at the various steps prevents a good plateau from being seen and this is also reflected in large errors in the (comparable) ages obtained from 40Ar/36Ar vs 39Ar/36Ar and 39Ar/40Ar vs 36Ar/40Ar diagrams (not shown). Very little evidence of partial Ar loss caused by secondary alteration is indicated for these samples, in accordance with petrographic observations. DISCUSSION Age and geotectonic setting of the analysed basement rocks The best constraints on the time of magmatic crystallization of the investigated granite and hornblende metagabbro are provided by the concordant U–Pb zircon age data. They are 3064 ± 20 and 3063 ± 5 Ma, respectively, identical within error (Fig. 11). Younger concordant and discordant U–Pb ages, in particular those obtained from the granite samples, point to a partial or complete resetting of the U–Pb system at <2720 Ma, perhaps by a pervasive alteration process. At this point it is worth noting that this alteration process left the original Hf isotope composition unchanged, as is reflected in the within-error identical 176Hf/177Hf (c. ±1·5 ϵ-units) ratios obtained for nearly all concordant and discordant zircon domains in the respective samples (Fig. 12). This clearly indicates that all the zircon grains or domains formed during the magmatic crystallization event at c. 3065 Ma, and not during later processes (e.g. during a Neoarchaean or Palaeoproterozoic metamorphic or metasomatic overprint). Clues about the age of such an overprint are, apart from the younger U–Pb zircon ages (Fig. 11), provided by the Rb–Sr isotope whole-rock data, which yield an errorchron age of c. 2633 Ma. However, the geological significance of this Rb–Sr age, which is predominantly constrained by the spread of the data from the granite sample, is not entirely clear. Judging from the petrographic observations (formation of secondary microcline, even in the metagabbro, and sericitization), it appears likely that the Rb–Sr errorchon age of 2633 Ma reflects the time of retrograde hydrothermal infiltration and alteration. In this context, the somewhat older ‘concordant’ U–Pb zircon age of c. 2720 Ma could be explained as a maximum age that results from incomplete Pb loss from altered (metamict), primary magmatic zircon domains at 2·63 Ga. Surprisingly, and in contrast to the Rb–Sr isotope data, a pervasive Neoarchaean hydrothermal alteration event is not indicated by the Ar–Ar data obtained on the hornblende from the metagabbro. As shown in Fig. 13, the hornblende separates yielded total fusion Ar–Ar ages, which are either within error of the zircon crystallization ages (3064 ± 19 Ma and 3078 ± 20 Ma) or slightly older (3108 ± 29 Ma and 3101 ± 26 Ma), and provide little evidence for a Neoarchaean or younger overprint. This finding points clearly to hornblende formation at, or close to, the time of magma crystallization. Judging from the very low Ti content of the hornblende in the metagabbro, and the fact that it occurs together with other Ti-bearing (buffer)-phases, it seems most likely that the magnesio-hornblende was not formed during magmatic crystallization, but rather resulted from post-magmatic, solid-state alteration either of primary magmatic (Ti-rich) hornblende or of other mafic magmatic phases (such as pyroxenes) immediately after magmatic crystallization. Possibly, this transformation was caused by pervasive autometasomatism. Such an explanation conforms to the U–Pb and Ar–Ar age data, but also to the very narrow range in Rb/Sr and 87Sr/86Sr in the five hornblende metagabbro samples analysed (Fig. 9, Table 4). Such an interpretation finds further support from recent O isotope data obtained on whole-rocks and mineral separates from the investigated core samples (M. Depiné et al., unpublished data, 2009). The slightly older 40Ar–39Ar ages obtained for some hornblende samples, or some steps, might result from the incorporation of excess Ar during secondary alteration of hornblende to actinolite and/or chlorite in the course of a later (Neoarchaean) low-grade metamorphic and/or hydrothermal overprint. Alternatively, the excess Ar may be due to infiltration and reaction of late-magmatic fluids that caused the autometasomatism suggested above. Such Ar-enriched fluids were perhaps released during crystallization of the associated granites, which formed nearly contemporaneously with the hornblende gabbro, as suggested by the within error-identical U–Pb zircon ages and Hf isotope data. In fact, this late- to post-magmatic infiltration process, which is related either to granite emplacement or to another unexposed source, could also account for the formation of the biotite-rich alteration zone between the hornblende metagabbro and the granite, and the presence of microcline throughout the metagabbro. This microcline, although only in minor proportions, is unusual and cannot be explained by fractional crystallization because of the low density of K-feldspar and its relatively late crystallization. As the same type of microcline is also found in locally very coarse-grained patches within the granite and concentrated along veins in the metagabbro, it is most probably related to the infiltration of a potassic fluid—possibly during a stage of pegmatite formation evident from surface outcrops (Fig. 3). Pegmatite formation was followed by a further retrograde, hydrothermal overprint as indicated by the chloritization of biotite, sericitization and saussuritization of microcline and plagioclase, respectively, and the formation of actinolite at the expense of magnesio-hornblende and the various veinlets, especially in the hornblende metagabbro. It appears likely that this final hydrothermal overprint was responsible for the resetting of the Rb–Sr isotope system, in particular of the granite, as reflected by the Rb–Sr whole-rock errorchron age of c. 2630 Ma. The reconstruction of the tectonic setting of the investigated magmatic rocks is hindered by their complex post-magmatic alteration. Some elements, such as the alkali elements, are certain to have been mobile. For instance, the (Na + K) concentrations shown in Fig. 4a are likely to be higher than those of the magmatic protoliths because of the later K-metasomatism. Consequently, the protoliths were even less alkaline than shown in that diagram and conform to the calc-alkaline series, as is also indicated by the relatively small proportion of total Fe relative to Mg at variable SiO2 concentrations (Fig. 4c). Despite the problems of some major element mobility, trends of petrogenetic significance can be derived from certain less mobile trace element distributions. Almost all geochemical indicators point to a calc-alkaline composition and crustal contamination typical of continental arc magmatism. This includes, for example, the Nb–Y and Ta–Yb relationships in the granite (Fig. 6), the overall enrichment of the metagabbro in the less compatible elements (e.g. Th), its relative depletion in Nb and its strong depletion in Ti relative to MORB (Fig. 7). Variable crustal contamination is also reflected by LREE enrichment and by chondritic to slightly subchondritic ϵNd 3·06Ga (mostly between +1·9 and −1·7) and ϵHf 3·06Ga values (+1·7 to −2·3; Fig. 10, Table 6). Notably, the ϵHft values are identical within error to those recently obtained for similarly old rocks from other parts of the Kaapvaal Craton by Zeh et al. (2009), comprising 3·1 Ga granitic rocks from the Swaziland, Witwatersrand and Pietersburg blocks (Fig. 1). The very low ϵNd 3·06Ga values of −4·8 and −10·4 obtained from two samples (Fig. 10) result either from post-intrusive alteration (e.g. Moorbath et al., 1997) or from the assimilation of much older crust, recycled during the intrusion of the granitic and gabbroic protoliths. Notably, similarly strong variations are not recorded by the Hf isotope data. Following Pearce (2008), we chose Yb-normalized Th and Nb as proxies for crustal input. Both Th and Nb have a similar geochemical behaviour in most petrogenetic processes and both are relatively immobile under conditions that range from weathering to medium-grade metamorphism. With regard to the Th–Nb–Yb relationships (Fig. 14a), the hornblende metagabbro samples plot in a position similar to enriched (E-)MORB with variable degrees of crustal contamination. In a similar way, Condie (2005) described the extent of crustal contamination versus mantle composition in terms of Nb/Th vs Zr/Nb and distinguished between plume and non-plume sources based on the Nb/Y vs Zr/Y relationships. As pointed out by Pearce (2008), the application of the latter diagram is problematic because of depth-dependent garnet fractionation. Thus the apparent plume source indicated in Fig. 14c may not be real. The low Nb/Th is in agreement with a volcanic arc setting, but the Zr/Nb is lower than expected for such a setting (Fig. 14b). Fig. 14. Open in new tabDownload slide (a) Th/Yb vs Nb/Yb plot for the hornblende metagabbro samples and the Dominion Group mafic volcanic rocks, utilizing the Th–Nb proxy for crustal contamination as suggested by Pearce (2008); (b) Zr/Nb vs Nb/Th, and (c) Nb/Y vs Zr/Y plots for the same samples. Compositional fields for different tectonic settings and effects of batch melting (BM) and subduction (SUB), as indicated by arrows, are from Condie (2005); PM, primitive mantle; DM, depleted mantle. The Cs/Nb and U/Nb ratios are supposed to be insensitive to magmatic differentiation by fractional crystallization and are therefore also used to assess the extent of crustal contamination (Fig. 15a). As can be seen from that figure, most samples plot away from primitive mantle ratios, thus testifying to considerable crustal contamination in both the mafic and felsic portions. One trend projects to particularly high U/Nb ratios at reasonable crustal Cs/Nb, whereas one sample plots far away from that trend, having a particularly high Cs/Nb ratio. The latter is rich in microcline and probably reflects a geochemical change during K-metasomatism. The U/Nb ratios are high for most samples, even for the mafic rocks, which show, on average, higher U/Nb ratios than typical of continental crust (Fig. 15b). Again, considerable crustal contamination can be inferred from this observation. Fig. 15. Open in new tabDownload slide Cs/Nb vs U/Nb (a) and U/Nb vs Ce/Pb (b) diagrams for the various lithotypes in the investigated drill core. The fields or points for oceanic island basalt (OIB), mid-oceanic ridge basalt (MORB), primitive mantle (PM), lower (LC), middle (MC) and upper continental crust (UC) are from Sun & McDonough (1989) and Rudnick & Fountain (1995). Also shown are fractional crystallization trends for amphibole (Amph), clinopyroxene (Cpx), plagioclase (Plag) and alkali feldspar (K-fsp), each arrow representing 50% fractional crystallization. The influence of fractional crystallization on the scale of Fig. 15a is negligible. The enrichment in U is also evident in Fig. 15b, in which the U/Nb ratio is plotted against Ce/Pb. The latter ratio is explicable in terms of extensive feldspar fractionation in the case of the granite and hornblende metagabbro, and conforms to the observed negative Eu anomalies (Fig. 8). This effect is most pronounced in the biotite-rich zone, because of the elevated, probably hydrothermal, allanite content therein. Formation and alteration of the basement next to the Witwatersrand Basin The U–Pb crystallization age of 3062 ± 5 Ma for the plutonic rocks adjacent to the ancient Witwatersrand Basin invariably invites a comparison to be made with the age of Dominion Group volcanism. The volcano-sedimentary Dominion Group underlies the Witwatersrand Supergroup rocks over probably a much larger area than indicated by the remnants of that group known from surface and subsurface exposures mainly along the northwestern margin of the Witwatersrand Basin (Fig. 2). The precise U–Pb single zircon age of 3074 ± 6 Ma obtained for a quartz–feldspar porphyry in the upper part of the Dominion Group (Armstrong et al., 1991) is only slightly older than the crystallization age obtained in this study. This raises the possibility that the intrusive rocks of this study are the plutonic expression of the same type of magmatism that formed most of the Dominion Group. A continental rift setting has been generally assumed for the Dominion Group (e.g. Clendenin et al., 1988; Marsh et al., 1989; Jackson, 1992) because of the bimodal nature of the volcanic suite and the tholeiitic affinity of the mafic rocks. The evidence is, however, not unequivocal. Geochemical data for the Dominion Group volcanic rocks were presented by Marsh et al. (1989) and Jackson (1994). The bimodal compositional distribution encompasses rocks with a range of SiO2 concentrations between 51 and 61%, with andesitic compositions dominating; however, there is a gap between 61 and 67% SiO2. A bimodal SiO2 distribution has been noted in many Archaean arcs and explained by shallow subduction caused by a higher mantle heat flow resulting in thicker oceanic crust (Abbott & Hofmann, 1984). According to Marsh et al. (1989) the Dominion Group mafic lavas are enriched in the less compatible trace elements shown in Fig. 7 in relation to MORB but they are depleted in Nb relative to Th, La, and Ce (no data for Ta available). Moreover, they are characterized by depletion in Ti relative to MORB and overall show a trace element pattern that resembles that of typical calc-alkaline basalts. The geochemistry of the Dominion Group volcanic rocks could thus be easily explained by a volcanic arc setting. Interestingly, Burke et al. (1986) previously suggested that the Dominion Group volcanic rocks erupted on the landward flank of an Andean-type continental margin. Closer inspection of the dataset of Marsh et al. (1989) reveals, however, that there are certain differences from the geochemical characteristics of the intrusive rocks studied by us. For example, the Nb and Y concentrations of their silicic rocks, the quartz–feldspar porphyries, correspond to those of volcanic arcs as well as syn-collisional granites but their Nb/Y ratios are slightly lower than for the granites investigated in this study (Fig. 6a). In terms of Th–Nb–Yb relationships, the Dominion Group volcanic rocks follow a trend from N- to E-MORB but lack evidence of major mantle–crust interaction (Fig. 14a). Minor crustal contamination might be indicated by their calculated Nd model ages (mean age 3233 ± 53 Ma, recalculated from the data of Marsh et al., 1989), which are slightly older than the age of emplacement. This is, however, subject to the previously mentioned uncertainty in the projection of the Archaean depleted mantle composition in general and that below the Kaapvaal Craton in particular. The Dominion Group rocks exhibit very different Zr–Nb–Y relationships that are more akin to typical volcanic arc compositions, except for a higher Nb/Th ratio (Fig. 14b and c). It is, therefore, assumed that the intrusive rocks of this study are not co-genetic with the Dominion Group lavas and might be slightly younger. In particular, the absence of evidence of significant crustal contamination in the Dominion Group rocks might indicate that they formed earlier, in a less mature arc above a thinner crust, whereas the 3062 ± 5 Ma intrusive rocks could reflect a more mature stage of arc development. Apart from our Nd and Hf isotope and Nb–Th–Yb–REE data, crustal contamination at 3·06 Ga seems likely in view of the presence of older crust in the neighbouring basement along the northern and northwestern margin of the Witwatersrand Basin and in the Vredefort Dome (for a summary of geochronological data see Poujol et al., 2003; Armstrong et al., 2006). The oldest reported zircon ages are 3480 ± 7 Ma from xenocrysts in a quartz porphyry of the upper Ventersdorp Supergroup (Armstrong et al., 1991) and c. 3245 Ma from paragneiss in the Vredefort Dome (Hart et al., 1999), respectively; these ages are similar to many of the Nd and Hf model ages obtained during this study (Table 6). Following trondhjemite–tonalite emplacement (the most precise age is 3340 ± 3 Ma, Poujol & Anhaeusser, 2001) in the Johannesburg Dome, the basement along the northern basin margin was affected by large-scale granodiorite emplacement with U–Pb single zircon ages of 3121 ± 5 Ma (southern Johannesburg Dome), 3120 ± 5 Ma (200 km SW of Johannesburg) and 3114 ± 2 Ma (southwestern Johannesburg Dome, Poujol & Anhaeusser, 2001). A slightly younger U–Pb zircon age of Ma was obtained on a granodiorite near Coligny (Robb et al., 1992). Our new data are comparable with previously obtained Rb–Sr, Pb–Pb and Sm–Nd data obtained on similar granitoids from the Johannesburg Dome. Granodiorite and granite from that area yielded Rb–Sr and Pb–Pb isochron ages of 3081 ± 33 and 3062 ± 26 Ma, respectively, and a Pb–Pb zircon age of 3093 ± 3 Ma (Barton et al., 1999). Significantly older Nd model ages obtained in the same study indicate involvement of older crustal components. Based on geochemical data for these calc-alkaline granitoids in the Johannesburg Dome, Anhaeusser (1999) concluded that these rocks formed in a volcanic arc setting. The age obtained here for the drilled granite and hornblende metagabbro (3062 ± 5 Ma) is identical within error to that reported by Armstrong et al. (2006) for an aplite dyke in the Vredefort Dome (3068 ± 6 Ma). The volumetrically minor aplite emplacement in the Vredefort Dome is the last of three magmatic episodes there, following tonalite–trondhjemite–granodiorite emplacement probably in an oceanic arc at 3·1 Ga and subsequent high-grade metamorphism and granite–granodiorite emplacement between 3·1 and 3·08 Ga (Armstrong et al., 2006). The latter episode has been interpreted by Armstrong et al. to reflect crustal thickening in response to accretion of the Vredefort rocks onto an older core of the Kaapvaal craton. Our new results reinforce the idea of Armstrong et al. (2006) that the central Kaapvaal Craton, which later became covered in places by the Witwatersrand sediments, had not already consolidated into a stable craton by 3·2 Ga, as suggested by de Wit et al. (1992) and subsequently adopted by many workers, but continued to grow through the accretion of magmatic arcs until about 3·06 Ga. On the basis of available data so far, all potential source rocks for the proximal Witwatersrand sediments seem to be of a magmatic arc affinity. The exact geometry of these magmatic arc systems remains uncertain, but an east-northeasterly trend along the northern margin of the Witwatersrand Block can be distinguished from a northwesterly trend in the Vredefort Dome and in the western part of the craton. It may be speculated that the voluminous potassic granites of similar age in the Barberton–Swaziland region (Poujol et al., 2003; Zeh et al., 2009), formed in response to extensive crustal heating within the continent behind an arc system. This conclusion is in good agreement with Hf isotope data recently obtained from granitoid rocks in the eastern part of the Kaapvaal Craton (Zeh et al., 2009). These data indicate that at c. 3·1 Ga new crust was added to the pre-existing basement of the Witwatersrand block, perhaps in response to roughly southward subduction during the accretion of the Pietersburg block onto the Witwatersrand block. In contrast to most previous models, we prefer an intra-arc position for the Dominion Group. This is notwithstanding the bimodal nature of the volcanism (similar to the bimodal character of the investigated drill core). In the light of mounting evidence for andesite magmas being formed by mixing between evolved, silicic melts and basic plutonic root components (e.g. Reubi & Blundy, 2008), the mere presence or absence of a bimodal magmatic suite might not be a reliable indicator of a specific tectonic setting. Instead, intra-arc extension might have provided suitable pathways for the ascent of felsic and mafic melts. The retrograde, hydrothermal alteration of the various basement rocks in the studied drill core is similar to that previously described for basement granites along the northern and northwestern margins of the Witwatersrand Basin (Klemd & Hallbauer, 1987; Robb & Meyer, 1987). As already pointed out by Klemd (1999), the similar fluid inclusion characteristics of the hydrothermal alteration of the basement granites and of the Witwatersrand Basin fill are suggestive of a relationship. The situation is, however, complicated because of the multistage alteration history recorded by the Witwatersrand metasedimentary rocks (Frimmel et al., 2005). This ranges from diagenetic dewatering to regional low-grade metamorphism, a thermal overprint by the Bushveld event, to brittle deformation and further hydrothermal alteration triggered by the Vredefort impact. Our zircon U–Pb and Rb–Sr isotope data are consistent with previous interpretations that the basement below the Witwatersrand Basin was affected by several alteration events between 2720 and 2630 Ga (Figs 6 and 11). For example, Kositcin et al. (2003) recognized, based on U–Pb sensitive high-resolution ion microprobe (SHRIMP) data on different hydrothermal xenotime generations, at least three stages of hydrothermal fluid infiltration. The oldest of these (2720 Ma) is close to the time of the outpouring of the voluminous Klipriviersberg lavas (lower Ventersdorp Supergroup) and is identical to the older alteration age suggested above for the pre-Witwatersrand basement. Considerable heating affected the Kaapvaal crust at that time, which is also evident from the contemporaneous ultrahigh-temperature metamorphism in the lower parts of that crust (Schmitz & Bowring, 2003). The cause of the 2714 ± 8 Ma Klipriviersberg volcanism (Armstrong et al., 1991) remains a matter of debate, with both a mantle plume and/or crustal thinning having been held responsible for it. More recently, Silver et al. (2006) suggested flood basalt eruption in a collisional rift. Their model involves short-term drainage of a molten basalt reservoir in the sublithospheric mantle during a change in the stress field in an overall collisional setting. In this case, the collision would be an early stage of amalgamation of the Kaapvaal Craton with crustal fragments now present in the Central Zone of the Limpopo Belt (see Zeh et al., 2009). A northward thrusting event that affected the greenstone belts along the northern flank of the craton, dated at 2729 ± 19 Ma (Passeraub et al., 1999), is likely to be an expression of this collision. Furthermore, several granites of comparable ages (Robb et al., 1992) in the immediate vicinity of the Witwatersrand may be related to the same tectonic stage. The maximum temperature experienced by the Witwatersrand Basin fill was attained at different times in different parts of the basin (Frimmel et al., 2005). At least in the upper parts of the basin fill it did not exceed lower greenschist-facies temperatures (350 ± 50°C; Frimmel, 1994; Phillips & Law, 1994). Along the northern margin of the Witwatersrand Basin, peak metamorphic conditions were already reached by the end of Ventersdorp Supergroup deposition, as indicated by kyanite- and pyrophyllite-bearing post-Platberg and pre-Transvaal thrust faults (Coetzee et al., 1995). The inferred younger alteration of the basement rocks investigated in this study could well be an expression of this collisional event in the Limpopo Belt because the obtained Rb–Sr errorchron ‘age’ of 2633 ± 50 Ma overlaps with the published ages of syntectonic granites in the Limpopo Belt (McCourt & Armstrong, 1998; Kröner et al., 1999; Zeh et al., 2007, 2009; Millonig et al., 2008). In addition, the high initial 87Sr/86Sr shown in Fig. 9 points to the involvement of crustal fluids. Further hydrothermal xenotime growth was noted by Kositcin et al. (2003) at 2210 Ma (Pretoria Group extension) and again at 2046–2061 Ma, the time of the Bushveld event. These younger events do not seem to have further disturbed significantly the isotopic composition of the investigated basement rocks and zircon grains therein. Similarly, the 2023 Ma Vredefort impact event, which undoubtedly triggered renewed fluid circulation through the Witwatersrand Basin and its surroundings (Frimmel et al., 1999), did not affect the isotope systems of the studied rocks, analogous to a previous finding by Barton et al. (1999) in the Johannesburg Dome. Significance for Witwatersrand gold genesis Mounting evidence exists for plate-tectonic processes having already been operative in Mesoarchaean times (e.g. de Wit, 1998; Moyen et al., 2006; Zeh et al., 2009). Thus comparison with the gold productivity in various post-Archaean plate-tectonic settings seems justified. The vast majority (∼87%) of known primary gold deposits (i.e. excluding secondary, placer deposits), appear to have been formed along active continental margins, where they are present as orogenic (including intrusion-related), Cu–Au porphyry and epithermal types (Frimmel, 2008). There is no doubt that active continental margins provide by far the best sites for the concentration of Au into ore bodies and there is no reason why this should have been any different in the Archaean. Of significance is not only our conclusion that at least some of the auriferous Witwatersrand sediments could have been sourced in an active continental margin but particularly the finding of ‘autometasomatized’ hornblende gabbro (or diorite) in that source area. The REE patterns of the studied mafic rocks are identical to those that are typically explained by the fractionation of middle and heavy REE-enriched hornblende. The abundance of hornblende in these rocks makes this a highly feasible explanation. A high magmatic oxidation state is unlikely to have been the reason for the noted lack of a pronounced negative Eu anomaly in the hornblende metagabbros because of the Mesoarchaean age of the system. This could also be due to the suppression of feldspar fractionation. Early plagioclase crystallization is suppressed when a magma contains sufficient H2O to stabilize hornblende as an early liquidus phase. By analogy with studies such as those of Rutherford & Devine (1993) and Richards et al. (2001) an H2O content of >4 wt % is required for the crystallization of near-liquidus hornblende. Such an elevated H2O content in the melt is a critical and typical ingredient of a number of productive magmatic–hydrothermal ore-forming systems worldwide, such as porphyry copper (–gold) or iron oxide–copper–gold (IOCG) deposits. Consequently, the investigated basement rocks contain the essential prerequisites for particularly high fertility in terms of primary magmatic–hydrothermal gold mineralization if the protolith of the studied mafic rocks was indeed a hornblende gabbro. In the light of a total absence of any pyroxene relics anywhere in the drilled hornblende metagabbro this is our preferred interpretation. Derivation of the Witwatersrand placer gold from an Au-enriched continental volcanic arc also would explain the previously noted wide range of generally orders of magnitude higher Os concentrations in the Witwatersrand gold compared with any other type of gold investigated so far, as noted by Kirk et al. (2002) and Frimmel et al. (2005). It has been suggested by these workers that the Witwatersrand gold was originally magmatic and extracted from a 3·1–3·3 Ga mantle source because of an initial 187Os/188Os value of 0·108 that corresponds to the projected depleted mantle composition at that time. These Os-depleted mantle extraction ages agree well with the Nd and Hf model ages obtained during this study. The wide range in Os concentrations would be explicable by a mixture of sources, including disseminated porphyry-style gold, intrusion-related, IOCG, epithermal or orogenic gold. The extent to which the gold was transported by hydrothermal fluids (as opposed to melts), which is vastly different between the above sources, would control the Os concentration of a particular type of gold because of the extremely low solubility of Os in aqueous fluids. Evidence of mesothermal (orogenic), potentially gold-bearing, quartz veins in the source area exists in the form of O isotope data on quartz pebbles in the host conglomerate (Vennemann et al., 1992, 1995). Such veins are, however, an unlikely major contributor to the Witwatersrand gold budget. The required abundance of typical Archaean mesothermal lode gold deposits in the source area to explain about 40% of all known gold would have to be unrealistically high. Hallbauer & Barton (1987) previously suggested that Archaean greenstone-hosted gold from quartz veins, as mined in the Barberton greenstone belt, could not have been a major source of the Witwatersrand gold because of compositional differences. Instead, they proposed that altered granites with as much as 80 ppb Au in the hinterland could be a more likely source. A magmatic–hydrothermal origin of the Witwatersrand gold solves the mass-balance problem and also explains the absence of vein quartz pebbles with visible gold inclusions in the Witwatersrand goldfields. It also explains the overall very small size of the Witwatersrand gold particles (see Minter et al., 1993) and the lack of reasonably sized gold nuggets (although the latter could simply be due to a lack of an oxidizing atmosphere). The comparison with magmatic–hydrothermal mineralizing systems, such as porphyry Cu–Au or IOCG systems, makes it tempting to interpret the earlier stages of the observed hydrothermal alteration as being part of such a mineralizing system. Late-stage magmatic alteration is additionally indicated by our combined datasets for the hornblende metagabbro, for which a late-magmatic stage of ‘hydrothermal’ autometasomatism immediately after emplacement at c. 3065 Ma is suggested by the very similar Ar–Ar ages of very low-Ti hornblende. In contrast, the Neoarchaean hydrothermal alteration at c. 2720 and 2630 Ma, as indicated by our Rb–Sr and U–Pb age data, might have nothing to do with enrichment in Au in the central Kaapvaal crust, because the bulk, if not all, of the gold in the Witwatersrand Supergroup had already been deposited within the largely conglomeratic host rocks prior to that time. The only effect this hydrothermal alteration had on the gold was that of local, short-range mobilization and dispersion within the host conglomerates. CONCLUSIONS New lithogeochemical, isotopic and geochronological data for hornblende metagabbro and granite from a basement horst at the northwestern margin of the Witwatersrand Basin provide new insights into the nature of some pre-Witwatersrand units that could have been potential source rocks for some of the auriferous conglomerates particularly in the Central Rand Basin. Both the hornblende metagabbro and the granite yielded indistinguishable U–Pb zircon ages of 3062 ± 5 Ma. It is concluded from widespread K-metasomatism and the alteration of presumably primary Ti-rich hornblende to Ti-poor magnesio-hornblende at effectively the same time that the granite intruded into the gabbro shortly after the latter's crystallization. Both rock types have geochemical signatures that are typical of calc-alkaline magmatism. This, combined with evidence for contamination by older crustal components based on Nd and Hf model ages for whole-rocks and zircon grains, respectively, leads to the conclusion that a continental volcanic arc is the most likely magmatic setting. Our new results support previous suggestions of successive arc accretion onto the northern and western margins of the Witwatersrand block prior to the development of the various basins that constitute the Witwatersrand ‘successor’ basin. With a previously published age of the Witwatersrand gold (and associated detrital pyrite) that is within error of the age obtained for the pre-Witwatersrand units of this study and the proximity of the studied rocks to the sites of deposition of the auriferous conglomerates, the investigated rocks could well be a potential source for some of the Witwatersrand gold. Inferred primary hornblende gabbro provides evidence for ascending water-rich melts that might have been particularly conducive for the transfer of Au into the crust. Gold enrichment in the Palaeo- to Mesoarchaean hinterland, similar to that found in younger active plate margins, might have been a major controlling factor for the unique extent of gold enrichment in the Witwatersrand Basin fill. ACKNOWLEDGEMENTS A. Gerdes is thanked for providing the infrastructure for the single zircon analyses. S. Govender assisted with the Nd isotope analyses, and H. Brätz and U. Schüßler helped with respectively the LA-ICPMS and XRF analyses. The Rb–Sr isotope analyses were kindly conducted by M. Bröcker. C. Anhaeusser, J. Barton and an anonymous reviewer provided constructive comments that helped to improve the original manuscript. Financial support by the Deutsche Forschungsgemeinschaft (DFG grant FR2183/3-1 and FR2183/3-2) is gratefully acknowledged. 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Geochemical and Geochronological Constraints on the Nature of the Immediate Basement next to the Mesoarchaean Auriferous Witwatersrand Basin, South Africa

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Abstract

Abstract A combined petrological, geochemical, and geochronological (Rb–Sr and Sm–Nd whole-rock, U–Pb and Lu–Hf zircon, and Ar–Ar hornblende) study on a section of pre-Witwatersrand basement drilled at the northwestern margin of the Witwatersrand Basin has revealed new insights into the nature and tectonic setting of the likely source area for some of the Mesoarchaean auriferous Witwatersrand sediments. The protoliths of intersected altered granite and hornblende metagabbro are of indistinguishable age (3062 ± 5 Ma) and have very similar geochemical signatures. Trace element characteristics typical of calc-alkaline magmatism and evidence of variable contamination with older crust (subchondritic ϵNd and ϵHf in zircon) point to an active continental margin setting. The Ar–Ar hornblende ages are within error of the magmatic crystallization age or slightly older. Alteration of presumably primary magmatic hornblende to magnesio-hornblende immediately after gabbro emplacement during late magmatic autometasomatism is suggested. The presence of hydrous melts (>4 wt % H2O), comparable with fertile Au-bearing magmatic–hydrothermal mineralizing systems in Phanerozoic volcanic arcs, is inferred. Thus, a kind of hinterland is proposed for the Witwatersrand that compares favourably with the tectonic domains that are known to host the majority of post-Archaean gold deposits. Later retrograde hydrothermal alteration at c. 2720 and 2630 Ma led to variable Pb loss in zircon and the resetting of the whole-rock Rb–Sr isotope system whereas the Ar–Ar and Lu–Hf isotope systems in the hornblende and zircon grains, respectively, were not significantly affected. Comparison with published data suggests that these alteration events are the same as those that affected the Witwatersrand Basin fill associated with major early Ventersdorp flood basalt volcanism and possibly a pre-Transvaal thrusting event in response to contractional deformation in the Limpopo Belt. INTRODUCTION The Mesoarchaean Witwatersrand Basin in South Africa accounts for about 40% of all known gold (Frimmel, 2008) and thus represents by far the single most important gold depository known in the Earth's crust. The genesis of the ore bodies in the Witwatersrand goldfields has been a matter of intense debate (for a relatively recent summary of the arguments see Muntean et al., 2005); however, consensus is emerging that the gold entered the conglomeratic host-rocks essentially in the form of detrital particles that were subsequently mobilized on a local scale by various post-depositional fluids (for a review and summary of arguments see Frimmel et al., 2005). A palaeoplacer model invariably raises the question of the source of all that gold and thus of the nature of the hinterland at the time of detrital gold transport and sedimentation. The term ‘Witwatersrand Basin’, although widely accepted in the literature, is misleading because it refers to tectonically different basins whose fills were stacked on top of each other. Most of the Witwatersrand gold (>80%) is concentrated in quartz-pebble conglomerates (reefs) of the <2·90 to >2·78 Ga Central Rand Group. In contrast to the underlying <2·98 to 2·91 Ga West Rand Group (contributing <5% of Witwatersrand gold production) for which a passive margin setting is indicated, the predominantly fluvial sediments of the Central Rand Group were deposited in a foreland basin whose shape was similar to the current subcrop outline, except for the later up-doming of the central part as a result of the 2023 Ma Vredefort impact event (Fig. 1; Coward et al., 1995; Kositcin & Krapez, 2004; Frimmel et al., 2005). Fig. 1. Open in new tabDownload slide Main Archaean stratigraphic units of the Kaapvaal Craton. The West Rand and Central Rand Groups constitute the Witwatersrand Supergroup. Bold dashed line outlines the boundary of the Kaapvaal Craton as inferred from aeromagnetic data. The crustal blocks (amalgamated by 2·8 Ga) are separated by major lineaments; modified from Eglington & Armstrong (2004) and Frimmel et al. (2005). Information on the nature of the source rocks may be obtained from the study of the clastic sediments. Derivation of the various allogenic components of the Witwatersrand sediments, including gold, from older granite–greenstone terranes, such as the Barberton Greenstone Belt, has been suggested repeatedly by several workers (e.g. Viljoen et al., 1970; Robb & Meyer, 1990). A number of undoubtedly allogenic minerals indicative of both felsic and mafic sources, most importantly zircon and chromite, respectively, are present in the siliciclastic Witwatersrand sediments (for a detailed list of minerals see Feather & Koen, 1975). From the distribution of relatively immobile elements in Witwatersrand shales, Wronkiewicz & Condie (1987) concluded that the proportion of granite, basalt and komatiite in the source areas increased with time at the expense of tonalite. Vennemann et al. (1992, 1995) showed that the oxygen isotopic composition of the Witwatersrand quartz pebbles conforms to that of mesothermal vein quartz as found in greenstone-hosted orogenic gold deposits. Detrital zircon age spectra from the Witwatersrand (Kositcin & Krapez, 2004) agree well with available ages from the various granitoid–greenstone belts to the east, north and west (Poujol et al., 2003; for a comparison see Frimmel et al., 2005). The basal contacts of the numerous auriferous and uraniferous conglomerate beds typically represent an erosional surface above which older intrabasinal sedimentary units have been partially reworked. The sedimentary units higher in the succession, for example the 2·71 Ga Ventersdorp Contact Reef (contributing c. 15% of the total Witwatersrand gold production) above the Witwatersrand Supergroup, reflect considerable reworking of the underlying sediments. This reworking of intrabasinal sediments forms a major obstacle in the reconstruction of the primary source rocks and our attention is, therefore, directed towards the potential source rocks directly. Interestingly, most zircon grains in the arenitic units below and above single reefs of the Central Rand Group have ages between 2·96 and 2·83 Ga, whereas those from the auriferous conglomeratic reefs indicate a dominance of 3·05–3·09 Ga sources (Ruiz et al., 2006; N. Koglin, unpublished data, 2009). An imprecise Re–Os age of 3016 ± 110 Ma obtained by Kirk et al. (2002) for the gold itself (3033 ± 21 Ma obtained for rounded pyrite and gold combined) is clearly older than the age of sediment deposition and thus provides strong support for the palaeoplacer model. More importantly, these ages conform to the detrital zircon age peaks mentioned above for the reef units. These data indicate that 3·05–3·09 Ga pre-Witwatersrand units are by far more important as a potential source of the gold than the various younger, syn-depositional granitoids in the hinterland and thus they will be at the centre of this study. Numerous studies have been conducted on the main granite–greenstone terranes of the Kaapvaal Craton, especially on the Barberton Belt (for recent reviews and further references see Brandl et al., 2006; Robb et al., 2006; Zeh et al., 2009). Today's exposed granite–greenstone belts on the craton (Fig. 1) cannot, however, be the source regions of the Witwatersrand sediments and the gold therein. They are the deeply eroded remnants of what might have been a potential 3·5–2·8 Ga hinterland. Although the finer grained metasedimentary rocks of the Witwatersrand Supergroup and its stratigraphic equivalent, the Pongola Supergroup (Fig. 1), may have been sourced in distal hinterlands, the allogenic components of the coarse-grained, auriferous and uraniniferous conglomerate beds are likely to be derived from proximal sources. Within a given reef, both Au and U concentrations decrease from the basin margin towards its centre, with a systematic increase in the U/Au ratio down the palaeoslope. This observation has been explained by hydraulic sorting of allogenic gold and uraninite particles (Minter et al., 1986; Frimmel et al., 2005). Thus the source rocks for the high-energy detritus, including the gold, have to be sought in proximity to the former basin margin. Unfortunately, data on such pre-Witwatersrand units in the immediate vicinity of the Witwatersrand Basin are sparse because of a lack of outcrop. So far, two types of basement have been recognized in the vicinity of the Witwatersrand Basin: bimodal volcanic rocks with minor siliciclastic sedimentary material of the Dominion Group as well as various granitoid domes (with minor greenstones). From the spatial distribution of both basement types (Fig. 2) it is apparent that both could have supplied material, particularly into the Central Rand Basin. Fig. 2. Open in new tabDownload slide Surface and subsurface distribution of the Witwatersrand Supergroup, Dominion Group and Mesoarchaean granite–greenstone basement domes. Also shown are the palaeocurrent directions during Central Rand Group times (small arrows) as well as the position of the producing goldfields (from Frimmel et al., 2005). Larger open arrow indicates approximate position of the studied drill site (see Fig. 3). The precisely dated 3074 ± 6 Ma (Armstrong et al., 1991) Dominion Group represents the first supracrustal volcano-sedimentary succession above the Palaeo- to Mesoarchaean granitoid–greenstone basement. Its tectonic setting has been a matter of controversy. Whereas some workers have argued for a continental margin setting because of the calc-alkaline andesitic character of the mafic rocks and depletion in Nb, Ti and Zr (Burke et al., 1986; Crow & Condie, 1987), others have considered this geochemical signature not conclusive, inherited from the mantle source rocks, and proposed deposition in a continental rift setting because of the bimodal nature of the volcanism and the tholeiitic affinity of the mafic rocks (Bowen et al., 1986; Marsh et al., 1989; Jackson, 1992). Fragments of greenstone belts in the vicinity of the Witwatersrand Basin have been known so far only from the Johannesburg Dome to the north (Anhaeusser, 1973) and from the eastern margin of the Vredefort Dome (Minnitt et al., 1994; Fig. 2). Ultramafic–mafic complexes dominate in the former area. No precise age data exist but these complexes, which have been interpreted as reflecting a former suture zone (Anhaeusser, 2006), are considered to be older than the 3200–3340 Ma intrusive tonalitic to trondhjemitic gneisses in the Johannesburg Dome (Poujol & Anhaeusser, 2001). Ultramafic–mafic intrusions in the core of the Vredefort Dome are metamorphosed at high grade and a xenocryst age of 3425 Ma has been reported from there (Hart et al., 1990). Some petrographic, geochronological and geochemical information is available from a variety of granitoid bodies that occur in several basement domes to the north and NW of the Witwatersrand Basin as well as in the Vredefort Dome (Anhaeusser, 1973, 1999; Klemd & Hallbauer, 1987; Robb & Meyer, 1987; Barton et al., 1999; Poujol & Anhaeusser, 2001; Armstrong et al., 2006; for a summary and further references see Robb et al., 2006). Their ages range from 3340 to 2777 Ma. Most of them, notably the majority of the granitoid bodies in the Johannesburg, Vredefort and Westerdam–Coligny Domes, have ages that cluster around 3·1 Ga, whereas the youngest of these intrusions post-dates the onset of sediment deposition in the Witwatersrand Basin. Whereas the focus of the previous studies has been on the granitoids along the perimeter of the Witwatersrand Basin, mafic rocks are at the centre of this study. Here we report the first geochemical and isotope data on mafic and associated felsic intrusive rocks that were intersected in exploration drill holes through a basement horst at the northwestern margin of the Witwatersrand Basin. The significance of this site lies not only in the abundance of mafic rocks but also in the fact that the pre-Witwatersrand basement is not covered by Witwatersrand sediments but is directly overlain by rocks of the lowermost Transvaal Supergroup, as well as in its proximity to the margin of the Central Rand Basin. Consequently, the drill core can provide insights into the nature and make-up of at least a section of what has been a potential source of proximal Central Rand Group sediments. Specifically, we provide petrographic, mineral-chemical and whole-rock geochemical data, U–Pb and Lu–Hf isotope data on zircon grains, Rb–Sr and Sm–Nd whole-rock isotope data, as well as Ar–Ar hornblende data, all of which are used to constrain the age and to formulate a petrogenetic model for this section of proximal source rocks. DRILL CORE SETTING AND PETROGRAPHY The investigated core comes from a borehole (BH1) that was drilled by the Chamber of Mines Research Organization (supervision: D. K. Hallbauer) on the farm Rooidraai 85 IQ at 26°21′47′′S, 27°06′54′′E, 30 km west of Carletonville (Fig. 2). The local geology of the site is dominated by a ridge that consists mainly of strongly weathered granodiorite with minor pegmatite and quartz veins, overlain by a 2–5 m thick cover of Black Reef Formation quartzite and conglomerate (the basal lithostratigraphic unit of the Transvaal Supergroup), and soil (Fig. 3). The ridge is part of an approximately north–south-trending horst structure within the northwestern Witwatersrand Basin. The structure was shaped during syn-Ventersdorp (2·71 Ga) extension. Block faulting at that time made it possible that locally the pre-Witwatersrand basement became elevated to shallow levels and was eventually exposed at or near the surface. The block-bounding faults have, however, a protracted history of repeated reactivation from syn-Central Rand Group compression to modification by the 2024 Ma Vredefort impact (Brink et al., 2000), leaving behind cataclasites and locally pseudotachylyte. Fig. 3. Open in new tabDownload slide Local geological map of the investigated borehole (BH1) site on the farm Rooidraai 85 IQ. The borehole intersected 35 m of pre-Witwatersrand basement rocks beneath 4 m of quartzite of the Black Reef Formation and overlying saprolite and lateritic soil. The top 10 m of the basement consists of coarse-grained altered granite underlain by massive, coarse-grained hornblende metagabbro (or metadiorite). The contact between the two rock types in the core is characterized by a transition zone with pink alkali feldspar disseminated in the otherwise mafic to intermediate rocks, and by a strong enrichment in largely chloritized biotite over a 1 m distance from the granite into the metagabbro (or metadiorite). Macroscopically visible hydrothermal alteration is restricted to quartz–calcite–sulphide veinlets that cross-cut particularly the mafic rocks in variable orientations. The altered granite is macroscopically highly heterogeneous with equigranular, medium-grained and fine-grained domains. In places, centimetre-size pinkish alkali feldspar grains give the rock an overall coarse-grained, inequigranular appearance. Under the microscope, the main phases are anhedral microcline (c. 36 vol. %), plagioclase (c. 29 vol. %), and quartz (29 vol. %) with irregular to serrated grain boundaries. Microcline displays typical tartan twinning and is, in places, altered to muscovite (2 vol. %). Plagioclase shows polysynthetic albite twinning and is partly saussuritized to epidote and white mica. Quartz is partly recrystallized. Variably chloritized biotite occurs in minor amounts (2 vol. %). Apatite, titanite, zircon, and magnetite are present as accessory phases (<1 vol. %). The metagabbro (or metadiorite) is, except for the contact with the overlying granite, mineralogically fairly uniform, but texturally variable. The overall dark green, coarse- to fine-grained rock consists mainly of amphiboles (56 vol. %) and plagioclase (20 vol. %). The generally subhedral amphibole grains are mainly hornblende and they are typically larger (up to several millimetres) than the interstitial, anhedral plagioclase. Variably chloritized biotite makes up as much as 13 vol. %. Irregular patches of the hornblende grains are replaced by actinolite. Epidote and sericite, derived from the saussuritization of plagioclase, occur in addition to actinolite and chlorite, and quartz (1 vol. %) as secondary, metamorphic–hydrothermal phases in minor amounts. Microcline is present in minor amounts (5 vol. %), irregularly distributed throughout the metagabbro (metadiorite) and especially along the contacts to veins. Zircon is relatively abundant as is apatite (1 vol. %). Accessory titanite is secondary and formed during the chloritization of biotite and the oxidation of presumably primary ilmenite. The biotite-rich zone in the mafic portion immediately below the granite is 1·15 m in thickness and occurs as a dark greenish-grey, fine-grained rock that contains irregularly distributed, anhedral, pink microcline grains, several centimetres in size. Largely chloritized biotite constitutes on average 71 vol. % of the rock, with the remainder being predominantly saussuritized plagioclase (15 vol. %). Apatite is present in unusually high amounts (9 vol. %), mainly as euhedral inclusions in plagioclase. Other minor phases include quartz (1 vol. %), zircon, secondary titanite and relics of a probably primary opaque phase (2 vol. %). Of special interest is a strong enrichment in allanite that displays oscillatory zonation and occurs concentrated as euhedral, several millimeter long grains at the bottom of this biotite-rich zone. All of the above lithotypes are cross-cut by various millimetre- to centimetre-thick veinlets that contain variable amounts of quartz, calcite, chlorite, microcline, pyrite and chalcopyrite. Depending on the mineralogy, their colour ranges from white, grey, green to pink. Where they cut across metabasite, the latter is typically affected by hydrothermal growth of microcline and pyrite. The spatial distribution of the microcline in the metagabbro (metadiorite) is suggestive of post-magmatic K-metasomatism that is probably related to the emplacement of the granite or that of pegmatite pods in the vicinity of the borehole. Thus, some of the microcline observed in the granite may be secondary, in which case the original composition of the granite would be granodioritic rather than granitic, analogous to the granodiorite that occurs in outcrops near the borehole locality (Fig. 3). MINERAL CHEMISTRY Mineral chemical data were obtained by electron microprobe analysis (EMPA) on 160 spots of amphiboles, feldspars, biotite and chlorite, using a CAMECA S50 instrument at the University of Würzburg (acceleration voltage 15 kV, beam current 15 nA). The analytical errors are less than 1%, except for Na (<2%). The amphibole compositions cover a wide range from magnesio-hornblende to actinolite, irrespective of the assumed Fe2+/Fe3+ ratio (Table 1), with 7·10–7·72 Si p.f.u. As expected, the Al content (Al2O3 6·56–2·19 wt %) is negatively correlated with Si and positively correlated with Ti (TiO2 0·74–0·08 wt %). Table 1: Representative analyses of amphibole grains in the metagabbro Core-metres . 14·10–14·35 . 14·10–14·35 . 22·70–22·80 . 24·20–24·40 . 24·20–24·40 . 32·00–32·35 . SiO2 54·35 49·08 53·13 50·32 52·52 52·60 TiO2 0·08 0·74 0·11 0·38 0·29 0·26 Al2O3 2·19 6·56 3·17 5·37 4·19 3·83 Cr2O3 0·14 0·13 0·19 0·14 0·17 0·12 FeO* 9·24 12·08 8·59 10·35 9·74 9·00 MgO 18·25 15·53 18·58 16·56 17·39 17·98 MnO 0·20 0·30 0·17 0·21 0·22 0·18 CaO 12·75 12·14 12·36 12·39 12·49 12·45 Na2O 0·32 1·04 0·49 0·92 0·58 0·75 K2O 0·08 0·61 0·19 0·35 0·23 0·22 Total 97·60 98·21 96·98 96·99 97·82 97·39 Core-metres . 14·10–14·35 . 14·10–14·35 . 22·70–22·80 . 24·20–24·40 . 24·20–24·40 . 32·00–32·35 . SiO2 54·35 49·08 53·13 50·32 52·52 52·60 TiO2 0·08 0·74 0·11 0·38 0·29 0·26 Al2O3 2·19 6·56 3·17 5·37 4·19 3·83 Cr2O3 0·14 0·13 0·19 0·14 0·17 0·12 FeO* 9·24 12·08 8·59 10·35 9·74 9·00 MgO 18·25 15·53 18·58 16·56 17·39 17·98 MnO 0·20 0·30 0·17 0·21 0·22 0·18 CaO 12·75 12·14 12·36 12·39 12·49 12·45 Na2O 0·32 1·04 0·49 0·92 0·58 0·75 K2O 0·08 0·61 0·19 0·35 0·23 0·22 Total 97·60 98·21 96·98 96·99 97·82 97·39 (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) Si 7·72 7·70 7·66 7·10 7·08 7·01 7·59 7·56 7·50 7·29 7·27 7·22 7·48 7·47 7·41 7·50 7·49 7·44 Al(IV) 0·28 0·30 0·34 0·90 0·92 0·99 0·41 0·44 0·50 0·71 0·73 0·78 0·52 0·53 0·59 0·50 0·51 0·56 Sum T 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 Al(VI) 0·09 0·07 0·03 0·22 0·19 0·12 0·13 0·10 0·02 0·21 0·18 0·13 0·19 0·17 0·11 0·14 0·13 0·08 Fe3+ — 0·09 0·32 — 0·15 0·58 — 0·17 0·53 — 0·13 0·42 — 0·08 0·40 — 0·06 0·39 Ti 0·01 0·01 0·01 0·08 0·08 0·08 0·01 0·01 0·01 0·04 0·04 0·04 0·04 0·04 0·04 0·03 0·03 0·03 Cr 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·01 0·01 Mg 3·86 3·85 3·84 3·35 3·34 3·31 3·95 3·94 3·91 3·58 3·57 3·54 3·69 3·68 3·66 3·83 3·82 3·79 Fe2+ 1·02 0·96 0·76 1·33 1·22 0·85 0·89 0·76 0·49 1·15 1·06 0·82 1·06 1·01 0·74 0·99 0·95 0·68 Mn — — 0·02 — — 0·04 — — 0·02 — — 0·03 — — 0·03 — — 0·02 Sum C 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 Fe2+ 0·08 0·04 — 0·13 0·08 — 0·13 0·10 — 0·10 0·05 — 0·10 0·07 — 0·08 0·08 — Mn 0·02 0·02 — 0·04 0·04 — 0·02 0·02 — 0·03 0·03 — 0·03 0·03 — 0·02 0·02 — Ca 1·94 1·94 1·93 1·88 1·88 1·86 1·89 1·88 1·87 1·92 1·92 1·91 1·91 1·90 1·89 1·90 1·90 1·89 Na 0·07 0·14 0·13 0·09 0·11 0·11 Sum B 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·01 2·00 2·00 Na 0·09 0·09 0·02 0·29 0·29 0·15 0·13 0·13 0·00 0·26 0·26 0·16 0·16 0·16 0·05 0·21 0·21 0·10 K 0·01 0·01 0·01 0·11 0·11 0·11 0·03 0·03 0·03 0·06 0·06 0·06 0·04 0·04 0·04 0·04 0·04 0·04 Sum A 0·10 0·10 0·03 0·40 0·40 0·26 0·16 0·16 0·03 0·32 0·32 0·22 0·20 0·20 0·09 0·25 0·25 0·14 Total 15·14 15·10 15·03 15·45 15·40 15·26 15·20 15·16 15·03 15·37 15·32 15·22 15·24 15·20 15·09 15·25 15·25 15·14 (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) Si 7·72 7·70 7·66 7·10 7·08 7·01 7·59 7·56 7·50 7·29 7·27 7·22 7·48 7·47 7·41 7·50 7·49 7·44 Al(IV) 0·28 0·30 0·34 0·90 0·92 0·99 0·41 0·44 0·50 0·71 0·73 0·78 0·52 0·53 0·59 0·50 0·51 0·56 Sum T 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 Al(VI) 0·09 0·07 0·03 0·22 0·19 0·12 0·13 0·10 0·02 0·21 0·18 0·13 0·19 0·17 0·11 0·14 0·13 0·08 Fe3+ — 0·09 0·32 — 0·15 0·58 — 0·17 0·53 — 0·13 0·42 — 0·08 0·40 — 0·06 0·39 Ti 0·01 0·01 0·01 0·08 0·08 0·08 0·01 0·01 0·01 0·04 0·04 0·04 0·04 0·04 0·04 0·03 0·03 0·03 Cr 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·01 0·01 Mg 3·86 3·85 3·84 3·35 3·34 3·31 3·95 3·94 3·91 3·58 3·57 3·54 3·69 3·68 3·66 3·83 3·82 3·79 Fe2+ 1·02 0·96 0·76 1·33 1·22 0·85 0·89 0·76 0·49 1·15 1·06 0·82 1·06 1·01 0·74 0·99 0·95 0·68 Mn — — 0·02 — — 0·04 — — 0·02 — — 0·03 — — 0·03 — — 0·02 Sum C 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 Fe2+ 0·08 0·04 — 0·13 0·08 — 0·13 0·10 — 0·10 0·05 — 0·10 0·07 — 0·08 0·08 — Mn 0·02 0·02 — 0·04 0·04 — 0·02 0·02 — 0·03 0·03 — 0·03 0·03 — 0·02 0·02 — Ca 1·94 1·94 1·93 1·88 1·88 1·86 1·89 1·88 1·87 1·92 1·92 1·91 1·91 1·90 1·89 1·90 1·90 1·89 Na 0·07 0·14 0·13 0·09 0·11 0·11 Sum B 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·01 2·00 2·00 Na 0·09 0·09 0·02 0·29 0·29 0·15 0·13 0·13 0·00 0·26 0·26 0·16 0·16 0·16 0·05 0·21 0·21 0·10 K 0·01 0·01 0·01 0·11 0·11 0·11 0·03 0·03 0·03 0·06 0·06 0·06 0·04 0·04 0·04 0·04 0·04 0·04 Sum A 0·10 0·10 0·03 0·40 0·40 0·26 0·16 0·16 0·03 0·32 0·32 0·22 0·20 0·20 0·09 0·25 0·25 0·14 Total 15·14 15·10 15·03 15·45 15·40 15·26 15·20 15·16 15·03 15·37 15·32 15·22 15·24 15·20 15·09 15·25 15·25 15·14 *All Fe expressed as FeO. (a) Normalization assuming all Fe to be FeO on the basis of 24 (O,OH); (b) normalization on the basis of 15eNK; (c) normalization on the basis of 13eCNK. Open in new tab Table 1: Representative analyses of amphibole grains in the metagabbro Core-metres . 14·10–14·35 . 14·10–14·35 . 22·70–22·80 . 24·20–24·40 . 24·20–24·40 . 32·00–32·35 . SiO2 54·35 49·08 53·13 50·32 52·52 52·60 TiO2 0·08 0·74 0·11 0·38 0·29 0·26 Al2O3 2·19 6·56 3·17 5·37 4·19 3·83 Cr2O3 0·14 0·13 0·19 0·14 0·17 0·12 FeO* 9·24 12·08 8·59 10·35 9·74 9·00 MgO 18·25 15·53 18·58 16·56 17·39 17·98 MnO 0·20 0·30 0·17 0·21 0·22 0·18 CaO 12·75 12·14 12·36 12·39 12·49 12·45 Na2O 0·32 1·04 0·49 0·92 0·58 0·75 K2O 0·08 0·61 0·19 0·35 0·23 0·22 Total 97·60 98·21 96·98 96·99 97·82 97·39 Core-metres . 14·10–14·35 . 14·10–14·35 . 22·70–22·80 . 24·20–24·40 . 24·20–24·40 . 32·00–32·35 . SiO2 54·35 49·08 53·13 50·32 52·52 52·60 TiO2 0·08 0·74 0·11 0·38 0·29 0·26 Al2O3 2·19 6·56 3·17 5·37 4·19 3·83 Cr2O3 0·14 0·13 0·19 0·14 0·17 0·12 FeO* 9·24 12·08 8·59 10·35 9·74 9·00 MgO 18·25 15·53 18·58 16·56 17·39 17·98 MnO 0·20 0·30 0·17 0·21 0·22 0·18 CaO 12·75 12·14 12·36 12·39 12·49 12·45 Na2O 0·32 1·04 0·49 0·92 0·58 0·75 K2O 0·08 0·61 0·19 0·35 0·23 0·22 Total 97·60 98·21 96·98 96·99 97·82 97·39 (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) Si 7·72 7·70 7·66 7·10 7·08 7·01 7·59 7·56 7·50 7·29 7·27 7·22 7·48 7·47 7·41 7·50 7·49 7·44 Al(IV) 0·28 0·30 0·34 0·90 0·92 0·99 0·41 0·44 0·50 0·71 0·73 0·78 0·52 0·53 0·59 0·50 0·51 0·56 Sum T 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 Al(VI) 0·09 0·07 0·03 0·22 0·19 0·12 0·13 0·10 0·02 0·21 0·18 0·13 0·19 0·17 0·11 0·14 0·13 0·08 Fe3+ — 0·09 0·32 — 0·15 0·58 — 0·17 0·53 — 0·13 0·42 — 0·08 0·40 — 0·06 0·39 Ti 0·01 0·01 0·01 0·08 0·08 0·08 0·01 0·01 0·01 0·04 0·04 0·04 0·04 0·04 0·04 0·03 0·03 0·03 Cr 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·01 0·01 Mg 3·86 3·85 3·84 3·35 3·34 3·31 3·95 3·94 3·91 3·58 3·57 3·54 3·69 3·68 3·66 3·83 3·82 3·79 Fe2+ 1·02 0·96 0·76 1·33 1·22 0·85 0·89 0·76 0·49 1·15 1·06 0·82 1·06 1·01 0·74 0·99 0·95 0·68 Mn — — 0·02 — — 0·04 — — 0·02 — — 0·03 — — 0·03 — — 0·02 Sum C 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 Fe2+ 0·08 0·04 — 0·13 0·08 — 0·13 0·10 — 0·10 0·05 — 0·10 0·07 — 0·08 0·08 — Mn 0·02 0·02 — 0·04 0·04 — 0·02 0·02 — 0·03 0·03 — 0·03 0·03 — 0·02 0·02 — Ca 1·94 1·94 1·93 1·88 1·88 1·86 1·89 1·88 1·87 1·92 1·92 1·91 1·91 1·90 1·89 1·90 1·90 1·89 Na 0·07 0·14 0·13 0·09 0·11 0·11 Sum B 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·01 2·00 2·00 Na 0·09 0·09 0·02 0·29 0·29 0·15 0·13 0·13 0·00 0·26 0·26 0·16 0·16 0·16 0·05 0·21 0·21 0·10 K 0·01 0·01 0·01 0·11 0·11 0·11 0·03 0·03 0·03 0·06 0·06 0·06 0·04 0·04 0·04 0·04 0·04 0·04 Sum A 0·10 0·10 0·03 0·40 0·40 0·26 0·16 0·16 0·03 0·32 0·32 0·22 0·20 0·20 0·09 0·25 0·25 0·14 Total 15·14 15·10 15·03 15·45 15·40 15·26 15·20 15·16 15·03 15·37 15·32 15·22 15·24 15·20 15·09 15·25 15·25 15·14 (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) (a) (b) (c) Si 7·72 7·70 7·66 7·10 7·08 7·01 7·59 7·56 7·50 7·29 7·27 7·22 7·48 7·47 7·41 7·50 7·49 7·44 Al(IV) 0·28 0·30 0·34 0·90 0·92 0·99 0·41 0·44 0·50 0·71 0·73 0·78 0·52 0·53 0·59 0·50 0·51 0·56 Sum T 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 8·00 Al(VI) 0·09 0·07 0·03 0·22 0·19 0·12 0·13 0·10 0·02 0·21 0·18 0·13 0·19 0·17 0·11 0·14 0·13 0·08 Fe3+ — 0·09 0·32 — 0·15 0·58 — 0·17 0·53 — 0·13 0·42 — 0·08 0·40 — 0·06 0·39 Ti 0·01 0·01 0·01 0·08 0·08 0·08 0·01 0·01 0·01 0·04 0·04 0·04 0·04 0·04 0·04 0·03 0·03 0·03 Cr 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·02 0·01 0·01 0·01 Mg 3·86 3·85 3·84 3·35 3·34 3·31 3·95 3·94 3·91 3·58 3·57 3·54 3·69 3·68 3·66 3·83 3·82 3·79 Fe2+ 1·02 0·96 0·76 1·33 1·22 0·85 0·89 0·76 0·49 1·15 1·06 0·82 1·06 1·01 0·74 0·99 0·95 0·68 Mn — — 0·02 — — 0·04 — — 0·02 — — 0·03 — — 0·03 — — 0·02 Sum C 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 5·00 Fe2+ 0·08 0·04 — 0·13 0·08 — 0·13 0·10 — 0·10 0·05 — 0·10 0·07 — 0·08 0·08 — Mn 0·02 0·02 — 0·04 0·04 — 0·02 0·02 — 0·03 0·03 — 0·03 0·03 — 0·02 0·02 — Ca 1·94 1·94 1·93 1·88 1·88 1·86 1·89 1·88 1·87 1·92 1·92 1·91 1·91 1·90 1·89 1·90 1·90 1·89 Na 0·07 0·14 0·13 0·09 0·11 0·11 Sum B 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·05 2·00 2·00 2·04 2·00 2·00 2·01 2·00 2·00 Na 0·09 0·09 0·02 0·29 0·29 0·15 0·13 0·13 0·00 0·26 0·26 0·16 0·16 0·16 0·05 0·21 0·21 0·10 K 0·01 0·01 0·01 0·11 0·11 0·11 0·03 0·03 0·03 0·06 0·06 0·06 0·04 0·04 0·04 0·04 0·04 0·04 Sum A 0·10 0·10 0·03 0·40 0·40 0·26 0·16 0·16 0·03 0·32 0·32 0·22 0·20 0·20 0·09 0·25 0·25 0·14 Total 15·14 15·10 15·03 15·45 15·40 15·26 15·20 15·16 15·03 15·37 15·32 15·22 15·24 15·20 15·09 15·25 15·25 15·14 *All Fe expressed as FeO. (a) Normalization assuming all Fe to be FeO on the basis of 24 (O,OH); (b) normalization on the basis of 15eNK; (c) normalization on the basis of 13eCNK. Open in new tab Plagioclase in the mafic rocks is oligoclase with XAn of 0·13. Considering the extent of saussuritization, the original anorthite content must have been considerably higher but can no longer be constrained. The plagioclase grains in the altered granite are all essentially stoichiometric albite. Again, a certain anorthite component has to be assumed for the original plagioclase grains in this rock, taking into account the extent of saussuritization. Overall, bearing in mind that both the felsic and mafic–intermediate rocks experienced the same post-intrusive alteration history, an originally more calcic plagioclase composition for the latter rocks compared with the granite can be inferred from the results obtained, but the initial anorthite content of the magmatic plagioclase remains elusive. Thus, based on purely petrographic criteria, it cannot be stated whether the protolith of the mafic drill core portion was a gabbro or a diorite. The alkali feldspar in the altered granite, as well as in the more mafic rocks and the veinlets, is almost pure K-feldspar throughout. Biotite is compositionally uniform and identical in both the hornblende-rich mafic portion and the biotite-rich zone. Its XFe is consistently 0·37; the TiO2 content is elevated and varies between 2·0 and 2·8 wt % (Table 2). Table 2: Representative compositions of biotite and chlorite in metagabbro . Biotite . . Chlorite . Core-metre: . 32·00–32·35 . 12·00–12·10 . . 22·70–22·80 . 14·10–14·35 . 14·10–14·35 . 5·65–5·75 . Rock type: . Hbl-gabbro . Bio-gabbro . . Hbl-gabbro . Vein . Bio-gabbro . Granite . SiO2 37·37 38·09 27·80 27·29 27·08 24·44 TiO2 1·97 2·76 0·05 0·03 0·01 0·01 Al2O3 15·24 13·88 19·87 19·73 18·51 18·65 Cr2O3 0·40 0·18 0·15 0·00 0·01 0·00 MgO 14·87 14·49 21·71 21·13 17·01 8·83 CaO 0·02 0·67 0·01 0·01 0·04 0·03 MnO 0·11 0·28 0·21 0·33 0·50 0·74 FeO* 15·68 15·49 17·98 18·48 24·63 36·12 BaO 0·01 0·00 0·00 0·00 0·00 0·00 Na2O 0·05 0·09 0·00 0·03 0·03 0·00 K2O 8·65 8·94 0·00 0·01 0·02 0·03 Total 94·37 94·87 87·79 87·03 87·83 88·85 Si 5·63 5·73 Si 5·62 5·59 5·69 5·44 AlIV 2·37 2·28 AlIV 2·38 2·41 2·31 2·57 T-site 8·00 8·00 T-site 8·00 8·00 8·00 8·00 AlVI 0·33 0·18 AlVI 2·35 2·35 2·27 2·32 Cr 0·05 0·02 Ti 0·01 0·01 0·00 0·00 Ti 0·22 0·31 Fe2+ 3·04 3·16 4·32 6·72 Fe2+ 1·97 1·95 Mn 0·04 0·06 0·09 0·14 Mn 0·01 0·04 Mg 6·54 6·45 5·33 2·93 Mg 3·34 3·25 Na 0·00 0·01 0·01 0·00 O-site 5·93 5·75 K 0·00 0·00 0·01 0·01 Ba 0·00 0·00 Sum 11·97 12·03 12·02 12·12 Ca 0·00 0·11 Na 0·02 0·03 K 1·66 1·71 A-site 1·68 1·85 Total 19·97 20·03 20·02 20·12 Total 15·61 15·59 XFe 0·32 0·33 0·45 0·70 XFe 0·37 0·37 T (°C)† 294 298 297 342 . Biotite . . Chlorite . Core-metre: . 32·00–32·35 . 12·00–12·10 . . 22·70–22·80 . 14·10–14·35 . 14·10–14·35 . 5·65–5·75 . Rock type: . Hbl-gabbro . Bio-gabbro . . Hbl-gabbro . Vein . Bio-gabbro . Granite . SiO2 37·37 38·09 27·80 27·29 27·08 24·44 TiO2 1·97 2·76 0·05 0·03 0·01 0·01 Al2O3 15·24 13·88 19·87 19·73 18·51 18·65 Cr2O3 0·40 0·18 0·15 0·00 0·01 0·00 MgO 14·87 14·49 21·71 21·13 17·01 8·83 CaO 0·02 0·67 0·01 0·01 0·04 0·03 MnO 0·11 0·28 0·21 0·33 0·50 0·74 FeO* 15·68 15·49 17·98 18·48 24·63 36·12 BaO 0·01 0·00 0·00 0·00 0·00 0·00 Na2O 0·05 0·09 0·00 0·03 0·03 0·00 K2O 8·65 8·94 0·00 0·01 0·02 0·03 Total 94·37 94·87 87·79 87·03 87·83 88·85 Si 5·63 5·73 Si 5·62 5·59 5·69 5·44 AlIV 2·37 2·28 AlIV 2·38 2·41 2·31 2·57 T-site 8·00 8·00 T-site 8·00 8·00 8·00 8·00 AlVI 0·33 0·18 AlVI 2·35 2·35 2·27 2·32 Cr 0·05 0·02 Ti 0·01 0·01 0·00 0·00 Ti 0·22 0·31 Fe2+ 3·04 3·16 4·32 6·72 Fe2+ 1·97 1·95 Mn 0·04 0·06 0·09 0·14 Mn 0·01 0·04 Mg 6·54 6·45 5·33 2·93 Mg 3·34 3·25 Na 0·00 0·01 0·01 0·00 O-site 5·93 5·75 K 0·00 0·00 0·01 0·01 Ba 0·00 0·00 Sum 11·97 12·03 12·02 12·12 Ca 0·00 0·11 Na 0·02 0·03 K 1·66 1·71 A-site 1·68 1·85 Total 19·97 20·03 20·02 20·12 Total 15·61 15·59 XFe 0·32 0·33 0·45 0·70 XFe 0·37 0·37 T (°C)† 294 298 297 342 Biotite normalization on the basis of 22 O, chlorite normalization on the basis of 36 (O,OH). *All Fe as FeO. †Temperature calculated according to Kranidiotis & McLean (1987) for chlorite with XFe < 0·50. Open in new tab Table 2: Representative compositions of biotite and chlorite in metagabbro . Biotite . . Chlorite . Core-metre: . 32·00–32·35 . 12·00–12·10 . . 22·70–22·80 . 14·10–14·35 . 14·10–14·35 . 5·65–5·75 . Rock type: . Hbl-gabbro . Bio-gabbro . . Hbl-gabbro . Vein . Bio-gabbro . Granite . SiO2 37·37 38·09 27·80 27·29 27·08 24·44 TiO2 1·97 2·76 0·05 0·03 0·01 0·01 Al2O3 15·24 13·88 19·87 19·73 18·51 18·65 Cr2O3 0·40 0·18 0·15 0·00 0·01 0·00 MgO 14·87 14·49 21·71 21·13 17·01 8·83 CaO 0·02 0·67 0·01 0·01 0·04 0·03 MnO 0·11 0·28 0·21 0·33 0·50 0·74 FeO* 15·68 15·49 17·98 18·48 24·63 36·12 BaO 0·01 0·00 0·00 0·00 0·00 0·00 Na2O 0·05 0·09 0·00 0·03 0·03 0·00 K2O 8·65 8·94 0·00 0·01 0·02 0·03 Total 94·37 94·87 87·79 87·03 87·83 88·85 Si 5·63 5·73 Si 5·62 5·59 5·69 5·44 AlIV 2·37 2·28 AlIV 2·38 2·41 2·31 2·57 T-site 8·00 8·00 T-site 8·00 8·00 8·00 8·00 AlVI 0·33 0·18 AlVI 2·35 2·35 2·27 2·32 Cr 0·05 0·02 Ti 0·01 0·01 0·00 0·00 Ti 0·22 0·31 Fe2+ 3·04 3·16 4·32 6·72 Fe2+ 1·97 1·95 Mn 0·04 0·06 0·09 0·14 Mn 0·01 0·04 Mg 6·54 6·45 5·33 2·93 Mg 3·34 3·25 Na 0·00 0·01 0·01 0·00 O-site 5·93 5·75 K 0·00 0·00 0·01 0·01 Ba 0·00 0·00 Sum 11·97 12·03 12·02 12·12 Ca 0·00 0·11 Na 0·02 0·03 K 1·66 1·71 A-site 1·68 1·85 Total 19·97 20·03 20·02 20·12 Total 15·61 15·59 XFe 0·32 0·33 0·45 0·70 XFe 0·37 0·37 T (°C)† 294 298 297 342 . Biotite . . Chlorite . Core-metre: . 32·00–32·35 . 12·00–12·10 . . 22·70–22·80 . 14·10–14·35 . 14·10–14·35 . 5·65–5·75 . Rock type: . Hbl-gabbro . Bio-gabbro . . Hbl-gabbro . Vein . Bio-gabbro . Granite . SiO2 37·37 38·09 27·80 27·29 27·08 24·44 TiO2 1·97 2·76 0·05 0·03 0·01 0·01 Al2O3 15·24 13·88 19·87 19·73 18·51 18·65 Cr2O3 0·40 0·18 0·15 0·00 0·01 0·00 MgO 14·87 14·49 21·71 21·13 17·01 8·83 CaO 0·02 0·67 0·01 0·01 0·04 0·03 MnO 0·11 0·28 0·21 0·33 0·50 0·74 FeO* 15·68 15·49 17·98 18·48 24·63 36·12 BaO 0·01 0·00 0·00 0·00 0·00 0·00 Na2O 0·05 0·09 0·00 0·03 0·03 0·00 K2O 8·65 8·94 0·00 0·01 0·02 0·03 Total 94·37 94·87 87·79 87·03 87·83 88·85 Si 5·63 5·73 Si 5·62 5·59 5·69 5·44 AlIV 2·37 2·28 AlIV 2·38 2·41 2·31 2·57 T-site 8·00 8·00 T-site 8·00 8·00 8·00 8·00 AlVI 0·33 0·18 AlVI 2·35 2·35 2·27 2·32 Cr 0·05 0·02 Ti 0·01 0·01 0·00 0·00 Ti 0·22 0·31 Fe2+ 3·04 3·16 4·32 6·72 Fe2+ 1·97 1·95 Mn 0·04 0·06 0·09 0·14 Mn 0·01 0·04 Mg 6·54 6·45 5·33 2·93 Mg 3·34 3·25 Na 0·00 0·01 0·01 0·00 O-site 5·93 5·75 K 0·00 0·00 0·01 0·01 Ba 0·00 0·00 Sum 11·97 12·03 12·02 12·12 Ca 0·00 0·11 Na 0·02 0·03 K 1·66 1·71 A-site 1·68 1·85 Total 19·97 20·03 20·02 20·12 Total 15·61 15·59 XFe 0·32 0·33 0·45 0·70 XFe 0·37 0·37 T (°C)† 294 298 297 342 Biotite normalization on the basis of 22 O, chlorite normalization on the basis of 36 (O,OH). *All Fe as FeO. †Temperature calculated according to Kranidiotis & McLean (1987) for chlorite with XFe < 0·50. Open in new tab Secondary chlorite in the hornblende metagabbro (metadiorite), where it replaces biotite, as well as in the cross-cutting veinlets has an XFe of c. 0·33 (Table 2). It is slightly more enriched in Fe in the biotite-rich zone (XFe = 0·45) and even more so where it replaced biotite in the altered granite (XFe = 0·70). CONSTRAINTS ON METAMORPHIC–HYDROTHERMAL OVERPRINT The original magmatic mineral assemblage K-feldspar–plagioclase–quartz–biotite in the granite or granodiorite shows evidence of potassic alteration in the form of secondary microcline growth and a low-grade metamorphic overprint in the form of partial recrystallization of quartz and the sericitization and saussuritization of microcline and plagioclase, respectively, as well as the chloritization of biotite. Qualitatively, a retrograde overprint at lower greenschist-facies conditions can be inferred. The primary mineral assemblage in the gabbroic or dioritic rock is not easily reconstructed. The actinolitic domains are evidently metamorphic but the dominant magnesio-hornblende could be either a primary magmatic or an earlier metamorphic phase. A magmatic origin for the hornblende seems indicated by a lack of any pyroxene relics anywhere in the samples as well as the lack of any other evidence of medium-grade metamorphism having affected the area. However, in the presence of Ti-minerals, such as titanite, in the rock a higher Ti content than detected would be expected in the hornblende if it were indeed magmatic. Similarly, the original magmatic plagioclase composition remains unknown because of retrograde alteration to oligoclase. The biotite remnants are presumed to be magmatic because of their elevated Ti content, indicative of a relatively high formation temperature. A retrograde overprint is reflected by chloritization of the biotite, formation of actinolite and the saussuritization of plagioclase, all of which indicate lower greenschist-facies conditions, also supported by the limited extent of quartz recrystallization in the adjacent altered granite. Assuming a relatively low pressure of c. 2 kbar (estimated from Na contents in the crystallographic B-site of amphibole), application of the plagioclase–amphibole thermometer (Holland & Blundy, 1994), specifically the edenite–richterite thermometer, yielded a mean temperature of 439 ± 48°C for 23 actinolite–plagioclase pairs, largely dependent on the XAb (0·86–0·99). The temperature at which the hydrothermal alteration, evident in the various veinlets, took place appears to have been similar to that of the pervasive retrograde overprint. Some of the microcline, notably the porphyroblastic textural variety and those grains that occur in the mafic–intermediate rocks adjacent to the hydrothermal veins, must be hydrothermal. The infiltrating fluid, whose pH is constrained by the stability of K-feldspar, was carbonic and S-bearing, as is evident from the precipitation of calcite, pyrite and chalcopyrite in the veinlets. GEOCHEMISTRY Major and trace elements Notwithstanding the low-grade pervasive metamorphic and local hydrothermal overprint, the whole-rock geochemical compositions of the three principal rock types, hornblende metagabbro (metadiorite), altered granite and the biotite-rich zone, have been used in an attempt to assess the genetic relationship between the felsic and more mafic rocks in the drill core and to constrain the likely tectonic setting of the magmatism. Altogether, 17 hornblende metagabbro (metadiorite) samples, five samples of the biotite-rich zone and seven altered granite samples were analysed for their major and trace element concentrations (Table 3). In addition, four samples of intensely veined metagabbro (metadiorite) were analysed to assess the effect of hydrothermal alteration on the overall geochemistry of the mafic–intermediate rocks. The major elements and selected trace element contents were determined on fusion disks by conventional X-ray fluorescence spectroscopy (XRF) using a Philips PW1480 instrument, whereas most of the trace element (including the rare earth elements, REE) concentrations were obtained by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS), using a Merchantek 266 LUV laser connected to an Agilent 7500i mass spectrometer, both at the Geodynamics and Geomaterials Research Division of the University of Würzburg. External calibration was carried out with the aid of the NIST 612 glass standard (Pearce et al., 1997). Reproducibility was tested by repeated analyses of the NIST 612 and 614 glass standards and the in-house standard BE-N. The lower limit of detection for most trace elements is below 0·02 ppm, except for Cr (0·54 ppm), Cu (0·18 ppm), Gd (0·07 ppm) and Pb (0·12 ppm). The results are given in Table 3. An additional 13 hornblende metagabbro (metadiorite) and four altered granite samples were analysed for both major and selected trace element concentrations by XRF only. The results are very similar to those above and are used in the data analysis but are not included in Table 3. Table 3: Geochemical analyses of different rock types in borehole BH1 on the farm Rooidraai Rock type: . Granite . Biotite-rich zone . Core- . 0·15– . 1·65– . 2·20– . 3·10– . 4·25– . 5·10– . 6·10– . 7·00– . 7·00– . 7·40– . 8·00– . 9·30– . metre: . 0·35 . 1·75 . 2·45 . 3·30 . 4·40 . 5·30 . 6·20 . 7·30A . 7·30B . 7·70 . 8·10 . 9·55 . Depth: . 4·25 . 5·7 . 6·3 . 7·2 . 8·3 . 9·20 . 10·15 . 11·15 . 11·15 . 11·55 . 12·05 . 13·4 . SiO2 (%) 72·12 74·25 70·81 72·97 63·32 70·29 73·26 47·69 44·96 51·55 50·73 53·42 TiO2 0·16 0·07 0·18 0·11 0·50 0·22 0·06 1·67 2·23 1·48 1·12 0·57 Al2O3 13·96 13·48 14·60 13·80 15·81 13·96 13·79 17·65 14·30 12·39 12·08 13·10 Fe2O3* 1·33 0·76 2·19 0·83 4·38 2·42 0·70 12·71 15·06 11·9 11·60 6·56 MnO 0·02 b.d. 0·04 0·02 0·10 0·04 0·01 0·15 0·24 0·23 0·22 0·16 MgO 0·30 0·17 0·90 0·24 1·50 0·59 0·18 6·30 11·14 9·91 11·82 7·95 CaO 0·93 0·59 0·75 1·25 3·08 1·74 0·72 2·91 3·07 3·98 3·05 7·13 Na2O 3·90 3·68 4·86 4·13 5·41 4·07 3·26 4·82 1·17 1·55 1·08 3·46 K2O 5·38 5·44 3·48 4·86 2·25 4·54 6·46 1·43 1·63 1·46 3·20 2·56 P2O5 0·06 0·02 0·09 0·03 0·28 0·06 0·03 0·70 0·66 0·52 0·67 0·37 LOI 0·97 0·76 1·32 1·31 2·86 1·73 0·81 3·51 5·87 5·17 4·26 4·35 Total 99·13 99·22 99·22 99·55 99·49 99·66 99·28 99·54 100·33 100·14 99·83 99·63 Sc (ppm) 3·02 2·02 3·23 2·32 10·22 3·43 1·48 23·84 42·58 36·93 32·30 23·24 V 20·00 b.d. 23·00 b.d. 45·00 11·00 11·00 169·0 145·0 118·0 134·0 103·0 Cr 6·55 7·46 7·35 6·42 8·58 6·76 6·52 104·1 634·8 744·3 872·8 440·6 Co 35·0 43·0 27·0 39·0 40·0 46·0 49·0 35·0 49·0 39·0 59·0 41·0 Ni 3·98 3·45 4·39 2·92 6·38 4·49 2·69 52·00 125·60 121·75 162·80 103·60 Cu 7·45 6·54 8·58 5·62 12·52 8·50 5·37 23·92 27·16 14·33 12·86 40·20 Zn 31·0 17·0 51·0 21·0 94·0 37·0 29·0 208·0 320·0 265·0 284·0 117·0 Ga 21·0 18·0 28·0 17·0 24·0 19·0 19·0 38·0 50·0 26·0 30·0 20·0 Rb 212 213 210 202 140 186 232 110 65 167 471 147 Sr 211 185 124 182 264 236 203 284 105 177 185 320 Y 17·64 9·03 18·40 11·84 50·12 28·76 7·02 98·74 111·00 93·68 151·80 95·24 Zr 183·6 62·7 226·6 99·9 205·2 321·6 53·4 330·0 47·3 85·3 71·8 306·4 Nb 13·00 11·03 13·42 9·57 50·10 14·10 8·09 94·02 165·20 110·00 135·60 41·58 Mo 0·64 0·63 1·35 6·28 0·74 7·43 0·56 0·49 0·42 0·61 0·37 0·53 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. 16·00 18·00 21·00 18·00 b.d. Cs 2·88 2·19 9·95 3·24 2·45 2·09 2·19 4·59 6·16 8·95 19·74 2·51 Ba 638 615 357 562 247 622 757 179 270 192 216 452 La 47·6 17·5 7·1 28·4 71·8 95·2 8·5 103·7 88·5 55·8 439·8 49·7 Ce 81·9 30·6 11·5 46·7 132·6 169·8 16·5 205·4 190·2 115·8 895·6 84·5 Pr 8·86 3·56 1·49 5·04 15·26 18·88 1·90 22·26 21·44 13·50 88·74 10·50 Nd 31·54 11·86 6·31 18·22 56·80 67·56 6·51 85·84 78·64 55·13 290·20 43·88 Sm 5·63 2·32 1·87 3·30 11·72 10·98 1·60 18·18 19·16 15·00 48·64 11·84 Eu 0·91 0·68 0·51 0·80 1·25 1·26 0·73 1·25 1·16 1·20 2·09 1·31 Gd 4·52 2·16 2·42 2·71 10·17 8·09 1·38 19·02 20·60 14·90 35·04 12·34 Tb 0·61 0·29 0·45 0·38 1·54 1·06 0·24 2·85 3·26 2·53 5·01 2·20 Dy 3·66 2·02 3·18 2·21 9·31 5·83 1·40 18·00 20·60 16·30 28·24 15·48 Ho 0·71 0·37 0·61 0·47 1·78 1·10 0·27 3·47 3·87 2·99 5·18 3·22 Er 1·70 1·09 1·71 1·35 4·94 2·81 0·82 8·67 10·14 8·46 13·20 9·89 Tm 0·25 0·16 0·25 0·16 0·68 0·43 0·14 1·05 1·26 1·18 1·78 1·58 Yb 1·65 1·12 1·58 1·28 4·41 2·90 0·99 6·25 7·64 7·28 10·33 11·74 Lu 0·24 0·15 0·26 0·19 0·59 0·41 0·13 0·80 0·98 0·96 1·40 1·55 Hf 4·94 2·13 5·97 3·06 5·47 8·11 1·92 8·37 1·23 2·14 1·72 7·45 Ta 1·32 1·04 0·60 0·57 3·22 1·64 1·03 2·67 5·02 3·79 5·43 2·14 Pb 21·74 17·56 10·56 17·60 8·58 15·62 17·30 7·10 7·14 5·06 17·44 4·90 Th 14·06 5·58 7·04 12·76 10·86 17·38 3·04 17·00 20·06 15·10 72·40 2·60 U 5·40 4·71 5·90 7·50 5·10 4·38 2·07 2·47 1·97 4·01 4·04 4·36 Rock type: . Granite . Biotite-rich zone . Core- . 0·15– . 1·65– . 2·20– . 3·10– . 4·25– . 5·10– . 6·10– . 7·00– . 7·00– . 7·40– . 8·00– . 9·30– . metre: . 0·35 . 1·75 . 2·45 . 3·30 . 4·40 . 5·30 . 6·20 . 7·30A . 7·30B . 7·70 . 8·10 . 9·55 . Depth: . 4·25 . 5·7 . 6·3 . 7·2 . 8·3 . 9·20 . 10·15 . 11·15 . 11·15 . 11·55 . 12·05 . 13·4 . SiO2 (%) 72·12 74·25 70·81 72·97 63·32 70·29 73·26 47·69 44·96 51·55 50·73 53·42 TiO2 0·16 0·07 0·18 0·11 0·50 0·22 0·06 1·67 2·23 1·48 1·12 0·57 Al2O3 13·96 13·48 14·60 13·80 15·81 13·96 13·79 17·65 14·30 12·39 12·08 13·10 Fe2O3* 1·33 0·76 2·19 0·83 4·38 2·42 0·70 12·71 15·06 11·9 11·60 6·56 MnO 0·02 b.d. 0·04 0·02 0·10 0·04 0·01 0·15 0·24 0·23 0·22 0·16 MgO 0·30 0·17 0·90 0·24 1·50 0·59 0·18 6·30 11·14 9·91 11·82 7·95 CaO 0·93 0·59 0·75 1·25 3·08 1·74 0·72 2·91 3·07 3·98 3·05 7·13 Na2O 3·90 3·68 4·86 4·13 5·41 4·07 3·26 4·82 1·17 1·55 1·08 3·46 K2O 5·38 5·44 3·48 4·86 2·25 4·54 6·46 1·43 1·63 1·46 3·20 2·56 P2O5 0·06 0·02 0·09 0·03 0·28 0·06 0·03 0·70 0·66 0·52 0·67 0·37 LOI 0·97 0·76 1·32 1·31 2·86 1·73 0·81 3·51 5·87 5·17 4·26 4·35 Total 99·13 99·22 99·22 99·55 99·49 99·66 99·28 99·54 100·33 100·14 99·83 99·63 Sc (ppm) 3·02 2·02 3·23 2·32 10·22 3·43 1·48 23·84 42·58 36·93 32·30 23·24 V 20·00 b.d. 23·00 b.d. 45·00 11·00 11·00 169·0 145·0 118·0 134·0 103·0 Cr 6·55 7·46 7·35 6·42 8·58 6·76 6·52 104·1 634·8 744·3 872·8 440·6 Co 35·0 43·0 27·0 39·0 40·0 46·0 49·0 35·0 49·0 39·0 59·0 41·0 Ni 3·98 3·45 4·39 2·92 6·38 4·49 2·69 52·00 125·60 121·75 162·80 103·60 Cu 7·45 6·54 8·58 5·62 12·52 8·50 5·37 23·92 27·16 14·33 12·86 40·20 Zn 31·0 17·0 51·0 21·0 94·0 37·0 29·0 208·0 320·0 265·0 284·0 117·0 Ga 21·0 18·0 28·0 17·0 24·0 19·0 19·0 38·0 50·0 26·0 30·0 20·0 Rb 212 213 210 202 140 186 232 110 65 167 471 147 Sr 211 185 124 182 264 236 203 284 105 177 185 320 Y 17·64 9·03 18·40 11·84 50·12 28·76 7·02 98·74 111·00 93·68 151·80 95·24 Zr 183·6 62·7 226·6 99·9 205·2 321·6 53·4 330·0 47·3 85·3 71·8 306·4 Nb 13·00 11·03 13·42 9·57 50·10 14·10 8·09 94·02 165·20 110·00 135·60 41·58 Mo 0·64 0·63 1·35 6·28 0·74 7·43 0·56 0·49 0·42 0·61 0·37 0·53 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. 16·00 18·00 21·00 18·00 b.d. Cs 2·88 2·19 9·95 3·24 2·45 2·09 2·19 4·59 6·16 8·95 19·74 2·51 Ba 638 615 357 562 247 622 757 179 270 192 216 452 La 47·6 17·5 7·1 28·4 71·8 95·2 8·5 103·7 88·5 55·8 439·8 49·7 Ce 81·9 30·6 11·5 46·7 132·6 169·8 16·5 205·4 190·2 115·8 895·6 84·5 Pr 8·86 3·56 1·49 5·04 15·26 18·88 1·90 22·26 21·44 13·50 88·74 10·50 Nd 31·54 11·86 6·31 18·22 56·80 67·56 6·51 85·84 78·64 55·13 290·20 43·88 Sm 5·63 2·32 1·87 3·30 11·72 10·98 1·60 18·18 19·16 15·00 48·64 11·84 Eu 0·91 0·68 0·51 0·80 1·25 1·26 0·73 1·25 1·16 1·20 2·09 1·31 Gd 4·52 2·16 2·42 2·71 10·17 8·09 1·38 19·02 20·60 14·90 35·04 12·34 Tb 0·61 0·29 0·45 0·38 1·54 1·06 0·24 2·85 3·26 2·53 5·01 2·20 Dy 3·66 2·02 3·18 2·21 9·31 5·83 1·40 18·00 20·60 16·30 28·24 15·48 Ho 0·71 0·37 0·61 0·47 1·78 1·10 0·27 3·47 3·87 2·99 5·18 3·22 Er 1·70 1·09 1·71 1·35 4·94 2·81 0·82 8·67 10·14 8·46 13·20 9·89 Tm 0·25 0·16 0·25 0·16 0·68 0·43 0·14 1·05 1·26 1·18 1·78 1·58 Yb 1·65 1·12 1·58 1·28 4·41 2·90 0·99 6·25 7·64 7·28 10·33 11·74 Lu 0·24 0·15 0·26 0·19 0·59 0·41 0·13 0·80 0·98 0·96 1·40 1·55 Hf 4·94 2·13 5·97 3·06 5·47 8·11 1·92 8·37 1·23 2·14 1·72 7·45 Ta 1·32 1·04 0·60 0·57 3·22 1·64 1·03 2·67 5·02 3·79 5·43 2·14 Pb 21·74 17·56 10·56 17·60 8·58 15·62 17·30 7·10 7·14 5·06 17·44 4·90 Th 14·06 5·58 7·04 12·76 10·86 17·38 3·04 17·00 20·06 15·10 72·40 2·60 U 5·40 4·71 5·90 7·50 5·10 4·38 2·07 2·47 1·97 4·01 4·04 4·36 Rock type: . Hbl-gabbro with veins . Hornblende gabbro . . . . . . . . . . dyke . . . . Core- . 10·10– . 11·00– . 12·50– . 28·00– . 14·70– . 15·60– . 18·70– . 18·80– . 19·50– . 19·60– . 20·40– . 21·10– . metre: . 10·35 . 11·25 . 12·70 . 28·35 . 15·00 . 15·80 . 18·80 . 19·00 . 19·60 . 20·00 . 20·60 . 21·40 . Depth: . 14·25 . 15·15 . 16·6 . 32·2 . 18·85 . 19·7 . 22·75 . 22·9 . 23·55 . 23·8 . 24·5 . 25·25 . SiO2 (%) 52·83 51·83 53·00 52·75 51·98 52·72 53·05 53·21 51·61 52·70 51·33 51·23 TiO2 0·53 0·52 0·49 0·43 0·47 0·41 0·33 0·32 0·29 0·34 0·42 0·50 Al2O3 10·02 9·61 8·84 8·45 8·64 9·02 8·78 8·69 7·64 8·49 8·40 8·91 Fe2O3* 9·06 9·32 9·07 8·49 9·05 8·47 8·25 8·42 8·93 8·70 8·94 9·42 MnO 0·17 0·18 0·17 0·16 0·16 0·15 0·15 0·15 0·20 0·17 0·17 0·17 MgO 12·60 13·18 13·32 13·70 13·83 13·82 13·35 13·68 13·73 13·85 14·71 13·57 CaO 8·10 9·00 9·18 9·47 9·23 9·02 8·81 8·99 9·31 8·95 9·04 9·27 Na2O 1·79 1·79 1·62 1·37 1·43 1·57 1·51 1·64 0·75 1·52 1·25 1·50 K2O 2·93 2·42 2·21 2·35 2·22 2·50 2·68 2·37 0·85 2·12 2·05 2·09 P2O5 0·40 0·40 0·39 0·32 0·35 0·32 0·20 0·23 0·18 0·23 0·30 0·38 LOI 2·08 2·67 2·52 2·88 2·20 2·04 2·13 2·04 6·09 2·10 2·49 2·36 Total 100·51 100·92 100·81 100·37 99·56 100·04 99·24 99·74 99·58 99·17 99·10 99·40 Sc (ppm) 29·66 37·72 29·90 32·00 31·16 29·54 28·58 27·90 36·68 29·18 31·84 32·82 V 143·00 144·00 142·00 128·00 122·00 115·00 96·00 98·00 105·00 108·00 116·00 142·00 Cr 891·80 1174·80 914·40 1382·60 1286·20 1204·40 1165·20 1230·00 1509·00 1231·80 1325·00 1059·80 Co 58·00 53·00 64·00 59·00 62·00 59·00 67·00 60·00 65·00 65·00 60·00 69·00 Ni 179·20 236·40 184·60 246·20 221·00 206·60 211·80 220·00 249·40 232·00 246·60 200·60 Cu 49·98 51·94 63·64 69·80 45·04 48·26 50·70 51·56 28·14 56·26 58·18 43·92 Zn 110·00 119·00 112·00 91·00 84·00 77·00 72·00 74·00 137·00 89·00 94·00 88·00 Ga 22·00 17·00 21·00 24·00 12·00 15·00 15·00 8·00 23·00 16·00 9·00 9·00 Rb 169·00 131·60 120·40 135·40 89·00 107·00 97·00 121·40 65·46 116·00 90·00 127·60 Sr 245·00 251·00 244·00 244·00 248·00 311·00 261·00 268·00 128·00 228·00 212·00 277·00 Y 30·22 25·54 18·82 18·52 18·24 16·00 15·90 15·76 48·68 23·04 15·48 17·16 Zr 111·80 120·00 95·64 63·04 99·80 97·84 118·80 112·80 150·80 86·76 70·86 105·00 Nb 28·22 21·78 13·80 14·44 12·58 10·78 10·27 9·51 20·38 15·14 10·51 8·30 Mo 0·47 0·96 9·27 0·68 0·57 0·46 0·43 0·73 0·48 0·55 0·76 0·59 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·45 1·53 2·04 2·05 1·43 1·53 1·13 1·37 3·51 2·09 2·10 2·16 Ba 687·00 591·00 513·00 624·00 482·00 531·00 592·00 533·00 171·00 538·00 553·00 557·00 La 26·54 31·20 32·16 23·80 23·74 23·12 18·08 21·04 14·26 26·38 21·56 26·14 Ce 67·42 54·76 57·68 47·42 52·62 49·52 43·92 46·26 39·24 45·72 48·86 52·22 Pr 8·33 6·40 6·53 5·52 6·12 5·72 5·30 5·49 5·64 5·77 5·56 6·30 Nd 31·48 25·50 26·14 22·48 23·36 22·00 20·92 21·68 24·90 23·74 21·26 25·68 Sm 6·40 5·48 5·58 5·06 4·77 4·56 4·38 4·56 6·48 5·49 3·64 5·84 Eu 1·54 1·26 1·37 1·19 1·05 1·05 1·05 1·13 0·99 1·14 1·05 1·38 Gd 5·28 5·05 4·99 4·41 4·28 3·63 3·43 3·94 6·32 4·76 3·45 4·66 Tb 0·76 0·69 0·66 0·53 0·57 0·50 0·48 0·54 1·13 0·72 0·47 0·65 Dy 4·75 3·50 3·90 3·33 3·58 2·95 2·67 3·19 8·33 4·11 2·82 3·74 Ho 0·92 0·64 0·73 0·60 0·66 0·60 0·53 0·55 1·71 0·80 0·52 0·71 Er 2·81 1·57 2·00 1·60 1·81 1·47 1·47 1·57 5·25 2·25 1·49 1·82 Tm 0·49 0·25 0·29 0·20 0·24 0·20 0·21 0·25 0·96 0·37 0·22 0·27 Yb 4·10 1·66 2·10 1·73 1·93 1·57 1·57 1·69 7·54 2·86 1·74 1·72 Lu 0·62 0·23 0·31 0·24 0·28 0·25 0·24 0·23 1·12 0·42 0·23 0·23 Hf 2·67 2·18 2·67 1·71 2·71 2·48 2·83 3·10 3·70 2·55 1·92 2·98 Ta 1·70 1·41 1·48 1·78 1·96 1·48 1·46 1·14 1·30 1·43 1·07 1·30 Pb 5·97 4·51 5·82 5·77 6·33 6·07 6·29 7·11 3·85 6·71 4·86 5·89 Th 1·93 1·52 2·42 2·00 2·04 1·90 1·80 1·89 1·63 1·70 1·54 6·11 U 2·48 1·58 2·49 1·95 2·47 2·23 1·60 2·28 6·55 2·02 1·89 2·29 Rock type: . Hbl-gabbro with veins . Hornblende gabbro . . . . . . . . . . dyke . . . . Core- . 10·10– . 11·00– . 12·50– . 28·00– . 14·70– . 15·60– . 18·70– . 18·80– . 19·50– . 19·60– . 20·40– . 21·10– . metre: . 10·35 . 11·25 . 12·70 . 28·35 . 15·00 . 15·80 . 18·80 . 19·00 . 19·60 . 20·00 . 20·60 . 21·40 . Depth: . 14·25 . 15·15 . 16·6 . 32·2 . 18·85 . 19·7 . 22·75 . 22·9 . 23·55 . 23·8 . 24·5 . 25·25 . SiO2 (%) 52·83 51·83 53·00 52·75 51·98 52·72 53·05 53·21 51·61 52·70 51·33 51·23 TiO2 0·53 0·52 0·49 0·43 0·47 0·41 0·33 0·32 0·29 0·34 0·42 0·50 Al2O3 10·02 9·61 8·84 8·45 8·64 9·02 8·78 8·69 7·64 8·49 8·40 8·91 Fe2O3* 9·06 9·32 9·07 8·49 9·05 8·47 8·25 8·42 8·93 8·70 8·94 9·42 MnO 0·17 0·18 0·17 0·16 0·16 0·15 0·15 0·15 0·20 0·17 0·17 0·17 MgO 12·60 13·18 13·32 13·70 13·83 13·82 13·35 13·68 13·73 13·85 14·71 13·57 CaO 8·10 9·00 9·18 9·47 9·23 9·02 8·81 8·99 9·31 8·95 9·04 9·27 Na2O 1·79 1·79 1·62 1·37 1·43 1·57 1·51 1·64 0·75 1·52 1·25 1·50 K2O 2·93 2·42 2·21 2·35 2·22 2·50 2·68 2·37 0·85 2·12 2·05 2·09 P2O5 0·40 0·40 0·39 0·32 0·35 0·32 0·20 0·23 0·18 0·23 0·30 0·38 LOI 2·08 2·67 2·52 2·88 2·20 2·04 2·13 2·04 6·09 2·10 2·49 2·36 Total 100·51 100·92 100·81 100·37 99·56 100·04 99·24 99·74 99·58 99·17 99·10 99·40 Sc (ppm) 29·66 37·72 29·90 32·00 31·16 29·54 28·58 27·90 36·68 29·18 31·84 32·82 V 143·00 144·00 142·00 128·00 122·00 115·00 96·00 98·00 105·00 108·00 116·00 142·00 Cr 891·80 1174·80 914·40 1382·60 1286·20 1204·40 1165·20 1230·00 1509·00 1231·80 1325·00 1059·80 Co 58·00 53·00 64·00 59·00 62·00 59·00 67·00 60·00 65·00 65·00 60·00 69·00 Ni 179·20 236·40 184·60 246·20 221·00 206·60 211·80 220·00 249·40 232·00 246·60 200·60 Cu 49·98 51·94 63·64 69·80 45·04 48·26 50·70 51·56 28·14 56·26 58·18 43·92 Zn 110·00 119·00 112·00 91·00 84·00 77·00 72·00 74·00 137·00 89·00 94·00 88·00 Ga 22·00 17·00 21·00 24·00 12·00 15·00 15·00 8·00 23·00 16·00 9·00 9·00 Rb 169·00 131·60 120·40 135·40 89·00 107·00 97·00 121·40 65·46 116·00 90·00 127·60 Sr 245·00 251·00 244·00 244·00 248·00 311·00 261·00 268·00 128·00 228·00 212·00 277·00 Y 30·22 25·54 18·82 18·52 18·24 16·00 15·90 15·76 48·68 23·04 15·48 17·16 Zr 111·80 120·00 95·64 63·04 99·80 97·84 118·80 112·80 150·80 86·76 70·86 105·00 Nb 28·22 21·78 13·80 14·44 12·58 10·78 10·27 9·51 20·38 15·14 10·51 8·30 Mo 0·47 0·96 9·27 0·68 0·57 0·46 0·43 0·73 0·48 0·55 0·76 0·59 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·45 1·53 2·04 2·05 1·43 1·53 1·13 1·37 3·51 2·09 2·10 2·16 Ba 687·00 591·00 513·00 624·00 482·00 531·00 592·00 533·00 171·00 538·00 553·00 557·00 La 26·54 31·20 32·16 23·80 23·74 23·12 18·08 21·04 14·26 26·38 21·56 26·14 Ce 67·42 54·76 57·68 47·42 52·62 49·52 43·92 46·26 39·24 45·72 48·86 52·22 Pr 8·33 6·40 6·53 5·52 6·12 5·72 5·30 5·49 5·64 5·77 5·56 6·30 Nd 31·48 25·50 26·14 22·48 23·36 22·00 20·92 21·68 24·90 23·74 21·26 25·68 Sm 6·40 5·48 5·58 5·06 4·77 4·56 4·38 4·56 6·48 5·49 3·64 5·84 Eu 1·54 1·26 1·37 1·19 1·05 1·05 1·05 1·13 0·99 1·14 1·05 1·38 Gd 5·28 5·05 4·99 4·41 4·28 3·63 3·43 3·94 6·32 4·76 3·45 4·66 Tb 0·76 0·69 0·66 0·53 0·57 0·50 0·48 0·54 1·13 0·72 0·47 0·65 Dy 4·75 3·50 3·90 3·33 3·58 2·95 2·67 3·19 8·33 4·11 2·82 3·74 Ho 0·92 0·64 0·73 0·60 0·66 0·60 0·53 0·55 1·71 0·80 0·52 0·71 Er 2·81 1·57 2·00 1·60 1·81 1·47 1·47 1·57 5·25 2·25 1·49 1·82 Tm 0·49 0·25 0·29 0·20 0·24 0·20 0·21 0·25 0·96 0·37 0·22 0·27 Yb 4·10 1·66 2·10 1·73 1·93 1·57 1·57 1·69 7·54 2·86 1·74 1·72 Lu 0·62 0·23 0·31 0·24 0·28 0·25 0·24 0·23 1·12 0·42 0·23 0·23 Hf 2·67 2·18 2·67 1·71 2·71 2·48 2·83 3·10 3·70 2·55 1·92 2·98 Ta 1·70 1·41 1·48 1·78 1·96 1·48 1·46 1·14 1·30 1·43 1·07 1·30 Pb 5·97 4·51 5·82 5·77 6·33 6·07 6·29 7·11 3·85 6·71 4·86 5·89 Th 1·93 1·52 2·42 2·00 2·04 1·90 1·80 1·89 1·63 1·70 1·54 6·11 U 2·48 1·58 2·49 1·95 2·47 2·23 1·60 2·28 6·55 2·02 1·89 2·29 Rock type: . Hornblende gabbro . Core- . 22·20– . 24·70– . 25·45– . 26·10– . 26·55– . 29·00– . 31·80– . 34·65– . metre: . 22·40 . 25·00 . 25·70 . 26·30 . 26·80 . 29·30 . 32·00 . 35·00 . Depth: . 26·3 . 28·85 . 29·6 . 30·2 . 30·7 . 33·15 . 35·9 . 38·8 . SiO2 (%) 51·38 51·12 50·66 50·52 51·07 51·78 52·72 52·53 TiO2 0·50 0·46 0·46 0·44 0·41 0·50 0·47 0·43 Al2O3 8·71 8·59 8·83 8·84 8·75 8·76 9·25 9·35 Fe2O3* 9·49 9·31 9·27 9·27 9·10 9·07 8·64 8·78 MnO 0·17 0·17 0·16 0·16 0·16 0·16 0·16 0·16 MgO 13·82 13·95 14·10 14·45 14·23 13·56 13·68 14·01 CaO 9·26 9·36 9·07 9·11 8·98 8·86 8·65 8·85 Na2O 1·41 1·50 1·51 1·48 1·36 1·48 1·52 1·61 K2O 2·17 1·80 1·80 1·44 1·86 2·21 2·67 2·51 P2O5 0·40 0·37 0·34 0·35 0·34 0·35 0·31 0·34 LOI 2·23 2·44 2·75 2·80 2·65 2·24 2·06 2·20 Total 99·54 99·07 98·95 98·86 98·91 98·97 100·13 100·77 Sc (ppm) 34·08 33·04 30·34 30·92 29·08 30·44 29·80 30·02 V 145·00 136·00 134·00 127·00 112·00 130·00 127·00 122·00 Cr 1035·50 1122·60 1163·80 1206·20 1218·20 1296·60 1203·60 1229·40 Co 64·00 57·00 70·00 74·00 67·00 67·00 67·00 59·00 Ni 196·00 212·40 230·40 232·80 239·00 228·00 209·40 214·60 Cu 54·98 40·92 46·88 54·32 38·68 54·58 54·24 54·34 Zn 93·00 120·00 81·00 73·00 75·00 91·00 91·00 94·00 Ga 11·00 11·00 16·00 19·00 17·00 10·00 9·00 14·00 Rb 93·00 83·00 110·60 75·00 101·48 101·00 115·00 96·00 Sr 266·00 257·00 277·00 289·00 277·00 255·00 324·00 280·00 Y 16·45 15·76 15·02 16·12 16·24 19·16 18·06 21·50 Zr 89·00 91·04 84·68 57·90 72·80 84·48 78·72 81·86 Nb 7·81 7·38 7·47 7·87 8·01 14·32 11·80 11·70 Mo 0·46 0·49 0·75 3·45 0·85 0·43 2·24 0·62 Sn b.d. 16·00 b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·28 1·21 2·19 2·46 1·89 2·78 2·09 1·60 Ba 573·00 458·00 520·00 353·00 599·00 579·00 751·00 665·00 La 22·98 20·28 23·48 23·46 24·00 21·94 19·98 22·32 Ce 48·45 44·54 45·24 50·26 50·52 49·22 45·58 51·92 Pr 5·81 5·48 5·43 6·02 6·08 5·81 5·34 6·12 Nd 24·20 22·38 22·68 23·24 24·84 23·68 21·28 24·30 Sm 5·03 5·11 4·62 5·38 5·26 5·53 4·72 5·25 Eu 1·25 1·12 1·19 1·20 1·20 1·18 1·12 1·23 Gd 4·59 4·30 3·99 4·49 4·04 4·57 4·03 4·50 Tb 0·61 0·62 0·56 0·59 0·58 0·64 0·58 0·65 Dy 3·39 3·23 3·37 3·21 3·38 3·72 3·27 3·99 Ho 0·62 0·59 0·57 0·62 0·59 0·70 0·58 0·75 Er 1·65 1·47 1·60 1·39 1·59 1·97 1·80 2·15 Tm 0·22 0·21 0·20 0·20 0·23 0·31 0·26 0·34 Yb 1·54 1·59 1·45 1·56 1·46 2·19 1·97 2·20 Lu 0·23 0·20 0·21 0·18 0·21 0·31 0·28 0·29 Hf 2·36 2·28 2·30 1·82 2·28 2·40 2·27 2·46 Ta 1·16 1·17 0·98 0·76 0·63 1·71 1·13 1·64 Pb 4·69 4·09 5·10 4·51 5·79 5·70 6·04 7·03 Th 2·39 1·94 2·65 2·56 1·92 2·71 1·75 2·47 U 1·82 2·10 2·12 1·65 1·63 3·02 1·88 2·20 Rock type: . Hornblende gabbro . Core- . 22·20– . 24·70– . 25·45– . 26·10– . 26·55– . 29·00– . 31·80– . 34·65– . metre: . 22·40 . 25·00 . 25·70 . 26·30 . 26·80 . 29·30 . 32·00 . 35·00 . Depth: . 26·3 . 28·85 . 29·6 . 30·2 . 30·7 . 33·15 . 35·9 . 38·8 . SiO2 (%) 51·38 51·12 50·66 50·52 51·07 51·78 52·72 52·53 TiO2 0·50 0·46 0·46 0·44 0·41 0·50 0·47 0·43 Al2O3 8·71 8·59 8·83 8·84 8·75 8·76 9·25 9·35 Fe2O3* 9·49 9·31 9·27 9·27 9·10 9·07 8·64 8·78 MnO 0·17 0·17 0·16 0·16 0·16 0·16 0·16 0·16 MgO 13·82 13·95 14·10 14·45 14·23 13·56 13·68 14·01 CaO 9·26 9·36 9·07 9·11 8·98 8·86 8·65 8·85 Na2O 1·41 1·50 1·51 1·48 1·36 1·48 1·52 1·61 K2O 2·17 1·80 1·80 1·44 1·86 2·21 2·67 2·51 P2O5 0·40 0·37 0·34 0·35 0·34 0·35 0·31 0·34 LOI 2·23 2·44 2·75 2·80 2·65 2·24 2·06 2·20 Total 99·54 99·07 98·95 98·86 98·91 98·97 100·13 100·77 Sc (ppm) 34·08 33·04 30·34 30·92 29·08 30·44 29·80 30·02 V 145·00 136·00 134·00 127·00 112·00 130·00 127·00 122·00 Cr 1035·50 1122·60 1163·80 1206·20 1218·20 1296·60 1203·60 1229·40 Co 64·00 57·00 70·00 74·00 67·00 67·00 67·00 59·00 Ni 196·00 212·40 230·40 232·80 239·00 228·00 209·40 214·60 Cu 54·98 40·92 46·88 54·32 38·68 54·58 54·24 54·34 Zn 93·00 120·00 81·00 73·00 75·00 91·00 91·00 94·00 Ga 11·00 11·00 16·00 19·00 17·00 10·00 9·00 14·00 Rb 93·00 83·00 110·60 75·00 101·48 101·00 115·00 96·00 Sr 266·00 257·00 277·00 289·00 277·00 255·00 324·00 280·00 Y 16·45 15·76 15·02 16·12 16·24 19·16 18·06 21·50 Zr 89·00 91·04 84·68 57·90 72·80 84·48 78·72 81·86 Nb 7·81 7·38 7·47 7·87 8·01 14·32 11·80 11·70 Mo 0·46 0·49 0·75 3·45 0·85 0·43 2·24 0·62 Sn b.d. 16·00 b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·28 1·21 2·19 2·46 1·89 2·78 2·09 1·60 Ba 573·00 458·00 520·00 353·00 599·00 579·00 751·00 665·00 La 22·98 20·28 23·48 23·46 24·00 21·94 19·98 22·32 Ce 48·45 44·54 45·24 50·26 50·52 49·22 45·58 51·92 Pr 5·81 5·48 5·43 6·02 6·08 5·81 5·34 6·12 Nd 24·20 22·38 22·68 23·24 24·84 23·68 21·28 24·30 Sm 5·03 5·11 4·62 5·38 5·26 5·53 4·72 5·25 Eu 1·25 1·12 1·19 1·20 1·20 1·18 1·12 1·23 Gd 4·59 4·30 3·99 4·49 4·04 4·57 4·03 4·50 Tb 0·61 0·62 0·56 0·59 0·58 0·64 0·58 0·65 Dy 3·39 3·23 3·37 3·21 3·38 3·72 3·27 3·99 Ho 0·62 0·59 0·57 0·62 0·59 0·70 0·58 0·75 Er 1·65 1·47 1·60 1·39 1·59 1·97 1·80 2·15 Tm 0·22 0·21 0·20 0·20 0·23 0·31 0·26 0·34 Yb 1·54 1·59 1·45 1·56 1·46 2·19 1·97 2·20 Lu 0·23 0·20 0·21 0·18 0·21 0·31 0·28 0·29 Hf 2·36 2·28 2·30 1·82 2·28 2·40 2·27 2·46 Ta 1·16 1·17 0·98 0·76 0·63 1·71 1·13 1·64 Pb 4·69 4·09 5·10 4·51 5·79 5·70 6·04 7·03 Th 2·39 1·94 2·65 2·56 1·92 2·71 1·75 2·47 U 1·82 2·10 2·12 1·65 1·63 3·02 1·88 2·20 *All Fe as Fe2O3. Open in new tab Table 3: Geochemical analyses of different rock types in borehole BH1 on the farm Rooidraai Rock type: . Granite . Biotite-rich zone . Core- . 0·15– . 1·65– . 2·20– . 3·10– . 4·25– . 5·10– . 6·10– . 7·00– . 7·00– . 7·40– . 8·00– . 9·30– . metre: . 0·35 . 1·75 . 2·45 . 3·30 . 4·40 . 5·30 . 6·20 . 7·30A . 7·30B . 7·70 . 8·10 . 9·55 . Depth: . 4·25 . 5·7 . 6·3 . 7·2 . 8·3 . 9·20 . 10·15 . 11·15 . 11·15 . 11·55 . 12·05 . 13·4 . SiO2 (%) 72·12 74·25 70·81 72·97 63·32 70·29 73·26 47·69 44·96 51·55 50·73 53·42 TiO2 0·16 0·07 0·18 0·11 0·50 0·22 0·06 1·67 2·23 1·48 1·12 0·57 Al2O3 13·96 13·48 14·60 13·80 15·81 13·96 13·79 17·65 14·30 12·39 12·08 13·10 Fe2O3* 1·33 0·76 2·19 0·83 4·38 2·42 0·70 12·71 15·06 11·9 11·60 6·56 MnO 0·02 b.d. 0·04 0·02 0·10 0·04 0·01 0·15 0·24 0·23 0·22 0·16 MgO 0·30 0·17 0·90 0·24 1·50 0·59 0·18 6·30 11·14 9·91 11·82 7·95 CaO 0·93 0·59 0·75 1·25 3·08 1·74 0·72 2·91 3·07 3·98 3·05 7·13 Na2O 3·90 3·68 4·86 4·13 5·41 4·07 3·26 4·82 1·17 1·55 1·08 3·46 K2O 5·38 5·44 3·48 4·86 2·25 4·54 6·46 1·43 1·63 1·46 3·20 2·56 P2O5 0·06 0·02 0·09 0·03 0·28 0·06 0·03 0·70 0·66 0·52 0·67 0·37 LOI 0·97 0·76 1·32 1·31 2·86 1·73 0·81 3·51 5·87 5·17 4·26 4·35 Total 99·13 99·22 99·22 99·55 99·49 99·66 99·28 99·54 100·33 100·14 99·83 99·63 Sc (ppm) 3·02 2·02 3·23 2·32 10·22 3·43 1·48 23·84 42·58 36·93 32·30 23·24 V 20·00 b.d. 23·00 b.d. 45·00 11·00 11·00 169·0 145·0 118·0 134·0 103·0 Cr 6·55 7·46 7·35 6·42 8·58 6·76 6·52 104·1 634·8 744·3 872·8 440·6 Co 35·0 43·0 27·0 39·0 40·0 46·0 49·0 35·0 49·0 39·0 59·0 41·0 Ni 3·98 3·45 4·39 2·92 6·38 4·49 2·69 52·00 125·60 121·75 162·80 103·60 Cu 7·45 6·54 8·58 5·62 12·52 8·50 5·37 23·92 27·16 14·33 12·86 40·20 Zn 31·0 17·0 51·0 21·0 94·0 37·0 29·0 208·0 320·0 265·0 284·0 117·0 Ga 21·0 18·0 28·0 17·0 24·0 19·0 19·0 38·0 50·0 26·0 30·0 20·0 Rb 212 213 210 202 140 186 232 110 65 167 471 147 Sr 211 185 124 182 264 236 203 284 105 177 185 320 Y 17·64 9·03 18·40 11·84 50·12 28·76 7·02 98·74 111·00 93·68 151·80 95·24 Zr 183·6 62·7 226·6 99·9 205·2 321·6 53·4 330·0 47·3 85·3 71·8 306·4 Nb 13·00 11·03 13·42 9·57 50·10 14·10 8·09 94·02 165·20 110·00 135·60 41·58 Mo 0·64 0·63 1·35 6·28 0·74 7·43 0·56 0·49 0·42 0·61 0·37 0·53 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. 16·00 18·00 21·00 18·00 b.d. Cs 2·88 2·19 9·95 3·24 2·45 2·09 2·19 4·59 6·16 8·95 19·74 2·51 Ba 638 615 357 562 247 622 757 179 270 192 216 452 La 47·6 17·5 7·1 28·4 71·8 95·2 8·5 103·7 88·5 55·8 439·8 49·7 Ce 81·9 30·6 11·5 46·7 132·6 169·8 16·5 205·4 190·2 115·8 895·6 84·5 Pr 8·86 3·56 1·49 5·04 15·26 18·88 1·90 22·26 21·44 13·50 88·74 10·50 Nd 31·54 11·86 6·31 18·22 56·80 67·56 6·51 85·84 78·64 55·13 290·20 43·88 Sm 5·63 2·32 1·87 3·30 11·72 10·98 1·60 18·18 19·16 15·00 48·64 11·84 Eu 0·91 0·68 0·51 0·80 1·25 1·26 0·73 1·25 1·16 1·20 2·09 1·31 Gd 4·52 2·16 2·42 2·71 10·17 8·09 1·38 19·02 20·60 14·90 35·04 12·34 Tb 0·61 0·29 0·45 0·38 1·54 1·06 0·24 2·85 3·26 2·53 5·01 2·20 Dy 3·66 2·02 3·18 2·21 9·31 5·83 1·40 18·00 20·60 16·30 28·24 15·48 Ho 0·71 0·37 0·61 0·47 1·78 1·10 0·27 3·47 3·87 2·99 5·18 3·22 Er 1·70 1·09 1·71 1·35 4·94 2·81 0·82 8·67 10·14 8·46 13·20 9·89 Tm 0·25 0·16 0·25 0·16 0·68 0·43 0·14 1·05 1·26 1·18 1·78 1·58 Yb 1·65 1·12 1·58 1·28 4·41 2·90 0·99 6·25 7·64 7·28 10·33 11·74 Lu 0·24 0·15 0·26 0·19 0·59 0·41 0·13 0·80 0·98 0·96 1·40 1·55 Hf 4·94 2·13 5·97 3·06 5·47 8·11 1·92 8·37 1·23 2·14 1·72 7·45 Ta 1·32 1·04 0·60 0·57 3·22 1·64 1·03 2·67 5·02 3·79 5·43 2·14 Pb 21·74 17·56 10·56 17·60 8·58 15·62 17·30 7·10 7·14 5·06 17·44 4·90 Th 14·06 5·58 7·04 12·76 10·86 17·38 3·04 17·00 20·06 15·10 72·40 2·60 U 5·40 4·71 5·90 7·50 5·10 4·38 2·07 2·47 1·97 4·01 4·04 4·36 Rock type: . Granite . Biotite-rich zone . Core- . 0·15– . 1·65– . 2·20– . 3·10– . 4·25– . 5·10– . 6·10– . 7·00– . 7·00– . 7·40– . 8·00– . 9·30– . metre: . 0·35 . 1·75 . 2·45 . 3·30 . 4·40 . 5·30 . 6·20 . 7·30A . 7·30B . 7·70 . 8·10 . 9·55 . Depth: . 4·25 . 5·7 . 6·3 . 7·2 . 8·3 . 9·20 . 10·15 . 11·15 . 11·15 . 11·55 . 12·05 . 13·4 . SiO2 (%) 72·12 74·25 70·81 72·97 63·32 70·29 73·26 47·69 44·96 51·55 50·73 53·42 TiO2 0·16 0·07 0·18 0·11 0·50 0·22 0·06 1·67 2·23 1·48 1·12 0·57 Al2O3 13·96 13·48 14·60 13·80 15·81 13·96 13·79 17·65 14·30 12·39 12·08 13·10 Fe2O3* 1·33 0·76 2·19 0·83 4·38 2·42 0·70 12·71 15·06 11·9 11·60 6·56 MnO 0·02 b.d. 0·04 0·02 0·10 0·04 0·01 0·15 0·24 0·23 0·22 0·16 MgO 0·30 0·17 0·90 0·24 1·50 0·59 0·18 6·30 11·14 9·91 11·82 7·95 CaO 0·93 0·59 0·75 1·25 3·08 1·74 0·72 2·91 3·07 3·98 3·05 7·13 Na2O 3·90 3·68 4·86 4·13 5·41 4·07 3·26 4·82 1·17 1·55 1·08 3·46 K2O 5·38 5·44 3·48 4·86 2·25 4·54 6·46 1·43 1·63 1·46 3·20 2·56 P2O5 0·06 0·02 0·09 0·03 0·28 0·06 0·03 0·70 0·66 0·52 0·67 0·37 LOI 0·97 0·76 1·32 1·31 2·86 1·73 0·81 3·51 5·87 5·17 4·26 4·35 Total 99·13 99·22 99·22 99·55 99·49 99·66 99·28 99·54 100·33 100·14 99·83 99·63 Sc (ppm) 3·02 2·02 3·23 2·32 10·22 3·43 1·48 23·84 42·58 36·93 32·30 23·24 V 20·00 b.d. 23·00 b.d. 45·00 11·00 11·00 169·0 145·0 118·0 134·0 103·0 Cr 6·55 7·46 7·35 6·42 8·58 6·76 6·52 104·1 634·8 744·3 872·8 440·6 Co 35·0 43·0 27·0 39·0 40·0 46·0 49·0 35·0 49·0 39·0 59·0 41·0 Ni 3·98 3·45 4·39 2·92 6·38 4·49 2·69 52·00 125·60 121·75 162·80 103·60 Cu 7·45 6·54 8·58 5·62 12·52 8·50 5·37 23·92 27·16 14·33 12·86 40·20 Zn 31·0 17·0 51·0 21·0 94·0 37·0 29·0 208·0 320·0 265·0 284·0 117·0 Ga 21·0 18·0 28·0 17·0 24·0 19·0 19·0 38·0 50·0 26·0 30·0 20·0 Rb 212 213 210 202 140 186 232 110 65 167 471 147 Sr 211 185 124 182 264 236 203 284 105 177 185 320 Y 17·64 9·03 18·40 11·84 50·12 28·76 7·02 98·74 111·00 93·68 151·80 95·24 Zr 183·6 62·7 226·6 99·9 205·2 321·6 53·4 330·0 47·3 85·3 71·8 306·4 Nb 13·00 11·03 13·42 9·57 50·10 14·10 8·09 94·02 165·20 110·00 135·60 41·58 Mo 0·64 0·63 1·35 6·28 0·74 7·43 0·56 0·49 0·42 0·61 0·37 0·53 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. 16·00 18·00 21·00 18·00 b.d. Cs 2·88 2·19 9·95 3·24 2·45 2·09 2·19 4·59 6·16 8·95 19·74 2·51 Ba 638 615 357 562 247 622 757 179 270 192 216 452 La 47·6 17·5 7·1 28·4 71·8 95·2 8·5 103·7 88·5 55·8 439·8 49·7 Ce 81·9 30·6 11·5 46·7 132·6 169·8 16·5 205·4 190·2 115·8 895·6 84·5 Pr 8·86 3·56 1·49 5·04 15·26 18·88 1·90 22·26 21·44 13·50 88·74 10·50 Nd 31·54 11·86 6·31 18·22 56·80 67·56 6·51 85·84 78·64 55·13 290·20 43·88 Sm 5·63 2·32 1·87 3·30 11·72 10·98 1·60 18·18 19·16 15·00 48·64 11·84 Eu 0·91 0·68 0·51 0·80 1·25 1·26 0·73 1·25 1·16 1·20 2·09 1·31 Gd 4·52 2·16 2·42 2·71 10·17 8·09 1·38 19·02 20·60 14·90 35·04 12·34 Tb 0·61 0·29 0·45 0·38 1·54 1·06 0·24 2·85 3·26 2·53 5·01 2·20 Dy 3·66 2·02 3·18 2·21 9·31 5·83 1·40 18·00 20·60 16·30 28·24 15·48 Ho 0·71 0·37 0·61 0·47 1·78 1·10 0·27 3·47 3·87 2·99 5·18 3·22 Er 1·70 1·09 1·71 1·35 4·94 2·81 0·82 8·67 10·14 8·46 13·20 9·89 Tm 0·25 0·16 0·25 0·16 0·68 0·43 0·14 1·05 1·26 1·18 1·78 1·58 Yb 1·65 1·12 1·58 1·28 4·41 2·90 0·99 6·25 7·64 7·28 10·33 11·74 Lu 0·24 0·15 0·26 0·19 0·59 0·41 0·13 0·80 0·98 0·96 1·40 1·55 Hf 4·94 2·13 5·97 3·06 5·47 8·11 1·92 8·37 1·23 2·14 1·72 7·45 Ta 1·32 1·04 0·60 0·57 3·22 1·64 1·03 2·67 5·02 3·79 5·43 2·14 Pb 21·74 17·56 10·56 17·60 8·58 15·62 17·30 7·10 7·14 5·06 17·44 4·90 Th 14·06 5·58 7·04 12·76 10·86 17·38 3·04 17·00 20·06 15·10 72·40 2·60 U 5·40 4·71 5·90 7·50 5·10 4·38 2·07 2·47 1·97 4·01 4·04 4·36 Rock type: . Hbl-gabbro with veins . Hornblende gabbro . . . . . . . . . . dyke . . . . Core- . 10·10– . 11·00– . 12·50– . 28·00– . 14·70– . 15·60– . 18·70– . 18·80– . 19·50– . 19·60– . 20·40– . 21·10– . metre: . 10·35 . 11·25 . 12·70 . 28·35 . 15·00 . 15·80 . 18·80 . 19·00 . 19·60 . 20·00 . 20·60 . 21·40 . Depth: . 14·25 . 15·15 . 16·6 . 32·2 . 18·85 . 19·7 . 22·75 . 22·9 . 23·55 . 23·8 . 24·5 . 25·25 . SiO2 (%) 52·83 51·83 53·00 52·75 51·98 52·72 53·05 53·21 51·61 52·70 51·33 51·23 TiO2 0·53 0·52 0·49 0·43 0·47 0·41 0·33 0·32 0·29 0·34 0·42 0·50 Al2O3 10·02 9·61 8·84 8·45 8·64 9·02 8·78 8·69 7·64 8·49 8·40 8·91 Fe2O3* 9·06 9·32 9·07 8·49 9·05 8·47 8·25 8·42 8·93 8·70 8·94 9·42 MnO 0·17 0·18 0·17 0·16 0·16 0·15 0·15 0·15 0·20 0·17 0·17 0·17 MgO 12·60 13·18 13·32 13·70 13·83 13·82 13·35 13·68 13·73 13·85 14·71 13·57 CaO 8·10 9·00 9·18 9·47 9·23 9·02 8·81 8·99 9·31 8·95 9·04 9·27 Na2O 1·79 1·79 1·62 1·37 1·43 1·57 1·51 1·64 0·75 1·52 1·25 1·50 K2O 2·93 2·42 2·21 2·35 2·22 2·50 2·68 2·37 0·85 2·12 2·05 2·09 P2O5 0·40 0·40 0·39 0·32 0·35 0·32 0·20 0·23 0·18 0·23 0·30 0·38 LOI 2·08 2·67 2·52 2·88 2·20 2·04 2·13 2·04 6·09 2·10 2·49 2·36 Total 100·51 100·92 100·81 100·37 99·56 100·04 99·24 99·74 99·58 99·17 99·10 99·40 Sc (ppm) 29·66 37·72 29·90 32·00 31·16 29·54 28·58 27·90 36·68 29·18 31·84 32·82 V 143·00 144·00 142·00 128·00 122·00 115·00 96·00 98·00 105·00 108·00 116·00 142·00 Cr 891·80 1174·80 914·40 1382·60 1286·20 1204·40 1165·20 1230·00 1509·00 1231·80 1325·00 1059·80 Co 58·00 53·00 64·00 59·00 62·00 59·00 67·00 60·00 65·00 65·00 60·00 69·00 Ni 179·20 236·40 184·60 246·20 221·00 206·60 211·80 220·00 249·40 232·00 246·60 200·60 Cu 49·98 51·94 63·64 69·80 45·04 48·26 50·70 51·56 28·14 56·26 58·18 43·92 Zn 110·00 119·00 112·00 91·00 84·00 77·00 72·00 74·00 137·00 89·00 94·00 88·00 Ga 22·00 17·00 21·00 24·00 12·00 15·00 15·00 8·00 23·00 16·00 9·00 9·00 Rb 169·00 131·60 120·40 135·40 89·00 107·00 97·00 121·40 65·46 116·00 90·00 127·60 Sr 245·00 251·00 244·00 244·00 248·00 311·00 261·00 268·00 128·00 228·00 212·00 277·00 Y 30·22 25·54 18·82 18·52 18·24 16·00 15·90 15·76 48·68 23·04 15·48 17·16 Zr 111·80 120·00 95·64 63·04 99·80 97·84 118·80 112·80 150·80 86·76 70·86 105·00 Nb 28·22 21·78 13·80 14·44 12·58 10·78 10·27 9·51 20·38 15·14 10·51 8·30 Mo 0·47 0·96 9·27 0·68 0·57 0·46 0·43 0·73 0·48 0·55 0·76 0·59 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·45 1·53 2·04 2·05 1·43 1·53 1·13 1·37 3·51 2·09 2·10 2·16 Ba 687·00 591·00 513·00 624·00 482·00 531·00 592·00 533·00 171·00 538·00 553·00 557·00 La 26·54 31·20 32·16 23·80 23·74 23·12 18·08 21·04 14·26 26·38 21·56 26·14 Ce 67·42 54·76 57·68 47·42 52·62 49·52 43·92 46·26 39·24 45·72 48·86 52·22 Pr 8·33 6·40 6·53 5·52 6·12 5·72 5·30 5·49 5·64 5·77 5·56 6·30 Nd 31·48 25·50 26·14 22·48 23·36 22·00 20·92 21·68 24·90 23·74 21·26 25·68 Sm 6·40 5·48 5·58 5·06 4·77 4·56 4·38 4·56 6·48 5·49 3·64 5·84 Eu 1·54 1·26 1·37 1·19 1·05 1·05 1·05 1·13 0·99 1·14 1·05 1·38 Gd 5·28 5·05 4·99 4·41 4·28 3·63 3·43 3·94 6·32 4·76 3·45 4·66 Tb 0·76 0·69 0·66 0·53 0·57 0·50 0·48 0·54 1·13 0·72 0·47 0·65 Dy 4·75 3·50 3·90 3·33 3·58 2·95 2·67 3·19 8·33 4·11 2·82 3·74 Ho 0·92 0·64 0·73 0·60 0·66 0·60 0·53 0·55 1·71 0·80 0·52 0·71 Er 2·81 1·57 2·00 1·60 1·81 1·47 1·47 1·57 5·25 2·25 1·49 1·82 Tm 0·49 0·25 0·29 0·20 0·24 0·20 0·21 0·25 0·96 0·37 0·22 0·27 Yb 4·10 1·66 2·10 1·73 1·93 1·57 1·57 1·69 7·54 2·86 1·74 1·72 Lu 0·62 0·23 0·31 0·24 0·28 0·25 0·24 0·23 1·12 0·42 0·23 0·23 Hf 2·67 2·18 2·67 1·71 2·71 2·48 2·83 3·10 3·70 2·55 1·92 2·98 Ta 1·70 1·41 1·48 1·78 1·96 1·48 1·46 1·14 1·30 1·43 1·07 1·30 Pb 5·97 4·51 5·82 5·77 6·33 6·07 6·29 7·11 3·85 6·71 4·86 5·89 Th 1·93 1·52 2·42 2·00 2·04 1·90 1·80 1·89 1·63 1·70 1·54 6·11 U 2·48 1·58 2·49 1·95 2·47 2·23 1·60 2·28 6·55 2·02 1·89 2·29 Rock type: . Hbl-gabbro with veins . Hornblende gabbro . . . . . . . . . . dyke . . . . Core- . 10·10– . 11·00– . 12·50– . 28·00– . 14·70– . 15·60– . 18·70– . 18·80– . 19·50– . 19·60– . 20·40– . 21·10– . metre: . 10·35 . 11·25 . 12·70 . 28·35 . 15·00 . 15·80 . 18·80 . 19·00 . 19·60 . 20·00 . 20·60 . 21·40 . Depth: . 14·25 . 15·15 . 16·6 . 32·2 . 18·85 . 19·7 . 22·75 . 22·9 . 23·55 . 23·8 . 24·5 . 25·25 . SiO2 (%) 52·83 51·83 53·00 52·75 51·98 52·72 53·05 53·21 51·61 52·70 51·33 51·23 TiO2 0·53 0·52 0·49 0·43 0·47 0·41 0·33 0·32 0·29 0·34 0·42 0·50 Al2O3 10·02 9·61 8·84 8·45 8·64 9·02 8·78 8·69 7·64 8·49 8·40 8·91 Fe2O3* 9·06 9·32 9·07 8·49 9·05 8·47 8·25 8·42 8·93 8·70 8·94 9·42 MnO 0·17 0·18 0·17 0·16 0·16 0·15 0·15 0·15 0·20 0·17 0·17 0·17 MgO 12·60 13·18 13·32 13·70 13·83 13·82 13·35 13·68 13·73 13·85 14·71 13·57 CaO 8·10 9·00 9·18 9·47 9·23 9·02 8·81 8·99 9·31 8·95 9·04 9·27 Na2O 1·79 1·79 1·62 1·37 1·43 1·57 1·51 1·64 0·75 1·52 1·25 1·50 K2O 2·93 2·42 2·21 2·35 2·22 2·50 2·68 2·37 0·85 2·12 2·05 2·09 P2O5 0·40 0·40 0·39 0·32 0·35 0·32 0·20 0·23 0·18 0·23 0·30 0·38 LOI 2·08 2·67 2·52 2·88 2·20 2·04 2·13 2·04 6·09 2·10 2·49 2·36 Total 100·51 100·92 100·81 100·37 99·56 100·04 99·24 99·74 99·58 99·17 99·10 99·40 Sc (ppm) 29·66 37·72 29·90 32·00 31·16 29·54 28·58 27·90 36·68 29·18 31·84 32·82 V 143·00 144·00 142·00 128·00 122·00 115·00 96·00 98·00 105·00 108·00 116·00 142·00 Cr 891·80 1174·80 914·40 1382·60 1286·20 1204·40 1165·20 1230·00 1509·00 1231·80 1325·00 1059·80 Co 58·00 53·00 64·00 59·00 62·00 59·00 67·00 60·00 65·00 65·00 60·00 69·00 Ni 179·20 236·40 184·60 246·20 221·00 206·60 211·80 220·00 249·40 232·00 246·60 200·60 Cu 49·98 51·94 63·64 69·80 45·04 48·26 50·70 51·56 28·14 56·26 58·18 43·92 Zn 110·00 119·00 112·00 91·00 84·00 77·00 72·00 74·00 137·00 89·00 94·00 88·00 Ga 22·00 17·00 21·00 24·00 12·00 15·00 15·00 8·00 23·00 16·00 9·00 9·00 Rb 169·00 131·60 120·40 135·40 89·00 107·00 97·00 121·40 65·46 116·00 90·00 127·60 Sr 245·00 251·00 244·00 244·00 248·00 311·00 261·00 268·00 128·00 228·00 212·00 277·00 Y 30·22 25·54 18·82 18·52 18·24 16·00 15·90 15·76 48·68 23·04 15·48 17·16 Zr 111·80 120·00 95·64 63·04 99·80 97·84 118·80 112·80 150·80 86·76 70·86 105·00 Nb 28·22 21·78 13·80 14·44 12·58 10·78 10·27 9·51 20·38 15·14 10·51 8·30 Mo 0·47 0·96 9·27 0·68 0·57 0·46 0·43 0·73 0·48 0·55 0·76 0·59 Sn b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·45 1·53 2·04 2·05 1·43 1·53 1·13 1·37 3·51 2·09 2·10 2·16 Ba 687·00 591·00 513·00 624·00 482·00 531·00 592·00 533·00 171·00 538·00 553·00 557·00 La 26·54 31·20 32·16 23·80 23·74 23·12 18·08 21·04 14·26 26·38 21·56 26·14 Ce 67·42 54·76 57·68 47·42 52·62 49·52 43·92 46·26 39·24 45·72 48·86 52·22 Pr 8·33 6·40 6·53 5·52 6·12 5·72 5·30 5·49 5·64 5·77 5·56 6·30 Nd 31·48 25·50 26·14 22·48 23·36 22·00 20·92 21·68 24·90 23·74 21·26 25·68 Sm 6·40 5·48 5·58 5·06 4·77 4·56 4·38 4·56 6·48 5·49 3·64 5·84 Eu 1·54 1·26 1·37 1·19 1·05 1·05 1·05 1·13 0·99 1·14 1·05 1·38 Gd 5·28 5·05 4·99 4·41 4·28 3·63 3·43 3·94 6·32 4·76 3·45 4·66 Tb 0·76 0·69 0·66 0·53 0·57 0·50 0·48 0·54 1·13 0·72 0·47 0·65 Dy 4·75 3·50 3·90 3·33 3·58 2·95 2·67 3·19 8·33 4·11 2·82 3·74 Ho 0·92 0·64 0·73 0·60 0·66 0·60 0·53 0·55 1·71 0·80 0·52 0·71 Er 2·81 1·57 2·00 1·60 1·81 1·47 1·47 1·57 5·25 2·25 1·49 1·82 Tm 0·49 0·25 0·29 0·20 0·24 0·20 0·21 0·25 0·96 0·37 0·22 0·27 Yb 4·10 1·66 2·10 1·73 1·93 1·57 1·57 1·69 7·54 2·86 1·74 1·72 Lu 0·62 0·23 0·31 0·24 0·28 0·25 0·24 0·23 1·12 0·42 0·23 0·23 Hf 2·67 2·18 2·67 1·71 2·71 2·48 2·83 3·10 3·70 2·55 1·92 2·98 Ta 1·70 1·41 1·48 1·78 1·96 1·48 1·46 1·14 1·30 1·43 1·07 1·30 Pb 5·97 4·51 5·82 5·77 6·33 6·07 6·29 7·11 3·85 6·71 4·86 5·89 Th 1·93 1·52 2·42 2·00 2·04 1·90 1·80 1·89 1·63 1·70 1·54 6·11 U 2·48 1·58 2·49 1·95 2·47 2·23 1·60 2·28 6·55 2·02 1·89 2·29 Rock type: . Hornblende gabbro . Core- . 22·20– . 24·70– . 25·45– . 26·10– . 26·55– . 29·00– . 31·80– . 34·65– . metre: . 22·40 . 25·00 . 25·70 . 26·30 . 26·80 . 29·30 . 32·00 . 35·00 . Depth: . 26·3 . 28·85 . 29·6 . 30·2 . 30·7 . 33·15 . 35·9 . 38·8 . SiO2 (%) 51·38 51·12 50·66 50·52 51·07 51·78 52·72 52·53 TiO2 0·50 0·46 0·46 0·44 0·41 0·50 0·47 0·43 Al2O3 8·71 8·59 8·83 8·84 8·75 8·76 9·25 9·35 Fe2O3* 9·49 9·31 9·27 9·27 9·10 9·07 8·64 8·78 MnO 0·17 0·17 0·16 0·16 0·16 0·16 0·16 0·16 MgO 13·82 13·95 14·10 14·45 14·23 13·56 13·68 14·01 CaO 9·26 9·36 9·07 9·11 8·98 8·86 8·65 8·85 Na2O 1·41 1·50 1·51 1·48 1·36 1·48 1·52 1·61 K2O 2·17 1·80 1·80 1·44 1·86 2·21 2·67 2·51 P2O5 0·40 0·37 0·34 0·35 0·34 0·35 0·31 0·34 LOI 2·23 2·44 2·75 2·80 2·65 2·24 2·06 2·20 Total 99·54 99·07 98·95 98·86 98·91 98·97 100·13 100·77 Sc (ppm) 34·08 33·04 30·34 30·92 29·08 30·44 29·80 30·02 V 145·00 136·00 134·00 127·00 112·00 130·00 127·00 122·00 Cr 1035·50 1122·60 1163·80 1206·20 1218·20 1296·60 1203·60 1229·40 Co 64·00 57·00 70·00 74·00 67·00 67·00 67·00 59·00 Ni 196·00 212·40 230·40 232·80 239·00 228·00 209·40 214·60 Cu 54·98 40·92 46·88 54·32 38·68 54·58 54·24 54·34 Zn 93·00 120·00 81·00 73·00 75·00 91·00 91·00 94·00 Ga 11·00 11·00 16·00 19·00 17·00 10·00 9·00 14·00 Rb 93·00 83·00 110·60 75·00 101·48 101·00 115·00 96·00 Sr 266·00 257·00 277·00 289·00 277·00 255·00 324·00 280·00 Y 16·45 15·76 15·02 16·12 16·24 19·16 18·06 21·50 Zr 89·00 91·04 84·68 57·90 72·80 84·48 78·72 81·86 Nb 7·81 7·38 7·47 7·87 8·01 14·32 11·80 11·70 Mo 0·46 0·49 0·75 3·45 0·85 0·43 2·24 0·62 Sn b.d. 16·00 b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·28 1·21 2·19 2·46 1·89 2·78 2·09 1·60 Ba 573·00 458·00 520·00 353·00 599·00 579·00 751·00 665·00 La 22·98 20·28 23·48 23·46 24·00 21·94 19·98 22·32 Ce 48·45 44·54 45·24 50·26 50·52 49·22 45·58 51·92 Pr 5·81 5·48 5·43 6·02 6·08 5·81 5·34 6·12 Nd 24·20 22·38 22·68 23·24 24·84 23·68 21·28 24·30 Sm 5·03 5·11 4·62 5·38 5·26 5·53 4·72 5·25 Eu 1·25 1·12 1·19 1·20 1·20 1·18 1·12 1·23 Gd 4·59 4·30 3·99 4·49 4·04 4·57 4·03 4·50 Tb 0·61 0·62 0·56 0·59 0·58 0·64 0·58 0·65 Dy 3·39 3·23 3·37 3·21 3·38 3·72 3·27 3·99 Ho 0·62 0·59 0·57 0·62 0·59 0·70 0·58 0·75 Er 1·65 1·47 1·60 1·39 1·59 1·97 1·80 2·15 Tm 0·22 0·21 0·20 0·20 0·23 0·31 0·26 0·34 Yb 1·54 1·59 1·45 1·56 1·46 2·19 1·97 2·20 Lu 0·23 0·20 0·21 0·18 0·21 0·31 0·28 0·29 Hf 2·36 2·28 2·30 1·82 2·28 2·40 2·27 2·46 Ta 1·16 1·17 0·98 0·76 0·63 1·71 1·13 1·64 Pb 4·69 4·09 5·10 4·51 5·79 5·70 6·04 7·03 Th 2·39 1·94 2·65 2·56 1·92 2·71 1·75 2·47 U 1·82 2·10 2·12 1·65 1·63 3·02 1·88 2·20 Rock type: . Hornblende gabbro . Core- . 22·20– . 24·70– . 25·45– . 26·10– . 26·55– . 29·00– . 31·80– . 34·65– . metre: . 22·40 . 25·00 . 25·70 . 26·30 . 26·80 . 29·30 . 32·00 . 35·00 . Depth: . 26·3 . 28·85 . 29·6 . 30·2 . 30·7 . 33·15 . 35·9 . 38·8 . SiO2 (%) 51·38 51·12 50·66 50·52 51·07 51·78 52·72 52·53 TiO2 0·50 0·46 0·46 0·44 0·41 0·50 0·47 0·43 Al2O3 8·71 8·59 8·83 8·84 8·75 8·76 9·25 9·35 Fe2O3* 9·49 9·31 9·27 9·27 9·10 9·07 8·64 8·78 MnO 0·17 0·17 0·16 0·16 0·16 0·16 0·16 0·16 MgO 13·82 13·95 14·10 14·45 14·23 13·56 13·68 14·01 CaO 9·26 9·36 9·07 9·11 8·98 8·86 8·65 8·85 Na2O 1·41 1·50 1·51 1·48 1·36 1·48 1·52 1·61 K2O 2·17 1·80 1·80 1·44 1·86 2·21 2·67 2·51 P2O5 0·40 0·37 0·34 0·35 0·34 0·35 0·31 0·34 LOI 2·23 2·44 2·75 2·80 2·65 2·24 2·06 2·20 Total 99·54 99·07 98·95 98·86 98·91 98·97 100·13 100·77 Sc (ppm) 34·08 33·04 30·34 30·92 29·08 30·44 29·80 30·02 V 145·00 136·00 134·00 127·00 112·00 130·00 127·00 122·00 Cr 1035·50 1122·60 1163·80 1206·20 1218·20 1296·60 1203·60 1229·40 Co 64·00 57·00 70·00 74·00 67·00 67·00 67·00 59·00 Ni 196·00 212·40 230·40 232·80 239·00 228·00 209·40 214·60 Cu 54·98 40·92 46·88 54·32 38·68 54·58 54·24 54·34 Zn 93·00 120·00 81·00 73·00 75·00 91·00 91·00 94·00 Ga 11·00 11·00 16·00 19·00 17·00 10·00 9·00 14·00 Rb 93·00 83·00 110·60 75·00 101·48 101·00 115·00 96·00 Sr 266·00 257·00 277·00 289·00 277·00 255·00 324·00 280·00 Y 16·45 15·76 15·02 16·12 16·24 19·16 18·06 21·50 Zr 89·00 91·04 84·68 57·90 72·80 84·48 78·72 81·86 Nb 7·81 7·38 7·47 7·87 8·01 14·32 11·80 11·70 Mo 0·46 0·49 0·75 3·45 0·85 0·43 2·24 0·62 Sn b.d. 16·00 b.d. b.d. b.d. b.d. b.d. b.d. Cs 2·28 1·21 2·19 2·46 1·89 2·78 2·09 1·60 Ba 573·00 458·00 520·00 353·00 599·00 579·00 751·00 665·00 La 22·98 20·28 23·48 23·46 24·00 21·94 19·98 22·32 Ce 48·45 44·54 45·24 50·26 50·52 49·22 45·58 51·92 Pr 5·81 5·48 5·43 6·02 6·08 5·81 5·34 6·12 Nd 24·20 22·38 22·68 23·24 24·84 23·68 21·28 24·30 Sm 5·03 5·11 4·62 5·38 5·26 5·53 4·72 5·25 Eu 1·25 1·12 1·19 1·20 1·20 1·18 1·12 1·23 Gd 4·59 4·30 3·99 4·49 4·04 4·57 4·03 4·50 Tb 0·61 0·62 0·56 0·59 0·58 0·64 0·58 0·65 Dy 3·39 3·23 3·37 3·21 3·38 3·72 3·27 3·99 Ho 0·62 0·59 0·57 0·62 0·59 0·70 0·58 0·75 Er 1·65 1·47 1·60 1·39 1·59 1·97 1·80 2·15 Tm 0·22 0·21 0·20 0·20 0·23 0·31 0·26 0·34 Yb 1·54 1·59 1·45 1·56 1·46 2·19 1·97 2·20 Lu 0·23 0·20 0·21 0·18 0·21 0·31 0·28 0·29 Hf 2·36 2·28 2·30 1·82 2·28 2·40 2·27 2·46 Ta 1·16 1·17 0·98 0·76 0·63 1·71 1·13 1·64 Pb 4·69 4·09 5·10 4·51 5·79 5·70 6·04 7·03 Th 2·39 1·94 2·65 2·56 1·92 2·71 1·75 2·47 U 1·82 2·10 2·12 1·65 1·63 3·02 1·88 2·20 *All Fe as Fe2O3. Open in new tab In a total alkalis–silica (TAS) diagram (Fig. 4a) the strong bimodal distribution of SiO2 contents reflects the polarity in felsic and mafic rock compositions in the drill core. Almost all of the felsic samples conform geochemically to metaluminous to peraluminous granitic compositions. According to the TAS classification as proposed by Middlemost (1994), specifically the SiO2 contents between 50·5 and 53·4 wt %, all of the mafic rocks would be classified as either gabbro or gabbroic diorite. For simplicity, they will be referred to in the following discussion as gabbro, with the understanding that a dioritic composition of the protolith cannot be excluded in all cases. In the TAS space all of the mafic samples, except for the biotite-rich zone, are subalkaline. In terms of K contents, both the mafic and felsic rocks could theoretically correspond to high-K calc-alkaline compositions. The measured K concentrations are unlikely to be representative of the protolith's composition, however, considering the evidence of hydrothermal overprinting and post-magmatic K-metasomatism. At least two stages of hydrothermal alteration can be distinguished: (1) a higher-temperature, more or less pervasive potassic alteration as evident from the distribution of microcline in the metagabbro and the formation of the biotite-rich zone near the contact between the metagabbro and the granite; (2) a lower temperature, fracture-controlled alteration along veins. Fig. 4. Open in new tabDownload slide Harker variation diagrams of Na2O + K2O (a), CaO + MgO (b), and FeO*/MgO (c) vs SiO2 for the various rock types in the investigated drill core. Distinction between calc-alkaline and tholeiitic compositions in (c) after Miyashiro (1974). The expected high concentration of K in what is referred to as the ‘biotite-rich zone’ is notably absent (Fig. 4a). This is due to almost complete chloritization of the biotite in that zone. Of interest is an enrichment in P and Ti in this previously biotite-rich zone (Fig. 5a and b), which reflects the relatively high proportion of apatite and titanite. Fig. 5. Open in new tabDownload slide TiO2 (a) and P2O5 vs SiO2 (b), and Cr vs Cu (c) for the various rock types in the investigated drill core. The mafic samples with the greatest extent of hydrothermal veining do not differ significantly in their major element concentrations from those samples that are not visibly veined (Fig. 4). Thus the lower temperature alteration did not pervasively alter the whole-rock composition. In contrast, the earlier K-metasomatism must have affected most, if not all, of the studied drill core and, consequently, any diagram relying on alkali element distribution for the characterization of the original melt composition cannot be reliable. A more reliable discriminant in this regard may be the relative proportion of Fe as shown in a FeO*/MgO vs SiO2 diagram (Miyashiro, 1974). All the hornblende metagabbro samples plot in a tight cluster in the field of calc-alkaline compositions (Fig. 4c), even those that contain hydrothermal veins. The granite samples show a wider spread but all of them, with the exception of two extreme outliers (highly altered granite samples), also plot in the calc-alkaline field. The Ni and Cr concentrations are higher by about two to three orders of magnitude in the relatively mafic rocks compared with the altered granite, with the biotite-rich zone samples plotting in an intermediate position (Fig. 5c). Elevated Cu contents in some samples are related to the hydrothermal alteration because the veined metagabbro samples contain an order of magnitude more Cu than the others. The hornblende metagabbro is enriched in Ni relative to normal mid-ocean ridge basalt (N-MORB) by a factor of 1·5, whereas the biotite-rich transition zone is depleted (Ni* = 0·75). All analysed mafic rocks (without visible veining) are depleted in Cu, with MORB-normalized Cu in the hornblende metagabbro being 0·73 and in the biotite-rich zone only 0·32. Considering the altered nature of the studied rocks, even the FeO–MgO–SiO2 distribution may no longer be representative of the original composition, and potentially more reliable information on the likely setting of the magmatic protoliths may be obtained from the relationships between the least mobile elements; that is, the REE and high field strength elements (HFSE). The relationships between Nb, Y, Ta, and Yb in all the granite samples with a narrow range in SiO2 that is within the range used by Pearce et al. (1984) for the construction of their discrimination diagrams conform to those of volcanic arc granites (Fig. 6). The least hydrothermally altered mafic compositions were normalized against mid-oceanic ridge basalt (N-MORB). The resulting diagram (Fig. 7) shows an overall enrichment in the less compatible elements, negative Nb, Zr and Hf, as well as a strong negative Ti anomalies and a positive Ho anomaly in all mafic samples. The trace element concentrations in the biotite-rich transition zone follow a similar pattern but this zone is markedly enriched in Th and the REE, which can be explained by the high concentration of allanite in this zone. The same zone is particularly strongly depleted in Zr and Hf, reflecting a low zircon content. Fig. 6. Open in new tabDownload slide Nb vs Y (a) and Ta vs Yb diagrams (b) for the granitic rocks; discrimination of genetic types according to tectonic setting from Pearce et al. (1984). For comparison, the composition of quartz–feldspar porphyries of the Dominion Group is also shown (from Marsh et al., 1989) as a grey shaded field in (a). Fig. 7. Open in new tabDownload slide N-MORB-normalized trace element patterns for mafic rocks in the investigated drill core. Elements are ordered according to decreasing compatibility. N-MORB composition from Hofmann (1988). All analysed samples are enriched in the light REE (LREE) relative to chondrite (Fig. 8) but also relative to N-MORB (not shown). The strongest enrichment is noted in the biotite-rich zone and some of the granite samples. The hornblende metagabbro samples display a very uniform REE distribution that is very similar to that of those metagabbro samples with hydrothermal vein-type alteration. Thus, the low-temperature hydrothermal alteration is regarded as not having significantly affected the overall REE patterns. Apart from the overall LREE enrichment, the gabbroic samples show no elemental anomalies. Notably, they lack any significant Eu anomaly. In contrast, the (chloritized) biotite-rich transition zone is characterized by a marked negative Eu anomaly, whereas most granite samples yielded a less pronounced negative Eu anomaly (Fig. 8). Fig. 8. Open in new tabDownload slide Chondrite-normalized rare earth element patterns in the principal rock types of the investigated drill core; normalization values from Sun & McDonough (1989). Rb–Sr and Sm–Nd isotopes Rb–Sr isotope analyses of five hornblende metagabbro and five metagranite samples (Table 4) were carried out at the Institute of Mineralogy, University of Münster. Whole-rock powders (c. 100 mg) were mixed with a 87Rb–84Sr spike in Teflon screw-top vials and dissolved in a HF–HNO3 (5:1) mixture on a hot plate overnight. After evaporation and drying, 6N HCl was added to the residue and mixed to homogenization. After a second evaporation to dryness, Rb and Sr were separated by standard ion-exchange procedures (AG 50W-X8 resin) on quartz glass columns using 2·5N and 6N HCl as eluents. Rb was loaded with H2O on Ta double filaments and Sr was loaded with TaF5 on W single filaments. The Rb and Sr isotope ratios were measured with a VG Sector 54 and a Finnigan Triton multicollector thermal ionization mass spectrometer, respectively. Correction for mass fractionation is based on a 86Sr/88Sr ratio of 0·1194. Rb ratios were corrected for mass fractionation using a factor deduced from multiple measurements of the Rb standard NBS 607. Total procedural blanks were less than 15 pg for Rb and less than 30 pg for Sr. Based on repeated measurements the 87Rb/86Sr ratios were assigned an uncertainty of 1% (2σ). In the course of this study, repeated runs of NBS standard 987 gave an average 87Sr/86Sr ratio of 0·710223 ± 0·000018 (2σ, n = 16). Table 4: Rb–Sr and Sm–Nd isotope data for hornblende metagabbro and metagranite samples Core length . Rock . Rb . Sr . 87Rb/86Sr . 87Sr/86Sr . Error (2σ) . Sm . Nd . 147Sm/144Nd . 143Nd/144Nd . Error (2σ) . TDM . ϵNd . . . (ppm) . (ppm) . . . . (ppm) . (ppm) . . . . (Ma) . (3062 Ma) . 22·20–22·40 Hbl-metagabbro 97·6 321 0·8842 0·745716 0·000013 5·03 24·20 0·12551 0·511292 0·000009 3106·2 1·85 18·70–18·80 Hbl-gabbro 98·1 293 0·9733 0·746338 0·000013 4·38 20·90 0·12967 0·511192 0·000009 3437·7 −1·78 15·60–15·80 Hbl-gabbro 102 322 0·9211 0·743865 0·000016 4·56 22·00 0·12929 0·511030 0·000008 3708 −4·80 31·80–32·00 Hbl-gabbro 110 342 0·9384 0·746976 0·000016 4·72 21·30 0·13381 0·511336 0·000010 3343·7 −0·59 19·60–20·00 Hbl-gabbro 85·8 257 0·9704 0·746410 0·000015 5·10–5·20 granite 187 231 2·3642 0·798783 0·000013 11·00 67·60 0·098243 0·510640 0·000011 3226·5 −0·13 3·10–3·30 granite 203 177 3·3529 0·837933 0·000018 3·30 18·20 0·109476 0·510859 0·000008 3256·2 −0·29 1·65–1·75 granite 203 179 3·3203 0·836689 0·000027 6·10–6·20 granite 235 203 3·4030 0·839558 0·000015 1·60 6·51 0·148403 0·511133 0·000009 4545 −10·38 0·15–0·35 granite 214 205 3·0502 0·826980 0·000013 5·63 31·50 0·10791 0·510753 0·000008 3358 −1·74 Core length . Rock . Rb . Sr . 87Rb/86Sr . 87Sr/86Sr . Error (2σ) . Sm . Nd . 147Sm/144Nd . 143Nd/144Nd . Error (2σ) . TDM . ϵNd . . . (ppm) . (ppm) . . . . (ppm) . (ppm) . . . . (Ma) . (3062 Ma) . 22·20–22·40 Hbl-metagabbro 97·6 321 0·8842 0·745716 0·000013 5·03 24·20 0·12551 0·511292 0·000009 3106·2 1·85 18·70–18·80 Hbl-gabbro 98·1 293 0·9733 0·746338 0·000013 4·38 20·90 0·12967 0·511192 0·000009 3437·7 −1·78 15·60–15·80 Hbl-gabbro 102 322 0·9211 0·743865 0·000016 4·56 22·00 0·12929 0·511030 0·000008 3708 −4·80 31·80–32·00 Hbl-gabbro 110 342 0·9384 0·746976 0·000016 4·72 21·30 0·13381 0·511336 0·000010 3343·7 −0·59 19·60–20·00 Hbl-gabbro 85·8 257 0·9704 0·746410 0·000015 5·10–5·20 granite 187 231 2·3642 0·798783 0·000013 11·00 67·60 0·098243 0·510640 0·000011 3226·5 −0·13 3·10–3·30 granite 203 177 3·3529 0·837933 0·000018 3·30 18·20 0·109476 0·510859 0·000008 3256·2 −0·29 1·65–1·75 granite 203 179 3·3203 0·836689 0·000027 6·10–6·20 granite 235 203 3·4030 0·839558 0·000015 1·60 6·51 0·148403 0·511133 0·000009 4545 −10·38 0·15–0·35 granite 214 205 3·0502 0·826980 0·000013 5·63 31·50 0·10791 0·510753 0·000008 3358 −1·74 Open in new tab Table 4: Rb–Sr and Sm–Nd isotope data for hornblende metagabbro and metagranite samples Core length . Rock . Rb . Sr . 87Rb/86Sr . 87Sr/86Sr . Error (2σ) . Sm . Nd . 147Sm/144Nd . 143Nd/144Nd . Error (2σ) . TDM . ϵNd . . . (ppm) . (ppm) . . . . (ppm) . (ppm) . . . . (Ma) . (3062 Ma) . 22·20–22·40 Hbl-metagabbro 97·6 321 0·8842 0·745716 0·000013 5·03 24·20 0·12551 0·511292 0·000009 3106·2 1·85 18·70–18·80 Hbl-gabbro 98·1 293 0·9733 0·746338 0·000013 4·38 20·90 0·12967 0·511192 0·000009 3437·7 −1·78 15·60–15·80 Hbl-gabbro 102 322 0·9211 0·743865 0·000016 4·56 22·00 0·12929 0·511030 0·000008 3708 −4·80 31·80–32·00 Hbl-gabbro 110 342 0·9384 0·746976 0·000016 4·72 21·30 0·13381 0·511336 0·000010 3343·7 −0·59 19·60–20·00 Hbl-gabbro 85·8 257 0·9704 0·746410 0·000015 5·10–5·20 granite 187 231 2·3642 0·798783 0·000013 11·00 67·60 0·098243 0·510640 0·000011 3226·5 −0·13 3·10–3·30 granite 203 177 3·3529 0·837933 0·000018 3·30 18·20 0·109476 0·510859 0·000008 3256·2 −0·29 1·65–1·75 granite 203 179 3·3203 0·836689 0·000027 6·10–6·20 granite 235 203 3·4030 0·839558 0·000015 1·60 6·51 0·148403 0·511133 0·000009 4545 −10·38 0·15–0·35 granite 214 205 3·0502 0·826980 0·000013 5·63 31·50 0·10791 0·510753 0·000008 3358 −1·74 Core length . Rock . Rb . Sr . 87Rb/86Sr . 87Sr/86Sr . Error (2σ) . Sm . Nd . 147Sm/144Nd . 143Nd/144Nd . Error (2σ) . TDM . ϵNd . . . (ppm) . (ppm) . . . . (ppm) . (ppm) . . . . (Ma) . (3062 Ma) . 22·20–22·40 Hbl-metagabbro 97·6 321 0·8842 0·745716 0·000013 5·03 24·20 0·12551 0·511292 0·000009 3106·2 1·85 18·70–18·80 Hbl-gabbro 98·1 293 0·9733 0·746338 0·000013 4·38 20·90 0·12967 0·511192 0·000009 3437·7 −1·78 15·60–15·80 Hbl-gabbro 102 322 0·9211 0·743865 0·000016 4·56 22·00 0·12929 0·511030 0·000008 3708 −4·80 31·80–32·00 Hbl-gabbro 110 342 0·9384 0·746976 0·000016 4·72 21·30 0·13381 0·511336 0·000010 3343·7 −0·59 19·60–20·00 Hbl-gabbro 85·8 257 0·9704 0·746410 0·000015 5·10–5·20 granite 187 231 2·3642 0·798783 0·000013 11·00 67·60 0·098243 0·510640 0·000011 3226·5 −0·13 3·10–3·30 granite 203 177 3·3529 0·837933 0·000018 3·30 18·20 0·109476 0·510859 0·000008 3256·2 −0·29 1·65–1·75 granite 203 179 3·3203 0·836689 0·000027 6·10–6·20 granite 235 203 3·4030 0·839558 0·000015 1·60 6·51 0·148403 0·511133 0·000009 4545 −10·38 0·15–0·35 granite 214 205 3·0502 0·826980 0·000013 5·63 31·50 0·10791 0·510753 0·000008 3358 −1·74 Open in new tab Both the Rb and Sr concentrations and respective isotope ratios are very similar for the hornblende metagabbro samples, whereas some spread and overall higher Rb concentrations, and thus more radiogenic Sr is noted for the granitic samples. Combining all 10 analyses yields an errorchron, the slope of which corresponds to an imprecise age of 2633 ± 50 Ma (Fig. 9) using a Rb decay constant of 1·42 × 10−11 (Steiger & Jäger, 1977). The calculated initial 87Sr/86Sr ratio is 0·7100 ± 0·0017. Using only the five granite data points would result in an errorchron ‘age’ of 2721 ± 150 Ma and an initial 87Sr/86Sr ratio of 0·7059 ± 0·0069 (MSWD = 226). Fig. 9. Open in new tabDownload slide Rb–Sr isochron plot for the granite and hornblende metagabbro. The Nd isotope ratios (Table 4) were measured on a VG Sector 7-collector mass spectrometer in multi-dynamic mode at the Department of Geological Sciences, University of Cape Town, following the standard chemical separation techniques described by le Roex & Lanyon (1998). A depleted mantle isotopic composition of 143Nd/144Nd = 0·5131 and 147Sm/144Nd = 0·2136 (Henry et al., 2000), and a 147Sm decay constant of 6·54 × 10−12 (Lugmair & Marti, 1978) were used for the calculation of the Nd model ages and the ϵNd values. The ϵNd values at the time of likely formation (3·07 Ga: see Fig. 10 and the subsequent section on geochronology) obtained for most of the granite and the hornblende metagabbro samples overlap and cluster between −1·8 and +1·9, but for one felsic and one mafic sample they are much lower, at −10·4 and −4·8, respectively. The subchondritic ϵNd values, corresponding to calculated TDM model ages that are higher than the age of magmatism (see below, Table 4), might be an indication of contamination by older crust. This argument is, however, not conclusive because the model age strongly depends on the preferred model for the evolution of the mantle composition in the Archaean—a controversial topic that is beyond the scope of this paper. Fig. 10. Open in new tabDownload slide ϵNd evolution diagram for the granite and hornblende metagabbro; for comparison the data field for the Dominion Group volcanic rocks (recalculated data from Marsh et al., 1989) is also shown. U–PB AND HF ISOTOPE DATA ON ZIRCON To constrain the age and to further characterize the source of the two principal magmatic rock types in the drill core, single zircon grains were separated from both the variably altered granite and the hornblende metagabbro by standard crushing and heavy mineral separation techniques. Polished zircon grain mounts were imaged by scanning electron microscope cathodoluminescence (CL) using a JEOL JSM-6400 electron microprobe at the Institute of Geosciences, University of Frankfurt. Selected zircon domains were analysed for their U–Pb isotopic composition by LA-ICPMS at the same institution, using a Thermo-Finnigan element II sector field ICPMS system coupled to a New Wave UP213 UV laser system. The results are listed in Table 5. Analytical details, data processing, and error calculations have been given by Gerdes & Zeh (2006, 2008). Concordia and upper intercept ages on concordia diagrams were calculated using the Isoplot/Ex 2.49 software (Ludwig, 2000). Table 5: U–Pb isotope data of single zircon domains Sample . U1 . Pb1 . Th/U1 . 206Pb/204Pb . 206Pb/238U2 . ±2σ . 207Pb/235U3 . ±2σ . 207Pb/206Pb2 . ±2σ . rho4 . Age (Ma) . no. . (ppm) . (ppm) . . . . (%) . . (%) . . (%) . . 206Pb/238U . ±2σ (%) . 207Pb/235U . ±2σ (%) . 207Pb/206Pb . ±2σ (%) . Con.5 (%) . BH1-metagranite A1 500 120 0·87 895·52 0·18910 3·2 4·888 3·6 0·18747 1·6 0·89 1116 33 1800 30 2720 27 41 A2 677 77 0·63 663·17 0·07754 10·5 1·721 11·4 0·16095 4·5 0·92 481 49 1016 76 2466 76 20 A3 311 147 0·78 2492·52 0·38819 9·5 11·548 9·8 0·21577 2·5 0·97 2114 173 2569 96 2949 40 72 A4 1088 186 0·46 541·40 0·12430 8·9 2·915 9·6 0·17011 3·4 0·93 755 64 1386 75 2559 57 30 A5 269 186 0·65 867·89 0·51473 8·6 13·515 9·4 0·19043 3·8 0·92 2677 192 2716 93 2746 62 97 A6 312 107 0·57 1294·63 0·23743 10·4 6·166 10·8 0·18835 2·9 0·96 1373 130 2000 99 2728 47 50 A7 303 93 0·84 4960·32 0·27662 5·6 7·148 6·5 0·18742 3·3 0·86 1574 79 2130 59 2720 54 58 A8 408 94 0·83 5374·14 0·20430 5·3 4·790 5·9 0·17006 2·6 0·90 1198 58 1783 51 2558 44 47 A9 426 67 0·85 6272·33 0·13968 5·5 2·776 6·2 0·14415 2·7 0·90 843 44 1349 47 2278 47 37 A10 341 116 0·92 906·62 0·22827 7·2 5·842 7·9 0·18561 3·1 0·92 1325 87 1953 71 2704 51 49 A11 198 114 0·60 1132·41 0·38801 3·3 11·750 3·7 0·21964 1·8 0·88 2114 59 2585 36 2978 29 71 A13 202 115 0·53 6420·44 0·48639 5·9 13·734 6·3 0·20479 2·4 0·93 2555 125 2732 62 2865 39 89 A14 67 47 0·49 21055·69 0·59171 4·0 19·133 4·3 0·23451 1·6 0·93 2996 96 3049 42 3083 25 97 A15 64 45 0·92 5691·13 0·60549 4·5 19·117 4·9 0·22898 1·9 0·92 3052 111 3048 48 3045 31 100 A16 163 92 1·03 30326·75 0·48851 4·2 14·399 5·3 0·21378 3·2 0·80 2564 89 2776 51 2934 51 87 A17 1316 80 0·53 407·67 0·04230 13·3 0·631 13·5 0·10811 2·1 0·99 267 35 496 54 1768 39 15 A18 183 47 1·58 5614·70 0·19251 7·1 5·283 7·3 0·19904 1·9 0·97 1135 74 1866 65 2818 30 40 BH1-hornblende metagabbro A1 331 66 0·71 5274·18 0·16279 6·4 4·624 6·6 0·20600 1·6 0·97 972 58 1754 56 2874 25 34 A2 335 81 1·82 1301·30 0·17600 3·9 4·430 4·3 0·18255 1·8 0·91 1045 38 1718 37 2676 30 39 A3 510 61 1·41 823·85 0·07949 2·4 1·650 3·4 0·15056 2·4 0·71 493 11 990 21 2352 40 21 A4 163 81 0·68 30538·03 0·44797 2·0 12·878 2·4 0·20849 1·4 0·81 2386 40 2671 23 2894 23 82 A5 358 82 1·11 17357·28 0·19269 2·4 5·836 2·6 0·21965 1·0 0·92 1136 25 1952 23 2978 16 38 A6 99 70 0·49 10869·06 0·60732 1·8 19·290 2·0 0·23036 1·0 0·87 3059 43 3056 20 3055 16 100 A7 92 68 0·64 7895·48 0·60824 1·3 19·335 1·6 0·23055 0·9 0·82 3063 32 3059 16 3056 15 100 A8 106 74 0·34 20377·72 0·60554 2·3 19·376 2·4 0·23207 0·8 0·94 3052 55 3061 23 3066 13 100 A9 128 89 0·36 34568·41 0·60691 2·4 19·397 2·6 0·23180 0·9 0·94 3058 60 3062 25 3065 14 100 A10 104 70 0·21 13214·50 0·60098 2·2 19·169 2·3 0·23133 0·8 0·94 3034 53 3050 23 3061 13 99 A11 116 84 0·35 2943·58 0·60482 2·3 19·320 2·6 0·23167 1·1 0·90 3049 56 3058 25 3064 18 100 A12 216 193 1·52 1579·22 0·61810 1·5 19·752 2·6 0·23177 2·1 0·59 3102 37 3079 25 3064 33 101 A13 302 45 0·66 4501·20 0·11856 3·3 2·994 3·5 0·18316 1·2 0·94 722 22 1406 27 2682 19 27 A14 407 53 0·51 1163·51 0·1071 3·4 2·741 4·1 0·1855 2·2 0·84 656 21 1340 31 2703 37 24 A15 374 93 0·87 1010·09 0·20086 1·9 5·0723 2·1 0·18315 1·0 0·89 1180 20 1831 18 2682 16 44 A16 220 118 0·71 1280·60 0·4381 2·9 13·393 3·2 0·2217 1·3 0·91 2342 57 2708 31 2993 21 78 A17 93 76 0·71 798·30 0·6289 2·3 20·089 2·7 0·2317 1·5 0·84 3145 57 3096 27 3064 24 103 A19 526 351 4·20 66·11 0·0187 19·9 0·53 20·0 0·2053 1·7 1·00 119 24 432 73 2869 28 4 A20 499 313 3·03 85·50 0·4148 3·5 10·94 5·1 0·1913 3·7 0·69 2237 67 2518 49 2754 60 81 A21 542 287 0·77 97·69 0·3263 2·7 7·416 4·3 0·1648 3·3 0·64 1820 44 2163 39 2506 56 73 A22 198 96 0·22 598·54 0·3938 4·9 12·0728 5·1 0·2223 1·6 0·95 2140 89 2610 49 2998 26 71 A23 325 237 0·90 155·36 0·5400 2·5 14·9350 2·9 0·2006 1·4 0·88 2783 57 2811 28 2831 23 98 A24 571 105 0·88 539·58 0·1434 2·6 3·8585 2·8 0·1951 0·9 0·94 864 21 1605 23 2786 15 31 A25 166 98 0·29 7837·33 0·5462 2·7 16·0264 2·8 0·2128 1·0 0·94 2810 61 2878 27 2927 15 96 A26 375 70 1·83 548·62 0·1385 2·1 3·2441 3·4 0·1699 2·6 0·64 836 17 1468 27 2556 44 33 A27 55 41 0·55 3577·39 0·6136 4·0 19·8287 4·2 0·2344 1·1 0·96 3085 99 3083 41 3082 18 100 A28 143 87 0·40 1039·91 0·5287 2·0 16·8013 2·1 0·2305 0·8 0·93 2736 44 2924 21 3055 12 90 A29 82 42 0·65 1804·93 0·3911 3·1 12·49 3·4 0·2315 1·4 0·91 2128 57 2642 33 3063 22 69 Zircon standard GJ1-4 178 82 1·09 12827·13 0·37576 3·6 11·364 4·2 0·21933 2·1 0·87 2056 64 2553 40 2976 34 69 Sample . U1 . Pb1 . Th/U1 . 206Pb/204Pb . 206Pb/238U2 . ±2σ . 207Pb/235U3 . ±2σ . 207Pb/206Pb2 . ±2σ . rho4 . Age (Ma) . no. . (ppm) . (ppm) . . . . (%) . . (%) . . (%) . . 206Pb/238U . ±2σ (%) . 207Pb/235U . ±2σ (%) . 207Pb/206Pb . ±2σ (%) . Con.5 (%) . BH1-metagranite A1 500 120 0·87 895·52 0·18910 3·2 4·888 3·6 0·18747 1·6 0·89 1116 33 1800 30 2720 27 41 A2 677 77 0·63 663·17 0·07754 10·5 1·721 11·4 0·16095 4·5 0·92 481 49 1016 76 2466 76 20 A3 311 147 0·78 2492·52 0·38819 9·5 11·548 9·8 0·21577 2·5 0·97 2114 173 2569 96 2949 40 72 A4 1088 186 0·46 541·40 0·12430 8·9 2·915 9·6 0·17011 3·4 0·93 755 64 1386 75 2559 57 30 A5 269 186 0·65 867·89 0·51473 8·6 13·515 9·4 0·19043 3·8 0·92 2677 192 2716 93 2746 62 97 A6 312 107 0·57 1294·63 0·23743 10·4 6·166 10·8 0·18835 2·9 0·96 1373 130 2000 99 2728 47 50 A7 303 93 0·84 4960·32 0·27662 5·6 7·148 6·5 0·18742 3·3 0·86 1574 79 2130 59 2720 54 58 A8 408 94 0·83 5374·14 0·20430 5·3 4·790 5·9 0·17006 2·6 0·90 1198 58 1783 51 2558 44 47 A9 426 67 0·85 6272·33 0·13968 5·5 2·776 6·2 0·14415 2·7 0·90 843 44 1349 47 2278 47 37 A10 341 116 0·92 906·62 0·22827 7·2 5·842 7·9 0·18561 3·1 0·92 1325 87 1953 71 2704 51 49 A11 198 114 0·60 1132·41 0·38801 3·3 11·750 3·7 0·21964 1·8 0·88 2114 59 2585 36 2978 29 71 A13 202 115 0·53 6420·44 0·48639 5·9 13·734 6·3 0·20479 2·4 0·93 2555 125 2732 62 2865 39 89 A14 67 47 0·49 21055·69 0·59171 4·0 19·133 4·3 0·23451 1·6 0·93 2996 96 3049 42 3083 25 97 A15 64 45 0·92 5691·13 0·60549 4·5 19·117 4·9 0·22898 1·9 0·92 3052 111 3048 48 3045 31 100 A16 163 92 1·03 30326·75 0·48851 4·2 14·399 5·3 0·21378 3·2 0·80 2564 89 2776 51 2934 51 87 A17 1316 80 0·53 407·67 0·04230 13·3 0·631 13·5 0·10811 2·1 0·99 267 35 496 54 1768 39 15 A18 183 47 1·58 5614·70 0·19251 7·1 5·283 7·3 0·19904 1·9 0·97 1135 74 1866 65 2818 30 40 BH1-hornblende metagabbro A1 331 66 0·71 5274·18 0·16279 6·4 4·624 6·6 0·20600 1·6 0·97 972 58 1754 56 2874 25 34 A2 335 81 1·82 1301·30 0·17600 3·9 4·430 4·3 0·18255 1·8 0·91 1045 38 1718 37 2676 30 39 A3 510 61 1·41 823·85 0·07949 2·4 1·650 3·4 0·15056 2·4 0·71 493 11 990 21 2352 40 21 A4 163 81 0·68 30538·03 0·44797 2·0 12·878 2·4 0·20849 1·4 0·81 2386 40 2671 23 2894 23 82 A5 358 82 1·11 17357·28 0·19269 2·4 5·836 2·6 0·21965 1·0 0·92 1136 25 1952 23 2978 16 38 A6 99 70 0·49 10869·06 0·60732 1·8 19·290 2·0 0·23036 1·0 0·87 3059 43 3056 20 3055 16 100 A7 92 68 0·64 7895·48 0·60824 1·3 19·335 1·6 0·23055 0·9 0·82 3063 32 3059 16 3056 15 100 A8 106 74 0·34 20377·72 0·60554 2·3 19·376 2·4 0·23207 0·8 0·94 3052 55 3061 23 3066 13 100 A9 128 89 0·36 34568·41 0·60691 2·4 19·397 2·6 0·23180 0·9 0·94 3058 60 3062 25 3065 14 100 A10 104 70 0·21 13214·50 0·60098 2·2 19·169 2·3 0·23133 0·8 0·94 3034 53 3050 23 3061 13 99 A11 116 84 0·35 2943·58 0·60482 2·3 19·320 2·6 0·23167 1·1 0·90 3049 56 3058 25 3064 18 100 A12 216 193 1·52 1579·22 0·61810 1·5 19·752 2·6 0·23177 2·1 0·59 3102 37 3079 25 3064 33 101 A13 302 45 0·66 4501·20 0·11856 3·3 2·994 3·5 0·18316 1·2 0·94 722 22 1406 27 2682 19 27 A14 407 53 0·51 1163·51 0·1071 3·4 2·741 4·1 0·1855 2·2 0·84 656 21 1340 31 2703 37 24 A15 374 93 0·87 1010·09 0·20086 1·9 5·0723 2·1 0·18315 1·0 0·89 1180 20 1831 18 2682 16 44 A16 220 118 0·71 1280·60 0·4381 2·9 13·393 3·2 0·2217 1·3 0·91 2342 57 2708 31 2993 21 78 A17 93 76 0·71 798·30 0·6289 2·3 20·089 2·7 0·2317 1·5 0·84 3145 57 3096 27 3064 24 103 A19 526 351 4·20 66·11 0·0187 19·9 0·53 20·0 0·2053 1·7 1·00 119 24 432 73 2869 28 4 A20 499 313 3·03 85·50 0·4148 3·5 10·94 5·1 0·1913 3·7 0·69 2237 67 2518 49 2754 60 81 A21 542 287 0·77 97·69 0·3263 2·7 7·416 4·3 0·1648 3·3 0·64 1820 44 2163 39 2506 56 73 A22 198 96 0·22 598·54 0·3938 4·9 12·0728 5·1 0·2223 1·6 0·95 2140 89 2610 49 2998 26 71 A23 325 237 0·90 155·36 0·5400 2·5 14·9350 2·9 0·2006 1·4 0·88 2783 57 2811 28 2831 23 98 A24 571 105 0·88 539·58 0·1434 2·6 3·8585 2·8 0·1951 0·9 0·94 864 21 1605 23 2786 15 31 A25 166 98 0·29 7837·33 0·5462 2·7 16·0264 2·8 0·2128 1·0 0·94 2810 61 2878 27 2927 15 96 A26 375 70 1·83 548·62 0·1385 2·1 3·2441 3·4 0·1699 2·6 0·64 836 17 1468 27 2556 44 33 A27 55 41 0·55 3577·39 0·6136 4·0 19·8287 4·2 0·2344 1·1 0·96 3085 99 3083 41 3082 18 100 A28 143 87 0·40 1039·91 0·5287 2·0 16·8013 2·1 0·2305 0·8 0·93 2736 44 2924 21 3055 12 90 A29 82 42 0·65 1804·93 0·3911 3·1 12·49 3·4 0·2315 1·4 0·91 2128 57 2642 33 3063 22 69 Zircon standard GJ1-4 178 82 1·09 12827·13 0·37576 3·6 11·364 4·2 0·21933 2·1 0·87 2056 64 2553 40 2976 34 69 1Calculated relative to GJ-1 reference zircon. 2Corrected for background, within-run Pb/U fractionation and common Pb using Stacey & Kramers (1975) model Pb composition and subsequently normalized to GJ-1 values. 3Calculated using 207Pb/206Pb/(238U/206Pb × 1/137·88). 4Rho is the error correlation; that is, error(206Pb/238U)/error(207Pb/235U). 5Degree of concordance. Open in new tab Table 5: U–Pb isotope data of single zircon domains Sample . U1 . Pb1 . Th/U1 . 206Pb/204Pb . 206Pb/238U2 . ±2σ . 207Pb/235U3 . ±2σ . 207Pb/206Pb2 . ±2σ . rho4 . Age (Ma) . no. . (ppm) . (ppm) . . . . (%) . . (%) . . (%) . . 206Pb/238U . ±2σ (%) . 207Pb/235U . ±2σ (%) . 207Pb/206Pb . ±2σ (%) . Con.5 (%) . BH1-metagranite A1 500 120 0·87 895·52 0·18910 3·2 4·888 3·6 0·18747 1·6 0·89 1116 33 1800 30 2720 27 41 A2 677 77 0·63 663·17 0·07754 10·5 1·721 11·4 0·16095 4·5 0·92 481 49 1016 76 2466 76 20 A3 311 147 0·78 2492·52 0·38819 9·5 11·548 9·8 0·21577 2·5 0·97 2114 173 2569 96 2949 40 72 A4 1088 186 0·46 541·40 0·12430 8·9 2·915 9·6 0·17011 3·4 0·93 755 64 1386 75 2559 57 30 A5 269 186 0·65 867·89 0·51473 8·6 13·515 9·4 0·19043 3·8 0·92 2677 192 2716 93 2746 62 97 A6 312 107 0·57 1294·63 0·23743 10·4 6·166 10·8 0·18835 2·9 0·96 1373 130 2000 99 2728 47 50 A7 303 93 0·84 4960·32 0·27662 5·6 7·148 6·5 0·18742 3·3 0·86 1574 79 2130 59 2720 54 58 A8 408 94 0·83 5374·14 0·20430 5·3 4·790 5·9 0·17006 2·6 0·90 1198 58 1783 51 2558 44 47 A9 426 67 0·85 6272·33 0·13968 5·5 2·776 6·2 0·14415 2·7 0·90 843 44 1349 47 2278 47 37 A10 341 116 0·92 906·62 0·22827 7·2 5·842 7·9 0·18561 3·1 0·92 1325 87 1953 71 2704 51 49 A11 198 114 0·60 1132·41 0·38801 3·3 11·750 3·7 0·21964 1·8 0·88 2114 59 2585 36 2978 29 71 A13 202 115 0·53 6420·44 0·48639 5·9 13·734 6·3 0·20479 2·4 0·93 2555 125 2732 62 2865 39 89 A14 67 47 0·49 21055·69 0·59171 4·0 19·133 4·3 0·23451 1·6 0·93 2996 96 3049 42 3083 25 97 A15 64 45 0·92 5691·13 0·60549 4·5 19·117 4·9 0·22898 1·9 0·92 3052 111 3048 48 3045 31 100 A16 163 92 1·03 30326·75 0·48851 4·2 14·399 5·3 0·21378 3·2 0·80 2564 89 2776 51 2934 51 87 A17 1316 80 0·53 407·67 0·04230 13·3 0·631 13·5 0·10811 2·1 0·99 267 35 496 54 1768 39 15 A18 183 47 1·58 5614·70 0·19251 7·1 5·283 7·3 0·19904 1·9 0·97 1135 74 1866 65 2818 30 40 BH1-hornblende metagabbro A1 331 66 0·71 5274·18 0·16279 6·4 4·624 6·6 0·20600 1·6 0·97 972 58 1754 56 2874 25 34 A2 335 81 1·82 1301·30 0·17600 3·9 4·430 4·3 0·18255 1·8 0·91 1045 38 1718 37 2676 30 39 A3 510 61 1·41 823·85 0·07949 2·4 1·650 3·4 0·15056 2·4 0·71 493 11 990 21 2352 40 21 A4 163 81 0·68 30538·03 0·44797 2·0 12·878 2·4 0·20849 1·4 0·81 2386 40 2671 23 2894 23 82 A5 358 82 1·11 17357·28 0·19269 2·4 5·836 2·6 0·21965 1·0 0·92 1136 25 1952 23 2978 16 38 A6 99 70 0·49 10869·06 0·60732 1·8 19·290 2·0 0·23036 1·0 0·87 3059 43 3056 20 3055 16 100 A7 92 68 0·64 7895·48 0·60824 1·3 19·335 1·6 0·23055 0·9 0·82 3063 32 3059 16 3056 15 100 A8 106 74 0·34 20377·72 0·60554 2·3 19·376 2·4 0·23207 0·8 0·94 3052 55 3061 23 3066 13 100 A9 128 89 0·36 34568·41 0·60691 2·4 19·397 2·6 0·23180 0·9 0·94 3058 60 3062 25 3065 14 100 A10 104 70 0·21 13214·50 0·60098 2·2 19·169 2·3 0·23133 0·8 0·94 3034 53 3050 23 3061 13 99 A11 116 84 0·35 2943·58 0·60482 2·3 19·320 2·6 0·23167 1·1 0·90 3049 56 3058 25 3064 18 100 A12 216 193 1·52 1579·22 0·61810 1·5 19·752 2·6 0·23177 2·1 0·59 3102 37 3079 25 3064 33 101 A13 302 45 0·66 4501·20 0·11856 3·3 2·994 3·5 0·18316 1·2 0·94 722 22 1406 27 2682 19 27 A14 407 53 0·51 1163·51 0·1071 3·4 2·741 4·1 0·1855 2·2 0·84 656 21 1340 31 2703 37 24 A15 374 93 0·87 1010·09 0·20086 1·9 5·0723 2·1 0·18315 1·0 0·89 1180 20 1831 18 2682 16 44 A16 220 118 0·71 1280·60 0·4381 2·9 13·393 3·2 0·2217 1·3 0·91 2342 57 2708 31 2993 21 78 A17 93 76 0·71 798·30 0·6289 2·3 20·089 2·7 0·2317 1·5 0·84 3145 57 3096 27 3064 24 103 A19 526 351 4·20 66·11 0·0187 19·9 0·53 20·0 0·2053 1·7 1·00 119 24 432 73 2869 28 4 A20 499 313 3·03 85·50 0·4148 3·5 10·94 5·1 0·1913 3·7 0·69 2237 67 2518 49 2754 60 81 A21 542 287 0·77 97·69 0·3263 2·7 7·416 4·3 0·1648 3·3 0·64 1820 44 2163 39 2506 56 73 A22 198 96 0·22 598·54 0·3938 4·9 12·0728 5·1 0·2223 1·6 0·95 2140 89 2610 49 2998 26 71 A23 325 237 0·90 155·36 0·5400 2·5 14·9350 2·9 0·2006 1·4 0·88 2783 57 2811 28 2831 23 98 A24 571 105 0·88 539·58 0·1434 2·6 3·8585 2·8 0·1951 0·9 0·94 864 21 1605 23 2786 15 31 A25 166 98 0·29 7837·33 0·5462 2·7 16·0264 2·8 0·2128 1·0 0·94 2810 61 2878 27 2927 15 96 A26 375 70 1·83 548·62 0·1385 2·1 3·2441 3·4 0·1699 2·6 0·64 836 17 1468 27 2556 44 33 A27 55 41 0·55 3577·39 0·6136 4·0 19·8287 4·2 0·2344 1·1 0·96 3085 99 3083 41 3082 18 100 A28 143 87 0·40 1039·91 0·5287 2·0 16·8013 2·1 0·2305 0·8 0·93 2736 44 2924 21 3055 12 90 A29 82 42 0·65 1804·93 0·3911 3·1 12·49 3·4 0·2315 1·4 0·91 2128 57 2642 33 3063 22 69 Zircon standard GJ1-4 178 82 1·09 12827·13 0·37576 3·6 11·364 4·2 0·21933 2·1 0·87 2056 64 2553 40 2976 34 69 Sample . U1 . Pb1 . Th/U1 . 206Pb/204Pb . 206Pb/238U2 . ±2σ . 207Pb/235U3 . ±2σ . 207Pb/206Pb2 . ±2σ . rho4 . Age (Ma) . no. . (ppm) . (ppm) . . . . (%) . . (%) . . (%) . . 206Pb/238U . ±2σ (%) . 207Pb/235U . ±2σ (%) . 207Pb/206Pb . ±2σ (%) . Con.5 (%) . BH1-metagranite A1 500 120 0·87 895·52 0·18910 3·2 4·888 3·6 0·18747 1·6 0·89 1116 33 1800 30 2720 27 41 A2 677 77 0·63 663·17 0·07754 10·5 1·721 11·4 0·16095 4·5 0·92 481 49 1016 76 2466 76 20 A3 311 147 0·78 2492·52 0·38819 9·5 11·548 9·8 0·21577 2·5 0·97 2114 173 2569 96 2949 40 72 A4 1088 186 0·46 541·40 0·12430 8·9 2·915 9·6 0·17011 3·4 0·93 755 64 1386 75 2559 57 30 A5 269 186 0·65 867·89 0·51473 8·6 13·515 9·4 0·19043 3·8 0·92 2677 192 2716 93 2746 62 97 A6 312 107 0·57 1294·63 0·23743 10·4 6·166 10·8 0·18835 2·9 0·96 1373 130 2000 99 2728 47 50 A7 303 93 0·84 4960·32 0·27662 5·6 7·148 6·5 0·18742 3·3 0·86 1574 79 2130 59 2720 54 58 A8 408 94 0·83 5374·14 0·20430 5·3 4·790 5·9 0·17006 2·6 0·90 1198 58 1783 51 2558 44 47 A9 426 67 0·85 6272·33 0·13968 5·5 2·776 6·2 0·14415 2·7 0·90 843 44 1349 47 2278 47 37 A10 341 116 0·92 906·62 0·22827 7·2 5·842 7·9 0·18561 3·1 0·92 1325 87 1953 71 2704 51 49 A11 198 114 0·60 1132·41 0·38801 3·3 11·750 3·7 0·21964 1·8 0·88 2114 59 2585 36 2978 29 71 A13 202 115 0·53 6420·44 0·48639 5·9 13·734 6·3 0·20479 2·4 0·93 2555 125 2732 62 2865 39 89 A14 67 47 0·49 21055·69 0·59171 4·0 19·133 4·3 0·23451 1·6 0·93 2996 96 3049 42 3083 25 97 A15 64 45 0·92 5691·13 0·60549 4·5 19·117 4·9 0·22898 1·9 0·92 3052 111 3048 48 3045 31 100 A16 163 92 1·03 30326·75 0·48851 4·2 14·399 5·3 0·21378 3·2 0·80 2564 89 2776 51 2934 51 87 A17 1316 80 0·53 407·67 0·04230 13·3 0·631 13·5 0·10811 2·1 0·99 267 35 496 54 1768 39 15 A18 183 47 1·58 5614·70 0·19251 7·1 5·283 7·3 0·19904 1·9 0·97 1135 74 1866 65 2818 30 40 BH1-hornblende metagabbro A1 331 66 0·71 5274·18 0·16279 6·4 4·624 6·6 0·20600 1·6 0·97 972 58 1754 56 2874 25 34 A2 335 81 1·82 1301·30 0·17600 3·9 4·430 4·3 0·18255 1·8 0·91 1045 38 1718 37 2676 30 39 A3 510 61 1·41 823·85 0·07949 2·4 1·650 3·4 0·15056 2·4 0·71 493 11 990 21 2352 40 21 A4 163 81 0·68 30538·03 0·44797 2·0 12·878 2·4 0·20849 1·4 0·81 2386 40 2671 23 2894 23 82 A5 358 82 1·11 17357·28 0·19269 2·4 5·836 2·6 0·21965 1·0 0·92 1136 25 1952 23 2978 16 38 A6 99 70 0·49 10869·06 0·60732 1·8 19·290 2·0 0·23036 1·0 0·87 3059 43 3056 20 3055 16 100 A7 92 68 0·64 7895·48 0·60824 1·3 19·335 1·6 0·23055 0·9 0·82 3063 32 3059 16 3056 15 100 A8 106 74 0·34 20377·72 0·60554 2·3 19·376 2·4 0·23207 0·8 0·94 3052 55 3061 23 3066 13 100 A9 128 89 0·36 34568·41 0·60691 2·4 19·397 2·6 0·23180 0·9 0·94 3058 60 3062 25 3065 14 100 A10 104 70 0·21 13214·50 0·60098 2·2 19·169 2·3 0·23133 0·8 0·94 3034 53 3050 23 3061 13 99 A11 116 84 0·35 2943·58 0·60482 2·3 19·320 2·6 0·23167 1·1 0·90 3049 56 3058 25 3064 18 100 A12 216 193 1·52 1579·22 0·61810 1·5 19·752 2·6 0·23177 2·1 0·59 3102 37 3079 25 3064 33 101 A13 302 45 0·66 4501·20 0·11856 3·3 2·994 3·5 0·18316 1·2 0·94 722 22 1406 27 2682 19 27 A14 407 53 0·51 1163·51 0·1071 3·4 2·741 4·1 0·1855 2·2 0·84 656 21 1340 31 2703 37 24 A15 374 93 0·87 1010·09 0·20086 1·9 5·0723 2·1 0·18315 1·0 0·89 1180 20 1831 18 2682 16 44 A16 220 118 0·71 1280·60 0·4381 2·9 13·393 3·2 0·2217 1·3 0·91 2342 57 2708 31 2993 21 78 A17 93 76 0·71 798·30 0·6289 2·3 20·089 2·7 0·2317 1·5 0·84 3145 57 3096 27 3064 24 103 A19 526 351 4·20 66·11 0·0187 19·9 0·53 20·0 0·2053 1·7 1·00 119 24 432 73 2869 28 4 A20 499 313 3·03 85·50 0·4148 3·5 10·94 5·1 0·1913 3·7 0·69 2237 67 2518 49 2754 60 81 A21 542 287 0·77 97·69 0·3263 2·7 7·416 4·3 0·1648 3·3 0·64 1820 44 2163 39 2506 56 73 A22 198 96 0·22 598·54 0·3938 4·9 12·0728 5·1 0·2223 1·6 0·95 2140 89 2610 49 2998 26 71 A23 325 237 0·90 155·36 0·5400 2·5 14·9350 2·9 0·2006 1·4 0·88 2783 57 2811 28 2831 23 98 A24 571 105 0·88 539·58 0·1434 2·6 3·8585 2·8 0·1951 0·9 0·94 864 21 1605 23 2786 15 31 A25 166 98 0·29 7837·33 0·5462 2·7 16·0264 2·8 0·2128 1·0 0·94 2810 61 2878 27 2927 15 96 A26 375 70 1·83 548·62 0·1385 2·1 3·2441 3·4 0·1699 2·6 0·64 836 17 1468 27 2556 44 33 A27 55 41 0·55 3577·39 0·6136 4·0 19·8287 4·2 0·2344 1·1 0·96 3085 99 3083 41 3082 18 100 A28 143 87 0·40 1039·91 0·5287 2·0 16·8013 2·1 0·2305 0·8 0·93 2736 44 2924 21 3055 12 90 A29 82 42 0·65 1804·93 0·3911 3·1 12·49 3·4 0·2315 1·4 0·91 2128 57 2642 33 3063 22 69 Zircon standard GJ1-4 178 82 1·09 12827·13 0·37576 3·6 11·364 4·2 0·21933 2·1 0·87 2056 64 2553 40 2976 34 69 1Calculated relative to GJ-1 reference zircon. 2Corrected for background, within-run Pb/U fractionation and common Pb using Stacey & Kramers (1975) model Pb composition and subsequently normalized to GJ-1 values. 3Calculated using 207Pb/206Pb/(238U/206Pb × 1/137·88). 4Rho is the error correlation; that is, error(206Pb/238U)/error(207Pb/235U). 5Degree of concordance. Open in new tab Subsequently, the same zircon domains were analysed for their Lu, Hf, and Yb isotopic composition (Table 6) using the same laser system, with a 40 μm spot size, and a Thermo-Finnigan Neptune multicollector (MC)-ICPMS system. The procedures for correction of isobaric interferences between Lu and Yb, instrumental mass fractionation, and comparison with standards have been detailed by Gerdes & Zeh (2006, 2008). Multiple analyses by LA-MC-ICPMS of the GJ1 zircon standard during the period of this study yielded 176Hf/177Hf of 0·281998 ± 0·000015 (n = 10). All uncertainties are reported at the 2σ level. Table 6: Lu–Hf isotope data for single zircon domains Sample . 176Yb/177Hf1 . ±2σ . 176Lu/177Hf1 . ±2σ . 178Hf/177Hf . 180Hf/177Hf . SigHf2 . 176Hf/177Hf . ±2σ3 . 176Hf/177Hf(t)4 . ϵHf(t)5 . ±2σ . TDM5 . Age5 . ±2σ . no. . . . . . . . (V) . . . . 3062 Ma . . (Ga) . (Ma) . . BH1-granite A1 0·1010 28 0·00241 4 1·46740 1·88685 7 0·281009 23 0·280883 1·7 0·8 3·31 2720 27 A2 0·0565 40 0·00137 7 1·46731 1·88673 8 0·280916 22 0·280851 0·6 0·8 3·37 2466 76 A3 0·0947 29 0·00223 7 1·46709 1·88610 8 0·280951 24 0·280825 0·1 0·9 3·40 2949 40 A4 0·1068 73 0·00257 15 1·46734 1·88681 7 0·280986 27 0·280861 0·6 0·9 3·37 2559 57 A5 0·1037 31 0·00247 6 1·46719 1·88654 7 0·280951 24 0·280821 −0·5 0·9 3·43 2746 62 A6 0·1078 36 0·00260 11 1·46723 1·88632 7 0·280958 25 0·280822 −0·5 0·9 3·43 2728 47 A7 0·0638 52 0·00160 12 1·46715 1·88631 9 0·280919 26 0·280836 0·2 0·9 3·39 2720 54 A8 0·0846 25 0·00208 8 1·46732 1·88638 9 0·280934 28 0·280833 −0·2 1·0 3·41 2558 44 A9 0·1225 40 0·00302 12 1·46704 1·88593 9 0·280974 26 0·280842 −0·8 0·9 3·45 2278 47 A10 0·09269 74 0·00230 19 1·46724 1·88647 8 0·280969 27 0·280837 0·5 1·0 3·37 2704 51 A11 0·0786 59 0·00200 15 1·46709 1·88607 8 0·280981 23 0·280866 1·6 0·8 3·31 2978 29 A13 0·0835 34 0·00209 9 1·46721 1·88631 9 0·280891 20 0·280776 −1·8 0·7 3·50 2865 39 A14* 0·0409 18 0·00108 4 1·46727 1·88693 7 0·280878 23 0·280814 −0·1 0·8 3·41 3083 25 A15* 0·0337 8 0·00090 2 1·46725 1·88671 7 0·280886 25 0·280833 0·5 0·9 3·37 3045 31 A18 0·0893 42 0·00215 11 1·46735 1·88680 8 0·280959 25 0·280843 0·5 0·9 3·37 2818 30 BH1-gabbro A1 0·0364 69 0·00114 21 1·46745 1·88674 8 0·280839 21 0·280777 −1·7 0·8 3·49 2874 25 A2 0·0615 49 0·00187 12 1·46711 1·88655 7 0·280865 23 0·280769 −2·3 0·8 3·53 2676 30 A3 0·0255 75 0·00072 18 1·46690 1·88588 8 0·280827 21 0·280795 −1·2 0·7 3·47 2352 40 A4 0·0159 12 0·00059 5 1·46720 1·88657 13 0·280810 19 0·280777 −1·6 0·7 3·49 2894 23 A5 0·0183 4 0·00066 1 1·46720 1·88649 13 0·280823 15 0·280785 −1·2 0·5 3·47 2978 16 A6* 0·0132 10 0·00042 3 1·46721 1·88628 8 0·280833 24 0·280809 −0·4 0·9 3·42 3055 16 A7* 0·0156 12 0·00049 2 1·46717 1·88625 8 0·280803 18 0·280775 −1·6 0·6 3·49 3056 15 A8* 0·0280 37 0·00090 12 1·46720 1·88669 7 0·280859 26 0·280806 −0·5 0·9 3·43 3066 13 A9* 0·0292 35 0·00094 12 1·46711 1·88631 8 0·280846 23 0·280791 −1·0 0·8 3·46 3065 14 A10* 0·0121 15 0·00040 5 1·46713 1·88650 8 0·280815 18 0·280791 −1·0 0·7 3·45 3061 13 A11* 0·0317 107 0·00091 31 1·46717 1·88649 10 0·280855 23 0·280801 −0·6 0·8 3·43 3064 18 A12* 0·0423 111 0·00129 34 1·46733 1·88661 7 0·280868 23 0·280792 −0·9 0·8 3·45 3064 33 A13 0·0415 54 0·00126 14 1·46732 1·88662 8 0·280855 20 0·280790 −1·3 0·7 3·48 2682 19 A14 0·0108 13 0·00035 3 1·46710 1·88654 9 0·280807 25 0·280789 −1·2 0·9 3·46 2703 37 A16 0·0296 23 0·00088 7 1·46704 1·88603 6 0·280826 23 0·280775 −1·6 0·8 3·49 2993 21 A17* 0·0311 108 0·00097 35 1·46738 1·88678 8 0·280826 22 0·280769 −1·8 0·8 3·50 3064 24 A19 0·1386 30 0·00389 6 1·46727 1·88667 8 0·281003 24 0·280789 −1·6 0·8 3·49 2869 28 A21 0·0659 24 0·00200 4 1·46739 1·88695 6 0·280899 31 0·280803 −1·3 1·1 3·48 2506 56 A22 0·0049 3 0·00015 1 1·46723 1·88660 12 0·280786 16 0·280777 −1·5 0·6 3·48 2998 26 A23 0·0655 47 0·00181 14 1·46710 1·88640 7 0·280910 25 0·280812 −0·5 0·9 3·43 2831 23 A24 0·0662 16 0·00181 6 1·46721 1·88664 6 0·280880 21 0·280784 −1·6 0·7 3·49 2786 15 A25 0·0106 4 0·00035 2 1·46714 1·88657 8 0·280812 15 0·280793 −1·0 0·5 3·45 2927 15 A26 0·0762 29 0·00227 5 1·46719 1·88676 7 0·280921 20 0·280810 −1·1 0·7 3·46 2556 44 A27* 0·0216 9 0·00067 4 1·46717 1·88657 7 0·280834 23 0·280794 −0·9 0·8 3·45 3082 18 A28* 0·0371 56 0·00115 19 1·46715 1·88680 7 0·280847 22 0·280780 −1·4 0·8 3·48 3055 12 A29* 0·0926 83 0·00277 24 1·46720 1·88638 7 0·280966 24 0·280803 −0·6 0·9 3·43 3063 22 JMC (50 ppb) (n = 7) 1·46718 1·88643 13 0·282162 14 GJ-1 0·0094 13 0·00031 6 1·46722 1·88656 9 0·282004 18 0·282000 −13·9 0·6 609 6 (n = 15) Sample . 176Yb/177Hf1 . ±2σ . 176Lu/177Hf1 . ±2σ . 178Hf/177Hf . 180Hf/177Hf . SigHf2 . 176Hf/177Hf . ±2σ3 . 176Hf/177Hf(t)4 . ϵHf(t)5 . ±2σ . TDM5 . Age5 . ±2σ . no. . . . . . . . (V) . . . . 3062 Ma . . (Ga) . (Ma) . . BH1-granite A1 0·1010 28 0·00241 4 1·46740 1·88685 7 0·281009 23 0·280883 1·7 0·8 3·31 2720 27 A2 0·0565 40 0·00137 7 1·46731 1·88673 8 0·280916 22 0·280851 0·6 0·8 3·37 2466 76 A3 0·0947 29 0·00223 7 1·46709 1·88610 8 0·280951 24 0·280825 0·1 0·9 3·40 2949 40 A4 0·1068 73 0·00257 15 1·46734 1·88681 7 0·280986 27 0·280861 0·6 0·9 3·37 2559 57 A5 0·1037 31 0·00247 6 1·46719 1·88654 7 0·280951 24 0·280821 −0·5 0·9 3·43 2746 62 A6 0·1078 36 0·00260 11 1·46723 1·88632 7 0·280958 25 0·280822 −0·5 0·9 3·43 2728 47 A7 0·0638 52 0·00160 12 1·46715 1·88631 9 0·280919 26 0·280836 0·2 0·9 3·39 2720 54 A8 0·0846 25 0·00208 8 1·46732 1·88638 9 0·280934 28 0·280833 −0·2 1·0 3·41 2558 44 A9 0·1225 40 0·00302 12 1·46704 1·88593 9 0·280974 26 0·280842 −0·8 0·9 3·45 2278 47 A10 0·09269 74 0·00230 19 1·46724 1·88647 8 0·280969 27 0·280837 0·5 1·0 3·37 2704 51 A11 0·0786 59 0·00200 15 1·46709 1·88607 8 0·280981 23 0·280866 1·6 0·8 3·31 2978 29 A13 0·0835 34 0·00209 9 1·46721 1·88631 9 0·280891 20 0·280776 −1·8 0·7 3·50 2865 39 A14* 0·0409 18 0·00108 4 1·46727 1·88693 7 0·280878 23 0·280814 −0·1 0·8 3·41 3083 25 A15* 0·0337 8 0·00090 2 1·46725 1·88671 7 0·280886 25 0·280833 0·5 0·9 3·37 3045 31 A18 0·0893 42 0·00215 11 1·46735 1·88680 8 0·280959 25 0·280843 0·5 0·9 3·37 2818 30 BH1-gabbro A1 0·0364 69 0·00114 21 1·46745 1·88674 8 0·280839 21 0·280777 −1·7 0·8 3·49 2874 25 A2 0·0615 49 0·00187 12 1·46711 1·88655 7 0·280865 23 0·280769 −2·3 0·8 3·53 2676 30 A3 0·0255 75 0·00072 18 1·46690 1·88588 8 0·280827 21 0·280795 −1·2 0·7 3·47 2352 40 A4 0·0159 12 0·00059 5 1·46720 1·88657 13 0·280810 19 0·280777 −1·6 0·7 3·49 2894 23 A5 0·0183 4 0·00066 1 1·46720 1·88649 13 0·280823 15 0·280785 −1·2 0·5 3·47 2978 16 A6* 0·0132 10 0·00042 3 1·46721 1·88628 8 0·280833 24 0·280809 −0·4 0·9 3·42 3055 16 A7* 0·0156 12 0·00049 2 1·46717 1·88625 8 0·280803 18 0·280775 −1·6 0·6 3·49 3056 15 A8* 0·0280 37 0·00090 12 1·46720 1·88669 7 0·280859 26 0·280806 −0·5 0·9 3·43 3066 13 A9* 0·0292 35 0·00094 12 1·46711 1·88631 8 0·280846 23 0·280791 −1·0 0·8 3·46 3065 14 A10* 0·0121 15 0·00040 5 1·46713 1·88650 8 0·280815 18 0·280791 −1·0 0·7 3·45 3061 13 A11* 0·0317 107 0·00091 31 1·46717 1·88649 10 0·280855 23 0·280801 −0·6 0·8 3·43 3064 18 A12* 0·0423 111 0·00129 34 1·46733 1·88661 7 0·280868 23 0·280792 −0·9 0·8 3·45 3064 33 A13 0·0415 54 0·00126 14 1·46732 1·88662 8 0·280855 20 0·280790 −1·3 0·7 3·48 2682 19 A14 0·0108 13 0·00035 3 1·46710 1·88654 9 0·280807 25 0·280789 −1·2 0·9 3·46 2703 37 A16 0·0296 23 0·00088 7 1·46704 1·88603 6 0·280826 23 0·280775 −1·6 0·8 3·49 2993 21 A17* 0·0311 108 0·00097 35 1·46738 1·88678 8 0·280826 22 0·280769 −1·8 0·8 3·50 3064 24 A19 0·1386 30 0·00389 6 1·46727 1·88667 8 0·281003 24 0·280789 −1·6 0·8 3·49 2869 28 A21 0·0659 24 0·00200 4 1·46739 1·88695 6 0·280899 31 0·280803 −1·3 1·1 3·48 2506 56 A22 0·0049 3 0·00015 1 1·46723 1·88660 12 0·280786 16 0·280777 −1·5 0·6 3·48 2998 26 A23 0·0655 47 0·00181 14 1·46710 1·88640 7 0·280910 25 0·280812 −0·5 0·9 3·43 2831 23 A24 0·0662 16 0·00181 6 1·46721 1·88664 6 0·280880 21 0·280784 −1·6 0·7 3·49 2786 15 A25 0·0106 4 0·00035 2 1·46714 1·88657 8 0·280812 15 0·280793 −1·0 0·5 3·45 2927 15 A26 0·0762 29 0·00227 5 1·46719 1·88676 7 0·280921 20 0·280810 −1·1 0·7 3·46 2556 44 A27* 0·0216 9 0·00067 4 1·46717 1·88657 7 0·280834 23 0·280794 −0·9 0·8 3·45 3082 18 A28* 0·0371 56 0·00115 19 1·46715 1·88680 7 0·280847 22 0·280780 −1·4 0·8 3·48 3055 12 A29* 0·0926 83 0·00277 24 1·46720 1·88638 7 0·280966 24 0·280803 −0·6 0·9 3·43 3063 22 JMC (50 ppb) (n = 7) 1·46718 1·88643 13 0·282162 14 GJ-1 0·0094 13 0·00031 6 1·46722 1·88656 9 0·282004 18 0·282000 −13·9 0·6 609 6 (n = 15) 1176Yb/177Hf = (176Yb/173Yb)true × 173Yb/177Hf)meas × [M173(Yb)/M177(Hf))]β(Hf). 176Lu/177Hf calculated in a similar way by using the 175Lu/177Hf. Quoted uncertainties (absolute) relate to the last quoted figure; Effect of inter-element fractionation on Lu/Hf is estimated to be about 6% or less based on analyses of the GJ-1 and Plesovice zircons. 2Mean Hf signal in volts. 3Uncertainties are quadratic additions of the within-run precision and the daily reproducibility of the 50 ppb JMC475 solution. Uncertainties for the JMC475 and GJ-1 are 2SD (2 standard deviations). 4Initial 176Hf/177Hf are calculated using the apparent Pb–Pb age determined by LA-ICPMS dating (see last two rows). 5All ϵHf and TDM are calculated for the emplacement age of 3062 Ma, TDM is the two-stage model age calculated by using the measured 176Hf/177Hf of each spot (first stage = emplacement age), a value of 0·0113 for the average continental crust (second stage), and a depleted mantle 176Lu/177Hf and 176Lu/177Hf of 0·0384 and 0·28325, respectively. *Most concordant analyses. Open in new tab Table 6: Lu–Hf isotope data for single zircon domains Sample . 176Yb/177Hf1 . ±2σ . 176Lu/177Hf1 . ±2σ . 178Hf/177Hf . 180Hf/177Hf . SigHf2 . 176Hf/177Hf . ±2σ3 . 176Hf/177Hf(t)4 . ϵHf(t)5 . ±2σ . TDM5 . Age5 . ±2σ . no. . . . . . . . (V) . . . . 3062 Ma . . (Ga) . (Ma) . . BH1-granite A1 0·1010 28 0·00241 4 1·46740 1·88685 7 0·281009 23 0·280883 1·7 0·8 3·31 2720 27 A2 0·0565 40 0·00137 7 1·46731 1·88673 8 0·280916 22 0·280851 0·6 0·8 3·37 2466 76 A3 0·0947 29 0·00223 7 1·46709 1·88610 8 0·280951 24 0·280825 0·1 0·9 3·40 2949 40 A4 0·1068 73 0·00257 15 1·46734 1·88681 7 0·280986 27 0·280861 0·6 0·9 3·37 2559 57 A5 0·1037 31 0·00247 6 1·46719 1·88654 7 0·280951 24 0·280821 −0·5 0·9 3·43 2746 62 A6 0·1078 36 0·00260 11 1·46723 1·88632 7 0·280958 25 0·280822 −0·5 0·9 3·43 2728 47 A7 0·0638 52 0·00160 12 1·46715 1·88631 9 0·280919 26 0·280836 0·2 0·9 3·39 2720 54 A8 0·0846 25 0·00208 8 1·46732 1·88638 9 0·280934 28 0·280833 −0·2 1·0 3·41 2558 44 A9 0·1225 40 0·00302 12 1·46704 1·88593 9 0·280974 26 0·280842 −0·8 0·9 3·45 2278 47 A10 0·09269 74 0·00230 19 1·46724 1·88647 8 0·280969 27 0·280837 0·5 1·0 3·37 2704 51 A11 0·0786 59 0·00200 15 1·46709 1·88607 8 0·280981 23 0·280866 1·6 0·8 3·31 2978 29 A13 0·0835 34 0·00209 9 1·46721 1·88631 9 0·280891 20 0·280776 −1·8 0·7 3·50 2865 39 A14* 0·0409 18 0·00108 4 1·46727 1·88693 7 0·280878 23 0·280814 −0·1 0·8 3·41 3083 25 A15* 0·0337 8 0·00090 2 1·46725 1·88671 7 0·280886 25 0·280833 0·5 0·9 3·37 3045 31 A18 0·0893 42 0·00215 11 1·46735 1·88680 8 0·280959 25 0·280843 0·5 0·9 3·37 2818 30 BH1-gabbro A1 0·0364 69 0·00114 21 1·46745 1·88674 8 0·280839 21 0·280777 −1·7 0·8 3·49 2874 25 A2 0·0615 49 0·00187 12 1·46711 1·88655 7 0·280865 23 0·280769 −2·3 0·8 3·53 2676 30 A3 0·0255 75 0·00072 18 1·46690 1·88588 8 0·280827 21 0·280795 −1·2 0·7 3·47 2352 40 A4 0·0159 12 0·00059 5 1·46720 1·88657 13 0·280810 19 0·280777 −1·6 0·7 3·49 2894 23 A5 0·0183 4 0·00066 1 1·46720 1·88649 13 0·280823 15 0·280785 −1·2 0·5 3·47 2978 16 A6* 0·0132 10 0·00042 3 1·46721 1·88628 8 0·280833 24 0·280809 −0·4 0·9 3·42 3055 16 A7* 0·0156 12 0·00049 2 1·46717 1·88625 8 0·280803 18 0·280775 −1·6 0·6 3·49 3056 15 A8* 0·0280 37 0·00090 12 1·46720 1·88669 7 0·280859 26 0·280806 −0·5 0·9 3·43 3066 13 A9* 0·0292 35 0·00094 12 1·46711 1·88631 8 0·280846 23 0·280791 −1·0 0·8 3·46 3065 14 A10* 0·0121 15 0·00040 5 1·46713 1·88650 8 0·280815 18 0·280791 −1·0 0·7 3·45 3061 13 A11* 0·0317 107 0·00091 31 1·46717 1·88649 10 0·280855 23 0·280801 −0·6 0·8 3·43 3064 18 A12* 0·0423 111 0·00129 34 1·46733 1·88661 7 0·280868 23 0·280792 −0·9 0·8 3·45 3064 33 A13 0·0415 54 0·00126 14 1·46732 1·88662 8 0·280855 20 0·280790 −1·3 0·7 3·48 2682 19 A14 0·0108 13 0·00035 3 1·46710 1·88654 9 0·280807 25 0·280789 −1·2 0·9 3·46 2703 37 A16 0·0296 23 0·00088 7 1·46704 1·88603 6 0·280826 23 0·280775 −1·6 0·8 3·49 2993 21 A17* 0·0311 108 0·00097 35 1·46738 1·88678 8 0·280826 22 0·280769 −1·8 0·8 3·50 3064 24 A19 0·1386 30 0·00389 6 1·46727 1·88667 8 0·281003 24 0·280789 −1·6 0·8 3·49 2869 28 A21 0·0659 24 0·00200 4 1·46739 1·88695 6 0·280899 31 0·280803 −1·3 1·1 3·48 2506 56 A22 0·0049 3 0·00015 1 1·46723 1·88660 12 0·280786 16 0·280777 −1·5 0·6 3·48 2998 26 A23 0·0655 47 0·00181 14 1·46710 1·88640 7 0·280910 25 0·280812 −0·5 0·9 3·43 2831 23 A24 0·0662 16 0·00181 6 1·46721 1·88664 6 0·280880 21 0·280784 −1·6 0·7 3·49 2786 15 A25 0·0106 4 0·00035 2 1·46714 1·88657 8 0·280812 15 0·280793 −1·0 0·5 3·45 2927 15 A26 0·0762 29 0·00227 5 1·46719 1·88676 7 0·280921 20 0·280810 −1·1 0·7 3·46 2556 44 A27* 0·0216 9 0·00067 4 1·46717 1·88657 7 0·280834 23 0·280794 −0·9 0·8 3·45 3082 18 A28* 0·0371 56 0·00115 19 1·46715 1·88680 7 0·280847 22 0·280780 −1·4 0·8 3·48 3055 12 A29* 0·0926 83 0·00277 24 1·46720 1·88638 7 0·280966 24 0·280803 −0·6 0·9 3·43 3063 22 JMC (50 ppb) (n = 7) 1·46718 1·88643 13 0·282162 14 GJ-1 0·0094 13 0·00031 6 1·46722 1·88656 9 0·282004 18 0·282000 −13·9 0·6 609 6 (n = 15) Sample . 176Yb/177Hf1 . ±2σ . 176Lu/177Hf1 . ±2σ . 178Hf/177Hf . 180Hf/177Hf . SigHf2 . 176Hf/177Hf . ±2σ3 . 176Hf/177Hf(t)4 . ϵHf(t)5 . ±2σ . TDM5 . Age5 . ±2σ . no. . . . . . . . (V) . . . . 3062 Ma . . (Ga) . (Ma) . . BH1-granite A1 0·1010 28 0·00241 4 1·46740 1·88685 7 0·281009 23 0·280883 1·7 0·8 3·31 2720 27 A2 0·0565 40 0·00137 7 1·46731 1·88673 8 0·280916 22 0·280851 0·6 0·8 3·37 2466 76 A3 0·0947 29 0·00223 7 1·46709 1·88610 8 0·280951 24 0·280825 0·1 0·9 3·40 2949 40 A4 0·1068 73 0·00257 15 1·46734 1·88681 7 0·280986 27 0·280861 0·6 0·9 3·37 2559 57 A5 0·1037 31 0·00247 6 1·46719 1·88654 7 0·280951 24 0·280821 −0·5 0·9 3·43 2746 62 A6 0·1078 36 0·00260 11 1·46723 1·88632 7 0·280958 25 0·280822 −0·5 0·9 3·43 2728 47 A7 0·0638 52 0·00160 12 1·46715 1·88631 9 0·280919 26 0·280836 0·2 0·9 3·39 2720 54 A8 0·0846 25 0·00208 8 1·46732 1·88638 9 0·280934 28 0·280833 −0·2 1·0 3·41 2558 44 A9 0·1225 40 0·00302 12 1·46704 1·88593 9 0·280974 26 0·280842 −0·8 0·9 3·45 2278 47 A10 0·09269 74 0·00230 19 1·46724 1·88647 8 0·280969 27 0·280837 0·5 1·0 3·37 2704 51 A11 0·0786 59 0·00200 15 1·46709 1·88607 8 0·280981 23 0·280866 1·6 0·8 3·31 2978 29 A13 0·0835 34 0·00209 9 1·46721 1·88631 9 0·280891 20 0·280776 −1·8 0·7 3·50 2865 39 A14* 0·0409 18 0·00108 4 1·46727 1·88693 7 0·280878 23 0·280814 −0·1 0·8 3·41 3083 25 A15* 0·0337 8 0·00090 2 1·46725 1·88671 7 0·280886 25 0·280833 0·5 0·9 3·37 3045 31 A18 0·0893 42 0·00215 11 1·46735 1·88680 8 0·280959 25 0·280843 0·5 0·9 3·37 2818 30 BH1-gabbro A1 0·0364 69 0·00114 21 1·46745 1·88674 8 0·280839 21 0·280777 −1·7 0·8 3·49 2874 25 A2 0·0615 49 0·00187 12 1·46711 1·88655 7 0·280865 23 0·280769 −2·3 0·8 3·53 2676 30 A3 0·0255 75 0·00072 18 1·46690 1·88588 8 0·280827 21 0·280795 −1·2 0·7 3·47 2352 40 A4 0·0159 12 0·00059 5 1·46720 1·88657 13 0·280810 19 0·280777 −1·6 0·7 3·49 2894 23 A5 0·0183 4 0·00066 1 1·46720 1·88649 13 0·280823 15 0·280785 −1·2 0·5 3·47 2978 16 A6* 0·0132 10 0·00042 3 1·46721 1·88628 8 0·280833 24 0·280809 −0·4 0·9 3·42 3055 16 A7* 0·0156 12 0·00049 2 1·46717 1·88625 8 0·280803 18 0·280775 −1·6 0·6 3·49 3056 15 A8* 0·0280 37 0·00090 12 1·46720 1·88669 7 0·280859 26 0·280806 −0·5 0·9 3·43 3066 13 A9* 0·0292 35 0·00094 12 1·46711 1·88631 8 0·280846 23 0·280791 −1·0 0·8 3·46 3065 14 A10* 0·0121 15 0·00040 5 1·46713 1·88650 8 0·280815 18 0·280791 −1·0 0·7 3·45 3061 13 A11* 0·0317 107 0·00091 31 1·46717 1·88649 10 0·280855 23 0·280801 −0·6 0·8 3·43 3064 18 A12* 0·0423 111 0·00129 34 1·46733 1·88661 7 0·280868 23 0·280792 −0·9 0·8 3·45 3064 33 A13 0·0415 54 0·00126 14 1·46732 1·88662 8 0·280855 20 0·280790 −1·3 0·7 3·48 2682 19 A14 0·0108 13 0·00035 3 1·46710 1·88654 9 0·280807 25 0·280789 −1·2 0·9 3·46 2703 37 A16 0·0296 23 0·00088 7 1·46704 1·88603 6 0·280826 23 0·280775 −1·6 0·8 3·49 2993 21 A17* 0·0311 108 0·00097 35 1·46738 1·88678 8 0·280826 22 0·280769 −1·8 0·8 3·50 3064 24 A19 0·1386 30 0·00389 6 1·46727 1·88667 8 0·281003 24 0·280789 −1·6 0·8 3·49 2869 28 A21 0·0659 24 0·00200 4 1·46739 1·88695 6 0·280899 31 0·280803 −1·3 1·1 3·48 2506 56 A22 0·0049 3 0·00015 1 1·46723 1·88660 12 0·280786 16 0·280777 −1·5 0·6 3·48 2998 26 A23 0·0655 47 0·00181 14 1·46710 1·88640 7 0·280910 25 0·280812 −0·5 0·9 3·43 2831 23 A24 0·0662 16 0·00181 6 1·46721 1·88664 6 0·280880 21 0·280784 −1·6 0·7 3·49 2786 15 A25 0·0106 4 0·00035 2 1·46714 1·88657 8 0·280812 15 0·280793 −1·0 0·5 3·45 2927 15 A26 0·0762 29 0·00227 5 1·46719 1·88676 7 0·280921 20 0·280810 −1·1 0·7 3·46 2556 44 A27* 0·0216 9 0·00067 4 1·46717 1·88657 7 0·280834 23 0·280794 −0·9 0·8 3·45 3082 18 A28* 0·0371 56 0·00115 19 1·46715 1·88680 7 0·280847 22 0·280780 −1·4 0·8 3·48 3055 12 A29* 0·0926 83 0·00277 24 1·46720 1·88638 7 0·280966 24 0·280803 −0·6 0·9 3·43 3063 22 JMC (50 ppb) (n = 7) 1·46718 1·88643 13 0·282162 14 GJ-1 0·0094 13 0·00031 6 1·46722 1·88656 9 0·282004 18 0·282000 −13·9 0·6 609 6 (n = 15) 1176Yb/177Hf = (176Yb/173Yb)true × 173Yb/177Hf)meas × [M173(Yb)/M177(Hf))]β(Hf). 176Lu/177Hf calculated in a similar way by using the 175Lu/177Hf. Quoted uncertainties (absolute) relate to the last quoted figure; Effect of inter-element fractionation on Lu/Hf is estimated to be about 6% or less based on analyses of the GJ-1 and Plesovice zircons. 2Mean Hf signal in volts. 3Uncertainties are quadratic additions of the within-run precision and the daily reproducibility of the 50 ppb JMC475 solution. Uncertainties for the JMC475 and GJ-1 are 2SD (2 standard deviations). 4Initial 176Hf/177Hf are calculated using the apparent Pb–Pb age determined by LA-ICPMS dating (see last two rows). 5All ϵHf and TDM are calculated for the emplacement age of 3062 Ma, TDM is the two-stage model age calculated by using the measured 176Hf/177Hf of each spot (first stage = emplacement age), a value of 0·0113 for the average continental crust (second stage), and a depleted mantle 176Lu/177Hf and 176Lu/177Hf of 0·0384 and 0·28325, respectively. *Most concordant analyses. Open in new tab The zircon grains in the granite are typically 0·1–0·2 mm in length and a number of them display complex internal structures. Some grains show a relic oscillatory zonation, overprinted by several patchy domains that point to recrystallization of the original magmatic zoning (Fig. 11a, insert). In some zircon grains, however, only compositional banding can be distinguished by bright and dull CL domains. Fig. 11. Open in new tabDownload slide Concordia diagrams for zircon grains from the granite (a) and the hornblende metagabbro (b). Data point error ellipses are 2σ. Also shown are cathodoluminescence images of typical zircon grains from each lithotype; scale bars represent 0·1 mm. Lines I represent discordia lines from the concordant crystallization age forced through the origin, lines II reflect a later thermal overprint (for further details see text). Many of the analysed zircon domains are rich in U, with contents up to 1316 ppm. These domains yielded the lowest 206Pb/204Pb ratios and the most discordant results. In contrast, those domains with the lowest U contents yielded perfectly concordant age data (Table 5). Two concordant domains (97 and 100% concordance) gave 207Pb–206Pb ages of 3083 and 3045 Ma, respectively. A third concordant analysis corresponds to a significantly younger age of 2746 ± 62 Ma. The rest of the analyses are variably discordant. In spite of this wide scatter of U–Pb age data, almost all (except for two) analysed domains yielded within error identical 176Hf/177Hf ratios (0·28084 ± 0·00003) calculated for the apparent 207Pb–206Pb age (Fig. 12a, Table 6). This indicates that all these zircon domains crystallized from the same magmatic source but underwent variable Pb loss as a result of later alteration (Gerdes & Zeh, 2008; Zeh et al., 2009). Two analyses deviate from the mean 176Hf/177Hf ratio. One of them has a lower 176Hf/177Hf and could reflect inheritance, whereas the other has a higher ratio, which might point to later incorporation of radiogenic Hf. Zircon formation during a later magmatic–metamorphic–hydrothermal event can be excluded on the basis of the combined U–Pb and Hf datasets shown in Fig. 12a. If such later zircon crystallization had taken place, the resulting domains should plot on, or above, the 176Lu/177Hf whole-rock evolution line. From the combined U–Pb and Lu–Hf isotope data it can be concluded that the oldest concordant ages best reflect the time of granite emplacement; that is, 3064 ± 20 Ma as calculated from the two most concordant analyses, A14 and A15 in Table 5 (Fig. 11a). Fig. 12. Open in new tabDownload slide Calculated 176Hf/177Hf ratio at the time of the apparent 207Pb–206Pb age vs apparent 207Pb–206Pb age for single zircon domains from the granite (a) and the hornblende metagabbro (b). The concordant analyses (97–103% concordance level) are shown as filled symbols. It should be noted that the corresponding ϵHf at the time of emplacement are all within a very narrow range close to zero, but slightly lower in the hornblende metagabbro. The zircon grains in the hornblende metagabbro are on average larger than (up to 0·8 mm in length) but of similar habit to those in the granite and also show complex internal structures with patchy bright and dull CL domains (Fig. 11b, insert). A large number of zircon domains, typically with U contents of much less than 200 ppm, yielded concordant results (Fig. 11b). Eight domains gave concordant (99–101%) age data that correspond to a concordia age of 3064 ± 7 Ma, including the error on the decay constant. This age is identical to an upper intercept age of 3063 ± 5 Ma obtained on 11 spot analyses (Fig. 11b), which is regarded as the best constraint on the time of emplacement. There are, however, also a number of other discordant zircon analyses that plot to the left of the discordia line as shown in Fig. 11b. All analyses, irrespective of the level of discordance, yielded within error identical 176Hf/177Hf ratios (0·28079 ± 0·000026) calculated for the apparent 207Pb–206Pb age (Fig. 12b, Table 6). This relationship is well reflected even by spot analyses obtained from different zircon domains that have different internal structure, U contents and 176Yb/177Hf ratios within the same grain (Fig. 12b, Tables 5 and 6). This indicates that all of these zircon grains were formed from an isotopically homogeneous magma. As with the zircon in the granite, multiple zircon growth can be excluded and the variable discordance, and 207Pb–206Pb ages, can be explained by multiple alteration events after zircon growth. Significantly, the U–Pb and Hf isotope data obtained on zircon domains from both the altered granite and the hornblende metagabbro appear very similar. Their respective emplacement ages and Hf isotope characteristics are identical within error. The ϵHf calculated for the time of emplacement (3063 Ma) ranges from −1·8 to +1·7 in the case of the granite and from −2·3 to −0·4 in the hornblende metagabbro. The mean ϵHf(t) values are slightly higher for the metagranite (+0·2) than for the hornblende metagabbro (–1·2). These values correspond to Hf model ages of 3·32 Ga for the metagranite and 3·47 Ga for the hornblende metagabbro, calculated by using the parameters as outlined in the legend of Table 6 and discussed by Zeh et al. (2007). The U–Pb isotope data indicate that the zircon grains in both rock types experienced multiple Pb loss. A major Pb loss event seemingly occurred at 2720 Ma, as reflected by one concordant data point obtained from the altered granite (spot A1, Table 5) and by several discordant analyses that yielded 207Pb–206Pb ages around 2700 Ma and plot on a reference line between the concordant datum and the origin (dashed line II in Fig. 11a). Subsequent Pb loss is further indicated by a few zircon analyses that yielded younger 207Pb–206Pb ages (Table 5). AR–AR AGE DATA FOR HORNBLENDE The above petrological and U–Pb zircon age data indicate that both the granitic and gabbroic protoliths intruded at about 3·07 Ga, and were altered afterwards, most probably at 2·7 Ga. At present, it is unclear whether the low-Ti composition of the magnesio-hornblende in the metagabbro is a result of a Neoarchaean alteration process or a primary feature related to the emplacement of the mafic intrusion. To assess the origin of the magnesio-hornblende in the metagabbro, we conducted 40Ar–39Ar analyses on hornblende separates from four positions within the mafic portion of the drill core. Between 60 and 120 mg of hornblende concentrates were enclosed in high-purity quartz vials and irradiated at the nuclear research reactor VR-1 in Prague, Czech Republic. Once cooled the samples were filled into annealed Ta capsules and subsequently analysed by stepwise heating experiments at the CEAL Laboratory of the Slovak Academy of Sciences in Bratislava. The analytical details for the fully automatic Ar-extraction and purification line are as described by Frimmel & Frank (1998; at that time the line was still housed at the former Institute of Geology, University of Vienna). The paper by Frimmel & Frank also includes details regarding corrections for mass discrimination and radioactive decay, as well as for the determination of the J-value (0·013552 ± 0·4%) and the definition of a plateau age. The K/Ca ratio was determined from the 39Ar/37Ar ratio (calculated for the end of irradiation) using a conversion factor of 0·247. The 40Ar/36Ar ratio of the line blank was close to air composition throughout the study (299 ± 1·0%). The errors of the calculated ages for single steps are given as 1σ. The errors of the plateau and total gas ages include an additional error of ±0·4% on the J-value. The hornblende separates from all four positions in the drill core yielded similar but not identical results with some significant subtle differences (Fig. 13, Table 7). The hornblende from position 15·1–15·5 m, nearest the overlying granite, gave a total gas age of 3064 ± 19 Ma. No distinct plateau can be recognized (Fig. 13a). Lower ages at the lowest temperature steps are most probably caused by secondary overgrowth of actinolite and/or chlorite. The highest ages were obtained on relatively low-temperature steps and this is regarded as reflecting the incorporation of excess Ar into the hornblende lattice as a consequence of partial chloritization. Hornblende in drill core 19·3–19·4 m yielded an identical total gas age of 3074 ± 21 Ma. Some Ar loss is indicated by lower apparent ages obtained for the lowest temperature steps, which are also characterized by elevated K/Ca (Fig. 13b). Most of the 39Ar released (65%) defines a very good plateau that corresponds to an age of 3078 ± 20 Ma. The analytical data were also used to construct 40Ar/36Ar vs 39Ar/36Ar as well as 39Ar/40Ar vs 36Ar/40Ar isotope correlation diagrams. The ages obtained from these diagrams (not shown) are all indistinguishable (3072 ± 8·5 and 3071 ± 11 Ma, respectively) from the plateau age and the total gas age, which is, therefore, regarded as dating the time of hornblende crystallization. Fig. 13. Open in new tabDownload slide 40Ar–39Ar incremental release spectra obtained for hornblende separates from different positions within the intersected hornblende metagabbro. Table 7: 40Ar–39Ar analytical data for incremental heating experiments on hornblende Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 15·1–15·5 m 1 800 4·3 478·71 97·9 0·631 1·56 302·58 0·9 2939·3 12·6 2 880 1·4 143·79 97·6 0·147 6·43 272·16 2·8 2787·3 40·1 3 950 9·6 1318·70 99·5 0·055 38·07 376·53 1·0 3263·0 15·2 4 980 34·8 4302·12 99·7 0·074 47·99 338·86 0·6 3105·4 9·4 5 1000 9·8 1116·66 99·7 0·079 51·34 313·06 1·0 2988·9 14·4 6 1015 4·9 556·58 99·8 0·083 52·70 308·58 0·7 2967·9 10·8 7 1045 6·5 730·72 99·9 0·097 64·59 306·97 0·7 2960·3 10·6 8 1095 3·3 374·96 99·8 0·090 55·77 311·53 0·6 2981·8 9·1 9 1160 9·6 1137·33 99·7 0·088 48·24 323·11 0·6 3035·2 8·5 10 1300 15·6 1869·61 99·6 0·079 41·29 329·39 0·6 3063·5 8·7 Total gas age 3063·7 18·9 1Measured; correcting factors: Daly/HF = 9·17 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 15·1–15·5 m 1 800 4·3 478·71 97·9 0·631 1·56 302·58 0·9 2939·3 12·6 2 880 1·4 143·79 97·6 0·147 6·43 272·16 2·8 2787·3 40·1 3 950 9·6 1318·70 99·5 0·055 38·07 376·53 1·0 3263·0 15·2 4 980 34·8 4302·12 99·7 0·074 47·99 338·86 0·6 3105·4 9·4 5 1000 9·8 1116·66 99·7 0·079 51·34 313·06 1·0 2988·9 14·4 6 1015 4·9 556·58 99·8 0·083 52·70 308·58 0·7 2967·9 10·8 7 1045 6·5 730·72 99·9 0·097 64·59 306·97 0·7 2960·3 10·6 8 1095 3·3 374·96 99·8 0·090 55·77 311·53 0·6 2981·8 9·1 9 1160 9·6 1137·33 99·7 0·088 48·24 323·11 0·6 3035·2 8·5 10 1300 15·6 1869·61 99·6 0·079 41·29 329·39 0·6 3063·5 8·7 Total gas age 3063·7 18·9 1Measured; correcting factors: Daly/HF = 9·17 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 19·3, 19·4 m 1 720 2·0 236·88 97·5 0·689 1·27 284·63 1·0 2851·2 13·9 2 760 1·3 142·03 97·7 1·195 0·88 264·65 1·7 2747·7 24·1 3 820 5·7 645·60 99·3 1·420 2·52 266·52 0·6 2757·7 8·9 4 850 1·2 142·48 98·0 0·366 3·09 270·54 1·5 2778·8 20·6 5 880 7·7 1081·27 99·2 0·093 21·20 332·69 0·7 3078·2 10·1 6 920 5·6 779·96 99·5 0·073 35·60 329·46 1·0 3063·8 14·4 7 950 4·7 654·01 99·1 0·071 23·12 330·10 1·2 3066·7 17·5 8 1000 35·9 5055·51 99·8 0·069 59·57 333·53 0·7 3081·9 10·9 9 1050 5·6 782·30 99·9 0·061 70·42 332·09 0·5 3075·5 7·6 10 1095 3·3 465·94 99·9 0·063 74·77 333·48 0·6 3081·7 8·4 11 1200 2·2 310·11 99·2 0·066 28·42 332·71 1·5 3078·3 22·6 12 1300 24·8 3742·31 99·7 0·057 55·23 357·17 1·2 3183·7 17·3 Total gas age 3073·7 21·3 65% plateau age 3078·1 19·6 1Measured; correcting factors: Daly/HF = 9·10 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 19·3, 19·4 m 1 720 2·0 236·88 97·5 0·689 1·27 284·63 1·0 2851·2 13·9 2 760 1·3 142·03 97·7 1·195 0·88 264·65 1·7 2747·7 24·1 3 820 5·7 645·60 99·3 1·420 2·52 266·52 0·6 2757·7 8·9 4 850 1·2 142·48 98·0 0·366 3·09 270·54 1·5 2778·8 20·6 5 880 7·7 1081·27 99·2 0·093 21·20 332·69 0·7 3078·2 10·1 6 920 5·6 779·96 99·5 0·073 35·60 329·46 1·0 3063·8 14·4 7 950 4·7 654·01 99·1 0·071 23·12 330·10 1·2 3066·7 17·5 8 1000 35·9 5055·51 99·8 0·069 59·57 333·53 0·7 3081·9 10·9 9 1050 5·6 782·30 99·9 0·061 70·42 332·09 0·5 3075·5 7·6 10 1095 3·3 465·94 99·9 0·063 74·77 333·48 0·6 3081·7 8·4 11 1200 2·2 310·11 99·2 0·066 28·42 332·71 1·5 3078·3 22·6 12 1300 24·8 3742·31 99·7 0·057 55·23 357·17 1·2 3183·7 17·3 Total gas age 3073·7 21·3 65% plateau age 3078·1 19·6 1Measured; correcting factors: Daly/HF = 9·10 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 26·8–26·9 m 1 800 5·5 281·94 97·3 0·262 2·98 295·721 1·0 2906·2 15·1 2 880 9·5 414·35 99·1 0·778 3·47 249·922 1·3 2667·5 18·1 3 950 10·9 598·29 99·0 0·073 21·79 316·749 0·9 3006·0 13·5 4 980 18·4 1194·83 99·7 0·057 50·42 376·066 1·3 3261·1 19·1 5 1000 19·6 1164·44 99·8 0·060 59·96 342·593 1·3 3121·6 19·4 6 1015 9·9 508·26 99·7 0·072 53·80 296·435 1·4 2909·7 20·9 7 1045 10·3 656·67 99·8 0·060 60·77 369·105 0·8 3233·0 12·5 8 1095 4·5 343·18 99·1 0·050 25·70 444·576 2·5 3516·8 38·6 9 1300 11·5 764·91 99·9 0·058 75·45 384·311 1·7 3293·9 26·2 Total gas age 3107·5 28·8 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 26·8–26·9 m 1 800 5·5 281·94 97·3 0·262 2·98 295·721 1·0 2906·2 15·1 2 880 9·5 414·35 99·1 0·778 3·47 249·922 1·3 2667·5 18·1 3 950 10·9 598·29 99·0 0·073 21·79 316·749 0·9 3006·0 13·5 4 980 18·4 1194·83 99·7 0·057 50·42 376·066 1·3 3261·1 19·1 5 1000 19·6 1164·44 99·8 0·060 59·96 342·593 1·3 3121·6 19·4 6 1015 9·9 508·26 99·7 0·072 53·80 296·435 1·4 2909·7 20·9 7 1045 10·3 656·67 99·8 0·060 60·77 369·105 0·8 3233·0 12·5 8 1095 4·5 343·18 99·1 0·050 25·70 444·576 2·5 3516·8 38·6 9 1300 11·5 764·91 99·9 0·058 75·45 384·311 1·7 3293·9 26·2 Total gas age 3107·5 28·8 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 30·1–30·2 m 1 800 7·1 831·15 98·63477 0·734 1·819598 319·415 0·7 3018·3 10·7 2 880 1·7 207·82 98·46037 0·144 7·334663 336·103 1·2 3093·3 18·4 3 950 14·6 1968·35 99·4128 0·066 29·60979 370·579 1·3 3239·0 19·6 4 980 16·3 1953·70 99·60721 0·077 37·92846 329·466 1·7 3063·8 24·3 5 1000 9·7 1086·63 99·49839 0·080 33·00942 306·726 1·3 2959·1 19·3 6 1015 4·1 468·07 99·20482 0·081 22·85102 313·544 1·5 2991·2 21·6 7 1045 3·7 412·32 99·13203 0·092 19·73829 304·459 2·6 2948·3 38·4 8 1095 4·6 533·32 98·68499 0·092 13·46074 316·918 1·3 3006·8 19·3 9 1160 13·7 1805·51 99·77105 0·072 50·79623 361·611 0·8 3202·2 12·4 10 1300 24·6 3087·86 99·70786 0·071 46·17452 344·218 0·8 3128·6 11·5 Total gas age 3101·4 26·1 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00020 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 30·1–30·2 m 1 800 7·1 831·15 98·63477 0·734 1·819598 319·415 0·7 3018·3 10·7 2 880 1·7 207·82 98·46037 0·144 7·334663 336·103 1·2 3093·3 18·4 3 950 14·6 1968·35 99·4128 0·066 29·60979 370·579 1·3 3239·0 19·6 4 980 16·3 1953·70 99·60721 0·077 37·92846 329·466 1·7 3063·8 24·3 5 1000 9·7 1086·63 99·49839 0·080 33·00942 306·726 1·3 2959·1 19·3 6 1015 4·1 468·07 99·20482 0·081 22·85102 313·544 1·5 2991·2 21·6 7 1045 3·7 412·32 99·13203 0·092 19·73829 304·459 2·6 2948·3 38·4 8 1095 4·6 533·32 98·68499 0·092 13·46074 316·918 1·3 3006·8 19·3 9 1160 13·7 1805·51 99·77105 0·072 50·79623 361·611 0·8 3202·2 12·4 10 1300 24·6 3087·86 99·70786 0·071 46·17452 344·218 0·8 3128·6 11·5 Total gas age 3101·4 26·1 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00020 Open in new tab Table 7: 40Ar–39Ar analytical data for incremental heating experiments on hornblende Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 15·1–15·5 m 1 800 4·3 478·71 97·9 0·631 1·56 302·58 0·9 2939·3 12·6 2 880 1·4 143·79 97·6 0·147 6·43 272·16 2·8 2787·3 40·1 3 950 9·6 1318·70 99·5 0·055 38·07 376·53 1·0 3263·0 15·2 4 980 34·8 4302·12 99·7 0·074 47·99 338·86 0·6 3105·4 9·4 5 1000 9·8 1116·66 99·7 0·079 51·34 313·06 1·0 2988·9 14·4 6 1015 4·9 556·58 99·8 0·083 52·70 308·58 0·7 2967·9 10·8 7 1045 6·5 730·72 99·9 0·097 64·59 306·97 0·7 2960·3 10·6 8 1095 3·3 374·96 99·8 0·090 55·77 311·53 0·6 2981·8 9·1 9 1160 9·6 1137·33 99·7 0·088 48·24 323·11 0·6 3035·2 8·5 10 1300 15·6 1869·61 99·6 0·079 41·29 329·39 0·6 3063·5 8·7 Total gas age 3063·7 18·9 1Measured; correcting factors: Daly/HF = 9·17 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 15·1–15·5 m 1 800 4·3 478·71 97·9 0·631 1·56 302·58 0·9 2939·3 12·6 2 880 1·4 143·79 97·6 0·147 6·43 272·16 2·8 2787·3 40·1 3 950 9·6 1318·70 99·5 0·055 38·07 376·53 1·0 3263·0 15·2 4 980 34·8 4302·12 99·7 0·074 47·99 338·86 0·6 3105·4 9·4 5 1000 9·8 1116·66 99·7 0·079 51·34 313·06 1·0 2988·9 14·4 6 1015 4·9 556·58 99·8 0·083 52·70 308·58 0·7 2967·9 10·8 7 1045 6·5 730·72 99·9 0·097 64·59 306·97 0·7 2960·3 10·6 8 1095 3·3 374·96 99·8 0·090 55·77 311·53 0·6 2981·8 9·1 9 1160 9·6 1137·33 99·7 0·088 48·24 323·11 0·6 3035·2 8·5 10 1300 15·6 1869·61 99·6 0·079 41·29 329·39 0·6 3063·5 8·7 Total gas age 3063·7 18·9 1Measured; correcting factors: Daly/HF = 9·17 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 19·3, 19·4 m 1 720 2·0 236·88 97·5 0·689 1·27 284·63 1·0 2851·2 13·9 2 760 1·3 142·03 97·7 1·195 0·88 264·65 1·7 2747·7 24·1 3 820 5·7 645·60 99·3 1·420 2·52 266·52 0·6 2757·7 8·9 4 850 1·2 142·48 98·0 0·366 3·09 270·54 1·5 2778·8 20·6 5 880 7·7 1081·27 99·2 0·093 21·20 332·69 0·7 3078·2 10·1 6 920 5·6 779·96 99·5 0·073 35·60 329·46 1·0 3063·8 14·4 7 950 4·7 654·01 99·1 0·071 23·12 330·10 1·2 3066·7 17·5 8 1000 35·9 5055·51 99·8 0·069 59·57 333·53 0·7 3081·9 10·9 9 1050 5·6 782·30 99·9 0·061 70·42 332·09 0·5 3075·5 7·6 10 1095 3·3 465·94 99·9 0·063 74·77 333·48 0·6 3081·7 8·4 11 1200 2·2 310·11 99·2 0·066 28·42 332·71 1·5 3078·3 22·6 12 1300 24·8 3742·31 99·7 0·057 55·23 357·17 1·2 3183·7 17·3 Total gas age 3073·7 21·3 65% plateau age 3078·1 19·6 1Measured; correcting factors: Daly/HF = 9·10 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 19·3, 19·4 m 1 720 2·0 236·88 97·5 0·689 1·27 284·63 1·0 2851·2 13·9 2 760 1·3 142·03 97·7 1·195 0·88 264·65 1·7 2747·7 24·1 3 820 5·7 645·60 99·3 1·420 2·52 266·52 0·6 2757·7 8·9 4 850 1·2 142·48 98·0 0·366 3·09 270·54 1·5 2778·8 20·6 5 880 7·7 1081·27 99·2 0·093 21·20 332·69 0·7 3078·2 10·1 6 920 5·6 779·96 99·5 0·073 35·60 329·46 1·0 3063·8 14·4 7 950 4·7 654·01 99·1 0·071 23·12 330·10 1·2 3066·7 17·5 8 1000 35·9 5055·51 99·8 0·069 59·57 333·53 0·7 3081·9 10·9 9 1050 5·6 782·30 99·9 0·061 70·42 332·09 0·5 3075·5 7·6 10 1095 3·3 465·94 99·9 0·063 74·77 333·48 0·6 3081·7 8·4 11 1200 2·2 310·11 99·2 0·066 28·42 332·71 1·5 3078·3 22·6 12 1300 24·8 3742·31 99·7 0·057 55·23 357·17 1·2 3183·7 17·3 Total gas age 3073·7 21·3 65% plateau age 3078·1 19·6 1Measured; correcting factors: Daly/HF = 9·10 ± 2·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 26·8–26·9 m 1 800 5·5 281·94 97·3 0·262 2·98 295·721 1·0 2906·2 15·1 2 880 9·5 414·35 99·1 0·778 3·47 249·922 1·3 2667·5 18·1 3 950 10·9 598·29 99·0 0·073 21·79 316·749 0·9 3006·0 13·5 4 980 18·4 1194·83 99·7 0·057 50·42 376·066 1·3 3261·1 19·1 5 1000 19·6 1164·44 99·8 0·060 59·96 342·593 1·3 3121·6 19·4 6 1015 9·9 508·26 99·7 0·072 53·80 296·435 1·4 2909·7 20·9 7 1045 10·3 656·67 99·8 0·060 60·77 369·105 0·8 3233·0 12·5 8 1095 4·5 343·18 99·1 0·050 25·70 444·576 2·5 3516·8 38·6 9 1300 11·5 764·91 99·9 0·058 75·45 384·311 1·7 3293·9 26·2 Total gas age 3107·5 28·8 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 26·8–26·9 m 1 800 5·5 281·94 97·3 0·262 2·98 295·721 1·0 2906·2 15·1 2 880 9·5 414·35 99·1 0·778 3·47 249·922 1·3 2667·5 18·1 3 950 10·9 598·29 99·0 0·073 21·79 316·749 0·9 3006·0 13·5 4 980 18·4 1194·83 99·7 0·057 50·42 376·066 1·3 3261·1 19·1 5 1000 19·6 1164·44 99·8 0·060 59·96 342·593 1·3 3121·6 19·4 6 1015 9·9 508·26 99·7 0·072 53·80 296·435 1·4 2909·7 20·9 7 1045 10·3 656·67 99·8 0·060 60·77 369·105 0·8 3233·0 12·5 8 1095 4·5 343·18 99·1 0·050 25·70 444·576 2·5 3516·8 38·6 9 1300 11·5 764·91 99·9 0·058 75·45 384·311 1·7 3293·9 26·2 Total gas age 3107·5 28·8 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0%, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00022 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 30·1–30·2 m 1 800 7·1 831·15 98·63477 0·734 1·819598 319·415 0·7 3018·3 10·7 2 880 1·7 207·82 98·46037 0·144 7·334663 336·103 1·2 3093·3 18·4 3 950 14·6 1968·35 99·4128 0·066 29·60979 370·579 1·3 3239·0 19·6 4 980 16·3 1953·70 99·60721 0·077 37·92846 329·466 1·7 3063·8 24·3 5 1000 9·7 1086·63 99·49839 0·080 33·00942 306·726 1·3 2959·1 19·3 6 1015 4·1 468·07 99·20482 0·081 22·85102 313·544 1·5 2991·2 21·6 7 1045 3·7 412·32 99·13203 0·092 19·73829 304·459 2·6 2948·3 38·4 8 1095 4·6 533·32 98·68499 0·092 13·46074 316·918 1·3 3006·8 19·3 9 1160 13·7 1805·51 99·77105 0·072 50·79623 361·611 0·8 3202·2 12·4 10 1300 24·6 3087·86 99·70786 0·071 46·17452 344·218 0·8 3128·6 11·5 Total gas age 3101·4 26·1 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00020 Step . T (°C) . 39Ar (%) . 40Ar1 (mV) . Rad (%) . 39Ar/37Ar . 36Ca (%) . 40Ar1/39Ar . ± (%) . Age (Ma) . ± (Ma) . Sample 30·1–30·2 m 1 800 7·1 831·15 98·63477 0·734 1·819598 319·415 0·7 3018·3 10·7 2 880 1·7 207·82 98·46037 0·144 7·334663 336·103 1·2 3093·3 18·4 3 950 14·6 1968·35 99·4128 0·066 29·60979 370·579 1·3 3239·0 19·6 4 980 16·3 1953·70 99·60721 0·077 37·92846 329·466 1·7 3063·8 24·3 5 1000 9·7 1086·63 99·49839 0·080 33·00942 306·726 1·3 2959·1 19·3 6 1015 4·1 468·07 99·20482 0·081 22·85102 313·544 1·5 2991·2 21·6 7 1045 3·7 412·32 99·13203 0·092 19·73829 304·459 2·6 2948·3 38·4 8 1095 4·6 533·32 98·68499 0·092 13·46074 316·918 1·3 3006·8 19·3 9 1160 13·7 1805·51 99·77105 0·072 50·79623 361·611 0·8 3202·2 12·4 10 1300 24·6 3087·86 99·70786 0·071 46·17452 344·218 0·8 3128·6 11·5 Total gas age 3101·4 26·1 1Measured; correcting factors: Daly/HF = 8·91 ± 5·0, 39Ca/37Ca = 0·00039, 36Ca/37Ca = 0·00020 Open in new tab The remaining two hornblende separates from the drill core sections 26·8–26·9 m and 30·1–30·2 m both yielded similar results with total gas ages of 3108 ± 29 and 3101 ± 26 Ma, respectively (Fig. 13c and d, Table 7). In both cases, a certain variability in the ages obtained at the various steps prevents a good plateau from being seen and this is also reflected in large errors in the (comparable) ages obtained from 40Ar/36Ar vs 39Ar/36Ar and 39Ar/40Ar vs 36Ar/40Ar diagrams (not shown). Very little evidence of partial Ar loss caused by secondary alteration is indicated for these samples, in accordance with petrographic observations. DISCUSSION Age and geotectonic setting of the analysed basement rocks The best constraints on the time of magmatic crystallization of the investigated granite and hornblende metagabbro are provided by the concordant U–Pb zircon age data. They are 3064 ± 20 and 3063 ± 5 Ma, respectively, identical within error (Fig. 11). Younger concordant and discordant U–Pb ages, in particular those obtained from the granite samples, point to a partial or complete resetting of the U–Pb system at <2720 Ma, perhaps by a pervasive alteration process. At this point it is worth noting that this alteration process left the original Hf isotope composition unchanged, as is reflected in the within-error identical 176Hf/177Hf (c. ±1·5 ϵ-units) ratios obtained for nearly all concordant and discordant zircon domains in the respective samples (Fig. 12). This clearly indicates that all the zircon grains or domains formed during the magmatic crystallization event at c. 3065 Ma, and not during later processes (e.g. during a Neoarchaean or Palaeoproterozoic metamorphic or metasomatic overprint). Clues about the age of such an overprint are, apart from the younger U–Pb zircon ages (Fig. 11), provided by the Rb–Sr isotope whole-rock data, which yield an errorchron age of c. 2633 Ma. However, the geological significance of this Rb–Sr age, which is predominantly constrained by the spread of the data from the granite sample, is not entirely clear. Judging from the petrographic observations (formation of secondary microcline, even in the metagabbro, and sericitization), it appears likely that the Rb–Sr errorchon age of 2633 Ma reflects the time of retrograde hydrothermal infiltration and alteration. In this context, the somewhat older ‘concordant’ U–Pb zircon age of c. 2720 Ma could be explained as a maximum age that results from incomplete Pb loss from altered (metamict), primary magmatic zircon domains at 2·63 Ga. Surprisingly, and in contrast to the Rb–Sr isotope data, a pervasive Neoarchaean hydrothermal alteration event is not indicated by the Ar–Ar data obtained on the hornblende from the metagabbro. As shown in Fig. 13, the hornblende separates yielded total fusion Ar–Ar ages, which are either within error of the zircon crystallization ages (3064 ± 19 Ma and 3078 ± 20 Ma) or slightly older (3108 ± 29 Ma and 3101 ± 26 Ma), and provide little evidence for a Neoarchaean or younger overprint. This finding points clearly to hornblende formation at, or close to, the time of magma crystallization. Judging from the very low Ti content of the hornblende in the metagabbro, and the fact that it occurs together with other Ti-bearing (buffer)-phases, it seems most likely that the magnesio-hornblende was not formed during magmatic crystallization, but rather resulted from post-magmatic, solid-state alteration either of primary magmatic (Ti-rich) hornblende or of other mafic magmatic phases (such as pyroxenes) immediately after magmatic crystallization. Possibly, this transformation was caused by pervasive autometasomatism. Such an explanation conforms to the U–Pb and Ar–Ar age data, but also to the very narrow range in Rb/Sr and 87Sr/86Sr in the five hornblende metagabbro samples analysed (Fig. 9, Table 4). Such an interpretation finds further support from recent O isotope data obtained on whole-rocks and mineral separates from the investigated core samples (M. Depiné et al., unpublished data, 2009). The slightly older 40Ar–39Ar ages obtained for some hornblende samples, or some steps, might result from the incorporation of excess Ar during secondary alteration of hornblende to actinolite and/or chlorite in the course of a later (Neoarchaean) low-grade metamorphic and/or hydrothermal overprint. Alternatively, the excess Ar may be due to infiltration and reaction of late-magmatic fluids that caused the autometasomatism suggested above. Such Ar-enriched fluids were perhaps released during crystallization of the associated granites, which formed nearly contemporaneously with the hornblende gabbro, as suggested by the within error-identical U–Pb zircon ages and Hf isotope data. In fact, this late- to post-magmatic infiltration process, which is related either to granite emplacement or to another unexposed source, could also account for the formation of the biotite-rich alteration zone between the hornblende metagabbro and the granite, and the presence of microcline throughout the metagabbro. This microcline, although only in minor proportions, is unusual and cannot be explained by fractional crystallization because of the low density of K-feldspar and its relatively late crystallization. As the same type of microcline is also found in locally very coarse-grained patches within the granite and concentrated along veins in the metagabbro, it is most probably related to the infiltration of a potassic fluid—possibly during a stage of pegmatite formation evident from surface outcrops (Fig. 3). Pegmatite formation was followed by a further retrograde, hydrothermal overprint as indicated by the chloritization of biotite, sericitization and saussuritization of microcline and plagioclase, respectively, and the formation of actinolite at the expense of magnesio-hornblende and the various veinlets, especially in the hornblende metagabbro. It appears likely that this final hydrothermal overprint was responsible for the resetting of the Rb–Sr isotope system, in particular of the granite, as reflected by the Rb–Sr whole-rock errorchron age of c. 2630 Ma. The reconstruction of the tectonic setting of the investigated magmatic rocks is hindered by their complex post-magmatic alteration. Some elements, such as the alkali elements, are certain to have been mobile. For instance, the (Na + K) concentrations shown in Fig. 4a are likely to be higher than those of the magmatic protoliths because of the later K-metasomatism. Consequently, the protoliths were even less alkaline than shown in that diagram and conform to the calc-alkaline series, as is also indicated by the relatively small proportion of total Fe relative to Mg at variable SiO2 concentrations (Fig. 4c). Despite the problems of some major element mobility, trends of petrogenetic significance can be derived from certain less mobile trace element distributions. Almost all geochemical indicators point to a calc-alkaline composition and crustal contamination typical of continental arc magmatism. This includes, for example, the Nb–Y and Ta–Yb relationships in the granite (Fig. 6), the overall enrichment of the metagabbro in the less compatible elements (e.g. Th), its relative depletion in Nb and its strong depletion in Ti relative to MORB (Fig. 7). Variable crustal contamination is also reflected by LREE enrichment and by chondritic to slightly subchondritic ϵNd 3·06Ga (mostly between +1·9 and −1·7) and ϵHf 3·06Ga values (+1·7 to −2·3; Fig. 10, Table 6). Notably, the ϵHft values are identical within error to those recently obtained for similarly old rocks from other parts of the Kaapvaal Craton by Zeh et al. (2009), comprising 3·1 Ga granitic rocks from the Swaziland, Witwatersrand and Pietersburg blocks (Fig. 1). The very low ϵNd 3·06Ga values of −4·8 and −10·4 obtained from two samples (Fig. 10) result either from post-intrusive alteration (e.g. Moorbath et al., 1997) or from the assimilation of much older crust, recycled during the intrusion of the granitic and gabbroic protoliths. Notably, similarly strong variations are not recorded by the Hf isotope data. Following Pearce (2008), we chose Yb-normalized Th and Nb as proxies for crustal input. Both Th and Nb have a similar geochemical behaviour in most petrogenetic processes and both are relatively immobile under conditions that range from weathering to medium-grade metamorphism. With regard to the Th–Nb–Yb relationships (Fig. 14a), the hornblende metagabbro samples plot in a position similar to enriched (E-)MORB with variable degrees of crustal contamination. In a similar way, Condie (2005) described the extent of crustal contamination versus mantle composition in terms of Nb/Th vs Zr/Nb and distinguished between plume and non-plume sources based on the Nb/Y vs Zr/Y relationships. As pointed out by Pearce (2008), the application of the latter diagram is problematic because of depth-dependent garnet fractionation. Thus the apparent plume source indicated in Fig. 14c may not be real. The low Nb/Th is in agreement with a volcanic arc setting, but the Zr/Nb is lower than expected for such a setting (Fig. 14b). Fig. 14. Open in new tabDownload slide (a) Th/Yb vs Nb/Yb plot for the hornblende metagabbro samples and the Dominion Group mafic volcanic rocks, utilizing the Th–Nb proxy for crustal contamination as suggested by Pearce (2008); (b) Zr/Nb vs Nb/Th, and (c) Nb/Y vs Zr/Y plots for the same samples. Compositional fields for different tectonic settings and effects of batch melting (BM) and subduction (SUB), as indicated by arrows, are from Condie (2005); PM, primitive mantle; DM, depleted mantle. The Cs/Nb and U/Nb ratios are supposed to be insensitive to magmatic differentiation by fractional crystallization and are therefore also used to assess the extent of crustal contamination (Fig. 15a). As can be seen from that figure, most samples plot away from primitive mantle ratios, thus testifying to considerable crustal contamination in both the mafic and felsic portions. One trend projects to particularly high U/Nb ratios at reasonable crustal Cs/Nb, whereas one sample plots far away from that trend, having a particularly high Cs/Nb ratio. The latter is rich in microcline and probably reflects a geochemical change during K-metasomatism. The U/Nb ratios are high for most samples, even for the mafic rocks, which show, on average, higher U/Nb ratios than typical of continental crust (Fig. 15b). Again, considerable crustal contamination can be inferred from this observation. Fig. 15. Open in new tabDownload slide Cs/Nb vs U/Nb (a) and U/Nb vs Ce/Pb (b) diagrams for the various lithotypes in the investigated drill core. The fields or points for oceanic island basalt (OIB), mid-oceanic ridge basalt (MORB), primitive mantle (PM), lower (LC), middle (MC) and upper continental crust (UC) are from Sun & McDonough (1989) and Rudnick & Fountain (1995). Also shown are fractional crystallization trends for amphibole (Amph), clinopyroxene (Cpx), plagioclase (Plag) and alkali feldspar (K-fsp), each arrow representing 50% fractional crystallization. The influence of fractional crystallization on the scale of Fig. 15a is negligible. The enrichment in U is also evident in Fig. 15b, in which the U/Nb ratio is plotted against Ce/Pb. The latter ratio is explicable in terms of extensive feldspar fractionation in the case of the granite and hornblende metagabbro, and conforms to the observed negative Eu anomalies (Fig. 8). This effect is most pronounced in the biotite-rich zone, because of the elevated, probably hydrothermal, allanite content therein. Formation and alteration of the basement next to the Witwatersrand Basin The U–Pb crystallization age of 3062 ± 5 Ma for the plutonic rocks adjacent to the ancient Witwatersrand Basin invariably invites a comparison to be made with the age of Dominion Group volcanism. The volcano-sedimentary Dominion Group underlies the Witwatersrand Supergroup rocks over probably a much larger area than indicated by the remnants of that group known from surface and subsurface exposures mainly along the northwestern margin of the Witwatersrand Basin (Fig. 2). The precise U–Pb single zircon age of 3074 ± 6 Ma obtained for a quartz–feldspar porphyry in the upper part of the Dominion Group (Armstrong et al., 1991) is only slightly older than the crystallization age obtained in this study. This raises the possibility that the intrusive rocks of this study are the plutonic expression of the same type of magmatism that formed most of the Dominion Group. A continental rift setting has been generally assumed for the Dominion Group (e.g. Clendenin et al., 1988; Marsh et al., 1989; Jackson, 1992) because of the bimodal nature of the volcanic suite and the tholeiitic affinity of the mafic rocks. The evidence is, however, not unequivocal. Geochemical data for the Dominion Group volcanic rocks were presented by Marsh et al. (1989) and Jackson (1994). The bimodal compositional distribution encompasses rocks with a range of SiO2 concentrations between 51 and 61%, with andesitic compositions dominating; however, there is a gap between 61 and 67% SiO2. A bimodal SiO2 distribution has been noted in many Archaean arcs and explained by shallow subduction caused by a higher mantle heat flow resulting in thicker oceanic crust (Abbott & Hofmann, 1984). According to Marsh et al. (1989) the Dominion Group mafic lavas are enriched in the less compatible trace elements shown in Fig. 7 in relation to MORB but they are depleted in Nb relative to Th, La, and Ce (no data for Ta available). Moreover, they are characterized by depletion in Ti relative to MORB and overall show a trace element pattern that resembles that of typical calc-alkaline basalts. The geochemistry of the Dominion Group volcanic rocks could thus be easily explained by a volcanic arc setting. Interestingly, Burke et al. (1986) previously suggested that the Dominion Group volcanic rocks erupted on the landward flank of an Andean-type continental margin. Closer inspection of the dataset of Marsh et al. (1989) reveals, however, that there are certain differences from the geochemical characteristics of the intrusive rocks studied by us. For example, the Nb and Y concentrations of their silicic rocks, the quartz–feldspar porphyries, correspond to those of volcanic arcs as well as syn-collisional granites but their Nb/Y ratios are slightly lower than for the granites investigated in this study (Fig. 6a). In terms of Th–Nb–Yb relationships, the Dominion Group volcanic rocks follow a trend from N- to E-MORB but lack evidence of major mantle–crust interaction (Fig. 14a). Minor crustal contamination might be indicated by their calculated Nd model ages (mean age 3233 ± 53 Ma, recalculated from the data of Marsh et al., 1989), which are slightly older than the age of emplacement. This is, however, subject to the previously mentioned uncertainty in the projection of the Archaean depleted mantle composition in general and that below the Kaapvaal Craton in particular. The Dominion Group rocks exhibit very different Zr–Nb–Y relationships that are more akin to typical volcanic arc compositions, except for a higher Nb/Th ratio (Fig. 14b and c). It is, therefore, assumed that the intrusive rocks of this study are not co-genetic with the Dominion Group lavas and might be slightly younger. In particular, the absence of evidence of significant crustal contamination in the Dominion Group rocks might indicate that they formed earlier, in a less mature arc above a thinner crust, whereas the 3062 ± 5 Ma intrusive rocks could reflect a more mature stage of arc development. Apart from our Nd and Hf isotope and Nb–Th–Yb–REE data, crustal contamination at 3·06 Ga seems likely in view of the presence of older crust in the neighbouring basement along the northern and northwestern margin of the Witwatersrand Basin and in the Vredefort Dome (for a summary of geochronological data see Poujol et al., 2003; Armstrong et al., 2006). The oldest reported zircon ages are 3480 ± 7 Ma from xenocrysts in a quartz porphyry of the upper Ventersdorp Supergroup (Armstrong et al., 1991) and c. 3245 Ma from paragneiss in the Vredefort Dome (Hart et al., 1999), respectively; these ages are similar to many of the Nd and Hf model ages obtained during this study (Table 6). Following trondhjemite–tonalite emplacement (the most precise age is 3340 ± 3 Ma, Poujol & Anhaeusser, 2001) in the Johannesburg Dome, the basement along the northern basin margin was affected by large-scale granodiorite emplacement with U–Pb single zircon ages of 3121 ± 5 Ma (southern Johannesburg Dome), 3120 ± 5 Ma (200 km SW of Johannesburg) and 3114 ± 2 Ma (southwestern Johannesburg Dome, Poujol & Anhaeusser, 2001). A slightly younger U–Pb zircon age of Ma was obtained on a granodiorite near Coligny (Robb et al., 1992). Our new data are comparable with previously obtained Rb–Sr, Pb–Pb and Sm–Nd data obtained on similar granitoids from the Johannesburg Dome. Granodiorite and granite from that area yielded Rb–Sr and Pb–Pb isochron ages of 3081 ± 33 and 3062 ± 26 Ma, respectively, and a Pb–Pb zircon age of 3093 ± 3 Ma (Barton et al., 1999). Significantly older Nd model ages obtained in the same study indicate involvement of older crustal components. Based on geochemical data for these calc-alkaline granitoids in the Johannesburg Dome, Anhaeusser (1999) concluded that these rocks formed in a volcanic arc setting. The age obtained here for the drilled granite and hornblende metagabbro (3062 ± 5 Ma) is identical within error to that reported by Armstrong et al. (2006) for an aplite dyke in the Vredefort Dome (3068 ± 6 Ma). The volumetrically minor aplite emplacement in the Vredefort Dome is the last of three magmatic episodes there, following tonalite–trondhjemite–granodiorite emplacement probably in an oceanic arc at 3·1 Ga and subsequent high-grade metamorphism and granite–granodiorite emplacement between 3·1 and 3·08 Ga (Armstrong et al., 2006). The latter episode has been interpreted by Armstrong et al. to reflect crustal thickening in response to accretion of the Vredefort rocks onto an older core of the Kaapvaal craton. Our new results reinforce the idea of Armstrong et al. (2006) that the central Kaapvaal Craton, which later became covered in places by the Witwatersrand sediments, had not already consolidated into a stable craton by 3·2 Ga, as suggested by de Wit et al. (1992) and subsequently adopted by many workers, but continued to grow through the accretion of magmatic arcs until about 3·06 Ga. On the basis of available data so far, all potential source rocks for the proximal Witwatersrand sediments seem to be of a magmatic arc affinity. The exact geometry of these magmatic arc systems remains uncertain, but an east-northeasterly trend along the northern margin of the Witwatersrand Block can be distinguished from a northwesterly trend in the Vredefort Dome and in the western part of the craton. It may be speculated that the voluminous potassic granites of similar age in the Barberton–Swaziland region (Poujol et al., 2003; Zeh et al., 2009), formed in response to extensive crustal heating within the continent behind an arc system. This conclusion is in good agreement with Hf isotope data recently obtained from granitoid rocks in the eastern part of the Kaapvaal Craton (Zeh et al., 2009). These data indicate that at c. 3·1 Ga new crust was added to the pre-existing basement of the Witwatersrand block, perhaps in response to roughly southward subduction during the accretion of the Pietersburg block onto the Witwatersrand block. In contrast to most previous models, we prefer an intra-arc position for the Dominion Group. This is notwithstanding the bimodal nature of the volcanism (similar to the bimodal character of the investigated drill core). In the light of mounting evidence for andesite magmas being formed by mixing between evolved, silicic melts and basic plutonic root components (e.g. Reubi & Blundy, 2008), the mere presence or absence of a bimodal magmatic suite might not be a reliable indicator of a specific tectonic setting. Instead, intra-arc extension might have provided suitable pathways for the ascent of felsic and mafic melts. The retrograde, hydrothermal alteration of the various basement rocks in the studied drill core is similar to that previously described for basement granites along the northern and northwestern margins of the Witwatersrand Basin (Klemd & Hallbauer, 1987; Robb & Meyer, 1987). As already pointed out by Klemd (1999), the similar fluid inclusion characteristics of the hydrothermal alteration of the basement granites and of the Witwatersrand Basin fill are suggestive of a relationship. The situation is, however, complicated because of the multistage alteration history recorded by the Witwatersrand metasedimentary rocks (Frimmel et al., 2005). This ranges from diagenetic dewatering to regional low-grade metamorphism, a thermal overprint by the Bushveld event, to brittle deformation and further hydrothermal alteration triggered by the Vredefort impact. Our zircon U–Pb and Rb–Sr isotope data are consistent with previous interpretations that the basement below the Witwatersrand Basin was affected by several alteration events between 2720 and 2630 Ga (Figs 6 and 11). For example, Kositcin et al. (2003) recognized, based on U–Pb sensitive high-resolution ion microprobe (SHRIMP) data on different hydrothermal xenotime generations, at least three stages of hydrothermal fluid infiltration. The oldest of these (2720 Ma) is close to the time of the outpouring of the voluminous Klipriviersberg lavas (lower Ventersdorp Supergroup) and is identical to the older alteration age suggested above for the pre-Witwatersrand basement. Considerable heating affected the Kaapvaal crust at that time, which is also evident from the contemporaneous ultrahigh-temperature metamorphism in the lower parts of that crust (Schmitz & Bowring, 2003). The cause of the 2714 ± 8 Ma Klipriviersberg volcanism (Armstrong et al., 1991) remains a matter of debate, with both a mantle plume and/or crustal thinning having been held responsible for it. More recently, Silver et al. (2006) suggested flood basalt eruption in a collisional rift. Their model involves short-term drainage of a molten basalt reservoir in the sublithospheric mantle during a change in the stress field in an overall collisional setting. In this case, the collision would be an early stage of amalgamation of the Kaapvaal Craton with crustal fragments now present in the Central Zone of the Limpopo Belt (see Zeh et al., 2009). A northward thrusting event that affected the greenstone belts along the northern flank of the craton, dated at 2729 ± 19 Ma (Passeraub et al., 1999), is likely to be an expression of this collision. Furthermore, several granites of comparable ages (Robb et al., 1992) in the immediate vicinity of the Witwatersrand may be related to the same tectonic stage. The maximum temperature experienced by the Witwatersrand Basin fill was attained at different times in different parts of the basin (Frimmel et al., 2005). At least in the upper parts of the basin fill it did not exceed lower greenschist-facies temperatures (350 ± 50°C; Frimmel, 1994; Phillips & Law, 1994). Along the northern margin of the Witwatersrand Basin, peak metamorphic conditions were already reached by the end of Ventersdorp Supergroup deposition, as indicated by kyanite- and pyrophyllite-bearing post-Platberg and pre-Transvaal thrust faults (Coetzee et al., 1995). The inferred younger alteration of the basement rocks investigated in this study could well be an expression of this collisional event in the Limpopo Belt because the obtained Rb–Sr errorchron ‘age’ of 2633 ± 50 Ma overlaps with the published ages of syntectonic granites in the Limpopo Belt (McCourt & Armstrong, 1998; Kröner et al., 1999; Zeh et al., 2007, 2009; Millonig et al., 2008). In addition, the high initial 87Sr/86Sr shown in Fig. 9 points to the involvement of crustal fluids. Further hydrothermal xenotime growth was noted by Kositcin et al. (2003) at 2210 Ma (Pretoria Group extension) and again at 2046–2061 Ma, the time of the Bushveld event. These younger events do not seem to have further disturbed significantly the isotopic composition of the investigated basement rocks and zircon grains therein. Similarly, the 2023 Ma Vredefort impact event, which undoubtedly triggered renewed fluid circulation through the Witwatersrand Basin and its surroundings (Frimmel et al., 1999), did not affect the isotope systems of the studied rocks, analogous to a previous finding by Barton et al. (1999) in the Johannesburg Dome. Significance for Witwatersrand gold genesis Mounting evidence exists for plate-tectonic processes having already been operative in Mesoarchaean times (e.g. de Wit, 1998; Moyen et al., 2006; Zeh et al., 2009). Thus comparison with the gold productivity in various post-Archaean plate-tectonic settings seems justified. The vast majority (∼87%) of known primary gold deposits (i.e. excluding secondary, placer deposits), appear to have been formed along active continental margins, where they are present as orogenic (including intrusion-related), Cu–Au porphyry and epithermal types (Frimmel, 2008). There is no doubt that active continental margins provide by far the best sites for the concentration of Au into ore bodies and there is no reason why this should have been any different in the Archaean. Of significance is not only our conclusion that at least some of the auriferous Witwatersrand sediments could have been sourced in an active continental margin but particularly the finding of ‘autometasomatized’ hornblende gabbro (or diorite) in that source area. The REE patterns of the studied mafic rocks are identical to those that are typically explained by the fractionation of middle and heavy REE-enriched hornblende. The abundance of hornblende in these rocks makes this a highly feasible explanation. A high magmatic oxidation state is unlikely to have been the reason for the noted lack of a pronounced negative Eu anomaly in the hornblende metagabbros because of the Mesoarchaean age of the system. This could also be due to the suppression of feldspar fractionation. Early plagioclase crystallization is suppressed when a magma contains sufficient H2O to stabilize hornblende as an early liquidus phase. By analogy with studies such as those of Rutherford & Devine (1993) and Richards et al. (2001) an H2O content of >4 wt % is required for the crystallization of near-liquidus hornblende. Such an elevated H2O content in the melt is a critical and typical ingredient of a number of productive magmatic–hydrothermal ore-forming systems worldwide, such as porphyry copper (–gold) or iron oxide–copper–gold (IOCG) deposits. Consequently, the investigated basement rocks contain the essential prerequisites for particularly high fertility in terms of primary magmatic–hydrothermal gold mineralization if the protolith of the studied mafic rocks was indeed a hornblende gabbro. In the light of a total absence of any pyroxene relics anywhere in the drilled hornblende metagabbro this is our preferred interpretation. Derivation of the Witwatersrand placer gold from an Au-enriched continental volcanic arc also would explain the previously noted wide range of generally orders of magnitude higher Os concentrations in the Witwatersrand gold compared with any other type of gold investigated so far, as noted by Kirk et al. (2002) and Frimmel et al. (2005). It has been suggested by these workers that the Witwatersrand gold was originally magmatic and extracted from a 3·1–3·3 Ga mantle source because of an initial 187Os/188Os value of 0·108 that corresponds to the projected depleted mantle composition at that time. These Os-depleted mantle extraction ages agree well with the Nd and Hf model ages obtained during this study. The wide range in Os concentrations would be explicable by a mixture of sources, including disseminated porphyry-style gold, intrusion-related, IOCG, epithermal or orogenic gold. The extent to which the gold was transported by hydrothermal fluids (as opposed to melts), which is vastly different between the above sources, would control the Os concentration of a particular type of gold because of the extremely low solubility of Os in aqueous fluids. Evidence of mesothermal (orogenic), potentially gold-bearing, quartz veins in the source area exists in the form of O isotope data on quartz pebbles in the host conglomerate (Vennemann et al., 1992, 1995). Such veins are, however, an unlikely major contributor to the Witwatersrand gold budget. The required abundance of typical Archaean mesothermal lode gold deposits in the source area to explain about 40% of all known gold would have to be unrealistically high. Hallbauer & Barton (1987) previously suggested that Archaean greenstone-hosted gold from quartz veins, as mined in the Barberton greenstone belt, could not have been a major source of the Witwatersrand gold because of compositional differences. Instead, they proposed that altered granites with as much as 80 ppb Au in the hinterland could be a more likely source. A magmatic–hydrothermal origin of the Witwatersrand gold solves the mass-balance problem and also explains the absence of vein quartz pebbles with visible gold inclusions in the Witwatersrand goldfields. It also explains the overall very small size of the Witwatersrand gold particles (see Minter et al., 1993) and the lack of reasonably sized gold nuggets (although the latter could simply be due to a lack of an oxidizing atmosphere). The comparison with magmatic–hydrothermal mineralizing systems, such as porphyry Cu–Au or IOCG systems, makes it tempting to interpret the earlier stages of the observed hydrothermal alteration as being part of such a mineralizing system. Late-stage magmatic alteration is additionally indicated by our combined datasets for the hornblende metagabbro, for which a late-magmatic stage of ‘hydrothermal’ autometasomatism immediately after emplacement at c. 3065 Ma is suggested by the very similar Ar–Ar ages of very low-Ti hornblende. In contrast, the Neoarchaean hydrothermal alteration at c. 2720 and 2630 Ma, as indicated by our Rb–Sr and U–Pb age data, might have nothing to do with enrichment in Au in the central Kaapvaal crust, because the bulk, if not all, of the gold in the Witwatersrand Supergroup had already been deposited within the largely conglomeratic host rocks prior to that time. The only effect this hydrothermal alteration had on the gold was that of local, short-range mobilization and dispersion within the host conglomerates. CONCLUSIONS New lithogeochemical, isotopic and geochronological data for hornblende metagabbro and granite from a basement horst at the northwestern margin of the Witwatersrand Basin provide new insights into the nature of some pre-Witwatersrand units that could have been potential source rocks for some of the auriferous conglomerates particularly in the Central Rand Basin. Both the hornblende metagabbro and the granite yielded indistinguishable U–Pb zircon ages of 3062 ± 5 Ma. It is concluded from widespread K-metasomatism and the alteration of presumably primary Ti-rich hornblende to Ti-poor magnesio-hornblende at effectively the same time that the granite intruded into the gabbro shortly after the latter's crystallization. Both rock types have geochemical signatures that are typical of calc-alkaline magmatism. This, combined with evidence for contamination by older crustal components based on Nd and Hf model ages for whole-rocks and zircon grains, respectively, leads to the conclusion that a continental volcanic arc is the most likely magmatic setting. Our new results support previous suggestions of successive arc accretion onto the northern and western margins of the Witwatersrand block prior to the development of the various basins that constitute the Witwatersrand ‘successor’ basin. With a previously published age of the Witwatersrand gold (and associated detrital pyrite) that is within error of the age obtained for the pre-Witwatersrand units of this study and the proximity of the studied rocks to the sites of deposition of the auriferous conglomerates, the investigated rocks could well be a potential source for some of the Witwatersrand gold. Inferred primary hornblende gabbro provides evidence for ascending water-rich melts that might have been particularly conducive for the transfer of Au into the crust. Gold enrichment in the Palaeo- to Mesoarchaean hinterland, similar to that found in younger active plate margins, might have been a major controlling factor for the unique extent of gold enrichment in the Witwatersrand Basin fill. ACKNOWLEDGEMENTS A. Gerdes is thanked for providing the infrastructure for the single zircon analyses. S. Govender assisted with the Nd isotope analyses, and H. Brätz and U. Schüßler helped with respectively the LA-ICPMS and XRF analyses. The Rb–Sr isotope analyses were kindly conducted by M. Bröcker. C. Anhaeusser, J. Barton and an anonymous reviewer provided constructive comments that helped to improve the original manuscript. Financial support by the Deutsche Forschungsgemeinschaft (DFG grant FR2183/3-1 and FR2183/3-2) is gratefully acknowledged. 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Published: Dec 1, 2009

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