TY - JOUR
AU1 - Di Vincenzo, G.
AU2 - Palmeri, R.
AU3 - Talarico, F.
AU4 - Andriessen, P.A.M.
AU5 - Ricci, G.A.
AB - Abstract The mafic eclogites of the Lanterman Range are the first record of a well-preserved high-pressure assemblage from the Pacific end of the Transantarctic Mountains. They occur among pods and lenses (from <1 to 30 m in size) of mafic and ultramafic metamorphic rocks that constitute a narrow zone intercalated with amphibolite-facies metasediments. This zone extends along the faulted contact between the Wilson Terrane and the Bowers Terrane, in northern Victoria Land. Most of the amphibolites and retrogressed eclogites analysed have geochemical compositions that resemble transitional to E-type mid-ocean ridge basalt (MORB) and Nd isotope data indicative of differentiation from a depleted mantle source. The age of the igneous precursors is not well determined, but Sm–Nd whole-rock data indicate a Neoproterozoic age, most probably around 700–750 Ma. For this group of metabasites a tectonic setting of an incipient ocean basin is proposed on geological and geochemical grounds. In contrast, the well-preserved eclogites are characterized by strong enrichment in more incompatible elements and pronounced negative Ta and Nb anomalies in MORB-normalized element patterns. Geochemical and Nd isotope data suggest that they are derived from a different mantle source with an enriched signature. The age of the protolith of the well-preserved eclogites, however, remains unconstrained. In the well-preserved eclogite samples the reaction textures testify to three main metamorphic states: (1) an eclogite facies stage, (2) a medium-pressure amphibolite facies stage, and (3) a low-pressure amphibolite facies stage. The high-pressure event occurred at temperatures of up to 850°C based on garnet and omphacite thermometry and at a minimum pressure of 15 kbar based on the jadeitic content of omphacites. Internal Sm–Nd isochrons from two well-preserved eclogites are 500±5 Ma (rutile, clinopyroxene, amphibole, whole rock and garnet) and 492±3 Ma (rutile, clinopyroxene, whole rock and garnet). Rutile–whole-rock 238U-206Pb ages ( 500 Ma) overlap the range of the Sm–Nd mineral ages. The inferred P–T path, the microtextural features and the overlap of the Sm–Nd garnet ages with the range of the 238U-206Pb rutile–whole-rock ages indicate fast cooling and suggest that the time of the high-pressure event was 500 Ma. These data place both the formation and exhumation of eclogite within a convergent plate margin setting, thus documenting the subduction–accretional nature of the early Palaeozoic Ross Orogen in northern Victoria Land. Introduction Metabasites with eclogite assemblages can occur as layers and lenses of variable size within medium- to high-grade metasediments. These rocks are of particular interest as they may reveal the sites of ancient subduction zones, and therefore provide information for geodynamic reconstructions. For kinetic and equilibrium reasons, mafic rocks tend to preserve high-pressure assemblages relative to non-mafic rocks (Koons & Thompson, 1985). This could be one of the reasons for the long-lasting controversy about the different tectonic interpretations of mafic eclogites intercalated in medium- to high-grade metasediments (see Smith, 1988), and in particular about whether the host-metasediments were present with the mafic rocks during the high-pressure event (the in situ eclogite model) or whether the mafic eclogites were tectonically emplaced in lower-pressure continental rocks (the foreign eclogite model). In spite of metamorphic and tectonic reworking during the high-pressure event and subsequent retrogression, most mafic eclogites also retain the geochemical signature of the protolith, thus providing information on the tectonic framework preceding the high-pressure stage. Nevertheless, the contribution of mafic eclogites to any plate tectonic reconstruction is complete only when the timing of the eclogite facies metamorphism is constrained. Different isotope systematics have been used for this purpose, and these have been reviewed by Vidal & Hunziker, (1985) and Gebauer, (1990). In the last decade, many papers have demonstrated that the Sm–Nd dating technique is one of the most feasible methods, as garnet is an abundant mineral phase in eclogites. However, Sm–Nd garnet ages appear to be strongly dependent on the cooling history. Thus in slowly cooled terranes the question arises as to whether the ages represent mineral growth or subsequent cooling (Mezger et al., 1992; Burton et al., 1995). In contrast, in eclogites that experienced very fast tectonic burial and uplift (Schmädicke et al., 1995), or in low-temperature eclogites (Thöni & Jagoutz, 1992), the lack of complete isotope homogenization during the metamorphism can yield spurious mineral ages. In some medium-temperature eclogites, the overlap of ages derived from different isotopic systems, with very different blocking temperatures, is strong evidence for fast cooling (Kalt et al., 1994; Chavagnac & Jahn, 1996), thus making the closure temperature of a specific isotopic system of secondary importance (Schmädicke et al., 1995). The purpose of this paper is twofold. One is to present the first available mineralogical, geochemical, petrological and geochronological data on newly discovered well-preserved eclogites and associated amphibolites from the Lanterman Range. The Lanterman Range is at the Pacific end of the Transantarctic Mountains, and these rocks may therefore place significant constraints on the reconstruction of the thermotectonic history of the Antarctic palaeo-Pacific margin of Gondwana. The second objective is to address, in a broad perspective, the problems inherent in dating high-pressure mafic rocks. Geological Setting Three tectonometamorphic terranes are recognized in northern Victoria Land (Fig. 1-Bradshaw & Laird, 1983): (1) the Robertson Bay Terrane, made up of a thick flysch-type sequence of Cambrian–lower Ordovician sedimentary rocks, (2) the Bowers Terrane, a Cambrian oceanic volcanic arc and related sediments (Weaver et al., 1984), and (3) the Wilson Terrane, comprising low- to high-grade metamorphic rocks extensively intruded by the Granite Harbour Intrusives, a calc-alkaline association with magmatic arc affinity and of Cambro-Ordovician age [Armienti et al., (1990) and references therein]. During the Early Palaeozoic Ross Orogeny, both the Bowers and Robertson Bay rocks experienced low-grade metamorphism (Buggisch & Kleinschmidt, 1989), whereas a much more complex metamorphic pattern is evident in the Wilson Terrane. In the Deep Freeze Range (Fig. 1), the Wilson Terrane comprises a metasedimentary sequence (including remnants of a polymetamorphic granulite complex—Castelli et al., 1991) which records a low- to high-grade metamorphism of low-pressure type (Grew et al., 1984; Talarico et al., 1992) with counter-clockwise P–T paths (Palmeri et al., 1994). In the Lanterman, Salamander and Mountaineer ranges (Fig. 1), an intermediate-pressure belt is characterized by scattered kyanite in metapelites (Grew et al., 1984; Ricci & Tessensohn, 1997b). At the boundary with the allochthonous Bowers Terrane, a chain of pods and lenses of mafic and/or ultramafic rocks occurs. This chain includes metabasites with eclogitic assemblages (Fig. 1-Ricci et al., 1996). Fig. 1. Open in new tabDownload slide (a) and (b), outcrop area (in grey) and main localities of northern Victoria Land and Central Transantarctic Mountains. RBT, Robertson Bay Terrane; BT, Bowers Terrane; WT, Wilson Terrane; LR, Lanterman Range; SR, Salamander Range; DR, Dessent Ridge; DFR, Deep Freeze Range; MR, Mountaineer Range. In black are indicated the main outcrop localities of mafic and/or ultramafic rocks. (c) Geological map of the Lanterman Range (after Roland et al., 1984) and sample locations. The Lanterman Range and the eclogite occurrence at Husky Pass The metamorphic sequence of the Wilson Terrane in the Lanterman Range comprises a wide variety of pelitic to quartzo-feldspathic gneisses, and minor calc-silicates, amphibolites and metamorphosed ultramafic rocks (Roland et al., 1984; Kleinschmidt et al., 1987). Large granitic to tonalitic plutons are apparently restricted to the western side of the range (Fig. 1) and have been assigned to the Granite Harbour Intrusives (Roland et al., 1984; Kreuzer et al., 1987). The metamorphic pattern of the Lanterman Range is mainly due to a major event that occurred under amphibolite facies conditions (Roland et al., 1984; Kleinschmidt et al., 1987). Pelitic gneisses that show incipient migmatization are confined to the northern part of the range (Mt Bernstein and Carnes Crags—Fig. 1). The Husky Pass eclogites (Ricci et al., 1996) occur as lenses of centimetric to metric thickness in the gneisses at the eastern margin of the Wilson Terrane (Fig. 1), ∼2 km from the tectonic contact with the Bowers Terrane. The mafic lenses show prominent zonation with increasing degrees of retrogression of the eclogite assemblage from the centre to the edge. Ultramafic rocks (mainly serpentinites) occur locally as levels of decimetric thickness within the retrogressed eclogites. The host-rocks are mainly augen quartzo-feldspathic and pelitic gneisses and minor garnet-bearing quartzites. The occurrence of thin lenses of quartzites and quartzo-feldspathic gneisses in the internal portions of the thickest mafic bodies may indicate primary intrusive relationships. In addition, thin layers of serpentinites in some mafic lenses may indicate local preservation of primary igneous layering. The main foliation in both gneisses and the mafic–ultramafic lenses is deformed by isoclinal folds with axial surfaces parallel to the Ross regional trend (Capponi et al., 1995). Petrography and Mineral Chemistry Eclogites The eclogites are fine- to medium-grained rocks with grano-nematoblastic textures. Three main metamorphic stages have been recognized: (1) an eclogite facies stage, (2) a medium-pressure amphibolite facies stage, and (3) a low-pressure amphibolite facies stage. The eclogitic mineral assemblage (M1) consists of omphacite (CpxI), garnet and rutile, together with accessory quartz, apatite and zircon. Phengite and ilmenite are rarely present. Omphacite occurs as aligned elongated crystals that impart a strong compositional layering and a penetrative lineation to the eclogite. Garnets occur as small subidioblastic grains, usually 0.3 mm in size; larger grains (up to ∼0.6 mm) are rare. Small inclusions of quartz, rutile, zircon and rare omphacite are enclosed in some garnets. The first retrogressive event affecting the eclogite (medium-pressure amphibolite facies stage—M2 assemblage) induced the breakdown of CpxI to a cryptocrystalline symplectitic intergrowth of an Na-poor clinopyroxene (CpxII) and albite-rich plagioclase. During this stage, garnet is still stable, rutile is usually associated with ilmenite and there is the first appearance of amphibole. Amphibole forms either isolated crystals in apparent equilibrium with the eclogite paragenesis or poikiloblasts on garnet and omphacite. The final stage of retrogression is characterized by extensive development of amphibole (low-pressure amphibolite facies stage—M3 assemblage). The symplectitic intergrowth of M2 assemblage was replaced by a micro- to medium-grained symplectite of amphibole + plagioclase, and garnet was rimmed by kelyphitic amphibole + epidote±Fe oxides±plagioclase. Mineral chemistry was performed on three well-preserved eclogites and selected microprobe analyses are given in Table 1. Garnet compositions show the following ranges: almandine 36–54 mol %, pyrope 25–44 mol %, grossular 12–22 mol %, spessartine 1–2 mol % and andradite 0–6 mol %. In the pyrope–(almandine + spessartine)–grossular ternary diagram (Fig. 2a), garnets mainly plot within the group B eclogite field of Coleman et al., (1965), whereas the outermost rim compositions plot in the C field. All grains show a complementary zoning pattern for Fe and Mg, and very slight zoning for Ca, whereas Mn is nearly constant. In the ∼0.3 mm sized garnets, Mg and, to a lesser degree, Ca generally decrease from the core to rim, whereas Fe shows an inverse trend (Fig. 3). The larger grains (up to ∼0.6 mm) are characterized by more complex patterns (Fig. 3) that are interpreted, on the basis of compositional maps, as the result of the coalescence of small garnet nuclei (coinciding with the highest Mg contents) growing during the eclogite climax and followed by later overgrowth around the coalesced centre. CpxI are unzoned omphacites (Fig. 2b) with a jadeite component of 30–42 mol %, acmite 1–14 mol % and augite 56–57 mol %. The correlation between acmite and jadeite contents is irregular and indicates different Fe3+/Fe2+ ratios at different points in the same grain. CpxII of the M2 assemblage is a sodic augite (Fig. 2b) with a jadeite component of 6–10 mol % and acmite 11–16 mol %. The first amphibole formed during early retrogression (M2 assemblage) is a barroisite (Leake, 1978) characterized by high AlIV and very low Ti contents (Table 1). Samples that experienced extensive retrogression show CpxII + albite–oligoclase symplectite replaced by large poikiloblasts of amphibole (M3 assemblage). The latter is barroisite in the core region, and tchermakitic-hornblende towards the rim (Table 1). Fig. 2. Open in new tabDownload slide Garnet (a) and pyroxene (b) compositions plotted after Coleman et al., (1965) and Essene & Fyfe, (1967), respectively. Pyroxene end-members are augite (Aug), jadeite (Jd) and acmite (Acm). Garnet end-members are almandine + spessartine (Alm + Sps), grossular (Grs) and pyrope (Prp). A, B and C fields represent the compositional variations of garnets in eclogites associated with kimberlites and peridotites (A), with gneisses (B) and with blueschists (C). Open symbols represent rim compositions. In the garnet diagram (a), square symbols refer to well-preserved eclogites and circles to retrogressed eclogite. Open circles in the pyroxene diagram (b) represent sodic augite in symplectite. Fig. 3. Open in new tabDownload slide Atomic profiles across two garnets of different size of a well-preserved eclogite (sample TC13). Table 1: Representative microprobe analyses of well-preserved eclogites ( TC13, TC16 and CP21), retrograded eclogite (CP28) and host-gneisses (CP37 and TC2) Garnet . . . Rock type: . Well-preserved eclogites . . . Sample no.: . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC16 . . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt2 . Grt2 . Grt2 . Grt1 . . rim . p3 . p8 . p11 . core . p22 . p24 . rim . core . p14 . core . SiO2 40.22 40.05 39.73 40.19 39.51 40.50 39.97 39.58 39.87 38.77 39.83 TiO2 – – – – – – – – – – – Al2O3 22.45 22.30 22.39 22.63 22.33 22.78 22.25 22.27 22.51 21.85 22.49 FeOt 19.92 19.05 21.77 18.50 21.59 19.24 22.05 23.20 19.76 23.17 20.89 MnO 0.72 0.70 0.65 0.69 0.67 0.55 0.83 0.70 0.69 0.67 0.61 MgO 10.94 11.55 9.08 10.41 9.79 11.72 9.99 8.61 9.99 8.45 10.40 CaO 6.51 6.82 7.34 8.30 6.81 6.46 6.43 6.63 7.63 6.56 6.44 Total 100.76 100.48 100.96 100.72 100.70 101.26 101.52 100.99 100.45 99.47 100.66 Si 3.00 2.99 2.99 3.00 2.98 2.99 2.99 3.00 2.99 2.98 2.99 Al 1.97 1.96 1.99 1.99 1.98 1.98 1.96 1.99 1.99 1.98 1.99 Ti – – – – – – – – – – – Fe3+ 0.03 0.06 0.03 0.01 0.06 0.04 0.06 0.01 0.03 0.06 0.03 Fe2+ 1.21 1.13 1.34 1.14 1.30 1.15 1.32 1.45 1.21 1.43 1.28 Mn 0.05 0.04 0.04 0.04 0.04 0.03 0.05 0.04 0.04 0.04 0.04 Mg 1.22 1.28 1.02 1.16 1.10 1.29 1.11 0.97 1.12 0.97 1.16 Ca 0.52 0.54 0.59 0.66 0.55 0.51 0.51 0.53 0.61 0.54 0.51 Total 8.00 8.00 8.00 8.00 8.01 8.00 8.00 8.00 7.99 8.00 8.00 Garnet . . . Rock type: . Well-preserved eclogites . . . Sample no.: . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC16 . . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt2 . Grt2 . Grt2 . Grt1 . . rim . p3 . p8 . p11 . core . p22 . p24 . rim . core . p14 . core . SiO2 40.22 40.05 39.73 40.19 39.51 40.50 39.97 39.58 39.87 38.77 39.83 TiO2 – – – – – – – – – – – Al2O3 22.45 22.30 22.39 22.63 22.33 22.78 22.25 22.27 22.51 21.85 22.49 FeOt 19.92 19.05 21.77 18.50 21.59 19.24 22.05 23.20 19.76 23.17 20.89 MnO 0.72 0.70 0.65 0.69 0.67 0.55 0.83 0.70 0.69 0.67 0.61 MgO 10.94 11.55 9.08 10.41 9.79 11.72 9.99 8.61 9.99 8.45 10.40 CaO 6.51 6.82 7.34 8.30 6.81 6.46 6.43 6.63 7.63 6.56 6.44 Total 100.76 100.48 100.96 100.72 100.70 101.26 101.52 100.99 100.45 99.47 100.66 Si 3.00 2.99 2.99 3.00 2.98 2.99 2.99 3.00 2.99 2.98 2.99 Al 1.97 1.96 1.99 1.99 1.98 1.98 1.96 1.99 1.99 1.98 1.99 Ti – – – – – – – – – – – Fe3+ 0.03 0.06 0.03 0.01 0.06 0.04 0.06 0.01 0.03 0.06 0.03 Fe2+ 1.21 1.13 1.34 1.14 1.30 1.15 1.32 1.45 1.21 1.43 1.28 Mn 0.05 0.04 0.04 0.04 0.04 0.03 0.05 0.04 0.04 0.04 0.04 Mg 1.22 1.28 1.02 1.16 1.10 1.29 1.11 0.97 1.12 0.97 1.16 Ca 0.52 0.54 0.59 0.66 0.55 0.51 0.51 0.53 0.61 0.54 0.51 Total 8.00 8.00 8.00 8.00 8.01 8.00 8.00 8.00 7.99 8.00 8.00 Garnet . . . Rock type: . Well-preserved eclogites . Retrogressed eclogites . . . . Sample no.: . TC16 . TC16 . TC16 . CP21 . CP21 . CP28 . CP28 . CP28 . CP28 . . Grt1 . Grt2 . Grt2 . Grt . Grt . Grt1 . Grt1 . Grt2 . Grt2 . . rim . core . rim . rim . core . rim . core . rim . core . SiO2 39.55 39.89 39.55 39.34 39.46 38.39 38.88 38.14 39.44 TiO2 – – – – – – 0.24 – – Al2O3 22.39 22.03 21.95 22.87 22.66 21.87 21.73 21.97 22.00 FeOt 22.79 20.26 22.34 22.76 22.17 24.32 22.33 26.03 22.08 MnO 0.51 0.58 0.64 0.64 0.74 2.38 2.28 1.78 2.05 MgO 9.07 10.87 9.49 7.87 9.29 5.31 6.87 5.46 8.08 CaO 6.33 6.62 6.40 7.89 6.95 8.03 8.39 7.51 7.07 Total 100.64 100.59 100.37 101.37 101.27 100.30 100.72 100.96 100.72 Si 2.99 2.99 3.00 2.97 2.96 2.98 2.98 2.95 3.00 Al 2.00 1.95 1.96 2.03 2.00 2.00 1.96 2.00 1.97 Ti – – – – – – 0.01 – – Fe3+ 0.02 0.07 0.04 0.00 0.03 0.03 0.06 0.10 0.03 Fe2+ 1.42 1.20 1.37 1.44 1.35 1.54 1.37 1.58 1.38 Mn 0.03 0.03 0.04 0.04 0.05 0.16 0.15 0.12 0.13 Mg 1.02 1.22 1.07 0.88 1.04 0.61 0.78 0.63 0.92 Ca 0.51 0.53 0.52 0.64 0.56 0.67 0.69 0.62 0.58 Total 8.00 8.00 8.00 8.00 7.99 7.99 8.00 8.00 8.00 Garnet . . . Rock type: . Well-preserved eclogites . Retrogressed eclogites . . . . Sample no.: . TC16 . TC16 . TC16 . CP21 . CP21 . CP28 . CP28 . CP28 . CP28 . . Grt1 . Grt2 . Grt2 . Grt . Grt . Grt1 . Grt1 . Grt2 . Grt2 . . rim . core . rim . rim . core . rim . core . rim . core . SiO2 39.55 39.89 39.55 39.34 39.46 38.39 38.88 38.14 39.44 TiO2 – – – – – – 0.24 – – Al2O3 22.39 22.03 21.95 22.87 22.66 21.87 21.73 21.97 22.00 FeOt 22.79 20.26 22.34 22.76 22.17 24.32 22.33 26.03 22.08 MnO 0.51 0.58 0.64 0.64 0.74 2.38 2.28 1.78 2.05 MgO 9.07 10.87 9.49 7.87 9.29 5.31 6.87 5.46 8.08 CaO 6.33 6.62 6.40 7.89 6.95 8.03 8.39 7.51 7.07 Total 100.64 100.59 100.37 101.37 101.27 100.30 100.72 100.96 100.72 Si 2.99 2.99 3.00 2.97 2.96 2.98 2.98 2.95 3.00 Al 2.00 1.95 1.96 2.03 2.00 2.00 1.96 2.00 1.97 Ti – – – – – – 0.01 – – Fe3+ 0.02 0.07 0.04 0.00 0.03 0.03 0.06 0.10 0.03 Fe2+ 1.42 1.20 1.37 1.44 1.35 1.54 1.37 1.58 1.38 Mn 0.03 0.03 0.04 0.04 0.05 0.16 0.15 0.12 0.13 Mg 1.02 1.22 1.07 0.88 1.04 0.61 0.78 0.63 0.92 Ca 0.51 0.53 0.52 0.64 0.56 0.67 0.69 0.62 0.58 Total 8.00 8.00 8.00 8.00 7.99 7.99 8.00 8.00 8.00 Garnet . Clinopyroxene . . . . . Rock type: . Host-gneisses . Rock type: . Well-preserved eclogites . . . . . Sample no.: . CP37 . CP37 . CP37 . TC2 . TC2 . Sample no.: . TC13 . TC13 . TC13 . . Grt1 . Grt1 . Grt1 . Grt . Grt . . Cpx1 . Cpx2 . Cpx3 . . rim . core . p22 . rim . core . . rim . core . . SiO2 37.34 37.84 37.06 38.44 38.45 SiO2 53.74 54.78 55.0 TiO2 – – – – – TiO2 – – – Al2O3 21.25 21.33 21.20 22.30 22.24 Al2O3 10.31 9.17 10.20 FeOt 25.32 23.46 24.27 23.80 23.72 Cr2O3 – – – MnO 12.85 13.82 12.58 0.27 0.41 FeOt 6.44 6.56 5.52 MgO 3.15 3.12 3.40 3.74 3.74 MnO – – – CaO 0.91 1.47 1.16 12.20 12.00 MgO 8.60 8.53 8.84 Total 100.82 101.05 99.67 100.75 100.56 CaO 14.07 14.29 14.33 Na2O 5.73 5.89 5.91 Si 2.98 3.00 2.99 2.97 2.98 K2O – – – Al 2.00 2.00 2.01 2.03 2.03 Total 98.89 99.22 99.85 Ti – – – – – Fe3+ 0.03 0.00 0.01 0.02 0.01 Si 1.95 1.99 1.97 Fe2+ 1.66 1.56 1.62 1.52 1.53 AlIV 0.05 0.01 0.03 Mn 0.87 0.93 0.86 0.02 0.03 AlVI 0.39 0.38 0.40 Mg 0.37 0.37 0.41 0.43 0.42 Ti – – – Ca 0.08 0.12 0.10 1.01 1.00 Cr – – – Total 7.99 7.98 8.00 8.00 8.00 Fe3+ 0.06 0.04 0.03 Fe2+ 0.13 0.16 0.14 Mn – – – Mg 0.46 0.46 0.47 Ca 0.55 0.55 0.55 Na 0.40 0.41 0.41 K – – – Total 4.00 4.00 4.00 Garnet . Clinopyroxene . . . . . Rock type: . Host-gneisses . Rock type: . Well-preserved eclogites . . . . . Sample no.: . CP37 . CP37 . CP37 . TC2 . TC2 . Sample no.: . TC13 . TC13 . TC13 . . Grt1 . Grt1 . Grt1 . Grt . Grt . . Cpx1 . Cpx2 . Cpx3 . . rim . core . p22 . rim . core . . rim . core . . SiO2 37.34 37.84 37.06 38.44 38.45 SiO2 53.74 54.78 55.0 TiO2 – – – – – TiO2 – – – Al2O3 21.25 21.33 21.20 22.30 22.24 Al2O3 10.31 9.17 10.20 FeOt 25.32 23.46 24.27 23.80 23.72 Cr2O3 – – – MnO 12.85 13.82 12.58 0.27 0.41 FeOt 6.44 6.56 5.52 MgO 3.15 3.12 3.40 3.74 3.74 MnO – – – CaO 0.91 1.47 1.16 12.20 12.00 MgO 8.60 8.53 8.84 Total 100.82 101.05 99.67 100.75 100.56 CaO 14.07 14.29 14.33 Na2O 5.73 5.89 5.91 Si 2.98 3.00 2.99 2.97 2.98 K2O – – – Al 2.00 2.00 2.01 2.03 2.03 Total 98.89 99.22 99.85 Ti – – – – – Fe3+ 0.03 0.00 0.01 0.02 0.01 Si 1.95 1.99 1.97 Fe2+ 1.66 1.56 1.62 1.52 1.53 AlIV 0.05 0.01 0.03 Mn 0.87 0.93 0.86 0.02 0.03 AlVI 0.39 0.38 0.40 Mg 0.37 0.37 0.41 0.43 0.42 Ti – – – Ca 0.08 0.12 0.10 1.01 1.00 Cr – – – Total 7.99 7.98 8.00 8.00 8.00 Fe3+ 0.06 0.04 0.03 Fe2+ 0.13 0.16 0.14 Mn – – – Mg 0.46 0.46 0.47 Ca 0.55 0.55 0.55 Na 0.40 0.41 0.41 K – – – Total 4.00 4.00 4.00 Clinopyroxene . . Amphibole . . . . . Rock type: . Well-preserved eclogites . Retrogressed eclogite . Well-preserveds . . . . . Sample no.: . TC13 . TC16 . TC16 . TC16 . TC16 . CP21 . CP28 . TC13 . TC13 . TC16 . . Cpx . Cpx1 . Cpx1 . Cpx2 . Cpx2 . Cpx . Cpx1 . Cam . Cam . Cam . . symplectite . rim . core . rim . core . core . matrix . core . rim . rim . SiO2 50.10 54.60 54.65 54.12 53.88 54.37 51.84 45.85 45.43 45.72 TiO2 0.24 – – – – 0.20 – 0.37 0.33 0.20 Al2O3 7.23 8.92 9.20 8.87 9.02 8.85 1.15 13.25 15.47 15.33 Cr2O3 – – – – – – – – 0.09 – FeOt 10.23 7.09 7.82 7.04 7.45 7.77 11.82 11.19 11.75 11.67 MnO – – – – – – – – – – MgO 11.33 8.86 8.38 9.14 8.42 8.71 11.39 12.95 11.78 11.46 CaO 17.83 13.96 13.95 13.90 13.35 14.91 22.99 8.52 8.72 8.62 Na2O 2.35 5.84 5.85 5.66 6.29 5.57 0.36 3.35 3.22 3.38 K2O – – – – – – – 0.36 0.21 0.20 Total 99.31 99.27 99.85 98.73 98.41 100.38 99.85 95.84 97.00 96.58 Si 1.87 1.97 1.97 1.97 1.96 1.96 1.97 6.66 6.46 6.55 AlIV 0.13 0.03 0.03 0.03 0.04 0.04 0.03 1.34 1.54 1.45 AlVI 0.19 0.35 0.36 0.35 0.35 0.34 0.02 0.88 1.06 1.14 Ti 0.01 – – – – 0.01 – 0.04 0.04 0.02 Cr – – – – – – – – 0.01 – Fe3+ 0.10 0.07 0.07 0.08 0.13 0.07 0.01 0.79 0.81 0.65 Fe2+ 0.19 0.14 0.16 0.13 0.10 0.14 0.36 0.54 0.59 0.74 Mn – – – – – – – – – – Mg 0.63 0.48 0.45 0.50 0.46 0.47 0.64 2.74 2.50 2.45 Ca 0.71 0.54 0.54 0.54 0.52 0.58 0.94 1.30 1.33 1.32 Na 0.17 0.41 0.41 0.40 0.44 0.39 0.03 0.92 0.89 0.94 K – – – – – – – 0.07 0.04 0.04 Total 4.00 3.99 3.99 4.00 4.00 4.00 4.00 15.29 15.26 15.30 Clinopyroxene . . Amphibole . . . . . Rock type: . Well-preserved eclogites . Retrogressed eclogite . Well-preserveds . . . . . Sample no.: . TC13 . TC16 . TC16 . TC16 . TC16 . CP21 . CP28 . TC13 . TC13 . TC16 . . Cpx . Cpx1 . Cpx1 . Cpx2 . Cpx2 . Cpx . Cpx1 . Cam . Cam . Cam . . symplectite . rim . core . rim . core . core . matrix . core . rim . rim . SiO2 50.10 54.60 54.65 54.12 53.88 54.37 51.84 45.85 45.43 45.72 TiO2 0.24 – – – – 0.20 – 0.37 0.33 0.20 Al2O3 7.23 8.92 9.20 8.87 9.02 8.85 1.15 13.25 15.47 15.33 Cr2O3 – – – – – – – – 0.09 – FeOt 10.23 7.09 7.82 7.04 7.45 7.77 11.82 11.19 11.75 11.67 MnO – – – – – – – – – – MgO 11.33 8.86 8.38 9.14 8.42 8.71 11.39 12.95 11.78 11.46 CaO 17.83 13.96 13.95 13.90 13.35 14.91 22.99 8.52 8.72 8.62 Na2O 2.35 5.84 5.85 5.66 6.29 5.57 0.36 3.35 3.22 3.38 K2O – – – – – – – 0.36 0.21 0.20 Total 99.31 99.27 99.85 98.73 98.41 100.38 99.85 95.84 97.00 96.58 Si 1.87 1.97 1.97 1.97 1.96 1.96 1.97 6.66 6.46 6.55 AlIV 0.13 0.03 0.03 0.03 0.04 0.04 0.03 1.34 1.54 1.45 AlVI 0.19 0.35 0.36 0.35 0.35 0.34 0.02 0.88 1.06 1.14 Ti 0.01 – – – – 0.01 – 0.04 0.04 0.02 Cr – – – – – – – – 0.01 – Fe3+ 0.10 0.07 0.07 0.08 0.13 0.07 0.01 0.79 0.81 0.65 Fe2+ 0.19 0.14 0.16 0.13 0.10 0.14 0.36 0.54 0.59 0.74 Mn – – – – – – – – – – Mg 0.63 0.48 0.45 0.50 0.46 0.47 0.64 2.74 2.50 2.45 Ca 0.71 0.54 0.54 0.54 0.52 0.58 0.94 1.30 1.33 1.32 Na 0.17 0.41 0.41 0.40 0.44 0.39 0.03 0.92 0.89 0.94 K – – – – – – – 0.07 0.04 0.04 Total 4.00 3.99 3.99 4.00 4.00 4.00 4.00 15.29 15.26 15.30 Amphibole . Biotite . . . . Rock type: . Well-preserved eclogites . Retrogressed eclogites . Host-gneisses . . . . . Sample no.: . TC16 . CP21 . CP28 . CP28 . CP28 . CP37 . CP37 . CP37 . TC2 . TC2 . . Cam . Cam . Cam3 . Cam1 . Cam2 . Bt1 . Bt3 . Bt4 . Bt1 . Bt2 . . core . core . matrix . kelyphite . kelyphite . . matrix . matrix . matrix . matrix . SiO2 46.48 46.37 42.42 42.85 42.40 36.11 35.97 36.30 37.39 37.56 TiO2 0.37 0.40 0.72 1.36 1.13 2.58 2.66 2.93 2.29 2.37 Al2O3 13.95 12.59 14.29 13.16 13.66 19.87 19.68 19.60 17.71 17.69 Cr2O3 – – – 0.13 0.05 – – – – – FeOt 10.85 14.88 17.12 17.28 16.74 17.22 17.93 17.28 16.76 16.56 MnO – 0.07 0.30 0.30 0.19 0.39 0.46 0.46 – – MgO 13.61 11.83 9.12 9.27 9.40 10.44 10.03 9.82 12.78 12.88 CaO 8.35 8.51 11.70 11.60 11.83 – – – – – Na2O 3.96 3.39 1.39 1.45 1.33 0.13 0.13 0.11 0.07 – K2O 0.42 0.10 0.95 0.87 0.87 10.07 10.00 9.97 9.27 9.16 Total 97.99 98.14 98.01 98.27 97.61 96.81 96.98 96.56 96.24 96.22 Si 6.53 6.67 6.27 6.33 6.29 5.36 5.36 5.41 5.54 5.56 AlIV 1.47 1.33 1.73 1.67 1.71 2.64 2.64 2.59 2.46 2.44 AlVI 0.84 0.81 0.76 0.62 0.68 0.84 0.82 0.85 0.63 0.64 Ti 0.04 0.04 0.08 0.15 0.13 0.29 0.30 0.33 0.25 0.26 Cr – – – 0.01 0.01 – – – – – Fe3+ 0.89 0.84 0.53 0.48 0.45 – – – – – Fe2+ 0.39 0.76 1.58 1.65 1.63 2.14 2.23 2.15 2.08 2.05 Mn – 0.01 0.04 0.04 0.02 0.05 0.06 0.06 – – Mg 2.85 2.54 2.01 2.04 2.08 2.31 2.23 2.18 2.82 2.84 Ca 1.26 1.31 1.85 1.84 1.88 – – – – – Na 1.08 0.95 0.40 0.41 0.38 0.04 0.04 0.03 0.02 – K 0.08 0.02 0.18 0.16 0.16 1.91 1.90 1.89 1.74 1.73 Total 15.41 15.28 15.43 15.41 15.43 15.58 15.58 15.49 15.54 15.52 Amphibole . Biotite . . . . Rock type: . Well-preserved eclogites . Retrogressed eclogites . Host-gneisses . . . . . Sample no.: . TC16 . CP21 . CP28 . CP28 . CP28 . CP37 . CP37 . CP37 . TC2 . TC2 . . Cam . Cam . Cam3 . Cam1 . Cam2 . Bt1 . Bt3 . Bt4 . Bt1 . Bt2 . . core . core . matrix . kelyphite . kelyphite . . matrix . matrix . matrix . matrix . SiO2 46.48 46.37 42.42 42.85 42.40 36.11 35.97 36.30 37.39 37.56 TiO2 0.37 0.40 0.72 1.36 1.13 2.58 2.66 2.93 2.29 2.37 Al2O3 13.95 12.59 14.29 13.16 13.66 19.87 19.68 19.60 17.71 17.69 Cr2O3 – – – 0.13 0.05 – – – – – FeOt 10.85 14.88 17.12 17.28 16.74 17.22 17.93 17.28 16.76 16.56 MnO – 0.07 0.30 0.30 0.19 0.39 0.46 0.46 – – MgO 13.61 11.83 9.12 9.27 9.40 10.44 10.03 9.82 12.78 12.88 CaO 8.35 8.51 11.70 11.60 11.83 – – – – – Na2O 3.96 3.39 1.39 1.45 1.33 0.13 0.13 0.11 0.07 – K2O 0.42 0.10 0.95 0.87 0.87 10.07 10.00 9.97 9.27 9.16 Total 97.99 98.14 98.01 98.27 97.61 96.81 96.98 96.56 96.24 96.22 Si 6.53 6.67 6.27 6.33 6.29 5.36 5.36 5.41 5.54 5.56 AlIV 1.47 1.33 1.73 1.67 1.71 2.64 2.64 2.59 2.46 2.44 AlVI 0.84 0.81 0.76 0.62 0.68 0.84 0.82 0.85 0.63 0.64 Ti 0.04 0.04 0.08 0.15 0.13 0.29 0.30 0.33 0.25 0.26 Cr – – – 0.01 0.01 – – – – – Fe3+ 0.89 0.84 0.53 0.48 0.45 – – – – – Fe2+ 0.39 0.76 1.58 1.65 1.63 2.14 2.23 2.15 2.08 2.05 Mn – 0.01 0.04 0.04 0.02 0.05 0.06 0.06 – – Mg 2.85 2.54 2.01 2.04 2.08 2.31 2.23 2.18 2.82 2.84 Ca 1.26 1.31 1.85 1.84 1.88 – – – – – Na 1.08 0.95 0.40 0.41 0.38 0.04 0.04 0.03 0.02 – K 0.08 0.02 0.18 0.16 0.16 1.91 1.90 1.89 1.74 1.73 Total 15.41 15.28 15.43 15.41 15.43 15.58 15.58 15.49 15.54 15.52 Muscovite . Plagioclase . . . . . Rock type: . Host-gneisses . Rock type: . Host-gneisses . . . . . Sample no.: . CP37 . CP37 . TC2 . TC2 . Sample no.: . CP37 . CP37 . CP37 . TC2 . TC2 . . Ms1 . Ms2 . Ms1 . Ms2 . . Pl1 . Pl1 . Pl2 . Pl . Pl . . . matrix . matrix . matrix . . rim . core . core . rim . core . SiO2 45.89 46.21 50.53 49.35 SiO2 65.27 65.55 65.04 59.68 59.25 TiO2 0.75 0.81 0.83 0.68 Al2O3 22.99 22.49 22.57 25.15 25.70 Al2O3 36.13 36.60 27.98 29.29 CaO 2.57 2.25 2.64 6.22 7.20 Cr2O3 – – – – Na2O 9.55 9.76 9.35 8.77 7.97 FeOt 1.86 1.74 3.65 3.45 K2O 0.07 0.22 0.30 0.26 – MnO – – – – Total 100.45 100.27 99.90 100.13 100.16 MgO 0.96 0.86 2.92 2.33 CaO – – – – Si 2.85 2.87 2.86 2.66 2.64 Na2O 0.33 0.55 – – Al 1.18 1.16 1.17 1.32 1.35 K2O 9.53 9.79 10.49 10.73 Ca 0.12 0.10 0.12 0.30 0.34 Total 95.46 96.59 96.40 95.83 Na 0.81 0.83 0.79 0.76 0.69 K 0.01 0.01 0.02 0.01 – Si 6.07 6.05 6.70 6.60 Total 4.97 4.97 4.96 5.05 5.02 AlIV 2.36 2.35 1.30 1.40 AlVI 3.27 3.30 3.08 3.21 Ti 0.07 0.08 0.08 0.07 Cr – – – – Fe3+ – – – – Fe2+ 0.21 0.19 0.40 0.38 Mn – – – – Mg 0.19 0.17 0.58 0.46 Ca – – – – Na 0.08 0.14 – – K 1.61 1.64 1.77 1.83 Total 13.43 13.52 13.91 13.95 Muscovite . Plagioclase . . . . . Rock type: . Host-gneisses . Rock type: . Host-gneisses . . . . . Sample no.: . CP37 . CP37 . TC2 . TC2 . Sample no.: . CP37 . CP37 . CP37 . TC2 . TC2 . . Ms1 . Ms2 . Ms1 . Ms2 . . Pl1 . Pl1 . Pl2 . Pl . Pl . . . matrix . matrix . matrix . . rim . core . core . rim . core . SiO2 45.89 46.21 50.53 49.35 SiO2 65.27 65.55 65.04 59.68 59.25 TiO2 0.75 0.81 0.83 0.68 Al2O3 22.99 22.49 22.57 25.15 25.70 Al2O3 36.13 36.60 27.98 29.29 CaO 2.57 2.25 2.64 6.22 7.20 Cr2O3 – – – – Na2O 9.55 9.76 9.35 8.77 7.97 FeOt 1.86 1.74 3.65 3.45 K2O 0.07 0.22 0.30 0.26 – MnO – – – – Total 100.45 100.27 99.90 100.13 100.16 MgO 0.96 0.86 2.92 2.33 CaO – – – – Si 2.85 2.87 2.86 2.66 2.64 Na2O 0.33 0.55 – – Al 1.18 1.16 1.17 1.32 1.35 K2O 9.53 9.79 10.49 10.73 Ca 0.12 0.10 0.12 0.30 0.34 Total 95.46 96.59 96.40 95.83 Na 0.81 0.83 0.79 0.76 0.69 K 0.01 0.01 0.02 0.01 – Si 6.07 6.05 6.70 6.60 Total 4.97 4.97 4.96 5.05 5.02 AlIV 2.36 2.35 1.30 1.40 AlVI 3.27 3.30 3.08 3.21 Ti 0.07 0.08 0.08 0.07 Cr – – – – Fe3+ – – – – Fe2+ 0.21 0.19 0.40 0.38 Mn – – – – Mg 0.19 0.17 0.58 0.46 Ca – – – – Na 0.08 0.14 – – K 1.61 1.64 1.77 1.83 Total 13.43 13.52 13.91 13.95 Ferrous and ferric iron contents for garnet and clinopyroxene were calculated according to Ryburn et al. (1976). Ferrous and ferric iron contents for amphibole were determined by charge balance. Structural formulae were calculated on the basis of 12 oxygens for garnet, 6 for clinopyroxene, 13 for epidote, 22 for micas, 8 for plagioclase and 23 oxygens and 13 cations excluding Ca+Na+K for amphibole. Open in new tab Table 1: Representative microprobe analyses of well-preserved eclogites ( TC13, TC16 and CP21), retrograded eclogite (CP28) and host-gneisses (CP37 and TC2) Garnet . . . Rock type: . Well-preserved eclogites . . . Sample no.: . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC16 . . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt2 . Grt2 . Grt2 . Grt1 . . rim . p3 . p8 . p11 . core . p22 . p24 . rim . core . p14 . core . SiO2 40.22 40.05 39.73 40.19 39.51 40.50 39.97 39.58 39.87 38.77 39.83 TiO2 – – – – – – – – – – – Al2O3 22.45 22.30 22.39 22.63 22.33 22.78 22.25 22.27 22.51 21.85 22.49 FeOt 19.92 19.05 21.77 18.50 21.59 19.24 22.05 23.20 19.76 23.17 20.89 MnO 0.72 0.70 0.65 0.69 0.67 0.55 0.83 0.70 0.69 0.67 0.61 MgO 10.94 11.55 9.08 10.41 9.79 11.72 9.99 8.61 9.99 8.45 10.40 CaO 6.51 6.82 7.34 8.30 6.81 6.46 6.43 6.63 7.63 6.56 6.44 Total 100.76 100.48 100.96 100.72 100.70 101.26 101.52 100.99 100.45 99.47 100.66 Si 3.00 2.99 2.99 3.00 2.98 2.99 2.99 3.00 2.99 2.98 2.99 Al 1.97 1.96 1.99 1.99 1.98 1.98 1.96 1.99 1.99 1.98 1.99 Ti – – – – – – – – – – – Fe3+ 0.03 0.06 0.03 0.01 0.06 0.04 0.06 0.01 0.03 0.06 0.03 Fe2+ 1.21 1.13 1.34 1.14 1.30 1.15 1.32 1.45 1.21 1.43 1.28 Mn 0.05 0.04 0.04 0.04 0.04 0.03 0.05 0.04 0.04 0.04 0.04 Mg 1.22 1.28 1.02 1.16 1.10 1.29 1.11 0.97 1.12 0.97 1.16 Ca 0.52 0.54 0.59 0.66 0.55 0.51 0.51 0.53 0.61 0.54 0.51 Total 8.00 8.00 8.00 8.00 8.01 8.00 8.00 8.00 7.99 8.00 8.00 Garnet . . . Rock type: . Well-preserved eclogites . . . Sample no.: . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC13 . TC16 . . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt1 . Grt2 . Grt2 . Grt2 . Grt1 . . rim . p3 . p8 . p11 . core . p22 . p24 . rim . core . p14 . core . SiO2 40.22 40.05 39.73 40.19 39.51 40.50 39.97 39.58 39.87 38.77 39.83 TiO2 – – – – – – – – – – – Al2O3 22.45 22.30 22.39 22.63 22.33 22.78 22.25 22.27 22.51 21.85 22.49 FeOt 19.92 19.05 21.77 18.50 21.59 19.24 22.05 23.20 19.76 23.17 20.89 MnO 0.72 0.70 0.65 0.69 0.67 0.55 0.83 0.70 0.69 0.67 0.61 MgO 10.94 11.55 9.08 10.41 9.79 11.72 9.99 8.61 9.99 8.45 10.40 CaO 6.51 6.82 7.34 8.30 6.81 6.46 6.43 6.63 7.63 6.56 6.44 Total 100.76 100.48 100.96 100.72 100.70 101.26 101.52 100.99 100.45 99.47 100.66 Si 3.00 2.99 2.99 3.00 2.98 2.99 2.99 3.00 2.99 2.98 2.99 Al 1.97 1.96 1.99 1.99 1.98 1.98 1.96 1.99 1.99 1.98 1.99 Ti – – – – – – – – – – – Fe3+ 0.03 0.06 0.03 0.01 0.06 0.04 0.06 0.01 0.03 0.06 0.03 Fe2+ 1.21 1.13 1.34 1.14 1.30 1.15 1.32 1.45 1.21 1.43 1.28 Mn 0.05 0.04 0.04 0.04 0.04 0.03 0.05 0.04 0.04 0.04 0.04 Mg 1.22 1.28 1.02 1.16 1.10 1.29 1.11 0.97 1.12 0.97 1.16 Ca 0.52 0.54 0.59 0.66 0.55 0.51 0.51 0.53 0.61 0.54 0.51 Total 8.00 8.00 8.00 8.00 8.01 8.00 8.00 8.00 7.99 8.00 8.00 Garnet . . . Rock type: . Well-preserved eclogites . Retrogressed eclogites . . . . Sample no.: . TC16 . TC16 . TC16 . CP21 . CP21 . CP28 . CP28 . CP28 . CP28 . . Grt1 . Grt2 . Grt2 . Grt . Grt . Grt1 . Grt1 . Grt2 . Grt2 . . rim . core . rim . rim . core . rim . core . rim . core . SiO2 39.55 39.89 39.55 39.34 39.46 38.39 38.88 38.14 39.44 TiO2 – – – – – – 0.24 – – Al2O3 22.39 22.03 21.95 22.87 22.66 21.87 21.73 21.97 22.00 FeOt 22.79 20.26 22.34 22.76 22.17 24.32 22.33 26.03 22.08 MnO 0.51 0.58 0.64 0.64 0.74 2.38 2.28 1.78 2.05 MgO 9.07 10.87 9.49 7.87 9.29 5.31 6.87 5.46 8.08 CaO 6.33 6.62 6.40 7.89 6.95 8.03 8.39 7.51 7.07 Total 100.64 100.59 100.37 101.37 101.27 100.30 100.72 100.96 100.72 Si 2.99 2.99 3.00 2.97 2.96 2.98 2.98 2.95 3.00 Al 2.00 1.95 1.96 2.03 2.00 2.00 1.96 2.00 1.97 Ti – – – – – – 0.01 – – Fe3+ 0.02 0.07 0.04 0.00 0.03 0.03 0.06 0.10 0.03 Fe2+ 1.42 1.20 1.37 1.44 1.35 1.54 1.37 1.58 1.38 Mn 0.03 0.03 0.04 0.04 0.05 0.16 0.15 0.12 0.13 Mg 1.02 1.22 1.07 0.88 1.04 0.61 0.78 0.63 0.92 Ca 0.51 0.53 0.52 0.64 0.56 0.67 0.69 0.62 0.58 Total 8.00 8.00 8.00 8.00 7.99 7.99 8.00 8.00 8.00 Garnet . . . Rock type: . Well-preserved eclogites . Retrogressed eclogites . . . . Sample no.: . TC16 . TC16 . TC16 . CP21 . CP21 . CP28 . CP28 . CP28 . CP28 . . Grt1 . Grt2 . Grt2 . Grt . Grt . Grt1 . Grt1 . Grt2 . Grt2 . . rim . core . rim . rim . core . rim . core . rim . core . SiO2 39.55 39.89 39.55 39.34 39.46 38.39 38.88 38.14 39.44 TiO2 – – – – – – 0.24 – – Al2O3 22.39 22.03 21.95 22.87 22.66 21.87 21.73 21.97 22.00 FeOt 22.79 20.26 22.34 22.76 22.17 24.32 22.33 26.03 22.08 MnO 0.51 0.58 0.64 0.64 0.74 2.38 2.28 1.78 2.05 MgO 9.07 10.87 9.49 7.87 9.29 5.31 6.87 5.46 8.08 CaO 6.33 6.62 6.40 7.89 6.95 8.03 8.39 7.51 7.07 Total 100.64 100.59 100.37 101.37 101.27 100.30 100.72 100.96 100.72 Si 2.99 2.99 3.00 2.97 2.96 2.98 2.98 2.95 3.00 Al 2.00 1.95 1.96 2.03 2.00 2.00 1.96 2.00 1.97 Ti – – – – – – 0.01 – – Fe3+ 0.02 0.07 0.04 0.00 0.03 0.03 0.06 0.10 0.03 Fe2+ 1.42 1.20 1.37 1.44 1.35 1.54 1.37 1.58 1.38 Mn 0.03 0.03 0.04 0.04 0.05 0.16 0.15 0.12 0.13 Mg 1.02 1.22 1.07 0.88 1.04 0.61 0.78 0.63 0.92 Ca 0.51 0.53 0.52 0.64 0.56 0.67 0.69 0.62 0.58 Total 8.00 8.00 8.00 8.00 7.99 7.99 8.00 8.00 8.00 Garnet . Clinopyroxene . . . . . Rock type: . Host-gneisses . Rock type: . Well-preserved eclogites . . . . . Sample no.: . CP37 . CP37 . CP37 . TC2 . TC2 . Sample no.: . TC13 . TC13 . TC13 . . Grt1 . Grt1 . Grt1 . Grt . Grt . . Cpx1 . Cpx2 . Cpx3 . . rim . core . p22 . rim . core . . rim . core . . SiO2 37.34 37.84 37.06 38.44 38.45 SiO2 53.74 54.78 55.0 TiO2 – – – – – TiO2 – – – Al2O3 21.25 21.33 21.20 22.30 22.24 Al2O3 10.31 9.17 10.20 FeOt 25.32 23.46 24.27 23.80 23.72 Cr2O3 – – – MnO 12.85 13.82 12.58 0.27 0.41 FeOt 6.44 6.56 5.52 MgO 3.15 3.12 3.40 3.74 3.74 MnO – – – CaO 0.91 1.47 1.16 12.20 12.00 MgO 8.60 8.53 8.84 Total 100.82 101.05 99.67 100.75 100.56 CaO 14.07 14.29 14.33 Na2O 5.73 5.89 5.91 Si 2.98 3.00 2.99 2.97 2.98 K2O – – – Al 2.00 2.00 2.01 2.03 2.03 Total 98.89 99.22 99.85 Ti – – – – – Fe3+ 0.03 0.00 0.01 0.02 0.01 Si 1.95 1.99 1.97 Fe2+ 1.66 1.56 1.62 1.52 1.53 AlIV 0.05 0.01 0.03 Mn 0.87 0.93 0.86 0.02 0.03 AlVI 0.39 0.38 0.40 Mg 0.37 0.37 0.41 0.43 0.42 Ti – – – Ca 0.08 0.12 0.10 1.01 1.00 Cr – – – Total 7.99 7.98 8.00 8.00 8.00 Fe3+ 0.06 0.04 0.03 Fe2+ 0.13 0.16 0.14 Mn – – – Mg 0.46 0.46 0.47 Ca 0.55 0.55 0.55 Na 0.40 0.41 0.41 K – – – Total 4.00 4.00 4.00 Garnet . Clinopyroxene . . . . . Rock type: . Host-gneisses . Rock type: . Well-preserved eclogites . . . . . Sample no.: . CP37 . CP37 . CP37 . TC2 . TC2 . Sample no.: . TC13 . TC13 . TC13 . . Grt1 . Grt1 . Grt1 . Grt . Grt . . Cpx1 . Cpx2 . Cpx3 . . rim . core . p22 . rim . core . . rim . core . . SiO2 37.34 37.84 37.06 38.44 38.45 SiO2 53.74 54.78 55.0 TiO2 – – – – – TiO2 – – – Al2O3 21.25 21.33 21.20 22.30 22.24 Al2O3 10.31 9.17 10.20 FeOt 25.32 23.46 24.27 23.80 23.72 Cr2O3 – – – MnO 12.85 13.82 12.58 0.27 0.41 FeOt 6.44 6.56 5.52 MgO 3.15 3.12 3.40 3.74 3.74 MnO – – – CaO 0.91 1.47 1.16 12.20 12.00 MgO 8.60 8.53 8.84 Total 100.82 101.05 99.67 100.75 100.56 CaO 14.07 14.29 14.33 Na2O 5.73 5.89 5.91 Si 2.98 3.00 2.99 2.97 2.98 K2O – – – Al 2.00 2.00 2.01 2.03 2.03 Total 98.89 99.22 99.85 Ti – – – – – Fe3+ 0.03 0.00 0.01 0.02 0.01 Si 1.95 1.99 1.97 Fe2+ 1.66 1.56 1.62 1.52 1.53 AlIV 0.05 0.01 0.03 Mn 0.87 0.93 0.86 0.02 0.03 AlVI 0.39 0.38 0.40 Mg 0.37 0.37 0.41 0.43 0.42 Ti – – – Ca 0.08 0.12 0.10 1.01 1.00 Cr – – – Total 7.99 7.98 8.00 8.00 8.00 Fe3+ 0.06 0.04 0.03 Fe2+ 0.13 0.16 0.14 Mn – – – Mg 0.46 0.46 0.47 Ca 0.55 0.55 0.55 Na 0.40 0.41 0.41 K – – – Total 4.00 4.00 4.00 Clinopyroxene . . Amphibole . . . . . Rock type: . Well-preserved eclogites . Retrogressed eclogite . Well-preserveds . . . . . Sample no.: . TC13 . TC16 . TC16 . TC16 . TC16 . CP21 . CP28 . TC13 . TC13 . TC16 . . Cpx . Cpx1 . Cpx1 . Cpx2 . Cpx2 . Cpx . Cpx1 . Cam . Cam . Cam . . symplectite . rim . core . rim . core . core . matrix . core . rim . rim . SiO2 50.10 54.60 54.65 54.12 53.88 54.37 51.84 45.85 45.43 45.72 TiO2 0.24 – – – – 0.20 – 0.37 0.33 0.20 Al2O3 7.23 8.92 9.20 8.87 9.02 8.85 1.15 13.25 15.47 15.33 Cr2O3 – – – – – – – – 0.09 – FeOt 10.23 7.09 7.82 7.04 7.45 7.77 11.82 11.19 11.75 11.67 MnO – – – – – – – – – – MgO 11.33 8.86 8.38 9.14 8.42 8.71 11.39 12.95 11.78 11.46 CaO 17.83 13.96 13.95 13.90 13.35 14.91 22.99 8.52 8.72 8.62 Na2O 2.35 5.84 5.85 5.66 6.29 5.57 0.36 3.35 3.22 3.38 K2O – – – – – – – 0.36 0.21 0.20 Total 99.31 99.27 99.85 98.73 98.41 100.38 99.85 95.84 97.00 96.58 Si 1.87 1.97 1.97 1.97 1.96 1.96 1.97 6.66 6.46 6.55 AlIV 0.13 0.03 0.03 0.03 0.04 0.04 0.03 1.34 1.54 1.45 AlVI 0.19 0.35 0.36 0.35 0.35 0.34 0.02 0.88 1.06 1.14 Ti 0.01 – – – – 0.01 – 0.04 0.04 0.02 Cr – – – – – – – – 0.01 – Fe3+ 0.10 0.07 0.07 0.08 0.13 0.07 0.01 0.79 0.81 0.65 Fe2+ 0.19 0.14 0.16 0.13 0.10 0.14 0.36 0.54 0.59 0.74 Mn – – – – – – – – – – Mg 0.63 0.48 0.45 0.50 0.46 0.47 0.64 2.74 2.50 2.45 Ca 0.71 0.54 0.54 0.54 0.52 0.58 0.94 1.30 1.33 1.32 Na 0.17 0.41 0.41 0.40 0.44 0.39 0.03 0.92 0.89 0.94 K – – – – – – – 0.07 0.04 0.04 Total 4.00 3.99 3.99 4.00 4.00 4.00 4.00 15.29 15.26 15.30 Clinopyroxene . . Amphibole . . . . . Rock type: . Well-preserved eclogites . Retrogressed eclogite . Well-preserveds . . . . . Sample no.: . TC13 . TC16 . TC16 . TC16 . TC16 . CP21 . CP28 . TC13 . TC13 . TC16 . . Cpx . Cpx1 . Cpx1 . Cpx2 . Cpx2 . Cpx . Cpx1 . Cam . Cam . Cam . . symplectite . rim . core . rim . core . core . matrix . core . rim . rim . SiO2 50.10 54.60 54.65 54.12 53.88 54.37 51.84 45.85 45.43 45.72 TiO2 0.24 – – – – 0.20 – 0.37 0.33 0.20 Al2O3 7.23 8.92 9.20 8.87 9.02 8.85 1.15 13.25 15.47 15.33 Cr2O3 – – – – – – – – 0.09 – FeOt 10.23 7.09 7.82 7.04 7.45 7.77 11.82 11.19 11.75 11.67 MnO – – – – – – – – – – MgO 11.33 8.86 8.38 9.14 8.42 8.71 11.39 12.95 11.78 11.46 CaO 17.83 13.96 13.95 13.90 13.35 14.91 22.99 8.52 8.72 8.62 Na2O 2.35 5.84 5.85 5.66 6.29 5.57 0.36 3.35 3.22 3.38 K2O – – – – – – – 0.36 0.21 0.20 Total 99.31 99.27 99.85 98.73 98.41 100.38 99.85 95.84 97.00 96.58 Si 1.87 1.97 1.97 1.97 1.96 1.96 1.97 6.66 6.46 6.55 AlIV 0.13 0.03 0.03 0.03 0.04 0.04 0.03 1.34 1.54 1.45 AlVI 0.19 0.35 0.36 0.35 0.35 0.34 0.02 0.88 1.06 1.14 Ti 0.01 – – – – 0.01 – 0.04 0.04 0.02 Cr – – – – – – – – 0.01 – Fe3+ 0.10 0.07 0.07 0.08 0.13 0.07 0.01 0.79 0.81 0.65 Fe2+ 0.19 0.14 0.16 0.13 0.10 0.14 0.36 0.54 0.59 0.74 Mn – – – – – – – – – – Mg 0.63 0.48 0.45 0.50 0.46 0.47 0.64 2.74 2.50 2.45 Ca 0.71 0.54 0.54 0.54 0.52 0.58 0.94 1.30 1.33 1.32 Na 0.17 0.41 0.41 0.40 0.44 0.39 0.03 0.92 0.89 0.94 K – – – – – – – 0.07 0.04 0.04 Total 4.00 3.99 3.99 4.00 4.00 4.00 4.00 15.29 15.26 15.30 Amphibole . Biotite . . . . Rock type: . Well-preserved eclogites . Retrogressed eclogites . Host-gneisses . . . . . Sample no.: . TC16 . CP21 . CP28 . CP28 . CP28 . CP37 . CP37 . CP37 . TC2 . TC2 . . Cam . Cam . Cam3 . Cam1 . Cam2 . Bt1 . Bt3 . Bt4 . Bt1 . Bt2 . . core . core . matrix . kelyphite . kelyphite . . matrix . matrix . matrix . matrix . SiO2 46.48 46.37 42.42 42.85 42.40 36.11 35.97 36.30 37.39 37.56 TiO2 0.37 0.40 0.72 1.36 1.13 2.58 2.66 2.93 2.29 2.37 Al2O3 13.95 12.59 14.29 13.16 13.66 19.87 19.68 19.60 17.71 17.69 Cr2O3 – – – 0.13 0.05 – – – – – FeOt 10.85 14.88 17.12 17.28 16.74 17.22 17.93 17.28 16.76 16.56 MnO – 0.07 0.30 0.30 0.19 0.39 0.46 0.46 – – MgO 13.61 11.83 9.12 9.27 9.40 10.44 10.03 9.82 12.78 12.88 CaO 8.35 8.51 11.70 11.60 11.83 – – – – – Na2O 3.96 3.39 1.39 1.45 1.33 0.13 0.13 0.11 0.07 – K2O 0.42 0.10 0.95 0.87 0.87 10.07 10.00 9.97 9.27 9.16 Total 97.99 98.14 98.01 98.27 97.61 96.81 96.98 96.56 96.24 96.22 Si 6.53 6.67 6.27 6.33 6.29 5.36 5.36 5.41 5.54 5.56 AlIV 1.47 1.33 1.73 1.67 1.71 2.64 2.64 2.59 2.46 2.44 AlVI 0.84 0.81 0.76 0.62 0.68 0.84 0.82 0.85 0.63 0.64 Ti 0.04 0.04 0.08 0.15 0.13 0.29 0.30 0.33 0.25 0.26 Cr – – – 0.01 0.01 – – – – – Fe3+ 0.89 0.84 0.53 0.48 0.45 – – – – – Fe2+ 0.39 0.76 1.58 1.65 1.63 2.14 2.23 2.15 2.08 2.05 Mn – 0.01 0.04 0.04 0.02 0.05 0.06 0.06 – – Mg 2.85 2.54 2.01 2.04 2.08 2.31 2.23 2.18 2.82 2.84 Ca 1.26 1.31 1.85 1.84 1.88 – – – – – Na 1.08 0.95 0.40 0.41 0.38 0.04 0.04 0.03 0.02 – K 0.08 0.02 0.18 0.16 0.16 1.91 1.90 1.89 1.74 1.73 Total 15.41 15.28 15.43 15.41 15.43 15.58 15.58 15.49 15.54 15.52 Amphibole . Biotite . . . . Rock type: . Well-preserved eclogites . Retrogressed eclogites . Host-gneisses . . . . . Sample no.: . TC16 . CP21 . CP28 . CP28 . CP28 . CP37 . CP37 . CP37 . TC2 . TC2 . . Cam . Cam . Cam3 . Cam1 . Cam2 . Bt1 . Bt3 . Bt4 . Bt1 . Bt2 . . core . core . matrix . kelyphite . kelyphite . . matrix . matrix . matrix . matrix . SiO2 46.48 46.37 42.42 42.85 42.40 36.11 35.97 36.30 37.39 37.56 TiO2 0.37 0.40 0.72 1.36 1.13 2.58 2.66 2.93 2.29 2.37 Al2O3 13.95 12.59 14.29 13.16 13.66 19.87 19.68 19.60 17.71 17.69 Cr2O3 – – – 0.13 0.05 – – – – – FeOt 10.85 14.88 17.12 17.28 16.74 17.22 17.93 17.28 16.76 16.56 MnO – 0.07 0.30 0.30 0.19 0.39 0.46 0.46 – – MgO 13.61 11.83 9.12 9.27 9.40 10.44 10.03 9.82 12.78 12.88 CaO 8.35 8.51 11.70 11.60 11.83 – – – – – Na2O 3.96 3.39 1.39 1.45 1.33 0.13 0.13 0.11 0.07 – K2O 0.42 0.10 0.95 0.87 0.87 10.07 10.00 9.97 9.27 9.16 Total 97.99 98.14 98.01 98.27 97.61 96.81 96.98 96.56 96.24 96.22 Si 6.53 6.67 6.27 6.33 6.29 5.36 5.36 5.41 5.54 5.56 AlIV 1.47 1.33 1.73 1.67 1.71 2.64 2.64 2.59 2.46 2.44 AlVI 0.84 0.81 0.76 0.62 0.68 0.84 0.82 0.85 0.63 0.64 Ti 0.04 0.04 0.08 0.15 0.13 0.29 0.30 0.33 0.25 0.26 Cr – – – 0.01 0.01 – – – – – Fe3+ 0.89 0.84 0.53 0.48 0.45 – – – – – Fe2+ 0.39 0.76 1.58 1.65 1.63 2.14 2.23 2.15 2.08 2.05 Mn – 0.01 0.04 0.04 0.02 0.05 0.06 0.06 – – Mg 2.85 2.54 2.01 2.04 2.08 2.31 2.23 2.18 2.82 2.84 Ca 1.26 1.31 1.85 1.84 1.88 – – – – – Na 1.08 0.95 0.40 0.41 0.38 0.04 0.04 0.03 0.02 – K 0.08 0.02 0.18 0.16 0.16 1.91 1.90 1.89 1.74 1.73 Total 15.41 15.28 15.43 15.41 15.43 15.58 15.58 15.49 15.54 15.52 Muscovite . Plagioclase . . . . . Rock type: . Host-gneisses . Rock type: . Host-gneisses . . . . . Sample no.: . CP37 . CP37 . TC2 . TC2 . Sample no.: . CP37 . CP37 . CP37 . TC2 . TC2 . . Ms1 . Ms2 . Ms1 . Ms2 . . Pl1 . Pl1 . Pl2 . Pl . Pl . . . matrix . matrix . matrix . . rim . core . core . rim . core . SiO2 45.89 46.21 50.53 49.35 SiO2 65.27 65.55 65.04 59.68 59.25 TiO2 0.75 0.81 0.83 0.68 Al2O3 22.99 22.49 22.57 25.15 25.70 Al2O3 36.13 36.60 27.98 29.29 CaO 2.57 2.25 2.64 6.22 7.20 Cr2O3 – – – – Na2O 9.55 9.76 9.35 8.77 7.97 FeOt 1.86 1.74 3.65 3.45 K2O 0.07 0.22 0.30 0.26 – MnO – – – – Total 100.45 100.27 99.90 100.13 100.16 MgO 0.96 0.86 2.92 2.33 CaO – – – – Si 2.85 2.87 2.86 2.66 2.64 Na2O 0.33 0.55 – – Al 1.18 1.16 1.17 1.32 1.35 K2O 9.53 9.79 10.49 10.73 Ca 0.12 0.10 0.12 0.30 0.34 Total 95.46 96.59 96.40 95.83 Na 0.81 0.83 0.79 0.76 0.69 K 0.01 0.01 0.02 0.01 – Si 6.07 6.05 6.70 6.60 Total 4.97 4.97 4.96 5.05 5.02 AlIV 2.36 2.35 1.30 1.40 AlVI 3.27 3.30 3.08 3.21 Ti 0.07 0.08 0.08 0.07 Cr – – – – Fe3+ – – – – Fe2+ 0.21 0.19 0.40 0.38 Mn – – – – Mg 0.19 0.17 0.58 0.46 Ca – – – – Na 0.08 0.14 – – K 1.61 1.64 1.77 1.83 Total 13.43 13.52 13.91 13.95 Muscovite . Plagioclase . . . . . Rock type: . Host-gneisses . Rock type: . Host-gneisses . . . . . Sample no.: . CP37 . CP37 . TC2 . TC2 . Sample no.: . CP37 . CP37 . CP37 . TC2 . TC2 . . Ms1 . Ms2 . Ms1 . Ms2 . . Pl1 . Pl1 . Pl2 . Pl . Pl . . . matrix . matrix . matrix . . rim . core . core . rim . core . SiO2 45.89 46.21 50.53 49.35 SiO2 65.27 65.55 65.04 59.68 59.25 TiO2 0.75 0.81 0.83 0.68 Al2O3 22.99 22.49 22.57 25.15 25.70 Al2O3 36.13 36.60 27.98 29.29 CaO 2.57 2.25 2.64 6.22 7.20 Cr2O3 – – – – Na2O 9.55 9.76 9.35 8.77 7.97 FeOt 1.86 1.74 3.65 3.45 K2O 0.07 0.22 0.30 0.26 – MnO – – – – Total 100.45 100.27 99.90 100.13 100.16 MgO 0.96 0.86 2.92 2.33 CaO – – – – Si 2.85 2.87 2.86 2.66 2.64 Na2O 0.33 0.55 – – Al 1.18 1.16 1.17 1.32 1.35 K2O 9.53 9.79 10.49 10.73 Ca 0.12 0.10 0.12 0.30 0.34 Total 95.46 96.59 96.40 95.83 Na 0.81 0.83 0.79 0.76 0.69 K 0.01 0.01 0.02 0.01 – Si 6.07 6.05 6.70 6.60 Total 4.97 4.97 4.96 5.05 5.02 AlIV 2.36 2.35 1.30 1.40 AlVI 3.27 3.30 3.08 3.21 Ti 0.07 0.08 0.08 0.07 Cr – – – – Fe3+ – – – – Fe2+ 0.21 0.19 0.40 0.38 Mn – – – – Mg 0.19 0.17 0.58 0.46 Ca – – – – Na 0.08 0.14 – – K 1.61 1.64 1.77 1.83 Total 13.43 13.52 13.91 13.95 Ferrous and ferric iron contents for garnet and clinopyroxene were calculated according to Ryburn et al. (1976). Ferrous and ferric iron contents for amphibole were determined by charge balance. Structural formulae were calculated on the basis of 12 oxygens for garnet, 6 for clinopyroxene, 13 for epidote, 22 for micas, 8 for plagioclase and 23 oxygens and 13 cations excluding Ca+Na+K for amphibole. Open in new tab Amphibolites They are fine to medium grained with a grano-nematoblastic texture. The mineral assemblage consists of hornblende, epidote, plagioclase and quartz, with titanite, ilmenite and rare rutile as accessory minerals. Garnet, clinopyroxene and cummingtonite are present in some samples. Garnet contains quartz and rutile inclusions and always shows resorbed edges surrounded by medium-grained amphibole±plagioclase±epidote kelyphite. Clinopyroxene together with plagioclase forms granoblastic sites surrounded by nematoblastic amphibole. These features, already observed in the M3 assemblage of eclogites, strongly suggest that garnet and clinopyroxene are relics of a high-pressure assemblage. Cummingtonite occurs as nematoblasts that grew together with hornblende in millimetric layers, sometimes forming poikiloblasts with inclusions of green amphibole. Rutile is generally enclosed in garnet and it and ilmenite are rimmed by titanite. Mineral chemistry was determined on an amphibolite sample with relics of the high-pressure stage (sample CP28—Table 1). Garnet consists of 45–55 mol % of almandine, 16–27 mol % of pyrope, 18–24 mol % of grossular, 4–9 mol % of spessartine and 0–5 mol % of andradite. It shows retrograde zoning with higher Mg and lower Fe contents in the core than in the rim, whereas Ca and Mn are nearly constant. Clinopyroxene is a salite with low acmite and jadeite components (∼1 mol %). Plagioclase ranges in composition from anorthite 48 mol % in the kelyphite to anorthite 24 mol % where it is associated with clinopyroxene. Amphibole is unzoned and ranges in composition from tschermakite hornblende to Mg-hornblende (Leake, 1978) (Table 1). Host-rocks The rocks hosting the metabasites are medium-grained gneisses with minor quartzites and are characterized by a grano-lepidoblastic texture consisting of quartz, plagioclase, biotite, muscovite, ±K-feldspar, ±epidote, ±amphibole, ±garnet, ±fibrolitic sillimanite. Accessory minerals include zircon, apatite, ilmenite, allanite and rutile. In the pelitic gneisses, garnet is euhedral, up to 3 mm in size, and generally shows inclusion-rich cores and inclusion-free rims. The inclusions consist of quartz, plagioclase, biotite, muscovite and ilmenite, which also constitute the matrix of the rocks. In the quartzo-feldspathic rocks and quartzites, garnet (up to 0.2 mm in size) is sub-euhedral to anhedral and is surrounded by fine-grained symplectite consisting of plagioclase + biotite + quartz. Similar symplectites are also found around relict phengite. Sillimanite shows wisp-like needles associated with biotite and quartz, and sometimes resembles ‘disthene-sillimanite’ (Marchand, 1974). Clear relict kyanite, however, has only been found in plagioclase (B. Ghiribelli, personal communication, 1996). Microprobe analyses (Table 1) were performed on garnet, biotite, muscovite and plagioclase from a pelitic gneiss (sample CP37) and a quartzo-feldspathic gneiss (sample TC2). In the pelitic gneiss, garnet is spessartine-rich almandine with slight zoning characterized by a higher spessartine and grossular component and lower almandine component in the core with respect to the rim (Table 1). In the quartzo-feldspathic gneiss, garnet is homogeneous spessartine-poor almandine with a high grossular component (Table 1). Biotite is compositionally homogeneous with XMg=0.50 in pelitic gneiss and 0.57 in the quartzo-feldspathic gneiss. Muscovite is characterized by low phengite and paragonite contents [Si up to 3.2–3.3 atoms on 11 oxygens and Na/(Na + K) up to 0.08]. Plagioclase is homogeneous and ranges in composition from anorthite 13 mol % in the pelitic gneiss to anorthite 30 mol % in the quartzo-feldspathic sample. Thermobarometry Mafic rocks Eclogite facies stage Temperatures were estimated using different geothermometer calibrations based on Fe–Mg exchange reactions between garnet and clinopyroxene (see Table 2). The thermometric data of the eclogitic stage were calculated using garnet and omphacite in well-preserved microtextural sites, where the absence of retrograde phases suggested that the eclogite paragenesis recorded the climax conditions. Moreover, the fact that the KD determined between garnet and omphacite varied in a narrow range in a single sample (Table 2), is indicative of chemical equilibrium in the sites where thermobarometry was evaluated. For garnet, core compositions of smaller crystals or the inner rim of larger garnets were used; for omphacites low acmite content omphacites were utilized. Table 2: Equilibrium temperatures ( °C) for well-preserved eclogites based on the Fe–Mg distribution in coexisting garnet–omphacite pairs ( P=16 kbar), using the calibrations of Ellis & Green (1979; EG); Krogh (1988; K); Ai Yang (1994; AI); Powell (1985; P) Sample . XMg . XMg . XCa . mg-no. . ln KD . EG . K . AI . P . . Grt . Cpx . Grt . Grt . . (°C) . (°C) . (°C) . (°C) . TC13 Grt1p24–Cpx1rim 0.46 0.78 0.14 45.68 1.44 817 739 702 796 Grt1p22–Cpx1rim 0.53 0.78 0.15 52.87 1.15 930 874 822 914 Grt1p11–Cpx1rim 0.50 0.78 0.22 50.43 1.25 956 940 867 943 Grt2rim–Cpx3 0.40 0.77 0.17 40.08 1.61 790 730 689 770 Grt2core–Cpx3 0.48 0.77 0.19 48.07 1.29 915 881 821 900 TC16 Grt1rim–Cpx1rim 0.42 0.77 0.16 41.80 1.56 795 729 690 775 Grt1core–Cpx1core 0.48 0.74 0.16 47.54 1.13 941 889 852 926 Grt2rim–Cpx2rim 0.44 0.79 0.15 43.85 1.13 779 706 664 757 Grt2core–Cpx2 0.50 0.79 0.14 50.41 1.33 854 782 734 835 CP21 Grtrim–Cpxrim 0.38 0.77 0.21 37.93 1.70 799 758 707 780 Grtcore–Cpxcore 0.44 0.77 0.17 43.51 1.47 834 779 733 815 Sample . XMg . XMg . XCa . mg-no. . ln KD . EG . K . AI . P . . Grt . Cpx . Grt . Grt . . (°C) . (°C) . (°C) . (°C) . TC13 Grt1p24–Cpx1rim 0.46 0.78 0.14 45.68 1.44 817 739 702 796 Grt1p22–Cpx1rim 0.53 0.78 0.15 52.87 1.15 930 874 822 914 Grt1p11–Cpx1rim 0.50 0.78 0.22 50.43 1.25 956 940 867 943 Grt2rim–Cpx3 0.40 0.77 0.17 40.08 1.61 790 730 689 770 Grt2core–Cpx3 0.48 0.77 0.19 48.07 1.29 915 881 821 900 TC16 Grt1rim–Cpx1rim 0.42 0.77 0.16 41.80 1.56 795 729 690 775 Grt1core–Cpx1core 0.48 0.74 0.16 47.54 1.13 941 889 852 926 Grt2rim–Cpx2rim 0.44 0.79 0.15 43.85 1.13 779 706 664 757 Grt2core–Cpx2 0.50 0.79 0.14 50.41 1.33 854 782 734 835 CP21 Grtrim–Cpxrim 0.38 0.77 0.21 37.93 1.70 799 758 707 780 Grtcore–Cpxcore 0.44 0.77 0.17 43.51 1.47 834 779 733 815 Mineral analyses are from Table 1. Open in new tab Table 2: Equilibrium temperatures ( °C) for well-preserved eclogites based on the Fe–Mg distribution in coexisting garnet–omphacite pairs ( P=16 kbar), using the calibrations of Ellis & Green (1979; EG); Krogh (1988; K); Ai Yang (1994; AI); Powell (1985; P) Sample . XMg . XMg . XCa . mg-no. . ln KD . EG . K . AI . P . . Grt . Cpx . Grt . Grt . . (°C) . (°C) . (°C) . (°C) . TC13 Grt1p24–Cpx1rim 0.46 0.78 0.14 45.68 1.44 817 739 702 796 Grt1p22–Cpx1rim 0.53 0.78 0.15 52.87 1.15 930 874 822 914 Grt1p11–Cpx1rim 0.50 0.78 0.22 50.43 1.25 956 940 867 943 Grt2rim–Cpx3 0.40 0.77 0.17 40.08 1.61 790 730 689 770 Grt2core–Cpx3 0.48 0.77 0.19 48.07 1.29 915 881 821 900 TC16 Grt1rim–Cpx1rim 0.42 0.77 0.16 41.80 1.56 795 729 690 775 Grt1core–Cpx1core 0.48 0.74 0.16 47.54 1.13 941 889 852 926 Grt2rim–Cpx2rim 0.44 0.79 0.15 43.85 1.13 779 706 664 757 Grt2core–Cpx2 0.50 0.79 0.14 50.41 1.33 854 782 734 835 CP21 Grtrim–Cpxrim 0.38 0.77 0.21 37.93 1.70 799 758 707 780 Grtcore–Cpxcore 0.44 0.77 0.17 43.51 1.47 834 779 733 815 Sample . XMg . XMg . XCa . mg-no. . ln KD . EG . K . AI . P . . Grt . Cpx . Grt . Grt . . (°C) . (°C) . (°C) . (°C) . TC13 Grt1p24–Cpx1rim 0.46 0.78 0.14 45.68 1.44 817 739 702 796 Grt1p22–Cpx1rim 0.53 0.78 0.15 52.87 1.15 930 874 822 914 Grt1p11–Cpx1rim 0.50 0.78 0.22 50.43 1.25 956 940 867 943 Grt2rim–Cpx3 0.40 0.77 0.17 40.08 1.61 790 730 689 770 Grt2core–Cpx3 0.48 0.77 0.19 48.07 1.29 915 881 821 900 TC16 Grt1rim–Cpx1rim 0.42 0.77 0.16 41.80 1.56 795 729 690 775 Grt1core–Cpx1core 0.48 0.74 0.16 47.54 1.13 941 889 852 926 Grt2rim–Cpx2rim 0.44 0.79 0.15 43.85 1.13 779 706 664 757 Grt2core–Cpx2 0.50 0.79 0.14 50.41 1.33 854 782 734 835 CP21 Grtrim–Cpxrim 0.38 0.77 0.21 37.93 1.70 799 758 707 780 Grtcore–Cpxcore 0.44 0.77 0.17 43.51 1.47 834 779 733 815 Mineral analyses are from Table 1. Open in new tab Temperatures estimated using the Ai Yang, (1994) calibration (Table 2) were in the range 720–850°C for smaller garnets and around 840°C for larger garnets. An important observation was that the temperatures obtained in the smallest garnets overlapped those in the largest ones. Minimum pressure estimations on the basis of the jadeitic content of omphacite in the presence of quartz (Holland, 1983) were ∼15 kbar at 800°C. Medium-pressure amphibolite facies stage Retrograde zoning in garnets suggests that rim compositions of garnet and omphacite in mutual contact, even in sites where eclogite assemblage is well preserved, can be used to constrain early retrograde conditions. Temperatures in the range 630–750°C were obtained in both small and large garnets [calibration of Ai Yang, (1992), Table 2]. Pressures were evaluated using the jadeitic content of clinopyroxene in symplectite (Holland, 1983) and by the plagioclase–clinopyroxene–garnet–quartz barometer (Newton & Perkins, 1982) using symplectite phases and garnet rim. However, the barometer could only be used in a few microstructural sites, which yielded pressures of 10 kbar at 700°C (Table 3). Pressures evaluated through the jadeitic content of CpxII at 700°C were 5–6 kbar, but the low jadeitic content of CpxII yields less precise and semiquantitative pressure estimations (Godard, 1988) compared with the eclogitic stage pressure. Table 3: Geothermobarometric data of the host-gneisses and of the retrogressed stages of eclogites Sample . Xpyr . Xgrs . Xphl . Xan . Si4+ . XK . XAlVI . XMg . XMg . T (°C) . T (°C) . T (°C) . T (°C) . T (°C) . P . P . P . P . P . . . . . . . . . . . . . . . . (kbar) . (kbar) . (kbar) . kbar) . (kbar) . . Grt . Grt . Bt . Pl . Ms . Ms . Ms . Hbl . Cpx . P . FS . IM . GS . GP . NP . P . NH . GS . MS . Host-gneisses CP37 Grtcore–Bt3 0.12 0.04 0.40 – – – – – – 622 693 790 – – – – – – – Grtcore–Bt3–Pl1core–Ms2 0.12 0.04 0.40 0.11 3.02 0.92 0.88 – – – – – 690 – – – – 6.9 – Grtcore–Bt4 0.12 0.04 0.40 – – – – – – 619 687 771 – – – – – – – Grtcore–Bt4–Pl2core–Ms2 0.12 0.04 0.40 0.13 3.02 0.92 0.88 – – – – – 682 – – – – 6.3 – Grtp22–Bt4 0.14 0.03 0.40 – – – – – – 627 716 771 – – – – – – – Grtp22–Bt4–Pl1core–Ms2 0.14 0.03 0.40 0.11 3.02 0.92 0.88 – – – – – 710 – – – – 6.7 – Grt1rim–Bt1 0.12 0.03 0.41 – – – – – – 584 636 708 – – – – – – – Grt1rim–Bt1–Pl1rim–Ms1 0.12 0.03 0.41 0.13 3.03 0.95 0.87 – – – – – 628 – – – – 4.9 – Grtcore–Pl1core 0.12 0.03 – 0.11 – – – – – – – – – – – 10 9.5 – – Grtcore–Pl2core 0.12 0.04 – 0.13 – – – – – – – – – – – 9 8.4 – – Grtp22–Pl1core 0.14 0.03 – 0.11 – – – – – – – – – – – 12 12 – – Grtrim–Pl1rim 0.12 0.03 – 0.13 – – – – – – – – – – – 9.9 9.9 – – TC2 Grtcore–Bt1 0.14 0.34 0.49 – – – – – – 730 631 745 – – – – – – – Grtcore–Bt1–Plcore–Ms1 0.14 0.34 0.49 0.33 3.3 1.00 0.74 – – – – – 638 – – – – 9.4 10 Grtrim–Bt2 0.14 0.34 0.49 – – – – – – – – – – – – – – – Grtrim–Bt2–Plrim–Ms2 0.14 0.34 0.49 0.28 3.3 1.00 0.78 – – 733 634 721 630 – – – – 10 10 Retrogressed eclogite CP28 Grt1rim–Cam1 0.20 0.21 – – – – – 0.55 – – – – – 730 – – – – – Grt2rim–Cam2 0.21 0.16 – – – – – 0.56 – – – – – 674 – – – – – Well–preserved eclogite TC13 Grt2rim–Cpx–Pl 0.32 0.17 – 0.22 – – – – 0.63 – – – – – 10 – – – – Grt2rim–Cpx–Pl 0.33 0.16 – 0.22 – – – – 0.63 – – – – – 10 – – – – Sample . Xpyr . Xgrs . Xphl . Xan . Si4+ . XK . XAlVI . XMg . XMg . T (°C) . T (°C) . T (°C) . T (°C) . T (°C) . P . P . P . P . P . . . . . . . . . . . . . . . . (kbar) . (kbar) . (kbar) . kbar) . (kbar) . . Grt . Grt . Bt . Pl . Ms . Ms . Ms . Hbl . Cpx . P . FS . IM . GS . GP . NP . P . NH . GS . MS . Host-gneisses CP37 Grtcore–Bt3 0.12 0.04 0.40 – – – – – – 622 693 790 – – – – – – – Grtcore–Bt3–Pl1core–Ms2 0.12 0.04 0.40 0.11 3.02 0.92 0.88 – – – – – 690 – – – – 6.9 – Grtcore–Bt4 0.12 0.04 0.40 – – – – – – 619 687 771 – – – – – – – Grtcore–Bt4–Pl2core–Ms2 0.12 0.04 0.40 0.13 3.02 0.92 0.88 – – – – – 682 – – – – 6.3 – Grtp22–Bt4 0.14 0.03 0.40 – – – – – – 627 716 771 – – – – – – – Grtp22–Bt4–Pl1core–Ms2 0.14 0.03 0.40 0.11 3.02 0.92 0.88 – – – – – 710 – – – – 6.7 – Grt1rim–Bt1 0.12 0.03 0.41 – – – – – – 584 636 708 – – – – – – – Grt1rim–Bt1–Pl1rim–Ms1 0.12 0.03 0.41 0.13 3.03 0.95 0.87 – – – – – 628 – – – – 4.9 – Grtcore–Pl1core 0.12 0.03 – 0.11 – – – – – – – – – – – 10 9.5 – – Grtcore–Pl2core 0.12 0.04 – 0.13 – – – – – – – – – – – 9 8.4 – – Grtp22–Pl1core 0.14 0.03 – 0.11 – – – – – – – – – – – 12 12 – – Grtrim–Pl1rim 0.12 0.03 – 0.13 – – – – – – – – – – – 9.9 9.9 – – TC2 Grtcore–Bt1 0.14 0.34 0.49 – – – – – – 730 631 745 – – – – – – – Grtcore–Bt1–Plcore–Ms1 0.14 0.34 0.49 0.33 3.3 1.00 0.74 – – – – – 638 – – – – 9.4 10 Grtrim–Bt2 0.14 0.34 0.49 – – – – – – – – – – – – – – – Grtrim–Bt2–Plrim–Ms2 0.14 0.34 0.49 0.28 3.3 1.00 0.78 – – 733 634 721 630 – – – – 10 10 Retrogressed eclogite CP28 Grt1rim–Cam1 0.20 0.21 – – – – – 0.55 – – – – – 730 – – – – – Grt2rim–Cam2 0.21 0.16 – – – – – 0.56 – – – – – 674 – – – – – Well–preserved eclogite TC13 Grt2rim–Cpx–Pl 0.32 0.17 – 0.22 – – – – 0.63 – – – – – 10 – – – – Grt2rim–Cpx–Pl 0.33 0.16 – 0.22 – – – – 0.63 – – – – – 10 – – – – Garnet–biotite thermometer: P, Perchuck et al. (1985); FS, Ferry & Spear (1978); IM, Indares & Martignole (1985). Garnet–plagioclase–muscovite–biotite therm-obarometer: GS, Ghent & Stout (1981). Garnet–plagioclase–sillimanite–quartz barometer: NH, Newton & Haselton (1981); P, Perchuk et al. (1985). Phengite barometry: MS, Massonne & Schreyer (1987). Garnet–plagioclase–clinopyroxene–quartz barometer on symplectites: NP, Newton & Perkins (1982). Garnet–hornblende thermometer: GP, Graham & Powell (1984). Analyses used in the calculation are reported in Table 1. Open in new tab Table 3: Geothermobarometric data of the host-gneisses and of the retrogressed stages of eclogites Sample . Xpyr . Xgrs . Xphl . Xan . Si4+ . XK . XAlVI . XMg . XMg . T (°C) . T (°C) . T (°C) . T (°C) . T (°C) . P . P . P . P . P . . . . . . . . . . . . . . . . (kbar) . (kbar) . (kbar) . kbar) . (kbar) . . Grt . Grt . Bt . Pl . Ms . Ms . Ms . Hbl . Cpx . P . FS . IM . GS . GP . NP . P . NH . GS . MS . Host-gneisses CP37 Grtcore–Bt3 0.12 0.04 0.40 – – – – – – 622 693 790 – – – – – – – Grtcore–Bt3–Pl1core–Ms2 0.12 0.04 0.40 0.11 3.02 0.92 0.88 – – – – – 690 – – – – 6.9 – Grtcore–Bt4 0.12 0.04 0.40 – – – – – – 619 687 771 – – – – – – – Grtcore–Bt4–Pl2core–Ms2 0.12 0.04 0.40 0.13 3.02 0.92 0.88 – – – – – 682 – – – – 6.3 – Grtp22–Bt4 0.14 0.03 0.40 – – – – – – 627 716 771 – – – – – – – Grtp22–Bt4–Pl1core–Ms2 0.14 0.03 0.40 0.11 3.02 0.92 0.88 – – – – – 710 – – – – 6.7 – Grt1rim–Bt1 0.12 0.03 0.41 – – – – – – 584 636 708 – – – – – – – Grt1rim–Bt1–Pl1rim–Ms1 0.12 0.03 0.41 0.13 3.03 0.95 0.87 – – – – – 628 – – – – 4.9 – Grtcore–Pl1core 0.12 0.03 – 0.11 – – – – – – – – – – – 10 9.5 – – Grtcore–Pl2core 0.12 0.04 – 0.13 – – – – – – – – – – – 9 8.4 – – Grtp22–Pl1core 0.14 0.03 – 0.11 – – – – – – – – – – – 12 12 – – Grtrim–Pl1rim 0.12 0.03 – 0.13 – – – – – – – – – – – 9.9 9.9 – – TC2 Grtcore–Bt1 0.14 0.34 0.49 – – – – – – 730 631 745 – – – – – – – Grtcore–Bt1–Plcore–Ms1 0.14 0.34 0.49 0.33 3.3 1.00 0.74 – – – – – 638 – – – – 9.4 10 Grtrim–Bt2 0.14 0.34 0.49 – – – – – – – – – – – – – – – Grtrim–Bt2–Plrim–Ms2 0.14 0.34 0.49 0.28 3.3 1.00 0.78 – – 733 634 721 630 – – – – 10 10 Retrogressed eclogite CP28 Grt1rim–Cam1 0.20 0.21 – – – – – 0.55 – – – – – 730 – – – – – Grt2rim–Cam2 0.21 0.16 – – – – – 0.56 – – – – – 674 – – – – – Well–preserved eclogite TC13 Grt2rim–Cpx–Pl 0.32 0.17 – 0.22 – – – – 0.63 – – – – – 10 – – – – Grt2rim–Cpx–Pl 0.33 0.16 – 0.22 – – – – 0.63 – – – – – 10 – – – – Sample . Xpyr . Xgrs . Xphl . Xan . Si4+ . XK . XAlVI . XMg . XMg . T (°C) . T (°C) . T (°C) . T (°C) . T (°C) . P . P . P . P . P . . . . . . . . . . . . . . . . (kbar) . (kbar) . (kbar) . kbar) . (kbar) . . Grt . Grt . Bt . Pl . Ms . Ms . Ms . Hbl . Cpx . P . FS . IM . GS . GP . NP . P . NH . GS . MS . Host-gneisses CP37 Grtcore–Bt3 0.12 0.04 0.40 – – – – – – 622 693 790 – – – – – – – Grtcore–Bt3–Pl1core–Ms2 0.12 0.04 0.40 0.11 3.02 0.92 0.88 – – – – – 690 – – – – 6.9 – Grtcore–Bt4 0.12 0.04 0.40 – – – – – – 619 687 771 – – – – – – – Grtcore–Bt4–Pl2core–Ms2 0.12 0.04 0.40 0.13 3.02 0.92 0.88 – – – – – 682 – – – – 6.3 – Grtp22–Bt4 0.14 0.03 0.40 – – – – – – 627 716 771 – – – – – – – Grtp22–Bt4–Pl1core–Ms2 0.14 0.03 0.40 0.11 3.02 0.92 0.88 – – – – – 710 – – – – 6.7 – Grt1rim–Bt1 0.12 0.03 0.41 – – – – – – 584 636 708 – – – – – – – Grt1rim–Bt1–Pl1rim–Ms1 0.12 0.03 0.41 0.13 3.03 0.95 0.87 – – – – – 628 – – – – 4.9 – Grtcore–Pl1core 0.12 0.03 – 0.11 – – – – – – – – – – – 10 9.5 – – Grtcore–Pl2core 0.12 0.04 – 0.13 – – – – – – – – – – – 9 8.4 – – Grtp22–Pl1core 0.14 0.03 – 0.11 – – – – – – – – – – – 12 12 – – Grtrim–Pl1rim 0.12 0.03 – 0.13 – – – – – – – – – – – 9.9 9.9 – – TC2 Grtcore–Bt1 0.14 0.34 0.49 – – – – – – 730 631 745 – – – – – – – Grtcore–Bt1–Plcore–Ms1 0.14 0.34 0.49 0.33 3.3 1.00 0.74 – – – – – 638 – – – – 9.4 10 Grtrim–Bt2 0.14 0.34 0.49 – – – – – – – – – – – – – – – Grtrim–Bt2–Plrim–Ms2 0.14 0.34 0.49 0.28 3.3 1.00 0.78 – – 733 634 721 630 – – – – 10 10 Retrogressed eclogite CP28 Grt1rim–Cam1 0.20 0.21 – – – – – 0.55 – – – – – 730 – – – – – Grt2rim–Cam2 0.21 0.16 – – – – – 0.56 – – – – – 674 – – – – – Well–preserved eclogite TC13 Grt2rim–Cpx–Pl 0.32 0.17 – 0.22 – – – – 0.63 – – – – – 10 – – – – Grt2rim–Cpx–Pl 0.33 0.16 – 0.22 – – – – 0.63 – – – – – 10 – – – – Garnet–biotite thermometer: P, Perchuck et al. (1985); FS, Ferry & Spear (1978); IM, Indares & Martignole (1985). Garnet–plagioclase–muscovite–biotite therm-obarometer: GS, Ghent & Stout (1981). Garnet–plagioclase–sillimanite–quartz barometer: NH, Newton & Haselton (1981); P, Perchuk et al. (1985). Phengite barometry: MS, Massonne & Schreyer (1987). Garnet–plagioclase–clinopyroxene–quartz barometer on symplectites: NP, Newton & Perkins (1982). Garnet–hornblende thermometer: GP, Graham & Powell (1984). Analyses used in the calculation are reported in Table 1. Open in new tab Low-pressure amphibolite facies stage Extensive development of amphibole in the mafic rocks allows temperatures and pressures to be estimated by the experimental calibration of Plyusnina, (1982) based on Ca and Al concentrations of coexisting plagioclase and amphibole. These results were compared with those obtained by the qualitative methods of Spear, (1980) and Brown, (1977). In the retrogressed eclogite CP28, temperature was also estimated through the Fe–Mg exchange between garnet rims and the kelyphitic amphibole (Graham & Powell, 1984). The Plyusnina, (1982) calibration yielded a wide range of values for temperature and pressure. Temperature ranged from 525 to 625°C and pressure from 4 to 8 kbar. A similar temperature range (500–650°C) was obtained using the method of Spear, (1980). Higher temperatures (670–730°C) were estimated through the Fe–Mg exchange between garnet and amphibole in kelyphites (Table 3). A qualitative pressure evaluation, based on AlIV and Na (M4) of amphibole (Brown, 1977) indicated higher pressures of formation (7 kbar) for early barroisitic amphibole (medium amphibolite stage) than for late tschermakitic and Mg-hornblende (3 kbar). The latter empirical estimate agrees with the microstructural and chemical data on barroisites and tschermakitic hornblende, as the increase in AlIV and Ca in the M4 site indicates that tschermakitic hornblende developed at relatively constant temperature and lower pressure than barroisites (Ungaretti et al., 1981; and personal communication, 1996). Host-rocks Temperatures were evaluated using the thermometer based on the Fe–Mg exchange between garnet and biotite (Ferry & Spear, 1978; Indares & Martignole, 1985; Perchuck et al., 1985). Pressures were calculated using the garnet–sillimanite–plagioclase–quartz barometer (Newton & Haselton, 1981; Perchuck et al., 1985), the phengite content of muscovite (Massonne & Schreyer, 1987), and the garnet–muscovite–biotite–plagioclase thermo-barometer (Ghent & Stout, 1981). The highest temperatures (625–730°C, Table 3) were obtained using core and/or inner rim compositions of the mineralogical phases of the matrix whereas the lowest temperatures (585–630°C, Table 3) were obtained using rim compositions of phases in mutual contact. The highest pressure (12 kbar, Table 3) was obtained with the garnet–sillimanite–plagioclase–quartz barometer in sample CP37. In the same sample, the Ghent & Stout, (1981) barometer gave lower pressures (∼7 kbar) indicating that muscovite recorded lower-pressure conditions. In quartzo-feldspathic rock (sample TC2), a pressure of 10 kbar was obtained for core and rim compositions of the mineral phases, using the barometers of Ghent & Stout, (1981) and Massonne & Schreyer, (1987). Geochemical and Isotope Data Major and trace elements Whole-rock major and trace element data for nine selected metabasites (two well-preserved eclogites, two retrogressed eclogites and five amphibolites) are reported in Table 4. They show normative parameters ranging from Ne-normative to Hy-normative basalts and, except for retrogressed eclogite CP20, are characterized by TiO2 enrichment correlated with Fe enrichment. Sample CP20 has anomalous compositions for most of the elements considered (e.g. low TiO2 and low Zr/Y ratio) and could have been involved in chemical modification phenomena during the metamorphic events. The other samples do not display collinear distribution for most of the immobile elements, suggesting that they are not cogenetic. Table 4: Major and trace element results Sample: . TC7D . TC13 . TC16 . CP10 . CP20 . CP24 . CP28 . CP34 . JT12 . . A . WPE . WPE . A . RE . A . RE . A . A . Major elements (wt %) SiO2 44.40 46.85 46.13 45.68 51.54 47.82 48.80 44.71 48.59 TiO2 1.73 1.43 1.72 2.03 0.29 1.14 1.61 1.25 0.75 Al2O3 14.67 14.22 14.45 13.88 14.47 14.73 14.04 12.77 14.99 Fe2O3t 13.70 12.96 14.46 16.45 11.02 11.07 11.92 10.55 10.20 MnO 0.26 0.20 0.19 0.30 0.19 0.16 0.18 0.16 0.16 MgO 8.84 8.49 8.45 6.68 7.84 7.67 6.55 8.06 8.56 CaO 9.48 11.82 10.89 10.73 9.57 11.11 11.25 17.40 12.59 Na2O 2.73 3.51 3.28 2.69 3.13 2.88 2.80 1.66 1.90 K2O 1.23 0.044 0.18 0.21 0.59 1.02 0.76 0.43 0.74 P2O5 0.32 0.36 0.26 0.25 0.09 0.16 0.21 0.18 0.12 LOI 1.64 0.16 0.22 0.70 0.75 2.08 1.31 2.13 0.60 Sum 99.00 100.04 100.23 99.60 99.48 99.84 99.43 99.30 99.20 mg-no. 54.2 61.0 56.4 52.7 59.8 63.3 61.8 73.9 67.9 Trace elements (p.p.m.) Ni 159 162 54 95 94 72 55 63 137 Co 50 51 51 36 43 41 38 34 42 Cr 442 468 118 161 303 245 149 152 322 V 233 271 270 430 238 285 259 237 270 Sc 24 32 35 32 31 27 26 27 29 Cu 22 18 17 18 9.6 37 7.6 57 47 Zn 90 108 114 95 71 78 84 67 57 Ga 20 14 15 20 13 17 18 16 15 Pb 4.8 4.2 1.5 2.3 6.1 1.7 13 4.2 2.9 Sr 66 49 32 103 36 84 136 155 164 Rb 41 2.0 9.1 1.9 24 47 25 11 23 Ba 305 3.2 15 6.9 65 42 60 45 109 Zr 118 93 107 127 14 70 114 82 43 Hf 3.0 2.4 2.9 3.4 0.4 1.9 2.9 2.1 1.2 Nb 15 5.0 6.0 2.0 1.2 2.7 9.1 5.3 2.1 Ta 0.97 0.38 0.48 0.16 0.12 0.20 0.70 0.38 0.17 Th 2.6 6.2 3.4 0.18 0.38 0.36 0.98 0.42 0.33 U 0.12 0.64 0.38 0.11 0.29 0.23 0.81 0.27 0.23 Y 28 26 32 54 15 29 32 29 16 La 12 18 12 2.9 1.3 2.9 9.4 5.1 2.5 Ce 34 41 27 9.3 2.9 8.1 22 13 6.7 Pr 4.2 5.4 3.4 1.8 0.35 1.3 3.1 2.0 1.0 Nd 19 22 15 11 1.8 7.4 15 9.9 5.1 Sm 4.3 5.3 3.4 4.8 0.65 2.7 4.9 3.5 1.7 Eu 1.1 1.1 0.68 1.9 0.27 1.0 1.5 1.16 0.72 Gd 3.6 3.4 2.4 6.2 1.0 3.3 4.63 3.6 2.0 Tb 0.64 0.49 0.49 1.4 0.26 0.68 0.91 0.76 0.4 Dy 4.6 3.6 4.2 8.8 2.1 4.4 5.5 4.8 2.5 Ho 1.1 1.0 1.3 2.0 0.53 1.1 1.2 1.1 0.64 Er 2.6 3.0 3.4 5.2 1.5 2.7 3.0 2.7 1.6 Tm 0.40 0.49 0.56 0.83 0.28 0.44 0.45 0.43 0.23 Yb 2.6 3.3 3.5 5.7 2.2 2.8 3.1 2.7 1.7 Lu 0.37 0.52 0.57 0.89 0.34 0.47 0.51 0.48 0.25 (La/Sm)N 1.84 2.19 2.22 0.39 1.25 0.69 1.24 0.95 0.95 (La/Yb)N 3.39 3.93 2.35 0.36 0.42 0.74 2.20 1.35 1.07 Eu/Eu* 0.87 0.77 0.72 1.06 1.03 1.04 0.96 1.00 1.19 Sample: . TC7D . TC13 . TC16 . CP10 . CP20 . CP24 . CP28 . CP34 . JT12 . . A . WPE . WPE . A . RE . A . RE . A . A . Major elements (wt %) SiO2 44.40 46.85 46.13 45.68 51.54 47.82 48.80 44.71 48.59 TiO2 1.73 1.43 1.72 2.03 0.29 1.14 1.61 1.25 0.75 Al2O3 14.67 14.22 14.45 13.88 14.47 14.73 14.04 12.77 14.99 Fe2O3t 13.70 12.96 14.46 16.45 11.02 11.07 11.92 10.55 10.20 MnO 0.26 0.20 0.19 0.30 0.19 0.16 0.18 0.16 0.16 MgO 8.84 8.49 8.45 6.68 7.84 7.67 6.55 8.06 8.56 CaO 9.48 11.82 10.89 10.73 9.57 11.11 11.25 17.40 12.59 Na2O 2.73 3.51 3.28 2.69 3.13 2.88 2.80 1.66 1.90 K2O 1.23 0.044 0.18 0.21 0.59 1.02 0.76 0.43 0.74 P2O5 0.32 0.36 0.26 0.25 0.09 0.16 0.21 0.18 0.12 LOI 1.64 0.16 0.22 0.70 0.75 2.08 1.31 2.13 0.60 Sum 99.00 100.04 100.23 99.60 99.48 99.84 99.43 99.30 99.20 mg-no. 54.2 61.0 56.4 52.7 59.8 63.3 61.8 73.9 67.9 Trace elements (p.p.m.) Ni 159 162 54 95 94 72 55 63 137 Co 50 51 51 36 43 41 38 34 42 Cr 442 468 118 161 303 245 149 152 322 V 233 271 270 430 238 285 259 237 270 Sc 24 32 35 32 31 27 26 27 29 Cu 22 18 17 18 9.6 37 7.6 57 47 Zn 90 108 114 95 71 78 84 67 57 Ga 20 14 15 20 13 17 18 16 15 Pb 4.8 4.2 1.5 2.3 6.1 1.7 13 4.2 2.9 Sr 66 49 32 103 36 84 136 155 164 Rb 41 2.0 9.1 1.9 24 47 25 11 23 Ba 305 3.2 15 6.9 65 42 60 45 109 Zr 118 93 107 127 14 70 114 82 43 Hf 3.0 2.4 2.9 3.4 0.4 1.9 2.9 2.1 1.2 Nb 15 5.0 6.0 2.0 1.2 2.7 9.1 5.3 2.1 Ta 0.97 0.38 0.48 0.16 0.12 0.20 0.70 0.38 0.17 Th 2.6 6.2 3.4 0.18 0.38 0.36 0.98 0.42 0.33 U 0.12 0.64 0.38 0.11 0.29 0.23 0.81 0.27 0.23 Y 28 26 32 54 15 29 32 29 16 La 12 18 12 2.9 1.3 2.9 9.4 5.1 2.5 Ce 34 41 27 9.3 2.9 8.1 22 13 6.7 Pr 4.2 5.4 3.4 1.8 0.35 1.3 3.1 2.0 1.0 Nd 19 22 15 11 1.8 7.4 15 9.9 5.1 Sm 4.3 5.3 3.4 4.8 0.65 2.7 4.9 3.5 1.7 Eu 1.1 1.1 0.68 1.9 0.27 1.0 1.5 1.16 0.72 Gd 3.6 3.4 2.4 6.2 1.0 3.3 4.63 3.6 2.0 Tb 0.64 0.49 0.49 1.4 0.26 0.68 0.91 0.76 0.4 Dy 4.6 3.6 4.2 8.8 2.1 4.4 5.5 4.8 2.5 Ho 1.1 1.0 1.3 2.0 0.53 1.1 1.2 1.1 0.64 Er 2.6 3.0 3.4 5.2 1.5 2.7 3.0 2.7 1.6 Tm 0.40 0.49 0.56 0.83 0.28 0.44 0.45 0.43 0.23 Yb 2.6 3.3 3.5 5.7 2.2 2.8 3.1 2.7 1.7 Lu 0.37 0.52 0.57 0.89 0.34 0.47 0.51 0.48 0.25 (La/Sm)N 1.84 2.19 2.22 0.39 1.25 0.69 1.24 0.95 0.95 (La/Yb)N 3.39 3.93 2.35 0.36 0.42 0.74 2.20 1.35 1.07 Eu/Eu* 0.87 0.77 0.72 1.06 1.03 1.04 0.96 1.00 1.19 WPE, well-preserved eclogites; RE, retrogressed eclogites; A, amphibolites; mg-no., molar Mg/(Mg+Fe2+) assuming Fe2O3/FeO=0.15. Open in new tab Table 4: Major and trace element results Sample: . TC7D . TC13 . TC16 . CP10 . CP20 . CP24 . CP28 . CP34 . JT12 . . A . WPE . WPE . A . RE . A . RE . A . A . Major elements (wt %) SiO2 44.40 46.85 46.13 45.68 51.54 47.82 48.80 44.71 48.59 TiO2 1.73 1.43 1.72 2.03 0.29 1.14 1.61 1.25 0.75 Al2O3 14.67 14.22 14.45 13.88 14.47 14.73 14.04 12.77 14.99 Fe2O3t 13.70 12.96 14.46 16.45 11.02 11.07 11.92 10.55 10.20 MnO 0.26 0.20 0.19 0.30 0.19 0.16 0.18 0.16 0.16 MgO 8.84 8.49 8.45 6.68 7.84 7.67 6.55 8.06 8.56 CaO 9.48 11.82 10.89 10.73 9.57 11.11 11.25 17.40 12.59 Na2O 2.73 3.51 3.28 2.69 3.13 2.88 2.80 1.66 1.90 K2O 1.23 0.044 0.18 0.21 0.59 1.02 0.76 0.43 0.74 P2O5 0.32 0.36 0.26 0.25 0.09 0.16 0.21 0.18 0.12 LOI 1.64 0.16 0.22 0.70 0.75 2.08 1.31 2.13 0.60 Sum 99.00 100.04 100.23 99.60 99.48 99.84 99.43 99.30 99.20 mg-no. 54.2 61.0 56.4 52.7 59.8 63.3 61.8 73.9 67.9 Trace elements (p.p.m.) Ni 159 162 54 95 94 72 55 63 137 Co 50 51 51 36 43 41 38 34 42 Cr 442 468 118 161 303 245 149 152 322 V 233 271 270 430 238 285 259 237 270 Sc 24 32 35 32 31 27 26 27 29 Cu 22 18 17 18 9.6 37 7.6 57 47 Zn 90 108 114 95 71 78 84 67 57 Ga 20 14 15 20 13 17 18 16 15 Pb 4.8 4.2 1.5 2.3 6.1 1.7 13 4.2 2.9 Sr 66 49 32 103 36 84 136 155 164 Rb 41 2.0 9.1 1.9 24 47 25 11 23 Ba 305 3.2 15 6.9 65 42 60 45 109 Zr 118 93 107 127 14 70 114 82 43 Hf 3.0 2.4 2.9 3.4 0.4 1.9 2.9 2.1 1.2 Nb 15 5.0 6.0 2.0 1.2 2.7 9.1 5.3 2.1 Ta 0.97 0.38 0.48 0.16 0.12 0.20 0.70 0.38 0.17 Th 2.6 6.2 3.4 0.18 0.38 0.36 0.98 0.42 0.33 U 0.12 0.64 0.38 0.11 0.29 0.23 0.81 0.27 0.23 Y 28 26 32 54 15 29 32 29 16 La 12 18 12 2.9 1.3 2.9 9.4 5.1 2.5 Ce 34 41 27 9.3 2.9 8.1 22 13 6.7 Pr 4.2 5.4 3.4 1.8 0.35 1.3 3.1 2.0 1.0 Nd 19 22 15 11 1.8 7.4 15 9.9 5.1 Sm 4.3 5.3 3.4 4.8 0.65 2.7 4.9 3.5 1.7 Eu 1.1 1.1 0.68 1.9 0.27 1.0 1.5 1.16 0.72 Gd 3.6 3.4 2.4 6.2 1.0 3.3 4.63 3.6 2.0 Tb 0.64 0.49 0.49 1.4 0.26 0.68 0.91 0.76 0.4 Dy 4.6 3.6 4.2 8.8 2.1 4.4 5.5 4.8 2.5 Ho 1.1 1.0 1.3 2.0 0.53 1.1 1.2 1.1 0.64 Er 2.6 3.0 3.4 5.2 1.5 2.7 3.0 2.7 1.6 Tm 0.40 0.49 0.56 0.83 0.28 0.44 0.45 0.43 0.23 Yb 2.6 3.3 3.5 5.7 2.2 2.8 3.1 2.7 1.7 Lu 0.37 0.52 0.57 0.89 0.34 0.47 0.51 0.48 0.25 (La/Sm)N 1.84 2.19 2.22 0.39 1.25 0.69 1.24 0.95 0.95 (La/Yb)N 3.39 3.93 2.35 0.36 0.42 0.74 2.20 1.35 1.07 Eu/Eu* 0.87 0.77 0.72 1.06 1.03 1.04 0.96 1.00 1.19 Sample: . TC7D . TC13 . TC16 . CP10 . CP20 . CP24 . CP28 . CP34 . JT12 . . A . WPE . WPE . A . RE . A . RE . A . A . Major elements (wt %) SiO2 44.40 46.85 46.13 45.68 51.54 47.82 48.80 44.71 48.59 TiO2 1.73 1.43 1.72 2.03 0.29 1.14 1.61 1.25 0.75 Al2O3 14.67 14.22 14.45 13.88 14.47 14.73 14.04 12.77 14.99 Fe2O3t 13.70 12.96 14.46 16.45 11.02 11.07 11.92 10.55 10.20 MnO 0.26 0.20 0.19 0.30 0.19 0.16 0.18 0.16 0.16 MgO 8.84 8.49 8.45 6.68 7.84 7.67 6.55 8.06 8.56 CaO 9.48 11.82 10.89 10.73 9.57 11.11 11.25 17.40 12.59 Na2O 2.73 3.51 3.28 2.69 3.13 2.88 2.80 1.66 1.90 K2O 1.23 0.044 0.18 0.21 0.59 1.02 0.76 0.43 0.74 P2O5 0.32 0.36 0.26 0.25 0.09 0.16 0.21 0.18 0.12 LOI 1.64 0.16 0.22 0.70 0.75 2.08 1.31 2.13 0.60 Sum 99.00 100.04 100.23 99.60 99.48 99.84 99.43 99.30 99.20 mg-no. 54.2 61.0 56.4 52.7 59.8 63.3 61.8 73.9 67.9 Trace elements (p.p.m.) Ni 159 162 54 95 94 72 55 63 137 Co 50 51 51 36 43 41 38 34 42 Cr 442 468 118 161 303 245 149 152 322 V 233 271 270 430 238 285 259 237 270 Sc 24 32 35 32 31 27 26 27 29 Cu 22 18 17 18 9.6 37 7.6 57 47 Zn 90 108 114 95 71 78 84 67 57 Ga 20 14 15 20 13 17 18 16 15 Pb 4.8 4.2 1.5 2.3 6.1 1.7 13 4.2 2.9 Sr 66 49 32 103 36 84 136 155 164 Rb 41 2.0 9.1 1.9 24 47 25 11 23 Ba 305 3.2 15 6.9 65 42 60 45 109 Zr 118 93 107 127 14 70 114 82 43 Hf 3.0 2.4 2.9 3.4 0.4 1.9 2.9 2.1 1.2 Nb 15 5.0 6.0 2.0 1.2 2.7 9.1 5.3 2.1 Ta 0.97 0.38 0.48 0.16 0.12 0.20 0.70 0.38 0.17 Th 2.6 6.2 3.4 0.18 0.38 0.36 0.98 0.42 0.33 U 0.12 0.64 0.38 0.11 0.29 0.23 0.81 0.27 0.23 Y 28 26 32 54 15 29 32 29 16 La 12 18 12 2.9 1.3 2.9 9.4 5.1 2.5 Ce 34 41 27 9.3 2.9 8.1 22 13 6.7 Pr 4.2 5.4 3.4 1.8 0.35 1.3 3.1 2.0 1.0 Nd 19 22 15 11 1.8 7.4 15 9.9 5.1 Sm 4.3 5.3 3.4 4.8 0.65 2.7 4.9 3.5 1.7 Eu 1.1 1.1 0.68 1.9 0.27 1.0 1.5 1.16 0.72 Gd 3.6 3.4 2.4 6.2 1.0 3.3 4.63 3.6 2.0 Tb 0.64 0.49 0.49 1.4 0.26 0.68 0.91 0.76 0.4 Dy 4.6 3.6 4.2 8.8 2.1 4.4 5.5 4.8 2.5 Ho 1.1 1.0 1.3 2.0 0.53 1.1 1.2 1.1 0.64 Er 2.6 3.0 3.4 5.2 1.5 2.7 3.0 2.7 1.6 Tm 0.40 0.49 0.56 0.83 0.28 0.44 0.45 0.43 0.23 Yb 2.6 3.3 3.5 5.7 2.2 2.8 3.1 2.7 1.7 Lu 0.37 0.52 0.57 0.89 0.34 0.47 0.51 0.48 0.25 (La/Sm)N 1.84 2.19 2.22 0.39 1.25 0.69 1.24 0.95 0.95 (La/Yb)N 3.39 3.93 2.35 0.36 0.42 0.74 2.20 1.35 1.07 Eu/Eu* 0.87 0.77 0.72 1.06 1.03 1.04 0.96 1.00 1.19 WPE, well-preserved eclogites; RE, retrogressed eclogites; A, amphibolites; mg-no., molar Mg/(Mg+Fe2+) assuming Fe2O3/FeO=0.15. Open in new tab When plotted on different discrimination diagrams (Pearce & Cann, 1973; Pearce & Norry, 1979; Wood et al., 1979; Pearce, 1982; Mullen, 1983; Meschede, 1986), most of the samples analysed fell in the mid-ocean ridge basalt field (both N- and E-type MORB). It is noteworthy that the two well-preserved eclogites (TC13 and TC16) plotted in the field representative of the volcanic-arc basalts or mafic rocks modified by continental crust, in the Th–Hf–Ta triangular diagram (Wood et al., 1979). However, the distribution of the data points in the Ti/Cr vs Ni (Beccaluva et al., 1979), Ti vs Cr (Pearce, 1975) and Ti vs V (Shervais, 1982) discrimination diagrams did not coroborate a volcanic-arc affinity. Most of the samples analysed had La/Ta ratios (Fig. 4) between 18 and 11 (∼18.5 and ∼10 are typical for N-MORB and E-MORB, respectively; Saunders, 1984). As for Th–Hf–Ta relationships, the two well-preserved eclogites strongly deviated from this range, with La/Ta ratios up to 47. Fig. 4. Open in new tabDownload slide La vs Ta diagram for well-preserved eclogites, retrogressed eclogites and amphibolites from Lanterman Range. The REE distribution patterns for retrogressed eclogites and amphibolites (Fig. 5b and c) ranged from light rare earth element (LREE) depleted (i.e. sample CP10) with (La/Sm)N=0.39 and unfractionated heavy REE (HREE), essentially unfractionated patterns (e.g. sample CP34) at abundances of ∼20 × chondrite, to LREE enriched (i.e. sample TC7D) with (La/Sm)N=1.84 and abundances of ∼50 × chondrite for La. Sample CP20 had low REE abundances (La 5.5 × chondritic value) and a pattern similar to some anomalous profiles as described for eclogite rocks elsewhere (Griffin & Brueckner, 1985; Paquette et al., 1989; Bernard-Griffiths et al., 1991). Most of the samples lacked pronounced Eu anomalies, except for sample TC7D, which showed a slight negative anomaly (Eu/Eu*=0.87), and sample JT12, characterized by a positive Eu anomaly (Eu/Eu*=1.19). The latter feature may be attributed to minor cumulate plagioclase. The two well-preserved eclogites (Fig. 5a) showed strong LREE enrichment [(La/Sm)N>2] with abundances of ∼50–75 × chondrite for La and negative Eu anomaly [(Eu/Eu*)=0.77–0.72]. Fig. 5. Open in new tabDownload slide REE chondritic patterns for well-preserved eclogites (a), retrogressed eclogites (b) and amphibolites (c). Chondritic values are from Sun & McDonough, (1989). In Fig. 6a-c expanded REE patterns and a range of incompatible elements are used to show geochemical anomalies with respect to normal MORB (Pearce, 1983). Even excluding Sr, K, Rb and Ba, which may have been mobile at any stage of the evolution of these rocks, only sample CP10 showed a pattern similar to normal MORB; most of the retrogressed eclogites and amphibolites showed variable enrichments for most of the more incompatible elements. These compositions resemble those of transitional- to E-type MORB. The two well-preserved eclogites (Fig. 6a) were characterized by strong enrichment in Th, Ce and P relative to Zr, and pronounced negative Ta and Nb anomalies. The latter feature is particularly important as it is indicative of crustal influence in the pre-metamorphic evolution of the protolith. Fig. 6. Open in new tabDownload slide MORB-normalized spidergrams for well-preserved eclogites (a), retrogressed eclogites (b) and amphibolites (c). Normalizing values are from Pearce, (1983). Isotope data Sm/Nd whole-rock data of metabasic rocks Seven whole-rock samples (two well-preserved eclogites, one retrogressed eclogite, two amphibolites and two migmatite gneisses from the Deep Freeze Range) were selected for Sm–Nd isotope analyses. The analytical results are shown in Table 5. No isochron relationships can be inferred from the five metabasites. Nevertheless, excluding the two well-preserved eclogites, the other three samples scatter along a line with a slope of ∼700 Ma and εNd(i) of 6.7. Present-day εNd(0) ranges from strongly negative values for the two well-preserved eclogites (-7.0 and -7.4) to positive values for the other samples (from 4.0 to 11.3). Nd model ages (Table 5) relative to a linear depleted mantle evolution (Michard et al., 1985) are around 1.5 Ga for the two well-preserved eclogites and between 0.50 and 0.93 Ga for the other samples. The two migmatite gneisses had exceedingly unradiogenic present-day composition and Nd model ages around 2.0 Ga (Table 5). Table 5: Sm–Nd analytical results Sample . Sm (p.p.m.) . Nd (p.p.m.) . 147Sm/144Nd . 143Nd/144Nd . ±2σm . εNd(0) . T(DM) (Ga) . Well-preserved eclogites TC13 Rt 0.5843 2.637 0.1339 0.512271 ±8 −7.2 TC13 Cpx 3.016 12.91 0.1413 0.512284 ±6 −6.9 TC13 Cpx/d 3.054 13.08 0.1412 0.512286 ±6 −6.9 TC13 Grt 0.2365 0.1312 1.091 0.515419 ±37 54.2 TC13 Grt/d 0.2371 0.1096 1.309 0.516080 ±50 67.1 TC13 Am 0.4374 1.550 0.1706 0.512383 ±40 −5.0 TC13 Am/d – – – 0.512427 ±39 −4.1 TC13 WR 5.278 23.02 0.1386 0.512254 ±40 −7.5 TC13 WR/d 5.308 23.20 0.1386 0.512280 ±6 −7.0 1.52 TC16 Rt 0.4456 2.091 0.1288 0.512243 ±9 −7.7 TC16 Cpx* 7.039 33.23 0.1281 0.512237 ±6 −7.8 TC16 Grt 0.2049 0.1186 1.045 0.515212 ±74 50.2 TC16 Grt/d 0.2014 0.1102 1.105 0.515384 ±14 53.6 TC16 Am 0.2382 0.7935 0.1812 0.512430 ±11 −4.1 TC16 Am/d – – – 0.512390 ±35 −4.9 TC16 WR 3.284 14.76 0.1345 0.512258 ±6 −7.4 1.49 Retrogressed eclogite CP28 WR 4.536 15.40 0.1781 0.512845 ±7 4.0 0.50 Amphibolites CP10 WR 4.859 11.60 0.2531 0.513216 ±6 11.3 0.93 CP34 WR 3.279 10.33 0.1918 0.512994 ±7 6.9 0.61 Migmatitic) Gneisses (Deep Freeze Range) L11A WR 6.871 38.47 0.1080 0.511518 ±6 −21.85 2.12 CF8 WR 7.488 42.10 0.1075 0.511637 ±10 −19.53 1.96 Sample . Sm (p.p.m.) . Nd (p.p.m.) . 147Sm/144Nd . 143Nd/144Nd . ±2σm . εNd(0) . T(DM) (Ga) . Well-preserved eclogites TC13 Rt 0.5843 2.637 0.1339 0.512271 ±8 −7.2 TC13 Cpx 3.016 12.91 0.1413 0.512284 ±6 −6.9 TC13 Cpx/d 3.054 13.08 0.1412 0.512286 ±6 −6.9 TC13 Grt 0.2365 0.1312 1.091 0.515419 ±37 54.2 TC13 Grt/d 0.2371 0.1096 1.309 0.516080 ±50 67.1 TC13 Am 0.4374 1.550 0.1706 0.512383 ±40 −5.0 TC13 Am/d – – – 0.512427 ±39 −4.1 TC13 WR 5.278 23.02 0.1386 0.512254 ±40 −7.5 TC13 WR/d 5.308 23.20 0.1386 0.512280 ±6 −7.0 1.52 TC16 Rt 0.4456 2.091 0.1288 0.512243 ±9 −7.7 TC16 Cpx* 7.039 33.23 0.1281 0.512237 ±6 −7.8 TC16 Grt 0.2049 0.1186 1.045 0.515212 ±74 50.2 TC16 Grt/d 0.2014 0.1102 1.105 0.515384 ±14 53.6 TC16 Am 0.2382 0.7935 0.1812 0.512430 ±11 −4.1 TC16 Am/d – – – 0.512390 ±35 −4.9 TC16 WR 3.284 14.76 0.1345 0.512258 ±6 −7.4 1.49 Retrogressed eclogite CP28 WR 4.536 15.40 0.1781 0.512845 ±7 4.0 0.50 Amphibolites CP10 WR 4.859 11.60 0.2531 0.513216 ±6 11.3 0.93 CP34 WR 3.279 10.33 0.1918 0.512994 ±7 6.9 0.61 Migmatitic) Gneisses (Deep Freeze Range) L11A WR 6.871 38.47 0.1080 0.511518 ±6 −21.85 2.12 CF8 WR 7.488 42.10 0.1075 0.511637 ±10 −19.53 1.96 Mineral symbols according to Kretz (1983). WR, whole rock; /d, full duplicate analysis. Epsilon notation was calculated with respect to a chondritic reservoir with present 147Sm/144Nd=0.1967 and 143Nd/144Nd=0.512638. The model ages were calculated assuming a linear depleted mantle evolution with present 147Sm/144Nd=0.222 and 143Nd/144Nd=0.513114 (Michard et al., 1985). * May be contaminated by epidote. Open in new tab Table 5: Sm–Nd analytical results Sample . Sm (p.p.m.) . Nd (p.p.m.) . 147Sm/144Nd . 143Nd/144Nd . ±2σm . εNd(0) . T(DM) (Ga) . Well-preserved eclogites TC13 Rt 0.5843 2.637 0.1339 0.512271 ±8 −7.2 TC13 Cpx 3.016 12.91 0.1413 0.512284 ±6 −6.9 TC13 Cpx/d 3.054 13.08 0.1412 0.512286 ±6 −6.9 TC13 Grt 0.2365 0.1312 1.091 0.515419 ±37 54.2 TC13 Grt/d 0.2371 0.1096 1.309 0.516080 ±50 67.1 TC13 Am 0.4374 1.550 0.1706 0.512383 ±40 −5.0 TC13 Am/d – – – 0.512427 ±39 −4.1 TC13 WR 5.278 23.02 0.1386 0.512254 ±40 −7.5 TC13 WR/d 5.308 23.20 0.1386 0.512280 ±6 −7.0 1.52 TC16 Rt 0.4456 2.091 0.1288 0.512243 ±9 −7.7 TC16 Cpx* 7.039 33.23 0.1281 0.512237 ±6 −7.8 TC16 Grt 0.2049 0.1186 1.045 0.515212 ±74 50.2 TC16 Grt/d 0.2014 0.1102 1.105 0.515384 ±14 53.6 TC16 Am 0.2382 0.7935 0.1812 0.512430 ±11 −4.1 TC16 Am/d – – – 0.512390 ±35 −4.9 TC16 WR 3.284 14.76 0.1345 0.512258 ±6 −7.4 1.49 Retrogressed eclogite CP28 WR 4.536 15.40 0.1781 0.512845 ±7 4.0 0.50 Amphibolites CP10 WR 4.859 11.60 0.2531 0.513216 ±6 11.3 0.93 CP34 WR 3.279 10.33 0.1918 0.512994 ±7 6.9 0.61 Migmatitic) Gneisses (Deep Freeze Range) L11A WR 6.871 38.47 0.1080 0.511518 ±6 −21.85 2.12 CF8 WR 7.488 42.10 0.1075 0.511637 ±10 −19.53 1.96 Sample . Sm (p.p.m.) . Nd (p.p.m.) . 147Sm/144Nd . 143Nd/144Nd . ±2σm . εNd(0) . T(DM) (Ga) . Well-preserved eclogites TC13 Rt 0.5843 2.637 0.1339 0.512271 ±8 −7.2 TC13 Cpx 3.016 12.91 0.1413 0.512284 ±6 −6.9 TC13 Cpx/d 3.054 13.08 0.1412 0.512286 ±6 −6.9 TC13 Grt 0.2365 0.1312 1.091 0.515419 ±37 54.2 TC13 Grt/d 0.2371 0.1096 1.309 0.516080 ±50 67.1 TC13 Am 0.4374 1.550 0.1706 0.512383 ±40 −5.0 TC13 Am/d – – – 0.512427 ±39 −4.1 TC13 WR 5.278 23.02 0.1386 0.512254 ±40 −7.5 TC13 WR/d 5.308 23.20 0.1386 0.512280 ±6 −7.0 1.52 TC16 Rt 0.4456 2.091 0.1288 0.512243 ±9 −7.7 TC16 Cpx* 7.039 33.23 0.1281 0.512237 ±6 −7.8 TC16 Grt 0.2049 0.1186 1.045 0.515212 ±74 50.2 TC16 Grt/d 0.2014 0.1102 1.105 0.515384 ±14 53.6 TC16 Am 0.2382 0.7935 0.1812 0.512430 ±11 −4.1 TC16 Am/d – – – 0.512390 ±35 −4.9 TC16 WR 3.284 14.76 0.1345 0.512258 ±6 −7.4 1.49 Retrogressed eclogite CP28 WR 4.536 15.40 0.1781 0.512845 ±7 4.0 0.50 Amphibolites CP10 WR 4.859 11.60 0.2531 0.513216 ±6 11.3 0.93 CP34 WR 3.279 10.33 0.1918 0.512994 ±7 6.9 0.61 Migmatitic) Gneisses (Deep Freeze Range) L11A WR 6.871 38.47 0.1080 0.511518 ±6 −21.85 2.12 CF8 WR 7.488 42.10 0.1075 0.511637 ±10 −19.53 1.96 Mineral symbols according to Kretz (1983). WR, whole rock; /d, full duplicate analysis. Epsilon notation was calculated with respect to a chondritic reservoir with present 147Sm/144Nd=0.1967 and 143Nd/144Nd=0.512638. The model ages were calculated assuming a linear depleted mantle evolution with present 147Sm/144Nd=0.222 and 143Nd/144Nd=0.513114 (Michard et al., 1985). * May be contaminated by epidote. Open in new tab Sm/Nd and U/Pb data for minerals from well-preserved eclogites Two well-preserved eclogites were selected for Sm–Nd isotope mineral analyses and for U–Pb analyses of rutile and whole rocks. One garnet fraction was also analysed for lead content and isotope compositions. The results are given in Tables 5 and 6. The garnet size (mostly ∼0.3 mm) of the samples was too small to allow separation of different portions, so whole grains were analysed. In contrast to what we found with duplicate analysis of the whole rock, clinopyroxene and amphibole, the reproducibility of the 147Sm/144Nd and 143Nd/144Nd ratios of replicate garnet analyses was well outside analytical uncertainty. Although the very low Sm and Nd contents (∼0.2 and ∼0.1 p.p.m., respectively—Table 5) of the garnets make this phase susceptible to contamination by the other mineral phases (e.g. clinopyroxene, epidote and apatite) during mineral purification, significant zoning in the Sm and Nd contents of garnet is likely (Getty et al., 1993; Brueckner et al., 1996), especially if we take into account the occurrence of chemical zoning for major cations. Table 6: U–Pb analytical results Sample . U (p.p.m.) . Pb (p.p.m.) . 235U/204Pb* . 238U/204Pb* . 208Pb/204Pb* . 207Pb/204Pb* . 206Pb/204Pb* . Well-preserved eclogites TC13 Rt 0.654 0.463 0.746 102.8 40.951 16.066 26.229 TC13 Grt 0.0632 – – – 38.965 15.675 18.655 TC13 WR 0.638 4.27 0.0710 9.79 40.394 15.683 18.808 TC16 Rt 1.23 0.476 1.51 208.2 40.440 16.702 34.948 TC16WR 0.377 1.40 0.133 18.3 42.305 15.728 19.537 Sample . U (p.p.m.) . Pb (p.p.m.) . 235U/204Pb* . 238U/204Pb* . 208Pb/204Pb* . 207Pb/204Pb* . 206Pb/204Pb* . Well-preserved eclogites TC13 Rt 0.654 0.463 0.746 102.8 40.951 16.066 26.229 TC13 Grt 0.0632 – – – 38.965 15.675 18.655 TC13 WR 0.638 4.27 0.0710 9.79 40.394 15.683 18.808 TC16 Rt 1.23 0.476 1.51 208.2 40.440 16.702 34.948 TC16WR 0.377 1.40 0.133 18.3 42.305 15.728 19.537 Mineral symbols according to Kretz (1983). WR, whole rock. * Corrected for blank and mass fractionation. Open in new tab Table 6: U–Pb analytical results Sample . U (p.p.m.) . Pb (p.p.m.) . 235U/204Pb* . 238U/204Pb* . 208Pb/204Pb* . 207Pb/204Pb* . 206Pb/204Pb* . Well-preserved eclogites TC13 Rt 0.654 0.463 0.746 102.8 40.951 16.066 26.229 TC13 Grt 0.0632 – – – 38.965 15.675 18.655 TC13 WR 0.638 4.27 0.0710 9.79 40.394 15.683 18.808 TC16 Rt 1.23 0.476 1.51 208.2 40.440 16.702 34.948 TC16WR 0.377 1.40 0.133 18.3 42.305 15.728 19.537 Sample . U (p.p.m.) . Pb (p.p.m.) . 235U/204Pb* . 238U/204Pb* . 208Pb/204Pb* . 207Pb/204Pb* . 206Pb/204Pb* . Well-preserved eclogites TC13 Rt 0.654 0.463 0.746 102.8 40.951 16.066 26.229 TC13 Grt 0.0632 – – – 38.965 15.675 18.655 TC13 WR 0.638 4.27 0.0710 9.79 40.394 15.683 18.808 TC16 Rt 1.23 0.476 1.51 208.2 40.440 16.702 34.948 TC16WR 0.377 1.40 0.133 18.3 42.305 15.728 19.537 Mineral symbols according to Kretz (1983). WR, whole rock. * Corrected for blank and mass fractionation. Open in new tab Figure 7a and b shows the Sm–Nd isotope data of whole rocks and minerals of samples TC13 and TC16 in isochron diagrams. For sample TC13, all mineral fractions and whole rock formed a well-defined isochron of age 500±5 Ma [mean square weighted deviation (MSWD)=1.13] with a εNd(i)=-3.28±0.14 (Fig. 7a). No significant variation in age and initial ratio was observed when the amphibole data point was excluded from the regression calculation. Least-squares linear regression of all minerals and whole rock from sample TC16 (Fig. 7b) yielded an age of 491±7 Ma (MSWD=2.89) with εNd(i)=-3.18±0.31. The MSWD of 2.89 indicates an excess of scattering according to the criterion of Wendt & Carl, (1991). If the amphibole data are excluded from the calculation, we obtain a better fit corresponding to an age of 492±3 Ma, εNd(i)=-3.28±0.14 and an MSWD of 0.21. Fig. 7. Open in new tabDownload slide Sm–Nd isochron diagram for the two well-preserved eclogite samples. Mineral symbols according to Kretz, (1983). WR, whole rock. Excluding the amphibole data, we obtained an age of 500±5 Ma (MSWD=1.35) and 492±3 Ma (MSWD=0.21) for samples TC13 and TC16, respectively. Regarding the U–Pb data, in contrast to other results in pelitic systems (Mezger et al., 1989a; Burton & O'Nions, 1991; Vance & O'Nions, 1992; Vance & Holland, 1993) no significant variations were observed between the Pb isotope ratios of the garnet and the respective whole rock (Table 6), making garnet in these rocks unsuitable for U–Pb age determination. In contrast to other documented findings [see Ludwig & Cooper, (1984), Corfu & Muir, (1989), Mezger et al., (1989b) or Tonarini et al., (1993) for mafic rocks], the rutile separates were characterized by low contents of radiogenic lead. Radiogenic Pb in the rutiles only accounted for ∼10% and ∼18% of total Pb in samples TC13 and TC16, respectively. Nevertheless, their U/Pb ratios were sufficiently higher than those of the whole rock, giving two-point isochrons with acceptable precision. The 235U-207Pb and 238U-206Pb two-point (rutile–whole-rock) ages of each sample were discordant, but the most reliable 238U-206Pb results (radiogenic 206Pb was up to 19 × higher than radiogenic 207Pb) yielded essentially identical ages of 495±6 and 503±6 Ma for TC13 and TC16, respectively (with a weighted mean of 499±4 Ma). All four data points of the two samples scattered in a 206Pb/204Pb vs 238U/204Pb diagram along a line with an age of 502±17 Ma (MSWD=5.7). Hence we consider the 238U-206Pb ages to be the best approximation of the age of cooling through the rutile closure temperature. It is noteworthy that these ages overlap the range of the Sm–Nd garnet ages. The 235U-207Pb ages of the two samples were younger and older than the 238U-206Pb age (∼500 Ma), yielding ages of 456±26 and 543±23 Ma for TC13 and TC16, respectively. Discussion and Petrogenesis Characteristics and age of the protoliths Characterization of the geochemical affinity of ancient metabasic rocks can sometimes be difficult because the geochemical features may be the result of complex interplay between the magmatic evolution of the igneous precursor and chemical modification promoted by recrystallization and deformational processes during metamorphism. For example, anomalous REE patterns for eclogite rocks have previously been described elsewhere (Griffin & Brueckner, 1985; Paquette et al., 1989; Bernard-Griffiths et al., 1991) and attributed to REE fractionation during metamorphism in a closed system (metamorphic layering) or in an open system with LREE loss (Bernard-Griffiths et al., 1991). Except for the retrogressed eclogite CP20, such anomalous patterns were not observed among the samples analysed, and the consistency shown by high field strength elements (HFSE) and REE in most samples suggests that their distribution and contents may reflect the magmatic characteristics of the protoliths. As stated above, the amphibolites and retrogressed eclogites constituted a heterogeneous group with a compositional range from N-MORB to E-MORB. Moreover, the occurrence of Eu anomalies in some samples may indicate shallow-depth fractionation involving plagioclase. Nd isotope data place further constraints on source compositions and the timing of these metabasites. Among the retrogressed eclogite and amphibolites, sample CP34 has an 147Sm/144Nd ratio of 0.192, close to that of chondrite, making its εNd value practically insensitive to age correction. This means that sample CP34, with a present-day εNd(0) of 6.9, must be derived from a mantle source that was depleted for a substantial period of time. As far as the age of these metabasites is concerned, unmetamorphosed mafic rocks older than the magmatic activity of the Ross Orogen are known from the Cotton Plateau (Nimrod Glacier area; Borg et al., 1990) and from the Skelton Glacier area (Rowell et al., 1993). Borg et al., (1990) reported an Sm–Nd three-point mineral–whole-rock isochron age of 762±24 Ma for the gabbro and basalt of the Cotton Plateau with εNd(i)=6.85. Rowell et al., (1993) inferred a maximum crystallization age of 700–800 Ma for the basalt of the Skelton Glaciers on the basis of Sm–Nd model ages. These mafic rocks were considered to be of oceanic type (Borg et al., 1990) or continental rift affinity (Rowell et al., 1993). Borsi et al., (1995) also described remnants of a mafic dyke swarm, recrystallized and metamorphosed during the Ross Orogeny, in the Deep Freeze Range (Wilson Terrane). They inferred an origin of the basaltic parental magma in a heterogeneous sub-continental lithosphere, and an emplacement age of 800–900 Ma. Figure 8 shows an εNd(0) vs 147Sm/144Nd diagram of the analysed metabasites from the Lanterman Range and the data reported by Borg et al., (1990), Rowell et al., (1993) and Borsi et al., (1995). Figure 8 also includes two migmatite gneisses from the Deep Freeze Range (Wilson Terrane). The inset shows the Nd evolution for the samples relative to the reference reservoir (CHUR) compared with the Depleted Mantle and migmatite gneisses. The overlap of the two amphibolites and the retrogressed eclogite from the Lanterman Range (Fig. 8a) with the data reported by Borg et al., (1990) and Rowell et al., (1993) is noteworthy. All whole-rock samples, including the three samples from the Lanterman Range, scatter along a line with a slope of ∼700 Ma and an εNd(i) of 6.8, whereas all 11 data points (including the plagioclase and the clinopyroxene separates from the gabbro of the Cotton Plateau and the three different concentrates from the basalt of the Skelton Glacier) yield an age of 725±89 and an εNd(i) of 6.5. In addition, samples CP28 and CP10 define a line of age 754±31 Ma and εNd(i)=5.9±0.9. This linear array of the samples on an isochron diagram is surprising because of their different geochemical signatures and the wide geographical distribution. If we consider that this correlation is not fortuitous, two alternative explanations are possible: (1) the correlation represents a mixing line or (2) it is geochronologically meaningful. The first possibility implies broad-scale mixing between a high εNd and Sm/Nd component and a low εNd and Sm/Nd end-member, and as a consequence, the linear array would indicate a maximum age. The second possibility requires that the retrogressed eclogite and amphibolite protoliths were derived from a mantle with a relatively uniform Nd isotope composition. In support of the latter interpretation is the fact that the ∼700 Ma array is similar to the mineral–whole-rock age of the mafic rock of the Cotton Plateau (Borg et al., 1990). The above arguments may therefore indicate that mafic magmatism with an affinity typical of a spreading setting and a depleted signature in terms of Nd isotope composition occurred in northern Victoria Land at ∼700–750 Ma. Fig. 8. Open in new tabDownload slide εNd(0) vs 147Sm/144Nd diagram for the metabasites from Lanterman Range. Data for basic rocks from the Nimrod Glacier area (Borg et al., 1990), Skelton Glacier area (Rowell et al., 1993) and for metabasic rocks from the Deep Freeze Range (Borsi et al., 1995) are plotted for comparison. Half-filled symbols refer to whole-rock data (see text). Depleted mantle compositions (DM) according to: (1) Goldstein et al., (1984); (2) Michard et al., (1985); (3) DePaolo, (1981). The inset shows the Nd evolution of the analysed sample. The upper limit for the depleted mantle evolution curve is from Goldstein et al., (1984) and the lower limit from DePaolo, (1981). The two well-preserved eclogites are enriched in more incompatible elements and have high La/Ta and Th/Ta ratios that distinguish them from the amphibolites and retrogressed eclogites. The pronounced negative Ta and Nb anomalies of these rocks strongly suggest crustal influence of the pre-metamorphic magmatic evolution by intracrustal contamination or the effects of a metasomatized sub-continental lithosphere on the generation of the parental magma. In addition, the two well-preserved eclogites display unradiogenic Nd isotope composition and low 147Sm/144Nd ratios (Fig. 8). These compositions, along with trace element evidence, cannot be simply attributed to different emplacement ages but are the product of a magmatic process. Simple mixing of an igneous protolith with a depleted signature (similar to that of the retrogressed eclogites and amphibolites analysed) and a crustal component (similar in composition to the migmatite gneisses) at 700–750 Ma, would require a crustal contribution of at least 20% to account for the unradiogenic composition of the well-preserved eclogites. These samples, however, have low silica and relatively high MgO and transition element contents, not so compatible with crustal contamination en route of an igneous protolith similar in composition to the retrogressed eclogites and amphibolites. These arguments suggest that the well-preserved eclogites could be derived from a different mantle source with an enriched signature. Consequently, the ∼1.5 Ga Nd model ages (Table 5) must be regarded as meaningless and not the true mantle separation age. The nature and time of the source enrichment as well as the age of the igneous precursor of the two well-preserved eclogites remain, at present, unconstrained. However, the evidence presented above indicates that along the suture between the Wilson Terrane and the allochthonous Bowers Terrane in the Lanterman Range, different protoliths, possibly of different ages, may have experienced eclogite facies metamorphism. Metamorphic evolution The petrological data and mineral chemistry of eclogites from the Lanterman Range provide information on only the retrograde part of the P–T path of the rocks. The metamorphic evolution from the eclogite stage to the amphibolite facies retrogression, inferred from the petrological data and the experimentally determined reaction curves, is summarized by the P–T path in Fig. 9. The box of the eclogite facies stage is based on estimated temperatures and on the jadeite content of omphacite. The first post-eclogite stage (medium-pressure amphibolite facies stage) is documented both by the destabilization of omphacite, giving rise to the symplectitic association CpxII + Pl, and by the first appearance of amphibole (barroisite). Roermund & van Boland, (1983) suggested that the degree of equilibration and the size of the retrometamorphic microstructures on omphacite are diffusion limited and temperature controlled. As the symplectite on omphacite is always cryptocrystalline, it presumably formed when the rocks were at relatively low temperatures. As the decompressional path after the eclogitic peak was therefore characterized by a decreasing temperature, any granulitic stage was excluded. The P–T box of this stage in Fig. 9 is based on estimated temperatures (630–750°C) and pressures (6–10 kbar). The subsequent low-pressure amphibolite facies stage characterized by the extensive development of tschermakitic hornblende is constrained by the experimental curves for amphibole (Plyusnina, 1982) and by the absence of chlorite (Fig. 9). Fig. 9. Open in new tabDownload slide P–T path (grey arrow) of the studied rocks. Aluminium silicate triple point is according to Holdaway, (1971). Albite=jadeite + quartz and stability of omphacite (jd=10, 30, 45) are after Holland, (1983). Isopleth of Si4+ content in phengite barometer is after Massonne & Schreyer, (1987). The curve representing the breakdown of amphibole is after Ellis & Thompson, (1986). Chlorite + plagioclase + quartz + calcite=hornblende + zoisite + CO2 + vapour and Altot content of hornblende are after Plyusnina, (1982). Reactions in pelitic gneisses involving biotite and muscovite are according to Le Breton & Thompson, (1988) and Vielzeuf & Holloway, (1988), respectively. Mineral symbols are according to Kretz, (1983). As far as the metamorphic evolution of the host-rocks is concerned, whether they were associated with mafic rocks during the high-pressure stage or had a different evolution is one of the most debated aspects of high-pressure terranes (Smith, 1988). In the Lanterman Range, field relationships such as lenses of quartzites and quartzo-feldspathic gneisses in large mafic bodies may reflect primary intrusive relations between the mafic rocks and part of the host-rocks. The petrographic data and P–T estimates in pelitic and quartz-feldspathic lithologies, however, contain no evidence of an eclogite stage. Nevertheless, the lack of high-pressure relics in pelitic and quartzo-feldspathic rocks is expected, as kinetic and equilibrium factors tend to preserve high-pressure mineralogy more in mafic rocks than in intermediate–felsic rocks (Koons & Thompson, 1985). However, the presence of relics of kyanite and phengite (Si4+ ∼3.3 atoms per formula unit), and the microstructural features in the quartzo-feldspathic gneisses and garnet-bearing quartzites, in which garnet and phengite are surrounded by the symplectitic intergrowth consisting of biotite + plagioclase + quartz, are consistent (Godard, 1988) with a drop in pressure, and the reaction phengite + omphacite = biotite + plagioclase + quartz can be regarded as responsible for symplectite development on phengite. We therefore believe that the ‘in situ eclogite model’, in which the metabasites and at least part of the host-rocks first underwent pervasive eclogite-facies metamorphism, is more likely for the Lanterman Range. Temperatures up to 850°C would even have generated melting in the KFMASH system of pelitic rocks under fluid-absent conditions (Le Breton & Thompson, 1988; Vielzeuf & Holloway, 1988). In quartzo-feldspathic rocks, because of the high thermal stability of the assemblage biotite + plagioclase + quartz at pressures >10 kbar, temperatures well above 850°C would have been required to produce melt (Vielzeuf & Montel, 1994). The absence of evidence of melting in the pelitic gneisses supports the interpretation that the mafic rocks could only have been associated with quartzites and quartzo-feldspathic rocks during the eclogite facies stage, whereas the pelitic gneisses were possibly juxtaposed at a later stage during exhumation of the high-pressure rocks. The estimated P–T conditions for the two amphibolite stages in the host-gneisses overlap the physical conditions of mafic rocks of the corresponding stages (Fig. 9). The formation of rare migmatites at Carnes Crags and Mt Bernstein is attributed to the medium- to low-pressure amphibolite stages. In conclusion, the post-peak path of eclogites from the Lanterman Range is characterized by decreasing temperature and pressure, and overlaps the path of all the host-rocks from medium- to low-pressure amphibolite facies stage. As discussed by Koons & Thompson, (1985), this path probably reflects rapid tectonic exhumation and would be appropriate to retain equilibration temperatures related to the depth of formation. Significance of Sm/Nd and U/Pb mineral ages and age of the high-pressure metamorphism In the last 10 years, many papers have demonstrated the potential of the Sm–Nd system to give high-precision ages in garnet-bearing metamorphic rocks [see Mezger et al., (1992) and references therein]. Garnet is also one of the main minerals used to establish P–T paths in medium- to high-grade metamorphic rocks, allowing direct correlation with age determinations. Nevertheless, large variations in the closure temperature (Tc) hypothesized for garnet, i.e. from ∼600°C to ∼900°C (Cohen et al., 1988; Mezger et al., 1992; Hensen & Zhou, 1995), require careful evaluation of whether garnet dating represents the mineral growth or cooling age. This is particularly true for slow cooling terranes (1–5°C/m.y.), such as erosionally exhumed granulites, in which the Sm–Nd garnet age may considerably postdate mineral growth ages (Mezger et al., 1992; Burton et al., 1995). On the other hand, many papers on mafic eclogites have shown that, as the eclogite facies stage is usually followed by fast cooling, the Sm–Nd system on garnet records ages that are close to the high-pressure event (Griffin & Brueckner, 1985; Brueckner et al., 1991; Becker, 1993; Miller & Thöni, 1995; Schmädicke et al., 1995). Furthermore, Sm–Nd dates on high-pressure mafic rocks could be spurious where temperature was not high enough (see, e.g. Thöni & Jagoutz, 1992) or when the high-temperature stage was too short (see, e.g. Schmädicke et al., 1995) for complete equilibration of garnet with the other mineral phases (mainly clinopyroxene). Garnet could also preserve zoned Sm–Nd dates when the host-rock experienced multistage histories (Jamtveit et al., 1991; Brueckner et al., 1996) and, as a consequence, yield mixed ages if garnet is analysed as a whole. As far as the present eclogites are concerned, the petrographic data do not indicate a polymetamorphic history, for which much more scatter would be expected between the garnet fractions of a single sample and/or between the internal isochrons of the two samples. Besides, incomplete isotopic equibilibration during the eclogitization process is not compatible with the good fit of the internal isochrons. The theoretical Tc (Dodson, 1979) can be calculated for garnet provided diffusion data for Sm and Nd and cooling rates are known. In our case, diffusion data for the Sm and Nd at temperatures relevant to the eclogite stage and independent estimates of the post-eclogitic cooling rate are not available. However, many workers [e.g. Chakraborty & Ganguly, (1990) and Jamtveit et al., (1991)] agree that the diffusion parameters of Mg may represent the upper limit of the diffusion rate of Sm and Nd in garnet. Regarding the cooling rate, Goodge & Dallmeyer, (1996) inferred a cooling rate as fast as 30°C/m.y. on the basis of 40Ar/39Ar mineral ages for metamorphic rocks of the southeastern Lanterman Range after the amphibolite facies metamorphism. This estimate may be a minimum value, as a higher cooling rate can be expected for the initial stage of cooling after the eclogite event. Assuming the self-diffusion data for Mg from the combined data set of Chakraborty & Ganguly, (1990), spherical geometry and a cooling rate of 30°C/m.y., garnet diameters of 0.3 and 0.6 mm yield Tc of ∼670°C and ∼710°C, respectively. These temperatures can be regarded as the lower limit for Sm and Nd diffusion in garnet of the present eclogites. As mentioned above, the two well-preserved eclogites yield essentially identical 238U-206Pb rutile–whole-rock ages of ∼500 Ma that overlap the range of Sm–Nd garnet ages. Experimental diffusion data of U and Pb in rutile are not available, but comparison with other decay systems and/or different mineral ages strongly indicates that U–Pb ages of rutile reflect cooling below the Tc rather than mineral growth age (Mezger et al., 1989b, 1991). On the basis of direct comparison between U–Pb ages of rutiles and K–Ar in hornblende and biotite in slow cooling terranes (0.5–1°C/m.y.), Mezger et al., (1989b) inferred a closure temperature of ∼420°C for rutile with a radius of 0.09–0.21 mm. Assuming activation energies for U and Pb in rutile similar to those of Fe and Ti (in the range of 40–60 kcal/mol; Mezger et al., 1989b), a cooling rate of 30°C/m.y. and the largest rutile grain size observed (up to ∼0.2 mm), then the extrapolated Tc for rutile of the studied eclogites is around 500°C. The Tc calculated for garnet (670–710°C) and rutile (∼500°C), and the overlap of the rutile and garnet ages strongly support a high initial cooling rate for the eclogites, even higher than the 30°C/m.y. estimated by Goodge & Dallmeyer, (1996). If we also consider the petrographic observations and the P–T trajectory, fast cooling must have been accomplished by concomitant rapid exhumation. The above arguments support the interpretation that the age of ∼500 Ma obtained by the Sm–Nd method on garnet and the U–Pb method on rutile closely approaches the time of the eclogite facies metamorphism. Palaeo-Tectonic Implications The eclogites of the Lanterman Range are the first reported record of a well-preserved high-pressure metamorphic event along the Antarctic palaeo-Pacific margin of Gondwana. Indeed, to our knowledge, the only evidence of eclogites in Antarctica, albeit cryptic, is from the Nimrod Group of the central Transantarctic Mountains (Goodge et al., 1992). In the Tasmanian and eastern Australian segments of the palaeo-Pacific margin of Gondwana, mafic rocks preserving eclogite facies relics are known from the Tyennan region and the New England Fold Belt, respectively. The eclogites of Tasmania and eastern Australia show strong similarities with the new findings from Antarctica, as they have similar field characteristics (i.e. lenses in continental type metasediments and/or blocks in serpentinite melange) and P–T conditions of metamorphism (14–17 kbar, ∼700°C). The Tasmanian eclogites (Franklin Metamorphics), previously considered Neoproterozoic (780 Ma; Raheim & Compston, 1977) were recently dated by SHRIMP and yielded a zircon age of ∼500 Ma (Turner et al., 1995). The age of the Australian eclogites was inferred to be Early Ordovician or, more likely, Cambrian (Watanabe et al., 1993) from K–Ar age determinations on phengites from surrounding glaucophane schists. The pre-orogenic setting The pre-orogenic history of Lanterman metabasites is constrained by independent lines of evidence, including (1) the compositional features of the enclosing gneisses, (2) possible primary intrusive relations with part of the surrounding gneisses and (3) their geochemical compositions. As stated above, excluding the well-preserved eclogites, the retrogressed eclogites and amphibolites show a prevailing transitional- to E-type MORB affinity. Although the ‘MORB’ geochemical signature may be consistent with different geotectonic settings (open ocean, embryonic narrow ocean, back-arc basin), the geological relationships inferred between sill-like mafic intrusions, together with continental-type sediments and the predominance of samples with a transitional- to E-type MORB affinity are more consistent with an incipient oceanic basin rather than a large, mature, oceanic domain. The age hypothesized for protolith formation of ‘MORB-type’ Lanterman metabasites (700–750 Ma) is similar to those of other mafic rocks with similar geochemical character scattered throughout the Transantarctic Mountains (Cotton Plateau, Skelton Glacier area). This evidence supports the view that diffuse mafic magmatism of spreading setting type occurred along the Pacific margin of eastern Gondwana during Neoproterozoic time. Nevertheless, the data do not clarify the meaning of the protoliths of the well-preserved eclogites and their tectonic setting. The orogenic evolution In stressing the particular structural position of the eclogites in the mafic-ultramafic belt at the eastern margin of the Wilson Terrane, Ricci et al., (1997a) proposed that the orogenic history of Lanterman eclogites could be wholly included in the Ross orogenic cycle. The data presented in this paper substantiate this scenario, especially the interpretation, supported by the mineralogical and compositional features of the eclogites, that the geochronological data closely approximate the age of the eclogite facies metamorphism. Additional arguments supporting a Ross-linkage for the formation and exhumation ages of the Lanterman eclogites arise from the reconstructed P–T-time path of eclogites in comparison with the metamorphic evolution recorded by other rock types from other localities in the eastern Wilson Terrane. Broad complex zonation in terms of P–T regimes and partly contrasting post-peak P–T-time trajectories are significant features of the metamorphic pattern of the whole eastern margin of the Wilson Terrane [Ricci et al., (1996), fig. 3b, and references]. Specifically, cooling and unloading after a stage of moderate crustal thickening are recorded at Mt Murchison and in the north-eastern part of the Lanterman Range, and near-isothermal or slightly T-decreasing decompressional paths after a much deeper burial at Dessent Ridge and for the eclogites included in this study. In summary, rocks units which recorded contrasting P–T metamorphic regimes and P–T-time path are juxtaposed in the same structural setting along the boundary between the Wilson Terrane and Bowers Terrane. The mafic rocks occurring along this boundary are heterogeneous in terms of geochemical affinity and possibly also in age of formation. All these considerations suggest that the Lanterman eclogites belong to a major structural zone which formed through tectonic imbrication of rock units from different domainal provenance, with different metamorphic evolution and a distinctive exhumation path. In this framework and on geochronological grounds, the most likely geotectonic setting for the formation and subsequent fast exhumation of eclogites is a convergent plate margin. These rocks consistently document the subduction–accretional nature of the early Palaeozoic Ross Orogen in northern Victoria Land (Kleinschmidt & Tessensohn, 1987; Borg & DePaolo, 1994). The chronological coincidence of the formation and exhumation of eclogite outboard the continental margin, and the generation and emplacement of calc-alkaline granitoids inboard the margin are consistent evidence that the Ross Orogen developed in the framework of a long-lasting subduction–accretion process which affected the Antarctic margin of Gondwana around 500 m.y. ago. Acknowledgements We thank G. Vaggelli for help with wavelength-dispersive spectrometry analyses, and S. G. Borg, C. Ghezzo and S. Tonarini for comments on an earlier version of the manuscript. G. D. expresses special thanks to R. J. Smeets and J. C. van Belle for assistance in the clean laboratory, and, in particular, to G. R. Davies and T. Elliott for invaluable suggestions on sample preparation for isotope analysis. The authors are indebted to M. Thirlwall for providing two NdO+ isotope analyses on garnet used in this work. The manuscript was improved by thorough reviews by D. Ellis, D. Jacob-Foley and B. M. Jahn. This research was carried out with the financial support of the Italian ‘Programma Nazionale di Ricerche in Antartide’. References Ai Yang . A revision of the garnet–clinopyroxene Fe2+-Mg exchange geothermometer , Contributions to Mineralogy and Petrology , 1994 , vol. 115 (pg. 467 - 473 ) Google Scholar Crossref Search ADS WorldCat Armienti P. , Ghezzo C. , Innocenti F. , Manetti P. , Rocchi S. , Tonarini S. . Isotope geochemistry of granitoid suites from Granite Harbour Intrusives of the Wilson Terrane, North Victoria Land, Antarctica , European Journal of Mineralogy , 1990 , vol. 2 (pg. 103 - 123 ) Google Scholar Crossref Search ADS WorldCat Beccaluva L. , Ohnenstetter D. , Ohnenstetter M. . Geochemical discrimination between ocean floor and island-arc tholeiites—application to some ophiolites , Canadian Journal of Earth Sciences , 1979 , vol. 16 (pg. 1874 - 1882 ) Google Scholar Crossref Search ADS WorldCat Becker H. . Garnet peridotite and eclogite Sm–Nd mineral ages from Lepontine dome (Swiss Alps): new evidence from Eocene high-pressure metamorphism in the central Alps , Geology , 1993 , vol. 21 (pg. 599 - 602 ) Google Scholar Crossref Search ADS WorldCat Bernard-Griffiths J. , Pecaut J. J. , Ménot R. P. . Isotopic (Rb–Sr, U–Pb and Sm–Nd) and trace element geochemistry of eclogites from the pan-African Belt: a case study of REE fractionation during high-grade metamorphism , Lithos , 1991 , vol. 27 (pg. 43 - 57 ) Google Scholar Crossref Search ADS WorldCat Borg S. G. , DePaolo D. J. . Laurentia, Australia, and Antarctica as a late Proterozoic supercontinent: constraints from isotopic mapping , Geology , 1994 , vol. 22 (pg. 307 - 310 ) Google Scholar Crossref Search ADS WorldCat Borg S. G. , DePaolo D. J. , Smith B. . Isotopic structure and tectonics of the Central Transantarctic Mountains , Journal of Geophysical Research , 1990 , vol. 95 (pg. 6647 - 6667 ) Google Scholar Crossref Search ADS WorldCat Borsi L. , Petrini R. , Talarico F. , Palmeri R. . Geochemistry and Sr-Nd isotopes of amphibolite dykes of northern Victoria Land, Antarctica , Lithos , 1995 , vol. 35 (pg. 245 - 259 ) Google Scholar Crossref Search ADS WorldCat Bradshaw J. D. , Laird M. G. . Oliver R. H. , James P. R. , Jago J. B. . The pre-Beacon geology of northern Victoria Land: a review , Antarctic Earth Science , 1983 Canberra, A.C.T. Australian Academy of Sciences (pg. 98 - 101 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Brown E. H. . The crossite content of Ca-amphibole as a guide to pressure of metamorphism , Journal of Petrology , 1977 , vol. 18 (pg. 53 - 72 ) Google Scholar Crossref Search ADS WorldCat Brueckner H. K. , Medaris L. G. , Bakun-Czubarow N. . Nd and Sr age isotope patterns from Variscan eclogites of the eastern Bohemian Massif , Neues Jahrbuch für Mineralogie, Abhandlungen , 1991 , vol. 163 (pg. 169 - 196 ) Google Scholar OpenURL Placeholder Text WorldCat Brueckner H. K. , Blusztajn J. , Bakun-Czubarow N. . Trace element and Sm–Nd ‘age’ zoning in garnets from peridotites of Caledonian and Variscan Mountains and tectonic implications , Journal of Metamorphic Geology , 1996 , vol. 14 (pg. 61 - 73 ) Google Scholar Crossref Search ADS WorldCat Buggisch W. , Kleinschmidt G. . Thompson M. R. A. , Crame J. A. , Thompson J. W. . Recovery and recrystallization of quartz and ‘crystallinity’ of illite in the Bowers and Robertson Bay Terranes (northern Victoria Land) , Geological Evolution of Antarctica , 1989 Cambridge University Press (pg. 155 - 160 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Burton K. W. , O'Nions R. K. . High resolution garnet chronometry and the rates of metamorphic processes , Earth and Planetary Science Letters , 1991 , vol. 107 (pg. 649 - 671 ) Google Scholar Crossref Search ADS WorldCat Burton K. W. , Kohn M. J. , Cohen A. S. , O'Nions R. K. . The relative diffusion of Pb, Nd, Sr and O in garnet , Earth and Planetary Science Letters , 1995 , vol. 133 (pg. 199 - 211 ) Google Scholar Crossref Search ADS WorldCat Capponi G. , Castelli D. , Fioretti A. M. , Oggiano G. . Geological mapping and field relationships of the eclogites from the Lanterman Range (northern Victoria Land, Antarctica) , VII International Symposium on Antarctic Earth Sciences, Siena, 10–15 September, Abstracts. Siena: Terra Antarctica , 1995 pg. 73 Castelli D. , Lombardo B. , Oggiano G. , Rossetti P. , Talarico F. . Granulite facies rocks of the Wilson Terrane (northern Victoria Land, Antarctica): Campbell Glacier , Memorie della Società Geologica Italiana , 1991 , vol. 46 (pg. 197 - 203 ) Google Scholar OpenURL Placeholder Text WorldCat Chakraborty S. , Ganguly J. . Ganguly J. . Compositional zoning and cation diffusion in aluminosilicate garnets , Diffusion, Atomic Ordering and Mass Transfer, Advances in Physical Geochemistry, 8 , 1990 Berlin Springer-Verlag (pg. 120 - 175 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Chavagnac V. , Jahn B. M. . Coesite-bearing eclogites from the Bixiling Complex, Diabe Mountains, China: Sm–Nd ages, geochemical characteristics and tectonic implications , Chemical Geology , 1996 , vol. 133 (pg. 29 - 51 ) Google Scholar Crossref Search ADS WorldCat Cohen A. S. , O'Nions R. K. , Siegenthaler R. , Griffin W. L. . Chronology of the pressure–temperature history recorded by granulite terrain , Contributions to Mineralogy and Petrology , 1988 , vol. 98 (pg. 303 - 311 ) Google Scholar Crossref Search ADS WorldCat Coleman R. G. , Beatty L. B. , Brannock W. W. . Eclogites and eclogites: their differences and similarities , Geological Society of America Bulletin , 1965 , vol. 76 (pg. 483 - 508 ) Google Scholar Crossref Search ADS WorldCat Corfu F. , Muir T. L. . The Hemlo–Heron Bay greenstone belt and Hemlo Au–Mo deposit, Superior Province, Ontario, Canada. 2. Timing of metamorphism, alteration and Au mineralization from titanite, rutile, and monazite U–Pb geochronology , Chemical Geology (Isotope Geosciences Section) , 1989 , vol. 79 (pg. 201 - 223 ) Google Scholar Crossref Search ADS WorldCat DePaolo D. J. . A neodymium and strontium isotope study of the Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California , Journal of Geophysical Research , 1981 , vol. 86 (pg. 10470 - 10488 ) Google Scholar Crossref Search ADS WorldCat Dodson M. H. . Jäger E. , Hunziger J. . Theory of cooling ages , Lectures in Isotope Geology , 1979 Berlin Springer-Verlag (pg. 194 - 202 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Ellis D. J. , Green D. H. . An experimental study of the effect of Ca upon garnet–clinopyroxene Fe–Mg exchange equilibria , Contributions to Mineralogy and Petrology , 1979 , vol. 71 (pg. 13 - 22 ) Google Scholar Crossref Search ADS WorldCat Ellis D. J. , Thompson A. B. . Subsolidus and partial melting reactions in the quartz-excess CaO + MgO + Al2O3 + SiO2 + H2O system under water-excess and water-deficient conditions to 10 kbar: some implications for the origin of peraluminous melts from mafic rocks , Journal of Petrology , 1986 , vol. 27 (pg. 91 - 121 ) Google Scholar Crossref Search ADS WorldCat Essene E. J. , Fyfe W. S. . Omphacite in California metamorphic rocks , Contributions to Mineralogy and Petrology , 1967 , vol. 15 (pg. 1 - 23 ) Google Scholar Crossref Search ADS WorldCat Ferry J. M. , Spear F. S. . Experimental calibration of the partitioning of Fe and Mg between biotite and garnet , Contributions to Mineralogy and Petrology , 1978 , vol. 66 (pg. 113 - 117 ) Google Scholar Crossref Search ADS WorldCat Gebauer D. . Carswell D. A. . Isotopic system–geochronology of eclogites , Eclogite Facies Rocks , 1990 Glasgow Blackie (pg. 141 - 160 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Getty S. R. , Selverstone J. , Wernicke B. P. , Jacobsen S. B. , Aliberti, E. Lux D. R. . Sm–Nd dating of multiple garnet growth events in an arc–continent collision zone, northwestern U.S. Cordillera , Contributions to Mineralogy and Petrology , 1993 , vol. 115 (pg. 45 - 57 ) Google Scholar Crossref Search ADS WorldCat Ghent E. D. , Stout M. Z. . Geobarometry and geothermometry of plagioclase–biotite–garnet–muscovite assemblages , Contributions to Mineralogy and Petrology , 1981 , vol. 76 (pg. 92 - 97 ) Google Scholar Crossref Search ADS WorldCat Godard G. . Smith D. C. . Petrology of some eclogites in the Hercynides: the eclogites from the southern Armorican Massif, France , Eclogites and Eclogite-facies Rocks , 1988 Amsterdam Elsevier (pg. 451 - 510 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Goldstein R. K. , O'Nions R. K. , Hamilton P. J. . A Sm–Nd isotopic study of atmospheric dusts and particulates from major river systems , Earth and Planetary Science Letters , 1984 , vol. 70 (pg. 221 - 236 ) Google Scholar Crossref Search ADS WorldCat Goodge J. W. , Dallmeyer R. D. . Contrasting thermal evolution within the Ross Orogen, Antarctica: evidence from mineral 40Ar/39Ar ages , Journal of Geology , 1996 , vol. 104 (pg. 435 - 458 ) Google Scholar Crossref Search ADS WorldCat Goodge J. W. , Hansen V. L. , Peacock S. M. . Yoshida Y. , Kaminuma K. , Shiraishi K. . Multiple petrotectonic events in high-grade metamorphic rocks of the Nimrod Group, central Transantarctic Mountains, Antarctica , Recent Progress in Antarctic Earth Science , 1992 Tokyo Terrapub (pg. 203 - 209 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Graham C. M. , Powell R. . A garnet–hornblende geothermometer: calibration testing and application to the Pelona schist, southern California , Journal of Metamorphic Geology , 1984 , vol. 2 (pg. 13 - 31 ) Google Scholar Crossref Search ADS WorldCat Grew E. S. , Kleinschmidt G. , Schubert W. . Contrasting metamorphic belts in north Victoria Land , Geologisches Jahrbuch , 1984 , vol. B60 (pg. 253 - 263 ) Google Scholar OpenURL Placeholder Text WorldCat Griffin W. L. , Brueckner H. K. . REE, Rb–Sr and Sm–Nd studies of Norwegian eclogites , Chemical Geology (Isotope Geosciences Section) , 1985 , vol. 52 (pg. 249 - 271 ) Google Scholar Crossref Search ADS WorldCat Hebeda E. H. , Andriessen P. A. M. , van Belle J. C. . Simultaneous isotope dilution analysis of Sm and Nd with fixed multicollector mass spectrometer , Fresenius’ Zeitschrift für Analytische Chemie , 1988 , vol. 331 (pg. 114 - 117 ) Google Scholar Crossref Search ADS WorldCat Hensen B. J. , Zhou B. . Retention of isotopic memory in garnets partially broken down during an overprinting granulite-facies metamorphism: implication for the Sm–Nd closure temperature , Geology , 1995 , vol. 23 (pg. 225 - 228 ) Google Scholar Crossref Search ADS WorldCat Holdaway M. J. . Stability of andalusite and the aluminium silicate phase diagram , American Journal of Science , 1971 , vol. 271 (pg. 97 - 131 ) Google Scholar Crossref Search ADS WorldCat Holland T. J. B. . The experimental determination of activities in disordered and short-range ordered jadeitic pyroxenes , Contributions to Mineralogy and Petrology , 1983 , vol. 82 (pg. 214 - 220 ) Google Scholar Crossref Search ADS WorldCat Indares A. , Martignole J. . Biotite–garnet geothermometry in the granulite facies: the influence of Ti and Al in biotite , American Mineralogist , 1985 , vol. 70 (pg. 272 - 278 ) Google Scholar OpenURL Placeholder Text WorldCat Jamtveit B. , Carswell D. A. , Mearns E. W. . Chronology of the high-pressure metamorphism of Norwegian garnet peridotites/pyroxenites , Journal of Metamorphic Geology , 1991 , vol. 9 (pg. 125 - 139 ) Google Scholar Crossref Search ADS WorldCat Kalt A. , Hanel M. , Schleicher H. , Kramm U. . Petrology and geochronology of eclogites from the Variscan Schwarzwald (F.R.G.) , Contributions to Mineralogy and Petrology , 1994 , vol. 115 (pg. 287 - 302 ) Google Scholar Crossref Search ADS WorldCat Kleinschmidt G. , Tessensohn F. . McKenzie G. D. . Early Paleozoic westward directed subduction of the Pacific margin of Antarctica , Gondwana Six: Structure, Tectonics, and Geophysics. Geophysical Monograph, American Geophysical Union , 1987 , vol. 40 (pg. 89 - 105 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Kleinschmidt G. , Schubert W. , Olesch M. , Rettmann E. S. . Ultramafic rocks of the Lanterman Range in north Victoria Land, Antarctica: petrology, geochemistry and geodynamic implications , Geologisches Jahrbuch , 1987 , vol. B66 (pg. 231 - 273 ) Google Scholar OpenURL Placeholder Text WorldCat Koons P. O. , Thompson A. B. . Non-mafic rocks in the greenschist, blueschist and eclogite facies , Chemical Geology , 1985 , vol. 50 (pg. 3 - 30 ) Google Scholar Crossref Search ADS WorldCat Kretz R. . Symbols for rock-forming minerals , American Mineralogist , 1983 , vol. 68 (pg. 277 - 279 ) Google Scholar OpenURL Placeholder Text WorldCat Kreuzer H. , Hohndorf A. , Lenz H. , Muller P. , Vetter U. . McKenzie G. D. . Radiometric ages of pre-Mesozoic rocks from northern Victoria Land, Antarctica , Gondwana Six: Structure, Tectonics, and Geophysics. Geophysical Monograph, American Geophysical Union , 1987 , vol. 40 (pg. 31 - 47 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Krogh E. J. . The garnet–clinopyroxene Fe–Mg geothermometer—a reinterpretation of existing experimental data , Contributions to Mineralogy and Petrology , 1988 , vol. 99 (pg. 44 - 48 ) Google Scholar Crossref Search ADS WorldCat Leake B. E. . Nomenclature of amphiboles , American Mineralogist , 1978 , vol. 63 (pg. 1023 - 1052 ) Google Scholar OpenURL Placeholder Text WorldCat Le Breton N. , Thompson A. B. . Fluid-absent (dehydration) melting of biotite in metapelites in the early stages of crustal anatexis , Contributions to Mineralogy and Petrology , 1988 , vol. 99 (pg. 226 - 237 ) Google Scholar Crossref Search ADS WorldCat Ludwig K. R. . ISOPLOT: a plotting and regression program for radiogenic isotope data, for IBM-PC compatible computers, Version 2.02 , US Geological Survey, Open-File Report , 1990 (pg. 88 - 557 ) Ludwig K. R. , Cooper J. A. . Geochronology of Precambrian granites and associated U–Ti-Th mineralization, northern Olary province, South Australia , Contributions to Mineralogy and Petrology , 1984 , vol. 86 (pg. 298 - 308 ) Google Scholar Crossref Search ADS WorldCat Marchand J. . Persistence d'une série granulitique au coeur du Massif central français (Haut Allier) , 1974 pg. 207 Les termes acides. Thèse 3 cycle, University of Nantes Massonne H. J. , Schreyer W. . Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz , Contributions to Mineralogy and Petrology , 1987 , vol. 96 (pg. 212 - 224 ) Google Scholar Crossref Search ADS WorldCat Meschede M. . A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with Nb–Zr–Y diagram , Chemical Geology , 1986 , vol. 56 (pg. 207 - 218 ) Google Scholar Crossref Search ADS WorldCat Mezger K. , Hanson G. N. , Bohlen S. R. . U–Pb systematics of garnet: dating of growth of garnet in the Late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes, Canada , Contributions to Mineralogy and Petrology , 1989 , vol. 101 (pg. 136 - 148 ) Google Scholar Crossref Search ADS WorldCat Mezger K. , Hanson G. N. , Bohlen S. R. . High precision U–Pb ages of metamorphic rutile: application to the cooling history of high-grade terranes , Earth and Planetary Science Letters , 1989 , vol. 96 (pg. 106 - 118 ) Google Scholar Crossref Search ADS WorldCat Mezger K. , Rawnsley C. M. , Bohlen S. R. , Hanson G. N. . U–Pb garnet, sphene, monazite and rutile ages: implications for the duration of high-grade metamorphism and cooling histories, Adirondacks Mts, New York , Journal of Geology , 1991 , vol. 99 (pg. 415 - 428 ) Google Scholar Crossref Search ADS WorldCat Mezger K. , Essene E. J. , Halliday A. N. . Closure temperatures of the Sm–Nd system in metamorphic garnets , Earth and Planetary Science Letters , 1992 , vol. 113 (pg. 397 - 409 ) Google Scholar Crossref Search ADS WorldCat Michard A. , Gurriet M. , Soudant M. , Albarede F. . Nd isotopes in French Phanerozoic shales: external vs internal aspects of crustal evolution , Geochimica et Cosmochimica Acta , 1985 , vol. 49 (pg. 601 - 610 ) Google Scholar Crossref Search ADS WorldCat Miller C. , Thöni M. . Origin of eclogites from the Austroalpine Öztal basement (Tyrol, Austria): geochemistry and Sm–Nd vs Rb–Sr isotope systematics , Chemical Geology (Isotope Geosciences Section) , 1995 , vol. 122 (pg. 199 - 225 ) Google Scholar OpenURL Placeholder Text WorldCat Mullen E. D. . MnO/TiO2/P2O5: a minor element discriminant for basaltic rocks of oceanic environments and its implications for petrogenesis , Earth and Planetary Science Letters , 1983 , vol. 62 (pg. 53 - 62 ) Google Scholar Crossref Search ADS WorldCat Newton R. C. , Haselton H. T. . Newton R. C. , Navrotsky A. , Wood. B. J. . Thermodynamics of the garnet plagioclase–Al2SiO5–quartz geobarometer , Thermodynamics of Minerals and Melts. Advances in Geochemistry, 1 , 1981 Berlin Springer-Verlag (pg. 129 - 145 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Newton R. C. , Perkins D. . Thermodynamic calibration of geobarometers based on the assemblages garnet–plagioclase–orthopyroxene–(clinopyroxene)-nquartz , American Mineralogist , 1982 , vol. 67 (pg. 203 - 222 ) Google Scholar OpenURL Placeholder Text WorldCat Palmeri R. , Pertusati P. C. , Ricci C. A. , Talarico F. . Late Proterozoic (?)–early Paleozoic evolution of the active Pacific margin of Gondwana: evidence from the southern Wilson Terrane (northern Victoria Land, Antarctica) , Terra Antartica , 1994 , vol. 1 (pg. 5 - 9 ) Google Scholar OpenURL Placeholder Text WorldCat Paquette J. L. , Menot R. P. , Peucat J. J. . REE, Sm–Nd and U–Pb zircon study of eclogites from the Alpine External Massifs (Western Alps): evidence for crustal contamination , Earth and Planetary Science Letters , 1989 , vol. 96 (pg. 181 - 198 ) Google Scholar Crossref Search ADS WorldCat Pearce J. A. . Basalt geochemistry used to investigate past tectonic environment on Cyprus , Tectonophysics , 1975 , vol. 25 (pg. 41 - 67 ) Google Scholar Crossref Search ADS WorldCat Pearce J. A. . Thorpe R. S. . Trace element characteristics of lavas from destructive plate boundaries , Andesite: Orogenic Andesites and Related Rocks , 1982 Chichester, UK John Wiley (pg. 525 - 548 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Pearce J. A. . Hawkesworth C. J. , Norry M. J. . Role of sub-continental lithosphere in magma genesis at active continental margins , Continental Basalts and Mantle Xenoliths , 1983 Nantwich, UK Shiva (pg. 209 - 229 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Pearce J. A. , Cann J. R. . Tectonic setting of basic volcanic rocks determined using trace element analysis , Earth and Planetary Science Letters , 1973 , vol. 19 (pg. 290 - 300 ) Google Scholar Crossref Search ADS WorldCat Pearce J. A. , Norry M. J. . Petrogenetic implications of Ti, Zr, Y and Nb variation in volcanic rocks , Contributions to Mineralogy and Petrology , 1979 , vol. 69 (pg. 33 - 47 ) Google Scholar Crossref Search ADS WorldCat Perchuck L. L. , Aranovich L. Y. A. , Podleskii K. K. , Lavrent'eva I. V. , Gerasimov V. Y. , Kitsul V. I. , Karsakov L. P. , Berokinov N. V. . Precambrian granulites of the shields, eastern Siberia, USSR , Journal of Metamorphic Geology , 1985 , vol. 3 (pg. 265 - 310 ) Google Scholar Crossref Search ADS WorldCat Plyusnina L. P. . Geothermometry and geobarometry of plagioclase–hornblende-bearing assemblages , Contributions to Mineralogy and Petrology , 1982 , vol. 80 (pg. 140 - 146 ) Google Scholar Crossref Search ADS WorldCat Powell R. . Regression diagnostic and robust regression in geothermometer/geobarometer calibration: the garnet–clinopyroxene geothermometer revisited , Journal of Metamorphic Geology , 1985 , vol. 3 (pg. 327 - 342 ) Google Scholar Crossref Search ADS WorldCat Raheim A. , Compston W. . Correlation between metamorphic events and Rb–Sr ages in metasediments and eclogites from western Tasmania , Lithos , 1977 , vol. 10 (pg. 271 - 289 ) Google Scholar Crossref Search ADS WorldCat Ricci C. A. , Talarico F. , Palmeri R. . Ricci C. A. . Tectonothermal evolution of the Antarctic palaeo-Pacific active margin of Gondwana: a northern Victoria Land perspective. The Antarctic Region: Geological Evolution and Processes , Siena: Terra Antarctica special issue in press Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Ricci C. A. , Tessensohn F. . The Lanterman–Mariner suture. Antarctic evidence for Paleozoic active margin of Gondwana , Geologisches Jahrbuch , 1997 in press Ricci C. A. , Talarico F. , Palmeri R. , Di Vincenzo G. , Pertusati P. C. . Eclogite at the Antarctic palaeo-Pacific margin of Gondwana (Lanterman Range, northern Victoria Land, Antarctica) , Antarctic Science , 1996 , vol. 8 3 (pg. 277 - 280 ) Google Scholar Crossref Search ADS Roermund H. L. M. , van Boland J. N. . Retrograde P–T trajectories of high temperature eclogites deduced from omphacite exsolution microstructures , Bulletin Minéralogique , 1983 , vol. 106 (pg. 723 - 726 ) Google Scholar OpenURL Placeholder Text WorldCat Roland R. W. , Gibson G. M. , Kleinschmidt G. , Schubert W. . Metamorphism and structural relations of the Lanterman metamorphics, north Victoria Land, Antarctica , Geologisches Jahrbuch , 1984 , vol. B60 (pg. 319 - 361 ) Google Scholar OpenURL Placeholder Text WorldCat Rowell A. J. , Rees M. N. , Duebendorfer E. M. , Wallin E. T. , van Schmus W. R. , Smith E. I. . An active Neoproterozoic margin: evidence from the Skelton Glacier area, Transantarctic Mountains , Journal of the Geological Society, London , 1993 , vol. 150 (pg. 677 - 682 ) Google Scholar Crossref Search ADS WorldCat Ryburn R. J. , Raheim A. , Green D. H. . Determination of the P–T paths of natural eclogites during metamorphism, a record of subduction. A correction , Lithos , 1976 , vol. 9 (pg. 161 - 165 ) Google Scholar Crossref Search ADS WorldCat Saunders A. D. . Henderson P. . The rare earth element characteristics of igneous rocks from the ocean basins , Rare Earth Element Geochemistry , 1984 Amsterdam Elsevier (pg. 205 - 236 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Schmädicke E. , Mezger K. , Cosca M. A. , Okrusch M. . Variscan Sm–Nd and Ar–Ar ages of eclogite facies rocks from the Erzgebirge, Bohemian Massif , Journal of Metamorphic Geology , 1995 , vol. 13 (pg. 537 - 552 ) Google Scholar Crossref Search ADS WorldCat Shervais J. K. . Ti–V plots and the petrogenesis of modern and ophiolitic lavas , Earth and Planetary Science Letters , 1982 , vol. 59 (pg. 101 - 118 ) Google Scholar Crossref Search ADS WorldCat Smith D. C. . Smith D. C. . A review of the peculiar mineralogy of the ‘Norwegian coesite–eclogite province’, with crystal-chemical, petrological, geochemical and geodynamical notes and an extensive bibliography , Eclogites and Eclogite-facies Rocks , 1988 Amsterdam Elsevier (pg. 1 - 178 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Spear F. S. . NaSi–CaAl exchange equilibrium between plagioclase and amphiboles. An empirical model , Contributions to Mineralogy and Petrology , 1980 , vol. 72 (pg. 33 - 41 ) Google Scholar Crossref Search ADS WorldCat Sun S. S. , McDonough W. F. . Saunders A. D. , Norry M. J. . Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes , Magmatism in the Ocean Basins. Geological Society, London, Special Publication , 1989 , vol. 42 (pg. 315 - 345 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Talarico F. , Franceschelli M. , Lombardo B. , Palmeri R. , Pertusati P. C. , Rastelli N. , Ricci C. A. . Yoshida Y. , Kaminuma K. , Shiraishi K. . Metamorphic facies of the Ross Orogeny in the southern Wilson Terrane of northern Victoria Land, Antarctica , Recent Progress in Antarctic Earth Science , 1992 Tokyo Terrapub (pg. 211 - 218 ) Google Scholar Google Preview OpenURL Placeholder Text WorldCat COPAC Thirlwall M. F. . High-precision multicollector isotopic analysis of low levels of Nd as oxide , Chemical Geology , 1991 , vol. 94 (pg. 13 - 22 ) Google Scholar Crossref Search ADS WorldCat Thöni M. , Jagoutz E. . Some new aspects of dating eclogites in orogenic belts: Sm–Nd, Rb–Sr and Pb–Pb isotopic results from the Austroalpine Saualpe and Koralpe type-locality (Carinthia/Styria, southern Austria) , Geochimica et Cosmochimica Acta , 1992 , vol. 56 (pg. 347 - 368 ) Google Scholar Crossref Search ADS WorldCat Tonarini S. , Villa I. M. , Oberli F. , Meier M. , Spencer D. A. , Pognante U. , Ramsay J. G. . Eocene age of eclogite metamorphism in Pakistan Himalaya: implications for India–Eurasia collision , Terra Nova , 1993 , vol. 5 (pg. 13 - 20 ) Google Scholar Crossref Search ADS WorldCat Turner N. J. , Black L. P. , Kamperman M. . Pre-middle Cambrian stratigraphy, orogenesis and geochronology in western Tasmania , Geological Society of Australia, Tasmanian Division, Contentious Issues in Tasmanian. Geology a Symposium , 1995 (pg. 51 - 56 ) Ungaretti L. , Smith D. C. , Rossi G. . Crystal-chemistry by X-ray structure refinement and electron microprobe analysis of a series of sodic–calcic to alkali-amphiboles from the Nybo eclogite pod, Norway , Bulletin Minéralogique , 1981 , vol. 104 (pg. 400 - 412 ) Google Scholar OpenURL Placeholder Text WorldCat Vance D. , Holland T. . A detailed isotopic and petrological study of a single garnet from the Gassets Schist, Vermont , Contributions to Mineralogy and Petrology , 1993 , vol. 114 (pg. 101 - 118 ) Google Scholar Crossref Search ADS WorldCat Vance D. , O'Nions R. K. . Prograde and retrograde thermal histories from the central Swiss Alps , Earth and Planetary Science Letters , 1992 , vol. 114 (pg. 113 - 129 ) Google Scholar Crossref Search ADS WorldCat Vidal P. , Hunziker J. C. . Systematics and problems in isotope work on eclogites , Chemical Geology (Isotope Geosciences Section) , 1985 , vol. 52 (pg. 129 - 141 ) Google Scholar Crossref Search ADS WorldCat Vielzeuf D. , Holloway J. R. . Experimental determination of the fluid-absent melting relations in the pelitic system , Contributions to Mineralogy and Petrology , 1988 , vol. 98 (pg. 257 - 276 ) Google Scholar Crossref Search ADS WorldCat Vielzuef D. , Montel J. M. . Partial melting of metagreywackes. Part I. Fluid-absent experiments and phase relationships , Contributions to Mineralogy and Petrology , 1994 , vol. 117 (pg. 375 - 393 ) Google Scholar Crossref Search ADS WorldCat Watanabe T. , Leicht E. C. , Itaya T. , Fukui S. . Eclogite and glaucophane schist from eastern Australia: evidence for early Paleozoic convergence at eastern Gondwana margin. Fourth International Eclogite Conference, Cosenza 6–9 September , Terra Abstract , 1993 , vol. 4 pg. 28 supplement to Terra Nova 5 Weaver S. D. , Bradshaw J. D. , Laird M. G. . Geochemistry of Cambrian volcanics of the Bowers Supergroup and implications for the early Paleozoic tectonic evolution of northern Victoria Land, Antarctica , Earth and Planetary Science Letters , 1984 , vol. 68 (pg. 128 - 140 ) Google Scholar Crossref Search ADS WorldCat Wendt I. , Carl C. . The statistical distribution of the mean squared weighted deviation , Chemical Geology (Isotope Geosciences Section) , 1991 , vol. 86 (pg. 275 - 285 ) Google Scholar Crossref Search ADS WorldCat Wood D. A. , Joron J. L. , Treuil M. . A re-appraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings , Earth and Planetary Science Letters , 1979 , vol. 45 (pg. 326 - 336 ) Google Scholar Crossref Search ADS WorldCat Appendix: Analytical Techniques Mineral analyses were performed on a JEOL JX 8600 electron microprobe fitted with four wavelength-dispersive spectrometers at the Department of Earth Sciences of Florence. Accelerating voltage was 15 kV and sample current 10 nA. Natural standards were used for calibration. Major and trace elements were determined by inductively coupled plasma (ICP)-emission spectrometry (major elements and Sc) and ICP-mass spectrometry at the CRPG (Vandœuvre-les-Nancy, France), except for K2O and loss on ignition, which were determined at the Department of Earth Sciences of Siena by atomic absorption spectrometry and by gravimetry at 1000°C after pre-heating at 110°C, respectively. Mineral and chemical separation was achieved at the Faculteit der Aardwetenschappen of the Vrije Universiteit of Amsterdam. Minerals were concentrated from the grain size fraction of 63–125 µm using a Frantz Isodynamic separator and heavy liquids, and were carefully purified by handpicking under a binocular microscope, cleaned ultrasonically and dried with ultrapure acetone. All mineral separates were washed in warm 2N HCl for about 30 min. Whole rocks and rutile were digested in PTF Teflon bombs at 220°C using a mixture of concentrated HF and HNO3. All the other mineral separates were digested with a mixture of concentrated HF and HNO3 in screw-top PFA Teflon beakers on a hot plate. Two separate aliquots were spiked with mixed 150Sm-148Nd and 235U-208Pb spikes, respectively. Single 235U and 208Pb spikes were used for whole rocks. The REE were separated as a group using TRU-SPEC (medium) chromatographic extraction material and 2N HNO3 as eluant, whereas Sm and Nd were separated by HDEHP Teflon columns. Pb was separated by HCl–HBr chemistry using AG1x8 (200–400 mesh) anion exchange resins. U was separated using U-TEVA SPEC (medium) chromatographic extraction material and 2N HNO3 as eluant. Sample weights were in the 0.2–0.5 g range for rutile, clinopyroxene, amphibole and whole rock, and 1.3–1.5 g and ∼0.5 g for garnet in the Sm–Nd and U–Pb determinations, respectively. Sm and Nd blanks were <50 pg and <100 pg for garnet, and <20 pg and <50 pg for the other minerals or the whole rocks, respectively. Pb blanks were 100 pg for whole rocks and ≤ 300 pg for rutile and garnet. U blank was <50 pg. All isotope analyses except for two garnet Nd isotope compositions (Grt/d, Table 5) that were run as NdO+ at the Department of Geology, Royal Holloway University of London, following the procedure of Thirlwall, (1991), were performed at the Faculteit der Aardwetenschappen of the Vrije Universiteit of Amsterdam. Mass spectometry analyses were performed with a Finnigan Mat 261 multicollector mass spectrometer. Nd was run as metal and the isotope ratios were normalized to 146Nd/144Nd=0.7219. During the course of this study La Jolla Nd standard gave an average of 0.511848±8 (2σ, n=6). For all isochron calculations a minimum uncertainty of ±0.002% (2σ) was assumed for isotope compositions. Sm and Nd concentrations were determined following the procedure of Hebeda et al., (1988). The uncertainty for the Sm/Nd ratio was taken to be ±0.4%. The Sm/Nd ratio for rock standard BHVO-1 was 0.2472 (Sm=6.121 p.p.m., Nd=24.76 p.p.m.). All measured Pb isotope ratios were corrected with a mass fractionation factor of 0.14±0.01% per mass unit based on repeated analyses of the NBS SRM 981 Pb standard, and for blank contribution. The blank correction was insignificant for whole-rock data. Reproducibility of the 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb ratios for the standard was within 0.12% (2σ), 0.02% (2σ) for the 207Pb/206Pb ratio and 0.03% (2σ) for the 208Pb/206Pb ratio, and was assumed as minimum uncertainty for age calculations. U was run with an electron multiplier and corrected with a mass fractionation factor of 0.21% per mass unit based on repeated analyses of the U500 standard. The estimated error on U/Pb ratios was less than ±1%. Isochron calculations were performed using the ISOPLOT program of Ludwig, (1990). © Oxford University Press 1997 © Oxford University Press 1997
TI - Petrology and Geochronology of Eclogites from the Lanterman Range, Antarctica
JF - Journal of Petrology
DO - 10.1093/petroj/38.10.1391
DA - 1997-10-01
UR - https://www.deepdyve.com/lp/oxford-university-press/petrology-and-geochronology-of-eclogites-from-the-lanterman-range-d2Pp87Cqrv
SP - 1391
EP - 1417
VL - 38
IS - 10
DP - DeepDyve
ER -