TY - JOUR
AU1 - Badenszki,, Eszter
AU2 - Daly, J, Stephen
AU3 - Whitehouse, Martin, J
AU4 - Kronz,, Andreas
AU5 - Upton, Brian G, J
AU6 - Horstwood, Matthew S, A
AB - Abstract Deep crustal felsic xenoliths from classic Scottish Midland Valley localities, carried to the surface by Permo-Carboniferous magmatism, are shown for the first time to include metaigneous varieties with dioritic and tonalitic protoliths. Four hypotheses regarding their origin have been evaluated: (1) Precambrian basement; (2) Permo-Carboniferous underplating; (3) ‘Newer Granite’ magmatism; (4) Ordovician arc magmatism. U–Pb zircon dating results rule out the Precambrian basement and Permo-Carboniferous underplating hypotheses, but establish that the meta-igneous xenoliths represent both ‘Newer Granite’ and Ordovician (to possibly Silurian) arc magmatism. The metadiorite xenoliths are shown to have protolith ages of c. 415 Ma with εHft zircon values ranging from +0·1 to +11·1. These are interpreted to represent unexposed ‘Newer Granite’ plutons, based on age, mineralogical, isotopic and geochemical data. This shows that Devonian ‘Newer Granite’ magmatism had a greater impact on the Midland Valley and Southern Uplands crust than previously realized. Clinopyroxene–plagioclase–quartz barometry on the metadiorites from the east and west of the Midland Valley yielded a similar pressure range of c. 5–10 kbar, and a metadiorite from the east yielded a minimum two-feldspar temperature estimate of c. 793–816°C. These results indicate that the metadiorites once resided in the middle–lower crust. In contrast, two metatonalite xenoliths have a Late Ordovician protolith age (c. 453 Ma), with zircon εHft values of +7·8 to +9·0. These are interpreted as samples of a buried Late Ordovician magmatic arc situated within the Midland Valley. Inherited zircons with similar Late Ordovician ages and εHft=453 values (+1·6 to +10·8) are present in the metadiorites, suggesting that the Devonian ‘Newer Granites’ intruded within or through this Late Ordovician Midland Valley arc. A younger protolith age of c. 430 Ma from one of the metatonalites suggests that arc activity continued until Silurian times. This validates the long-standing ‘arc collision’ hypothesis for the development of the Caledonian Orogen. Based on U–Pb zircon dating, the metatonalite and metadiorite xenoliths have both experienced metamorphism between c. 400 and c. 391 Ma, probably linked to the Acadian Orogeny. An older phase of metamorphism at c. 411 Ma was possibly triggered by the combined effects of heating owing to the emplacement of the ‘Newer Granite’ plutons and the overthrusting of the Southern Uplands terrane onto the southern margin of the Midland Valley terrane. INTRODUCTION The Midland Valley terrane (Fig. 1) has played a key role in the early Palaeozoic tectonic evolution of the Caledonian Orogen. During the Grampian Orogeny (the c. 480–470 Ma, early phase of the Caledonian Orogeny), the Midland Valley terrane is thought to have collided with the passive continental margin of Laurentia (e.g. Bluck et al., 1980), after which it has been proposed to have acted as an arc terrane at least until the Middle Ordovician (Bluck, 1983, 2013; Halliday, 1984; Stephens & Halliday, 1984; Thirlwall, 1988; Haughton & Halliday, 1991; Phillips et al., 2004, 2009). The putative arc is not exposed, as the Midland Valley terrane is mostly covered by Upper Palaeozoic rocks and younger sediments (Fig. 2). However, the arc hypothesis remains an enduring paradigm to explain the evolution of the Caledonian Orogen (e.g. Hollis et al., 2012). The classic Scottish xenolith suite from the Midland Valley and Southern Uplands includes highly diverse protolith compositions (ultramafic to felsic; igneous; metasedimentary) with a range of depths of origin (mantle to deep crust). This valuable archive has been the subject of intense study for the past four decades (e.g. Upton et al., 1976, 1983, 1999; Graham & Upton, 1978; Hunter et al., 1984; Halliday et al., 1993; Lee et al., 1993; Downes et al., 2001, 2007). Early studies were not always able to distinguish the nature of the xenolith protoliths, although several studies did recognize mafic lower crustal and mantle xenoliths (e.g. Upton et al., 1983; Hunter et al., 1984). The East Lothian and Ayrshire xenoliths (Fig. 2), carried to the surface by Permo-Carboniferous volcanism, were among the earliest described (Upton et al., 1976; Graham & Upton, 1978). Previous studies have focused on their mineral and whole-rock geochemistry and radiogenic (Sr, Nd, Pb) isotopic compositions (Hunter et al., 1984; Halliday et al., 1993; Lee et al., 1993; Downes et al., 2001). Fig. 1 Open in new tabDownload slide Sketch map of part of Ireland and northern Britain showing Caledonian terranes (terrane names in bold italics), terrane boundaries, selected major structural lines, geological features discussed in the text, outcrops of the Siluro-Devonian ‘Newer Granite’ plutons (Brown et al., 2008; Oliver et al., 2008; Chew & Stillman, 2009), significant xenolith localities, and the LISPB (Bamford et al., 1978) and WINCH seismic profiles (Hall et al., 1984). Fig. 1 Open in new tabDownload slide Sketch map of part of Ireland and northern Britain showing Caledonian terranes (terrane names in bold italics), terrane boundaries, selected major structural lines, geological features discussed in the text, outcrops of the Siluro-Devonian ‘Newer Granite’ plutons (Brown et al., 2008; Oliver et al., 2008; Chew & Stillman, 2009), significant xenolith localities, and the LISPB (Bamford et al., 1978) and WINCH seismic profiles (Hall et al., 1984). Fig. 2 Open in new tabDownload slide Simplified geological map of southern Scotland showing the Lower Palaeozoic geology, modified after the Bedrock Geology Map (British Geological Survey, 2007) and Floyd (1994). Deep crustal xenolith localities discussed in this paper (PC, Partan Craig; HSH, Horseshoe Vent; BC, Beggar’s Cap; TH, Tollis Hill; BdH, Baidland Hill; HoA, Heads of Ayr, HN, Hawk’s Nib) are shown together with relevant published isotopic ages of Siluro-Devonian ‘Newer Granite’ and related volcanic rocks. Dotted lines, county boundaries of Ayrshire and East Lothian; dashed line, LISPB seismic profile (Bamford et al., 1978). Fig. 2 Open in new tabDownload slide Simplified geological map of southern Scotland showing the Lower Palaeozoic geology, modified after the Bedrock Geology Map (British Geological Survey, 2007) and Floyd (1994). Deep crustal xenolith localities discussed in this paper (PC, Partan Craig; HSH, Horseshoe Vent; BC, Beggar’s Cap; TH, Tollis Hill; BdH, Baidland Hill; HoA, Heads of Ayr, HN, Hawk’s Nib) are shown together with relevant published isotopic ages of Siluro-Devonian ‘Newer Granite’ and related volcanic rocks. Dotted lines, county boundaries of Ayrshire and East Lothian; dashed line, LISPB seismic profile (Bamford et al., 1978). Badenszki (2014) distinguished three xenolith protoliths: metasediments and two meta-igneous varieties (metatonalites and metadiorites), of which the meta-igneous types are the subject of this study. The majority of the metadiorites investigated here (Table 1) are from East Lothian and Ayrshire, although a single xenolith comes from Tollis Hill (Fig. 2). Metatonalites were found exclusively in East Lothian (Fig. 2, Table 1). The East Lothian metasedimentary xenoliths (Graham & Upton, 1978; Van Breemen & Hawkesworth, 1980; Badenszki, 2014) will be discussed separately (Badenszki et al., in preparation). Table 1 Mineral modes and analytical techniques applied on metadiorite and metatonalite xenoliths Xenolith name Locality* Region† Mineral modes‡ Applied analytical techniques Alteration U–(Th)–Pb zircon geochronology Pl Qtz Cpx Fe–Ti oxide Zrn Ap Bt Kfs Cal Srp Zeo Gl Total alt. Total XRF whole-rock EPMA MC-LA-ICP-MS SIMS Zircon Lu–Hf isotopic analyses Sm–Nd and Rb–Sr isotopic analyses Pressure Temperature Metadiorites PC-00 PC EL 74 9 <1 2 <1 <1 — — 14 1 — — 15 100 — √ — — — — √ — PCW-13¶ PC EL 60 <1 <1 3 <1 <1 — — 31 6 — — 37 100 — √ — — — — √ — PCW-54¶ PC EL 65 20 <1 4 <1 <1 — — 10 1 — — 11 100 — — √ √ √ — — — PCW-70¶ PC EL 61 8 <1 3 <1 <1 — — 25 3 — — 28 100 √ — √ √ √ √ — — PCW-97|¶ PC EL 56 13 <1 4 <1 <1 — — 24 3 — — 27 100 — √ — — — — √ — PC-355§ PC EL 74 — — 5 <1 <1 1 — 3 17 — — 20 100 √ √ √ — √ √ — — HSH-1 HSH EL 74 — — <1 <1 <1 — — 25 1 — — 26 100 √ — √ √ — — — — HSH-3 HSH EL 46 <1 <1 <1 <1 <1 — — 41 13 — — 54 100 — — — — — — — — HSH-48 HSH EL 59 — — 2 1 <1 — 7 31 0 — — 31 100 — √ √ — — — — √ HSHe-27|¶ HSH EL 66 1 3 2 <1 <1 — — 27 1 — — 28 100 — √ — — — — √ — BC-7 BC EL 34 31 — — <1 <1 — — 32 3 — — 35 100 — √ √ — — — — — BdHe-16¶ BdH Ay 52 — 33 5 <1 <1 — — — — — 10 10 100 — √ — √ √ — — — HoA-3¶ HoA Ay 46 45 2 1 <1 <1 — — 1 5 — — 6 100 √ √ √ √ √ √ √ — TH-25 TH CB 58 — — 7 <1 <1 — — 35 0 — — 35 100 — — √ √ √ — — — Metatonalites PC-378§ PC EL 41 52 — 1 <1 <1 — 2 4 — — — 4 100 √ — √ — — √ — — PC-502 PC EL 57 34 — <1 <1 <1 — 3 6 — — — 6 100 √ — √ √ √ √ — — PC-503 PC EL 42 50 — <1 <1 <1 — 3 5 — — — 5 100 √ — √ √ √ √ — — BC-9 BC EL 60 30 — <1 <1 <1 — — 10 — <1 — 10 100 √ √ √ √ — √ — — Xenolith name Locality* Region† Mineral modes‡ Applied analytical techniques Alteration U–(Th)–Pb zircon geochronology Pl Qtz Cpx Fe–Ti oxide Zrn Ap Bt Kfs Cal Srp Zeo Gl Total alt. Total XRF whole-rock EPMA MC-LA-ICP-MS SIMS Zircon Lu–Hf isotopic analyses Sm–Nd and Rb–Sr isotopic analyses Pressure Temperature Metadiorites PC-00 PC EL 74 9 <1 2 <1 <1 — — 14 1 — — 15 100 — √ — — — — √ — PCW-13¶ PC EL 60 <1 <1 3 <1 <1 — — 31 6 — — 37 100 — √ — — — — √ — PCW-54¶ PC EL 65 20 <1 4 <1 <1 — — 10 1 — — 11 100 — — √ √ √ — — — PCW-70¶ PC EL 61 8 <1 3 <1 <1 — — 25 3 — — 28 100 √ — √ √ √ √ — — PCW-97|¶ PC EL 56 13 <1 4 <1 <1 — — 24 3 — — 27 100 — √ — — — — √ — PC-355§ PC EL 74 — — 5 <1 <1 1 — 3 17 — — 20 100 √ √ √ — √ √ — — HSH-1 HSH EL 74 — — <1 <1 <1 — — 25 1 — — 26 100 √ — √ √ — — — — HSH-3 HSH EL 46 <1 <1 <1 <1 <1 — — 41 13 — — 54 100 — — — — — — — — HSH-48 HSH EL 59 — — 2 1 <1 — 7 31 0 — — 31 100 — √ √ — — — — √ HSHe-27|¶ HSH EL 66 1 3 2 <1 <1 — — 27 1 — — 28 100 — √ — — — — √ — BC-7 BC EL 34 31 — — <1 <1 — — 32 3 — — 35 100 — √ √ — — — — — BdHe-16¶ BdH Ay 52 — 33 5 <1 <1 — — — — — 10 10 100 — √ — √ √ — — — HoA-3¶ HoA Ay 46 45 2 1 <1 <1 — — 1 5 — — 6 100 √ √ √ √ √ √ √ — TH-25 TH CB 58 — — 7 <1 <1 — — 35 0 — — 35 100 — — √ √ √ — — — Metatonalites PC-378§ PC EL 41 52 — 1 <1 <1 — 2 4 — — — 4 100 √ — √ — — √ — — PC-502 PC EL 57 34 — <1 <1 <1 — 3 6 — — — 6 100 √ — √ √ √ √ — — PC-503 PC EL 42 50 — <1 <1 <1 — 3 5 — — — 5 100 √ — √ √ √ √ — — BC-9 BC EL 60 30 — <1 <1 <1 — — 10 — <1 — 10 100 √ √ √ √ — √ — — * Localities: PC, Partan Craig; HSH, Horseshoe Vent; BC, Beggar’s Cap; BdH, Baidland Hill; HoA, Heads of Ayr; TH, Tollis Hill. † Regions: EL, East Lothian; Ay, Ayrshire; CB, Central Belt of the Southern Uplands. ‡ Mineral modes were measured by point counting thin sections, n = 1000, with the exception of BC-9 where mineral modes are estimated as xenolith was too fine-grained for point counting. Mineral abbreviations: Pl, plagioclase; Qtz, quartz; Cpx, clinopyroxene; Zrn, zircon; Ap, apatite; Bt, biotite; Kfs, K-feldspar; Cal, calcite; Srp, serpentine; Zeo, zeolite; Gl, glass. § Xenoliths analysed for whole-rock Rb–Sr, Sm–Nd and Pb by Halliday et al. (1993). ¶ Samples collected by the authors. The rest of the samples are from the collections of Brian Upton stored at the Rock Collection of the British Geological Survey, Edinburgh. Open in new tab Table 1 Mineral modes and analytical techniques applied on metadiorite and metatonalite xenoliths Xenolith name Locality* Region† Mineral modes‡ Applied analytical techniques Alteration U–(Th)–Pb zircon geochronology Pl Qtz Cpx Fe–Ti oxide Zrn Ap Bt Kfs Cal Srp Zeo Gl Total alt. Total XRF whole-rock EPMA MC-LA-ICP-MS SIMS Zircon Lu–Hf isotopic analyses Sm–Nd and Rb–Sr isotopic analyses Pressure Temperature Metadiorites PC-00 PC EL 74 9 <1 2 <1 <1 — — 14 1 — — 15 100 — √ — — — — √ — PCW-13¶ PC EL 60 <1 <1 3 <1 <1 — — 31 6 — — 37 100 — √ — — — — √ — PCW-54¶ PC EL 65 20 <1 4 <1 <1 — — 10 1 — — 11 100 — — √ √ √ — — — PCW-70¶ PC EL 61 8 <1 3 <1 <1 — — 25 3 — — 28 100 √ — √ √ √ √ — — PCW-97|¶ PC EL 56 13 <1 4 <1 <1 — — 24 3 — — 27 100 — √ — — — — √ — PC-355§ PC EL 74 — — 5 <1 <1 1 — 3 17 — — 20 100 √ √ √ — √ √ — — HSH-1 HSH EL 74 — — <1 <1 <1 — — 25 1 — — 26 100 √ — √ √ — — — — HSH-3 HSH EL 46 <1 <1 <1 <1 <1 — — 41 13 — — 54 100 — — — — — — — — HSH-48 HSH EL 59 — — 2 1 <1 — 7 31 0 — — 31 100 — √ √ — — — — √ HSHe-27|¶ HSH EL 66 1 3 2 <1 <1 — — 27 1 — — 28 100 — √ — — — — √ — BC-7 BC EL 34 31 — — <1 <1 — — 32 3 — — 35 100 — √ √ — — — — — BdHe-16¶ BdH Ay 52 — 33 5 <1 <1 — — — — — 10 10 100 — √ — √ √ — — — HoA-3¶ HoA Ay 46 45 2 1 <1 <1 — — 1 5 — — 6 100 √ √ √ √ √ √ √ — TH-25 TH CB 58 — — 7 <1 <1 — — 35 0 — — 35 100 — — √ √ √ — — — Metatonalites PC-378§ PC EL 41 52 — 1 <1 <1 — 2 4 — — — 4 100 √ — √ — — √ — — PC-502 PC EL 57 34 — <1 <1 <1 — 3 6 — — — 6 100 √ — √ √ √ √ — — PC-503 PC EL 42 50 — <1 <1 <1 — 3 5 — — — 5 100 √ — √ √ √ √ — — BC-9 BC EL 60 30 — <1 <1 <1 — — 10 — <1 — 10 100 √ √ √ √ — √ — — Xenolith name Locality* Region† Mineral modes‡ Applied analytical techniques Alteration U–(Th)–Pb zircon geochronology Pl Qtz Cpx Fe–Ti oxide Zrn Ap Bt Kfs Cal Srp Zeo Gl Total alt. Total XRF whole-rock EPMA MC-LA-ICP-MS SIMS Zircon Lu–Hf isotopic analyses Sm–Nd and Rb–Sr isotopic analyses Pressure Temperature Metadiorites PC-00 PC EL 74 9 <1 2 <1 <1 — — 14 1 — — 15 100 — √ — — — — √ — PCW-13¶ PC EL 60 <1 <1 3 <1 <1 — — 31 6 — — 37 100 — √ — — — — √ — PCW-54¶ PC EL 65 20 <1 4 <1 <1 — — 10 1 — — 11 100 — — √ √ √ — — — PCW-70¶ PC EL 61 8 <1 3 <1 <1 — — 25 3 — — 28 100 √ — √ √ √ √ — — PCW-97|¶ PC EL 56 13 <1 4 <1 <1 — — 24 3 — — 27 100 — √ — — — — √ — PC-355§ PC EL 74 — — 5 <1 <1 1 — 3 17 — — 20 100 √ √ √ — √ √ — — HSH-1 HSH EL 74 — — <1 <1 <1 — — 25 1 — — 26 100 √ — √ √ — — — — HSH-3 HSH EL 46 <1 <1 <1 <1 <1 — — 41 13 — — 54 100 — — — — — — — — HSH-48 HSH EL 59 — — 2 1 <1 — 7 31 0 — — 31 100 — √ √ — — — — √ HSHe-27|¶ HSH EL 66 1 3 2 <1 <1 — — 27 1 — — 28 100 — √ — — — — √ — BC-7 BC EL 34 31 — — <1 <1 — — 32 3 — — 35 100 — √ √ — — — — — BdHe-16¶ BdH Ay 52 — 33 5 <1 <1 — — — — — 10 10 100 — √ — √ √ — — — HoA-3¶ HoA Ay 46 45 2 1 <1 <1 — — 1 5 — — 6 100 √ √ √ √ √ √ √ — TH-25 TH CB 58 — — 7 <1 <1 — — 35 0 — — 35 100 — — √ √ √ — — — Metatonalites PC-378§ PC EL 41 52 — 1 <1 <1 — 2 4 — — — 4 100 √ — √ — — √ — — PC-502 PC EL 57 34 — <1 <1 <1 — 3 6 — — — 6 100 √ — √ √ √ √ — — PC-503 PC EL 42 50 — <1 <1 <1 — 3 5 — — — 5 100 √ — √ √ √ √ — — BC-9 BC EL 60 30 — <1 <1 <1 — — 10 — <1 — 10 100 √ √ √ √ — √ — — * Localities: PC, Partan Craig; HSH, Horseshoe Vent; BC, Beggar’s Cap; BdH, Baidland Hill; HoA, Heads of Ayr; TH, Tollis Hill. † Regions: EL, East Lothian; Ay, Ayrshire; CB, Central Belt of the Southern Uplands. ‡ Mineral modes were measured by point counting thin sections, n = 1000, with the exception of BC-9 where mineral modes are estimated as xenolith was too fine-grained for point counting. Mineral abbreviations: Pl, plagioclase; Qtz, quartz; Cpx, clinopyroxene; Zrn, zircon; Ap, apatite; Bt, biotite; Kfs, K-feldspar; Cal, calcite; Srp, serpentine; Zeo, zeolite; Gl, glass. § Xenoliths analysed for whole-rock Rb–Sr, Sm–Nd and Pb by Halliday et al. (1993). ¶ Samples collected by the authors. The rest of the samples are from the collections of Brian Upton stored at the Rock Collection of the British Geological Survey, Edinburgh. Open in new tab Early studies of the meta-igneous xenoliths (e.g. Upton et al., 1976) interpreted them as samples from the presumed Precambrian basement of the Midland Valley terrane, the possible existence of which persists in the recent literature (e.g. Phillips et al., 2009; Strachan, 2012). The East Lothian meta-igneous xenoliths (e.g. from Fidra; Fig. 2) have been interpreted as the product of Permo-Carboniferous magmatic underplating, generated by extensional tectonics (Upton et al., 2004). This was based on similarities in Sr and Nd isotopic compositions between the xenoliths and their host magma (Downes et al., 2001). Alternatively, based on trace element data, Halliday et al. (1993) suggested that at least some xenoliths may have formed in connection with late Caledonian magmatism (i.e. ‘Newer Granites’; c. 430–390 Ma). Early attempts to date the East Lothian xenoliths analysed only metasediments (Van Breemen & Hawkesworth, 1980; Davies et al., 1984; Halliday et al., 1984). A preliminary U–Pb zircon study of a meta-igneous xenolith (Badenszki et al., 2009) first raised the possibility of a late Ordovician arc-related origin. The aims of this study were to test four hypotheses for the origin of the meta-igneous xenoliths: (1) Precambrian basement; (2) Carboniferous or Permo-Carboniferous underplating; (3) ‘Newer Granite’ magmatism; (4) Ordovician arc magmatism. Our approach has been to investigate the xenolith protolith and metamorphic ages using in situ zircon U–Pb geochronology, together with whole-rock geochemistry, thermobarometry and zircon Hf isotope geochemistry. Details of the investigated samples are given in Table 1. GEOLOGICAL SETTING Late Ordovician to Middle Devonian geology and tectonics of southern Scotland The southern region of Scotland, including the Midland Valley and Southern Uplands terranes, has a complex Early Palaeozoic geological history (Fig. 3). Its oldest exposed component is the Ballantrae Ophiolite Complex (Fig. 1), interpreted as a dismembered ophiolite, with a U–Pb zircon crystallization age of 483 ± 4 Ma (Bluck et al., 1980). A garnet Sm–Nd age of 477·6 ± 1·9 Ma from a mafic granulite from the metamorphic sole of the complex has been interpreted to date obduction and the onset of the Grampian Orogeny (Stewart et al., 2017). Fig. 3 Open in new tabDownload slide Schematic illustrations (after Strachan, 2012; Tanner, 2014) depicting the plate-tectonic setting of southern Scotland from Late Cambrian to Early Devonian times. Fig. 3 Open in new tabDownload slide Schematic illustrations (after Strachan, 2012; Tanner, 2014) depicting the plate-tectonic setting of southern Scotland from Late Cambrian to Early Devonian times. In Middle Ordovician times the Iapetus Ocean was already closing. Based on indirect evidence (granitoid clasts derived from the north in the Middle Ordovician sedimentary record from the SW Midland Valley), a volcanic–plutonic arc-complex (± basement?) was suggested to have been active within the Midland Valley between c. 480 and 455 Ma (Bluck, 1983, 1984, 2013). These arc rocks are not exposed, but the existence of this arc has been widely accepted and has contributed significantly to models for the geotectonic evolution of the Scottish Caledonides (e.g. Bluck, 2013; Chew & Strachan, 2014). Northward subduction of the Iapetus oceanic lithosphere underneath Laurentia generated a mid-Ordovician to mid-Silurian accretionary prism complex (McKerrow et al., 1977; Floyd, 2001), forming the Southern Uplands terrane. Clast compositions (Smith et al., 2000; Stone & Evans, 2000) and U–Pb zircon provenance studies (Waldron et al., 2008, 2014) suggest that the Southern Uplands sediments were derived from the Precambrian rocks of Laurentia. However, the youngest detrital zircons are Ordovician in age with 206Pb/238U ages of 462–446 Ma (Waldron et al., 2008, 2014) implying the presence of a younger component. During the final mid-Silurian (Wenlock) stage of the closure of the Iapetus Ocean, Laurentia collided with a peri-Gondwanan terrane, possibly Ganderia (Waldron et al., 2014). Subduction under the Southern Uplands was probably oblique, which resulted in a generally sinistral, transpressional tectonic regime to the north, affecting the Midland Valley terrane (Dewey & Strachan, 2003). Bluck (1983, 2013) suggested that during the final stage of Iapetus closure the northern margin of the Southern Uplands was overthrust onto the southern margin of the Midland Valley. This idea is also supported by the Western Isles–North Channel (‘WINCH’) deep seismic reflection profile, which runs through the North Channel, crossing the Midland Valley and the Southern Uplands terranes offshore SW Scotland (Fig. 1; Hall et al., 1984) on which the overthrust is visible. From c. 420 to 400 Ma the tectonic regime switched to sinistral transtension (Dewey & Strachan, 2003) and was followed by the Acadian Orogeny (c. 400–390 Ma; Soper & Woodcock, 2003) and an accompanying transpressional tectonic regime, possibly as a consequence of the closing of the Rheic Ocean to the south of Avalonia. The Silurian to mid-Devonian period (435–390 Ma) was characterized by the intrusion of the ‘Newer Granites’ over a wide region of Scotland and in the Lakesman terrane of northern England (Fig. 1; Oliver et al., 2008). Atherton & Ghani (2002) suggested a slab break-off model in which the asthenospheric replacement of the detached slab led to partial melting of the overlying lithospheric mantle. This model was also reiterated by Neilson et al. (2009). Brown et al. (2008) proposed that lamprophyres generated during the transtensional period (c. 420–400 Ma) could have produced the S-type component of the Trans-Suture Zone (TSZ; see below) suite granites, an idea further endorsed by Miles et al. (2014). O’Reilly et al. (2012) suggested the ‘incipient delamination’ model to explain the formation of the ‘Newer Granites’. Oliver et al. (2008) proposed double slab break-off under the Midland Valley and Southern Uplands, as a result of which heating owing to the consequent asthenospheric upwelling led to granitoid formation. Miles et al. (2016) introduced the slab drop-off model in which the slab delaminated following the Iapetus closure below the Iapetus Suture and for c. 100 km farther south. South of the Highland Boundary Fault the ‘Newer Granite’ have been divided into the South of Scotland Suite (SSS) and ‘Trans-Suture Zone’ granites (TSZ) based on their petrology and chemical and isotopic compositions (Stephens & Halliday, 1984). Within this region granitoids of the SSS range in age from c. 413 to 408 Ma (Halliday et al., 1980a; Thirlwall, 1988; Fig. 2). Within the Midland Valley, the Distinkhorn granodiorite (Fig. 2) is the only exposed SSS pluton. However, broadly contemporaneous volcanism occurred widely within the Early Devonian Lower Old Red Sandstone sequence (Thirlwall, 1988; Fig. 2). In addition, dating of granite boulders in Devonian conglomerates from the Strathmore basin (NE Midland Valley), interpreted to have had a sedimentary provenance within the Midland Valley, indicates that ‘Newer Granite’ plutons were more widespread within this terrane than can be determined from the surface geology (Haughton & Halliday, 1991). Following the suggestion of Stone et al. (1997) that the southernmost plutons of the originally defined SSS (Stephens & Halliday, 1984) (south of the Moniaive Shear zone–Orlock Bridge Fault; Fig. 2) belonged to a separate group, Brown et al. (2008), included them with the granitoids in the Lakesman terrane and introduced the term ‘Trans-Suture Zone’ granites. The TSZ granites range in age from c. 416 to 387 Ma (Dunham & Wilson, 1985; Thirlwall, 1988; Rundle, 1992; Kimbell et al., 2010; Miles et al., 2014). The East Lothian region (Fig. 2) lies on the boundary of two major terranes, the Midland Valley and the Southern Uplands, which are separated by the Southern Uplands Fault (SUF). The SUF was most active during the Late Ordovician to Silurian (Barnes et al., 1989), and it was subsequently reactivated during the Devonian, Carboniferous and later times (Floyd, 1994). Floyd (1994) demonstrated that the East Lothian region is surrounded by minor faults considered as derivatives of the Southern Uplands Fault (sensu stricto). Hence, based on the surface geology, the East Lothian region can be considered as part of the Southern Uplands (Max, 1976; Floyd, 1994). However, the surface geology of East Lothian does not necessarily designate which terrane the xenoliths have sampled. The magnitude of the displacements on the parallel fractures of the SUF (sensu lato) is not well established (Anderson, 2000) and the LISPB seismic profile (Lithospheric Seismic Profile in Britain), a deep (crust and upper mantle) seismic reflection line (Bamford et al., 1977, 1978; Barton, 1992) that crosses all the major Scottish tectonic terranes (Figs 1 and 2), has revealed a complex deep crustal structure underneath the region. However, the overthrusting of the Southern Uplands onto the southern margin of the Midland Valley suggested by Bluck (1983) can be seen on the WINCH deep seismic profile (Fig. 1; Hall et al., 1984). Hence, we consider that the East Lothian xenoliths have been derived from the Midland Valley terrane at depth below the thrust. Xenolith host rocks Xenoliths from Partan Craig, Horseshoe Vent and Beggar’s Cap (East Lothian, Fig. 2) are hosted by early to mid-Viséan agglomerate vents related to the Garleton Hill Volcanic Formation, which represents an early episode in the widespread, continental, intra-plate, extension-related Permo-Carboniferous volcanism (Upton et al., 2004). The Tollis Hill xenolith locality is a lamprophyre dyke of uncertain age intruding foliated Early Silurian greywackes in the Central Belt of the Southern Uplands about 25 km south of the East Lothian localities (Fig. 2). The Baidland Hill xenoliths are hosted by a Lower Carboniferous basaltic pyroclastic volcanic neck (British Geological Survey, 2005), which cuts the Viséan Clyde Plateau Volcanic Formation (Upton et al., 2004). The Heads of Ayr xenoliths are hosted by basaltic pyroclastic rocks in a volcanic neck of presumed early Permian age (e.g. British Geological Survey, 2008), although Upton et al. (2004) suggested that the Heads of Ayr volcano could be an isolated expression of lower to mid-Viséan volcanism, similar to the Clyde Plateau Volcanic Formation. PREVIOUS WORK This section summarizes the previous work on meta-igneous xenoliths from East Lothian (Partan Craig, Horseshoe Vent and Beggar’s Cap) and Ayrshire (Baidland Hill, Heads of Ayr) localities. For comparison, deep crustal xenoliths of igneous origin from two other well-studied localities, Fidra and Hawks Nib (Fig. 2), will also be reviewed. In the literature, a confusing variety of petrographic terms has been used to describe the meta-igneous xenoliths from East Lothian and Ayrshire. As set out below, the samples discussed in this study have been classified as metadiorite and metatonalite, based mainly on their modal mineralogy (Table 1). Some of these samples were discussed in previous studies; for example, PC-355 and PC-378 from Partan Craig (East Lothian, Fig. 2), and described as ‘mafic granulite’ and ‘felsic granulite’ respectively (Halliday et al., 1993). The metadiorites, then referred to as ‘quartz-bearing plagioclase–pyroxene gneisses’, were first described from Partan Craig, East Lothian by Upton et al. (1976) and considered to be samples of Precambrian basement. Graham & Upton (1978) described both pyroxene granulite (i.e. metadiorite) and quartzofeldspathic granulite (i.e. metatonalite) xenoliths from Partan Craig and Baidland Hill. With the aid of the LISPB profile (Bamford et al., 1977, 1978; Fig. 1), it was suggested that these samples were derived from the basement of the Midland Valley from a depth of at least 7–8 km (Graham & Upton, 1978). However, this was based on P–T estimates carried out entirely on metasedimentary xenoliths, which were not recognized as such at the time. In review papers by Upton et al. (1983, 1984), plagioclase–pyroxene (± quartz) rocks or basic granulites (including metadiorites) were considered to be samples of the lower crust, whereas tonalitic and trondhjemitic gneisses (i.e. metatonalite) were viewed as middle crustal. The coexistence of the two lithologies in composite xenoliths suggests that the metatonalites were also derived from the lower crust (Upton et al., 1984). The crystallization ages of both xenolith types remained unresolved. However, their previously suggested Precambrian age (Upton et al., 1976) was never questioned or investigated. Halliday et al. (1993) reported whole-rock Rb–Sr, Sm–Nd and Pb isotopic data from various types of xenoliths from several Scottish localities, including the PC-355 (metadiorite) and PC-378 (metatonalite) xenoliths from East Lothian (Table 1). These have Sm–Nd depleted mantle model ages of 0·90 Ga and 0·79 Ga, respectively, which led Halliday et al. (1993) to suggest that these xenoliths represent Late Precambrian or Palaeozoic crust. The first (preliminary) results of in situ zircon U–Pb and Lu–Hf isotopic analyses from East Lothian meta-igneous xenoliths suggested an Early Devonian protolith age (c. 416 Ma) with εHf(t) values of 0 to +4 and also reported a single zircon analysis from a metatonalite with a concordia age of 440 ± 8 Ma, indicating Late Ordovician or Early Silurian magmatism (Badenszki et al., 2009). U–Pb dating of zircon rims in the same xenolith was interpreted to date metamorphism at c. 397 ± 2 Ma, with a possible younger phase at c. 384 Ma. Mafic granulite xenoliths from Fidra, East Lothian (Fig. 2), have a quartz-free mineral assemblage of Pl + Cpx + Mag ± Opx, which clearly distinguishes them from the metadiorites and metatonalites of this study. These mafic granulites yielded temperatures of c. 840–890°C, using the clinopyroxene–orthopyroxene geothermometer of Wells (1977), and pressures of c. 5·5–9 kbar, based on sub-solidus phase relations and mineral chemistry (Hunter et al., 1984). The calculated pressure range is equivalent to a depth of c. 20–35 km (Hunter et al., 1984) and corresponds to the lower crustal layer on the LISPB profile (Bamford et al., 1978). Downes et al. (2001) interpreted the mafic granulite xenoliths from Fidra (Fig. 2) as cumulates formed during Permian magmatism, based on the similarities between their whole-rock Rb–Sr, Sm–Nd and oxygen isotopic compositions and the host volcanic rocks. Although one group of Fidra xenoliths has clear mantle affinities, the overall compositional range was interpreted to indicate significant crustal contamination, with δ18O ranging from 5·5 to 8·4‰ and εNd(t) from +4 to -14. Gabbro, gabbronorite and anorthosite xenoliths from a late Carboniferous to early Permian olivine basaltic intrusion at Hawk’s Nib (Fig. 2) have a cumulate texture and their Sr–Nd isotopic compositions plot in or near the depleted mantle field. Hence, Downes et al. (2007) also considered them as products of magmatic underplating related to the host magmatism. SAMPLES AND ANALYTICAL METHODS Meta-igneous xenolith samples For this study, 18 deep crustal meta-igneous samples (Table 1) were selected from more than 200 samples described by Badenszki (2014). Eleven meta-igneous xenoliths were obtained from Brian Upton’s Scottish xenolith collection stored at the British Geological Survey, Edinburgh (Table 1). The remaining seven samples were collected by Badenszki and Daly. Most of the investigated samples came from East Lothian localities (nine from Partan Craig, four from Horseshoe Vent and two from Beggars Cap; Table 1; Fig. 2) as these localities are the richest in deep crustal xenoliths. Deep crustal xenoliths are less common in Tollis Hill, Heads of Ayr and Baidland Hill, from which single samples were selected (Table 1). Metadiorite xenoliths are present in all of the studied localities, whereas metatonalites are known only from East Lothian. Standard, 30 µm thick polished thin sections were made from each sample and were described using transmitted light microscopy. Mineral analyses were obtained from 10 representative xenoliths (Table 1) using a Jeol JXA 8900RL electron microprobe at the Department of Geochemistry, Geowissenschaftliches Zentrum der Universität Göttingen (GZG), with a 15 nA beam current and a variable 5–10 µm beam diameter (10 µm for feldspars and 5–7 µm for the rest of the minerals). Raw signals were collected using five wavelength-dispersive spectrometers, equipped with standard diffracting analyser crystals: LIF, PET and TAP. Mineral standards and synthetic compounds (Supplementary Data Electronic Appendix 3; supplementary data are available for downloading at http://www.petrology.oxfordjournals.org) were used for calibration. Counting times of the X-ray signals were varied between 15 and 30 s, depending the concentration range of the element. The Phi–rho–z matrix correction model after Armstrong (1995) was applied for raw data correction. Representative mineral chemical data together with standards run during analytical sessions are reported in Supplementary Data Electronic Appendix 3. Whole-rock powders of eight xenoliths, crushed in agate (Table 1), were analysed by X-ray fluorescence spectrometry (XRF, Table 2) at the Department of Geology, University of Leicester using a PANalytical Axios-Advanced XRF spectrometer, calibrated using international and in-house reference materials (Supplementary Data Electronic Appendix 4). Major and trace element compositions were determined on fused glass beads and pressed powder pellets, respectively. Table 2 Whole-rock major and trace element analyses and loss on ignition values for meta-igneous xenoliths analysed by XRF at Department of Geology, University of Leicester Sample: HoA-3 PCW-70 HSH-1 PC-355 PC-355* BC-9 PC-502 PC-503 PC-378 PC-378* Locality:† HoA PC HSH PC PC BC PC PC PC PC Rock type:‡ D D D D D T T T T T Oxides (wt %) SiO2 70·83 54·56 44·90 45·86 44·50 72·54 64·41 74·22 71·49 70·00 TiO2 0·30 0·56 1·29 1·92 n.a. 0·16 0·38 0·18 0·19 n.a. Al2O3 14·30 17·01 17·10 21·98 21·40 14·04 14·99 12·78 13·69 13·20 Fe2 O3tot 3·86 4·02 1·69 8·80 n.a. 1·10 0·73 0·94 1·59 n.a. MnO 0·04 0·11 0·08 0·05 n.a. 0·02 0·07 0·03 0·05 n.a. MgO 1·65 2·19 0·39 4·70 4·87 0·83 0·29 0·41 0·40 0·85 CaO 4·27 10·67 17·34 8·46 n.a. 4·94 9·70 5·28 5·56 n.a. Na2O 3·76 4·67 5·72 4·06 n.a. 3·27 4·07 3·70 4·21 n.a. K2O 0·34 0·46 0·73 0·70 0·68 1·38 0·72 1·03 0·70 0·65 P2O5 0·03 0·07 0·24 0·57 n.a. 0·06 0·04 0·01 0·03 n.a. LOI 1·27 6·25 10·85 3·32 n.a. 1·97 5·01 2·23 2·40 n.a. Total 100·66 100·55 100·32 100·42 n.a. 100·31 100·42 100·81 100·31 n.a. Trace elements (ppm) Ba 46·6 100·1 169·0 136·6 171·0 491·6 320·6 226·6 138·0 146·0 Ce 7·4 27·9 31·6 56·8 n.a. 21·9 19·0 10·5 14·3 n.a. Co 9·2 41·9 340·7 52·2 n.a. 5·1 84·1 91·1 74·4 n.a. Cr <0·6 44·3 <0·7 11·5 n.a. 16·3 26·9 <0·6 <0·6 n.a. Cs <1·5 <1·7 <1·9 2·8 n.a. <1·6 <1·7 <1·6 <1·6 n.a. Cu 6·0 52·6 175·8 45·6 n.a. 5·3 19·0 96·0 18·8 n.a. Ga 13·8 14·1 16·5 22·2 n.a. 12·1 10·8 8·6 9·4 n.a. La 4·1 21·7 16·5 22·3 n.a. 13·5 12·0 9·3 10·8 n.a. Mo 0·9 1·2 3·1 1·0 n.a. 0·8 1·5 1·9 1·2 n.a. Nb 0·4 6·0 6·8 10·4 n.a. 0·8 1·5 0·6 0·3 n.a. Sm§ 1·19 1·40 — 6·92 2·05 1·15 1·17 0·62 1·14 1·4 Nd§ 3·85 8·62 19·0 30·95 10·80 7·16 6·84 3·95 5·16 8·1 Ni 21·6 67·8 217·5 76·3 n.a. 26·3 22·8 86·5 16·2 n.a. Pb 1·6 11·8 11·0 6·6 1·9 9·6 4·3 19·4 5·3 6·1 Rb§ 2·70 3·82 6·8 2·52 13 8·00 2·86 3·47 2·13 13·0 Sc 13·1 20·0 27·6 20·0 n.a. 5·6 20·4 5·3 8·6 n.a. Sr§ 157·76 283·65 405·9 750·11 705 399·45 255·01 166·15 319·25 418·0 Th <0·4 <0·4 0·5 <0·5 n.a. <0·4 <0·4 <0·4 <0·4 n.a. U 0·5 0·5 1·2 0·9 0·2 0·5 <0·3 0·4 0·5 0·5 V 68·4 120·4 73·7 205·1 n.a. 24·6 71·3 23·0 35·1 n.a. Y 8·4 7·3 34·3 23·6 n.a. 4·2 5·7 4·2 11·2 n.a. Zn 23·6 37·8 160·6 99·9 n.a. 139·1 18·4 60·9 13·4 n.a. Zr 77·1 65·9 112·9 221·9 321·0 185·8 88·4 140·7 80·5 99·0 Sample: HoA-3 PCW-70 HSH-1 PC-355 PC-355* BC-9 PC-502 PC-503 PC-378 PC-378* Locality:† HoA PC HSH PC PC BC PC PC PC PC Rock type:‡ D D D D D T T T T T Oxides (wt %) SiO2 70·83 54·56 44·90 45·86 44·50 72·54 64·41 74·22 71·49 70·00 TiO2 0·30 0·56 1·29 1·92 n.a. 0·16 0·38 0·18 0·19 n.a. Al2O3 14·30 17·01 17·10 21·98 21·40 14·04 14·99 12·78 13·69 13·20 Fe2 O3tot 3·86 4·02 1·69 8·80 n.a. 1·10 0·73 0·94 1·59 n.a. MnO 0·04 0·11 0·08 0·05 n.a. 0·02 0·07 0·03 0·05 n.a. MgO 1·65 2·19 0·39 4·70 4·87 0·83 0·29 0·41 0·40 0·85 CaO 4·27 10·67 17·34 8·46 n.a. 4·94 9·70 5·28 5·56 n.a. Na2O 3·76 4·67 5·72 4·06 n.a. 3·27 4·07 3·70 4·21 n.a. K2O 0·34 0·46 0·73 0·70 0·68 1·38 0·72 1·03 0·70 0·65 P2O5 0·03 0·07 0·24 0·57 n.a. 0·06 0·04 0·01 0·03 n.a. LOI 1·27 6·25 10·85 3·32 n.a. 1·97 5·01 2·23 2·40 n.a. Total 100·66 100·55 100·32 100·42 n.a. 100·31 100·42 100·81 100·31 n.a. Trace elements (ppm) Ba 46·6 100·1 169·0 136·6 171·0 491·6 320·6 226·6 138·0 146·0 Ce 7·4 27·9 31·6 56·8 n.a. 21·9 19·0 10·5 14·3 n.a. Co 9·2 41·9 340·7 52·2 n.a. 5·1 84·1 91·1 74·4 n.a. Cr <0·6 44·3 <0·7 11·5 n.a. 16·3 26·9 <0·6 <0·6 n.a. Cs <1·5 <1·7 <1·9 2·8 n.a. <1·6 <1·7 <1·6 <1·6 n.a. Cu 6·0 52·6 175·8 45·6 n.a. 5·3 19·0 96·0 18·8 n.a. Ga 13·8 14·1 16·5 22·2 n.a. 12·1 10·8 8·6 9·4 n.a. La 4·1 21·7 16·5 22·3 n.a. 13·5 12·0 9·3 10·8 n.a. Mo 0·9 1·2 3·1 1·0 n.a. 0·8 1·5 1·9 1·2 n.a. Nb 0·4 6·0 6·8 10·4 n.a. 0·8 1·5 0·6 0·3 n.a. Sm§ 1·19 1·40 — 6·92 2·05 1·15 1·17 0·62 1·14 1·4 Nd§ 3·85 8·62 19·0 30·95 10·80 7·16 6·84 3·95 5·16 8·1 Ni 21·6 67·8 217·5 76·3 n.a. 26·3 22·8 86·5 16·2 n.a. Pb 1·6 11·8 11·0 6·6 1·9 9·6 4·3 19·4 5·3 6·1 Rb§ 2·70 3·82 6·8 2·52 13 8·00 2·86 3·47 2·13 13·0 Sc 13·1 20·0 27·6 20·0 n.a. 5·6 20·4 5·3 8·6 n.a. Sr§ 157·76 283·65 405·9 750·11 705 399·45 255·01 166·15 319·25 418·0 Th <0·4 <0·4 0·5 <0·5 n.a. <0·4 <0·4 <0·4 <0·4 n.a. U 0·5 0·5 1·2 0·9 0·2 0·5 <0·3 0·4 0·5 0·5 V 68·4 120·4 73·7 205·1 n.a. 24·6 71·3 23·0 35·1 n.a. Y 8·4 7·3 34·3 23·6 n.a. 4·2 5·7 4·2 11·2 n.a. Zn 23·6 37·8 160·6 99·9 n.a. 139·1 18·4 60·9 13·4 n.a. Zr 77·1 65·9 112·9 221·9 321·0 185·8 88·4 140·7 80·5 99·0 Fe2 O3tot ⁠, total iron as Fe2O3; n.a., data not available; —, not analysed; <, value below given detection limit. * Xenolith whole-rock major and trace element data from Halliday et al. (1993). † Localities: HoA, Heads of Ayr; PC, Partan Craig; HSH, Horseshoe Vent; BC, Beggar’s Cap. ‡ Rock types: D, metadiorite; T, metatonalite. § Rb, Sr, Sm and Nd concentrations by isotope dilution at University College Dublin, School of Earth Sciences. Open in new tab Table 2 Whole-rock major and trace element analyses and loss on ignition values for meta-igneous xenoliths analysed by XRF at Department of Geology, University of Leicester Sample: HoA-3 PCW-70 HSH-1 PC-355 PC-355* BC-9 PC-502 PC-503 PC-378 PC-378* Locality:† HoA PC HSH PC PC BC PC PC PC PC Rock type:‡ D D D D D T T T T T Oxides (wt %) SiO2 70·83 54·56 44·90 45·86 44·50 72·54 64·41 74·22 71·49 70·00 TiO2 0·30 0·56 1·29 1·92 n.a. 0·16 0·38 0·18 0·19 n.a. Al2O3 14·30 17·01 17·10 21·98 21·40 14·04 14·99 12·78 13·69 13·20 Fe2 O3tot 3·86 4·02 1·69 8·80 n.a. 1·10 0·73 0·94 1·59 n.a. MnO 0·04 0·11 0·08 0·05 n.a. 0·02 0·07 0·03 0·05 n.a. MgO 1·65 2·19 0·39 4·70 4·87 0·83 0·29 0·41 0·40 0·85 CaO 4·27 10·67 17·34 8·46 n.a. 4·94 9·70 5·28 5·56 n.a. Na2O 3·76 4·67 5·72 4·06 n.a. 3·27 4·07 3·70 4·21 n.a. K2O 0·34 0·46 0·73 0·70 0·68 1·38 0·72 1·03 0·70 0·65 P2O5 0·03 0·07 0·24 0·57 n.a. 0·06 0·04 0·01 0·03 n.a. LOI 1·27 6·25 10·85 3·32 n.a. 1·97 5·01 2·23 2·40 n.a. Total 100·66 100·55 100·32 100·42 n.a. 100·31 100·42 100·81 100·31 n.a. Trace elements (ppm) Ba 46·6 100·1 169·0 136·6 171·0 491·6 320·6 226·6 138·0 146·0 Ce 7·4 27·9 31·6 56·8 n.a. 21·9 19·0 10·5 14·3 n.a. Co 9·2 41·9 340·7 52·2 n.a. 5·1 84·1 91·1 74·4 n.a. Cr <0·6 44·3 <0·7 11·5 n.a. 16·3 26·9 <0·6 <0·6 n.a. Cs <1·5 <1·7 <1·9 2·8 n.a. <1·6 <1·7 <1·6 <1·6 n.a. Cu 6·0 52·6 175·8 45·6 n.a. 5·3 19·0 96·0 18·8 n.a. Ga 13·8 14·1 16·5 22·2 n.a. 12·1 10·8 8·6 9·4 n.a. La 4·1 21·7 16·5 22·3 n.a. 13·5 12·0 9·3 10·8 n.a. Mo 0·9 1·2 3·1 1·0 n.a. 0·8 1·5 1·9 1·2 n.a. Nb 0·4 6·0 6·8 10·4 n.a. 0·8 1·5 0·6 0·3 n.a. Sm§ 1·19 1·40 — 6·92 2·05 1·15 1·17 0·62 1·14 1·4 Nd§ 3·85 8·62 19·0 30·95 10·80 7·16 6·84 3·95 5·16 8·1 Ni 21·6 67·8 217·5 76·3 n.a. 26·3 22·8 86·5 16·2 n.a. Pb 1·6 11·8 11·0 6·6 1·9 9·6 4·3 19·4 5·3 6·1 Rb§ 2·70 3·82 6·8 2·52 13 8·00 2·86 3·47 2·13 13·0 Sc 13·1 20·0 27·6 20·0 n.a. 5·6 20·4 5·3 8·6 n.a. Sr§ 157·76 283·65 405·9 750·11 705 399·45 255·01 166·15 319·25 418·0 Th <0·4 <0·4 0·5 <0·5 n.a. <0·4 <0·4 <0·4 <0·4 n.a. U 0·5 0·5 1·2 0·9 0·2 0·5 <0·3 0·4 0·5 0·5 V 68·4 120·4 73·7 205·1 n.a. 24·6 71·3 23·0 35·1 n.a. Y 8·4 7·3 34·3 23·6 n.a. 4·2 5·7 4·2 11·2 n.a. Zn 23·6 37·8 160·6 99·9 n.a. 139·1 18·4 60·9 13·4 n.a. Zr 77·1 65·9 112·9 221·9 321·0 185·8 88·4 140·7 80·5 99·0 Sample: HoA-3 PCW-70 HSH-1 PC-355 PC-355* BC-9 PC-502 PC-503 PC-378 PC-378* Locality:† HoA PC HSH PC PC BC PC PC PC PC Rock type:‡ D D D D D T T T T T Oxides (wt %) SiO2 70·83 54·56 44·90 45·86 44·50 72·54 64·41 74·22 71·49 70·00 TiO2 0·30 0·56 1·29 1·92 n.a. 0·16 0·38 0·18 0·19 n.a. Al2O3 14·30 17·01 17·10 21·98 21·40 14·04 14·99 12·78 13·69 13·20 Fe2 O3tot 3·86 4·02 1·69 8·80 n.a. 1·10 0·73 0·94 1·59 n.a. MnO 0·04 0·11 0·08 0·05 n.a. 0·02 0·07 0·03 0·05 n.a. MgO 1·65 2·19 0·39 4·70 4·87 0·83 0·29 0·41 0·40 0·85 CaO 4·27 10·67 17·34 8·46 n.a. 4·94 9·70 5·28 5·56 n.a. Na2O 3·76 4·67 5·72 4·06 n.a. 3·27 4·07 3·70 4·21 n.a. K2O 0·34 0·46 0·73 0·70 0·68 1·38 0·72 1·03 0·70 0·65 P2O5 0·03 0·07 0·24 0·57 n.a. 0·06 0·04 0·01 0·03 n.a. LOI 1·27 6·25 10·85 3·32 n.a. 1·97 5·01 2·23 2·40 n.a. Total 100·66 100·55 100·32 100·42 n.a. 100·31 100·42 100·81 100·31 n.a. Trace elements (ppm) Ba 46·6 100·1 169·0 136·6 171·0 491·6 320·6 226·6 138·0 146·0 Ce 7·4 27·9 31·6 56·8 n.a. 21·9 19·0 10·5 14·3 n.a. Co 9·2 41·9 340·7 52·2 n.a. 5·1 84·1 91·1 74·4 n.a. Cr <0·6 44·3 <0·7 11·5 n.a. 16·3 26·9 <0·6 <0·6 n.a. Cs <1·5 <1·7 <1·9 2·8 n.a. <1·6 <1·7 <1·6 <1·6 n.a. Cu 6·0 52·6 175·8 45·6 n.a. 5·3 19·0 96·0 18·8 n.a. Ga 13·8 14·1 16·5 22·2 n.a. 12·1 10·8 8·6 9·4 n.a. La 4·1 21·7 16·5 22·3 n.a. 13·5 12·0 9·3 10·8 n.a. Mo 0·9 1·2 3·1 1·0 n.a. 0·8 1·5 1·9 1·2 n.a. Nb 0·4 6·0 6·8 10·4 n.a. 0·8 1·5 0·6 0·3 n.a. Sm§ 1·19 1·40 — 6·92 2·05 1·15 1·17 0·62 1·14 1·4 Nd§ 3·85 8·62 19·0 30·95 10·80 7·16 6·84 3·95 5·16 8·1 Ni 21·6 67·8 217·5 76·3 n.a. 26·3 22·8 86·5 16·2 n.a. Pb 1·6 11·8 11·0 6·6 1·9 9·6 4·3 19·4 5·3 6·1 Rb§ 2·70 3·82 6·8 2·52 13 8·00 2·86 3·47 2·13 13·0 Sc 13·1 20·0 27·6 20·0 n.a. 5·6 20·4 5·3 8·6 n.a. Sr§ 157·76 283·65 405·9 750·11 705 399·45 255·01 166·15 319·25 418·0 Th <0·4 <0·4 0·5 <0·5 n.a. <0·4 <0·4 <0·4 <0·4 n.a. U 0·5 0·5 1·2 0·9 0·2 0·5 <0·3 0·4 0·5 0·5 V 68·4 120·4 73·7 205·1 n.a. 24·6 71·3 23·0 35·1 n.a. Y 8·4 7·3 34·3 23·6 n.a. 4·2 5·7 4·2 11·2 n.a. Zn 23·6 37·8 160·6 99·9 n.a. 139·1 18·4 60·9 13·4 n.a. Zr 77·1 65·9 112·9 221·9 321·0 185·8 88·4 140·7 80·5 99·0 Fe2 O3tot ⁠, total iron as Fe2O3; n.a., data not available; —, not analysed; <, value below given detection limit. * Xenolith whole-rock major and trace element data from Halliday et al. (1993). † Localities: HoA, Heads of Ayr; PC, Partan Craig; HSH, Horseshoe Vent; BC, Beggar’s Cap. ‡ Rock types: D, metadiorite; T, metatonalite. § Rb, Sr, Sm and Nd concentrations by isotope dilution at University College Dublin, School of Earth Sciences. Open in new tab Rb–Sr and Sm–Nd isotopic ratios and concentrations are reported in Table 3. Prior to digestion in closed digestion vessels, whole-rock powders were spiked with mixed 147Sm–150Nd and 84Sr–85Rb isotopic tracers. Powders were not leached prior to analysis. This could possibly compromise the Sr data for sample PCW-70, which exhibits 25 modal % secondary calcite alteration. However, Sr isotope analyses of unleached powders of both PC-355 and PC-378 reproduced the 87Sr/86Sr ratios reported by Halliday et al. (1993) on leached samples. Chemical separation of Rb, Sr, Sm and Nd for isotopic analysis followed Pin et al. (1994) using Sr.Spec, TRU.Spec and LN resins. Analyses of Nd (142Nd, 143Nd, 144Nd, 145Nd, 148 Nd, 150Nd and 147Sm), Sr (84Sr, 85Sr, 86Sr and 88Sr), and Sm (147Sm, 149Sm and 152Sm) were carried out at University College Dublin by multiple-collector thermal ionization mass spectrometry (MC-TIMS) on a Thermo-Scientific Triton system using Faraday cups. Mass fractionation was corrected applying 88Sr/86Sr values of 8·3752, 146Nd/144Nd values of 0·7219 and 149Sm/152Sm values of 0·51686 using an exponential fractionation law. Measurement quality was monitored using the La Jolla Nd standard (143Nd/144Nd = 0·5118403 ± 0·0000053, n = 19) and SRM987 (87Sr/86Sr = 0·7102542 ± 0·0000134; n = 14). Isotopic dilution analyses of Rb (Table 3) were carried out at University College Dublin by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) on a Thermo-Scientific Neptune system on Zr-doped solutions to correct for mass bias, using a 90Zr/91Zr isotopic ratio of 4·588. Table 3 Whole-rock Rb–Sr and Sm–Nd isotope data for the Midland Valley meta-igneous xenoliths Sample Locality* Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr ±2σ Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd ±2σ εNd0 εNdt† TDMNd (Ga) Metadiorites HoA-3 HoA 2·70 157·76 0·0495 0·706329 0·000005 1·19 3·85 0·1862 0·512888 0·000005 4·9 5·4 1·00 PCW-70 PC 3·82 283·65 0·0390 0·705928 0·000005 1·40 8·62 0·0983 0·512518 0·000005 –2·3 2·9 0·70 PC-355 PC 2·52 750·11 0·0097 0·704560 0·000005 6·92 30·95 0·1352 0·512466 0·000005 –3·3 −0·1 1·12 PC-355‡ PC 13 705 0·0533 0·70452 0·00004 2·05 10·8 0·1148 0·51246 0·00002 –3·5 0·9 0·90 Metatonalites BC-9 BC 8·00 399·45 0·0579 0·705851 0·000005 1·15 7·16 0·0970 0·512446 0·000005 –3·7 2·0 0·79 PC-502 PC 2·86 255·01 0·0324 0·706310 0·000004 1·17 6·84 0·1035 0·512442 0·000005 –3·8 1·5 0·84 PC-503 PC 3·47 166·15 0·0603 0·706284 0·000005 0·62 3·95 0·0947 0·512528 0·000005 –2·1 3·7 0·67 PC-378 PC 2·13 319·25 0·0193 0·706372 0·000004 1·14 5·16 0·1337 0·512618 0·000005 –0·4 3·2 0·82 PC-378‡ PC 13 418 0·0900 0·70635 0·00003 1·41 8·1 0·1052 0·51249 0·00001 −2·9 2·4 0·79 Sample Locality* Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr ±2σ Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd ±2σ εNd0 εNdt† TDMNd (Ga) Metadiorites HoA-3 HoA 2·70 157·76 0·0495 0·706329 0·000005 1·19 3·85 0·1862 0·512888 0·000005 4·9 5·4 1·00 PCW-70 PC 3·82 283·65 0·0390 0·705928 0·000005 1·40 8·62 0·0983 0·512518 0·000005 –2·3 2·9 0·70 PC-355 PC 2·52 750·11 0·0097 0·704560 0·000005 6·92 30·95 0·1352 0·512466 0·000005 –3·3 −0·1 1·12 PC-355‡ PC 13 705 0·0533 0·70452 0·00004 2·05 10·8 0·1148 0·51246 0·00002 –3·5 0·9 0·90 Metatonalites BC-9 BC 8·00 399·45 0·0579 0·705851 0·000005 1·15 7·16 0·0970 0·512446 0·000005 –3·7 2·0 0·79 PC-502 PC 2·86 255·01 0·0324 0·706310 0·000004 1·17 6·84 0·1035 0·512442 0·000005 –3·8 1·5 0·84 PC-503 PC 3·47 166·15 0·0603 0·706284 0·000005 0·62 3·95 0·0947 0·512528 0·000005 –2·1 3·7 0·67 PC-378 PC 2·13 319·25 0·0193 0·706372 0·000004 1·14 5·16 0·1337 0·512618 0·000005 –0·4 3·2 0·82 PC-378‡ PC 13 418 0·0900 0·70635 0·00003 1·41 8·1 0·1052 0·51249 0·00001 −2·9 2·4 0·79 * Locality abbreviations: HoA, Heads of Ayr; PC, Partan Craig; BC, Beggar’s Cap. † t = 415 Ma for metadiorites; t = 453 Ma for metatonalites. ‡ Data from Halliday et al. (1993), recalculated. Open in new tab Table 3 Whole-rock Rb–Sr and Sm–Nd isotope data for the Midland Valley meta-igneous xenoliths Sample Locality* Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr ±2σ Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd ±2σ εNd0 εNdt† TDMNd (Ga) Metadiorites HoA-3 HoA 2·70 157·76 0·0495 0·706329 0·000005 1·19 3·85 0·1862 0·512888 0·000005 4·9 5·4 1·00 PCW-70 PC 3·82 283·65 0·0390 0·705928 0·000005 1·40 8·62 0·0983 0·512518 0·000005 –2·3 2·9 0·70 PC-355 PC 2·52 750·11 0·0097 0·704560 0·000005 6·92 30·95 0·1352 0·512466 0·000005 –3·3 −0·1 1·12 PC-355‡ PC 13 705 0·0533 0·70452 0·00004 2·05 10·8 0·1148 0·51246 0·00002 –3·5 0·9 0·90 Metatonalites BC-9 BC 8·00 399·45 0·0579 0·705851 0·000005 1·15 7·16 0·0970 0·512446 0·000005 –3·7 2·0 0·79 PC-502 PC 2·86 255·01 0·0324 0·706310 0·000004 1·17 6·84 0·1035 0·512442 0·000005 –3·8 1·5 0·84 PC-503 PC 3·47 166·15 0·0603 0·706284 0·000005 0·62 3·95 0·0947 0·512528 0·000005 –2·1 3·7 0·67 PC-378 PC 2·13 319·25 0·0193 0·706372 0·000004 1·14 5·16 0·1337 0·512618 0·000005 –0·4 3·2 0·82 PC-378‡ PC 13 418 0·0900 0·70635 0·00003 1·41 8·1 0·1052 0·51249 0·00001 −2·9 2·4 0·79 Sample Locality* Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr ±2σ Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd ±2σ εNd0 εNdt† TDMNd (Ga) Metadiorites HoA-3 HoA 2·70 157·76 0·0495 0·706329 0·000005 1·19 3·85 0·1862 0·512888 0·000005 4·9 5·4 1·00 PCW-70 PC 3·82 283·65 0·0390 0·705928 0·000005 1·40 8·62 0·0983 0·512518 0·000005 –2·3 2·9 0·70 PC-355 PC 2·52 750·11 0·0097 0·704560 0·000005 6·92 30·95 0·1352 0·512466 0·000005 –3·3 −0·1 1·12 PC-355‡ PC 13 705 0·0533 0·70452 0·00004 2·05 10·8 0·1148 0·51246 0·00002 –3·5 0·9 0·90 Metatonalites BC-9 BC 8·00 399·45 0·0579 0·705851 0·000005 1·15 7·16 0·0970 0·512446 0·000005 –3·7 2·0 0·79 PC-502 PC 2·86 255·01 0·0324 0·706310 0·000004 1·17 6·84 0·1035 0·512442 0·000005 –3·8 1·5 0·84 PC-503 PC 3·47 166·15 0·0603 0·706284 0·000005 0·62 3·95 0·0947 0·512528 0·000005 –2·1 3·7 0·67 PC-378 PC 2·13 319·25 0·0193 0·706372 0·000004 1·14 5·16 0·1337 0·512618 0·000005 –0·4 3·2 0·82 PC-378‡ PC 13 418 0·0900 0·70635 0·00003 1·41 8·1 0·1052 0·51249 0·00001 −2·9 2·4 0·79 * Locality abbreviations: HoA, Heads of Ayr; PC, Partan Craig; BC, Beggar’s Cap. † t = 415 Ma for metadiorites; t = 453 Ma for metatonalites. ‡ Data from Halliday et al. (1993), recalculated. Open in new tab Zircons for in situ analysis were separated from 13 xenoliths (nine metadiorites and four metatonalites; Table 1). The selected samples were disaggregated in a tungsten carbide TEMA mill, and subsequently sieved. Heavy and light fractions from the 100–250 µm and 250–500 µm grain-size intervals were separated by flotation in methylene iodide. Zircons were handpicked from the heavy mineral fraction. The separated zircons were cast in epoxy mounts and polished in the NORDSIM laboratory at the Swedish Museum of Natural History (Stockholm, Sweden). Zircon interiors were imaged by scanning electron microscopy (SEM) at the University of Leicester and at the Swedish Museum of Natural History, in cathodoluminescence (CL) and secondary electron (SE) modes to target different zircon growth domains for analysis and to avoid cracks and inclusions. In situ zircon analyses were carried out by laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS; Horstwood et al., 2003) and by secondary ion mass spectrometry (SIMS; Whitehouse & Kamber, 2005). Laser ablation U–Pb analyses at the NERC Isotope Geosciences Laboratories, Keyworth were undertaken on a Nu-Plasma HR MC-ICP-MS instrument coupled with a solid-state 193 nm wavelength Nd:YAG laser ablation system (UP193SS, New Wave Research) using 20 µm or 35 µm spot diameters, depending on the U content of the zircon. The detailed analytical protocol is described in Supplementary Data Electronic Appendix 1. Ion-microprobe U–Th–Pb analyses were carried out at the NORDSIM laboratory at the Swedish Museum of Natural History (Stockholm, Sweden) using a Cameca IMS 1280 ion microprobe. Reference zircon 91500 (Wiedenbeck et al., 1995) was used to calibrate U/Pb ratios. The raw U–Th–Pb data were processed following Whitehouse & Kamber (2005). A correction for common lead (Stacey & Kramers, 1975) was made, assuming that all common Pb present was a result of surface contamination. All plots and age calculations were made using the Isoplot 3.0 (Ludwig, 2003) add-in for Microsoft Excel. Unless stated otherwise, U–Pb ages discussed in the text are 206Pb/238U ages and the associated uncertainties are given at the 2σ level. Zircons from five samples were analysed only by LA-MC-ICP-MS. One sample was analysed only by SIMS and both analytical techniques were used on eight xenoliths (Table 1). The complete dataset is available in Supplementary Data Electronic Appendix 6. Lu–Hf zircon analyses are presented in Supplementary Data Electronic Appendix 7. Lu–Hf data from metadiorite xenoliths PCW-70, PCW-54, and PC-355 were obtained at the NERC Isotope Geosciences Laboratory (NIGL) in Nottingham, using a Nu Instruments Nu-Plasma HR MC-ICP-MS system coupled to a solid-state 193 nm wavelength Nd:YAG laser ablation system (UP193SS, New Wave Research). Metadiorites TH-25 and BdHe-16 and metatonalites PC-502 and PC-503 were analysed by LA-MC-ICP-MS at the National Centre for Isotope Geochemistry at University College Dublin (UCD), following the method of Hawkesworth & Kemp (2006), using a Thermo Fisher Scientific Neptune instrument coupled with a New Wave 193 nm excimer laser. Metatonalite HoA-3 was analysed for Hf isotopes in both laboratories and replicate analyses reproduce within analytical error. Laser ablation spots were 50 µm in diameter with a 5–10 Hz laser pulse repetition rate in both analytical setups. The detailed analytical protocols are presented in Supplementary Data Electronic Appendix 1. RESULTS Sample descriptions and petrography Two types of metamorphosed igneous xenoliths, metadiorite and metatonalite, are distinguished (Figs 4 and 5, Table 1, and see below) on the basis of petrography, whole-rock geochemistry and geochronology. Both varieties of xenolith are rounded and vary in size from 3 to 20 cm in diameter. All have been modified by metamorphism with variable preservation of igneous textures, and show varying degrees of foliation. Many of the xenoliths, especially those from East Lothian and Tollis Hill, are intensely altered. Representative mineral chemical analyses, also used for pressure and temperature calculations, are presented in Supplementary Data Electronic Appendix 3. Fig. 4 Open in new tabDownload slide Representative photomicrographs of metadiorite (a–d) and metatonalite (e, f) xenoliths are labelled using mineral abbreviations after Kretz (1983). (a) Granoblastic texture with calcite alteration, the majority of which is in the form of pseudomorphs after clinopyroxene (PC-355, East Lothian). (b) Heavily altered metadiorite with plagioclase showing antiperthite exsolution (HSH-48, East Lothian). (c) Fresh metadiorite with 45 modal % of quartz (HoA-3, Ayrshire). (d) Fresh metadiorite with clinopyroxene exhibiting fine exsolution lamellae (BdHe-16, Ayrshire). (e) Metatonalites usually have a bimodal grain-size distribution, in which larger quartz and plagioclase crystals can be up to 4 mm, surrounding smaller, c. 0·5 mm, crystals of plagioclase, and quartz (PC-502, East Lothian). (f) Larger grains are often elongated with an aspect ratio up to six, defining a weak foliation (BC-9, East Lothian). Fig. 4 Open in new tabDownload slide Representative photomicrographs of metadiorite (a–d) and metatonalite (e, f) xenoliths are labelled using mineral abbreviations after Kretz (1983). (a) Granoblastic texture with calcite alteration, the majority of which is in the form of pseudomorphs after clinopyroxene (PC-355, East Lothian). (b) Heavily altered metadiorite with plagioclase showing antiperthite exsolution (HSH-48, East Lothian). (c) Fresh metadiorite with 45 modal % of quartz (HoA-3, Ayrshire). (d) Fresh metadiorite with clinopyroxene exhibiting fine exsolution lamellae (BdHe-16, Ayrshire). (e) Metatonalites usually have a bimodal grain-size distribution, in which larger quartz and plagioclase crystals can be up to 4 mm, surrounding smaller, c. 0·5 mm, crystals of plagioclase, and quartz (PC-502, East Lothian). (f) Larger grains are often elongated with an aspect ratio up to six, defining a weak foliation (BC-9, East Lothian). Fig. 5 Open in new tabDownload slide QAP diagram showing selected meta-igneous xenoliths. All metatonalites fall within the tonalite field. Metadiorites show some scatter; most of them are within the diorite and quartz diorite fields with two exceptions: the quartz-rich HoA-3 and PCW-54 plot together with the metatonalites. Fig. 5 Open in new tabDownload slide QAP diagram showing selected meta-igneous xenoliths. All metatonalites fall within the tonalite field. Metadiorites show some scatter; most of them are within the diorite and quartz diorite fields with two exceptions: the quartz-rich HoA-3 and PCW-54 plot together with the metatonalites. Metadiorite Metadiorite xenoliths (Pl ≫ Cpx + Qtz + Fe–Ti oxide + Ap + Zrn ± Bt, Table 1) from East Lothian and Ayrshire have been referred to previously as ‘mafic’ (Graham & Upton, 1978; Upton et al., 1983, 1984) or ‘basic’ xenoliths (Halliday et al., 1993). Metadiorite xenoliths from Tollis Hill (TH-25) have not been described previously. Metadiorite xenoliths from different localities have very similar textures and mineralogy (Fig. 4a–d). These xenoliths have a grain size ranging from c. 0·1 to 4 mm, mainly between 0·5 and 1 mm, and a granular texture in which grain boundaries are strongly curved and only rarely show triple junctions (Fig. 4a;Upton et al., 1984). A weak foliation is occasionally present and it is primarily defined by quartz and plagioclase. The dominant phase is plagioclase (oligoclase–labradorite), which is only rarely antiperthitic (e.g. in HSH-48; see below). The modal proportion of quartz varies widely from <1 to c. 45% (Table 1). The HoA-3 metadiorite from Ayrshire exhibits the highest amount of quartz with c. 45 modal % and it is considered to represent a quartz-rich domain from a generally more mafic region (Fig. 4c). Clinopyroxenes have similar, apparently homogeneous, diopsidic compositions (Supplementary Data Electronic Appendix 3), regardless of locality, with low TiO2 and Al2O3 (<0·5 wt % and 2·1–4·5 wt %, respectively) and relatively low Na2O contents (<1·4 wt %). However, owing to the heavy alteration of the East Lothian metadiorites, analyses of both clinopyroxene cores and rims were possible only in two xenoliths, PCW-13 and HSHe-27. In the others, only clinopyroxene core analyses were feasible. Weak clinopyroxene zoning is present in the Horseshoe Vent metadiorite (HSHe-27), where the core is slightly richer in Al2O3 and FeO than the rim (see below). Biotite is rare and was analysed only in metadiorite PC-355. Biotite in this sample is homogeneous with a relatively high Ti content (c. 5·2 wt %). The Ayrshire xenoliths (HoA-3 and BdHe-16, Fig. 4c and d, respectively) are relatively free of alteration, whereas, despite great effort, no fresh metadiorite xenoliths were found in the Tollis Hill and East Lothian localities (Fig. 4a and b), where all display at least 10 modal % alteration (Table 1). The degree of alteration strongly depends on the original mineralogy. Metadiorites containing mafic minerals such as clinopyroxene and magnetite are more heavily altered than the metatonalites, which consist mainly of quartz and plagioclase. The main alteration products are calcite and serpentine with occasional minor (<1 modal %) amounts of zeolite (Table 1). If the alteration is not advanced, the major alteration product, calcite, primarily replaces clinopyroxene, partially or fully, (Fig. 4a), whereas other minerals remain intact. Where alteration is more intense, calcite can completely replace clinopyroxene and is also present in centimetre-scale veins replacing quartz and plagioclase, as well as clinopyroxene (Fig. 4b). Serpentine/chlorite alteration is usually less abundant as an alteration product than calcite. It replaces oxides and biotite and can surround calcite patches as a thin film (maximum few tens of microns; Fig. 4a). In heavily altered xenoliths, the main surviving mineral is plagioclase, and only very rarely quartz, which gives the misleading impression that these samples have an anorthositic composition (e.g. PC-355, HSH-48). TH-25 from Tollis Hill is composed of fresh plagioclase and magnetite with isolated patches of calcite alteration, still preserving the former grain boundaries as pseudomorphs. Based on the preservation of clinopyroxene inclusions in zircon (see section on zircon petrography, below) it is suggested that the mineral replaced by calcite alteration was clinopyroxene. Metatonalite In the literature, the metatonalite xenoliths (Pl + Qtz + Ap + Zrn ± Kfs ± Fe–Ti oxide, Table 1) have generally been described as ‘quartzo-feldspathic gneisses’ (Graham & Upton, 1978; Upton et al., 1983, 1984) or ‘felsic granulites’ (Halliday et al., 1993). Quartz and plagioclase (oligoclase–andesine, only rarely exhibiting antiperthitic exsolution) are the main mineral phases with modal proportions ranging from 30 to 52% and from 41 to 60%, respectively. Metatonalites usually have strongly curved grain boundaries and a granoblastic texture (Fig. 4e–f), similar to the metadiorites. A bimodal grain-size distribution is common, in which quartz, and more rarely plagioclase, can form large porphyroblasts appearing as elongated crystals up to 4 mm in diameter, with aspect ratios up to six, that define a weak foliation (Fig. 4f). These large crystals surround smaller plagioclase, quartz and K-feldspar grains with an average size of 0·5–1 mm. Metatonalites usually exhibit very little alteration (<10 modal %), which is primarily of calcite accompanied by minor amounts of serpentine and zeolite (Fig. 4f). The alteration is always present along grain boundaries or forms veins equally replacing all mineral phases. Whole-rock chemistry Whole-rock major and trace element composition Whole-rock chemical analyses were carried out on eight xenoliths: four metadiorites and four metatonalites (Table 2). The metatonalites are generally richer in SiO2 and K2O and poorer in MgO, Fe2O3 and TiO2 compared with the metadiorites. Owing to the intense alteration in the East Lothian metadiorites (Table 1), which is also reflected in the generally high loss on ignition (LOI) values, it is difficult to use their whole-rock geochemical compositions to characterize their protoliths. In an AFM diagram (Fig. 6) both the metadiorites and metatonalites plot together with the ‘Newer Granites‘ within the calc-alkaline field. Metadiorites, with the exception of HSH-1, which is intensely altered, tend to have more ferromagnesian compositions, similarly to the SSS plutons, whereas the tonalites are more alkaline. The Ayrshire metadiorites tend to be fresh, from which HoA-3 was analysed for whole-rock major and trace elements. This sample has an unusually high SiO2 content (71 wt %), reflected in its high quartz abundance. Also, apart from its Fe2 O3total and MgO, the values of which are similar to those for the other metadiorites, the TiO2, Al2O3, CaO and Na2O contents are within the same low range as for the metatonalites (Table 2). Fig. 6 Open in new tabDownload slide AFM diagram comparing the metadiorite xenoliths with the South of Scotland and Trans-Suture Zone plutons of the ‘Newer Granites‘ (Stephens & Halliday, 1984). The boundary between the tholeiitic and calc-alkaline fields is from Irvine & Baragar (1971). Fig. 6 Open in new tabDownload slide AFM diagram comparing the metadiorite xenoliths with the South of Scotland and Trans-Suture Zone plutons of the ‘Newer Granites‘ (Stephens & Halliday, 1984). The boundary between the tholeiitic and calc-alkaline fields is from Irvine & Baragar (1971). Metatonalites and metadiorites have generally similar mantle-normalized trace element patterns (Fig. 7). However, the metadiorites tend to be more enriched compared with the metatonalites. The metatonalites display a distinct negative Nb anomaly (Fig. 7), a feature also seen in the only fresh metadiorite, HoA-3, the overall trace element pattern of which resembles more the metatonalites, rather than the metadiorites. Both types of xenolith have strongly similar trace element patterns to the ‘Newer Granite’ SSS and TSZ plutons (Fig. 7a), although the ‘Newer Granites’ are generally more enriched in large ion lithophile elements (Fig. 7a). This similarity is especially the case for the East Lothian metadiorites, in that both they and the ‘Newer Granites’ lack a pronounced negative Nb anomaly (Stephens & Halliday, 1984; Fig. 7a). Fig. 7 Open in new tabDownload slide Primitive mantle-normalized (McDonough & Sun, 1995) trace element diagram for the metatonalite and metadiorite xenoliths compared with (a) ‘Newer Granite’ plutons of the South of Scotland and Trans-Suture Zone suites (Stephens & Halliday, 1984) and (b) Fidra mafic granulites (Downes et al., 2001) and Hawk’s Nib mafic xenoliths (Downes et al., 2007). Fig. 7 Open in new tabDownload slide Primitive mantle-normalized (McDonough & Sun, 1995) trace element diagram for the metatonalite and metadiorite xenoliths compared with (a) ‘Newer Granite’ plutons of the South of Scotland and Trans-Suture Zone suites (Stephens & Halliday, 1984) and (b) Fidra mafic granulites (Downes et al., 2001) and Hawk’s Nib mafic xenoliths (Downes et al., 2007). Trace element patterns of the Fidra xenoliths are also comparable with the trends shown by the metadiorite and metatonalite xenoliths (Fig. 7b). However, as the Hawk’s Nib mafic xenoliths are more depleted, they show only a broad resemblance to the xenoliths of this study (Fig. 7b). Whole-rock Rb–Sr and Sm–Nd isotopic compositions Whole-rock Rb–Sr and Sm–Nd compositions have been determined for three metadiorite and three metatonalite xenoliths (Table 3). All six of these have been dated by the U–Pb zircon method (see below). PC-355 metadiorite and PC-378 metatonalite were previously analysed for Sr, Nd and Pb isotopes by Halliday et al. (1993; Table 3). Compared with previously studied lower crustal meta-igneous xenoliths from the Fidra and Hawk’s Nib localities (Fig. 2), the East Lothian metatonalites cluster tightly on a present-day 143Nd/144Nd vs 87Sr/86Sr diagram (Fig. 8a). Their 143Nd/144Nd ratios are indistinguishable from most of the mafic granulite xenoliths from Fidra (Fig. 2; Downes et al., 2001) but are less radiogenic than the gabbroic and anorthositic xenoliths from Hawk’s Nib (Fig. 2; shown to be cumulates by Downes et al., 2007). The East Lothian metatonalites have slightly more radiogenic Sr than the Hawk’s Nib xenoliths and all but two of those from Fidra (Fig. 8a). Fig. 8 Open in new tabDownload slide (a) Present-day 143Nd/144Nd vs 87Sr/86Sr diagram for the metatonalite and metadiorite xenoliths compared with mafic lower crustal xenoliths from Fidra (Downes et al., 2001, including data from Halliday et al., 1993) and gabbroic and anorthositic xenoliths from Hawk’s Nib (Downes et al., 2007, including data from Halliday et al., 1993). (b) εNdt vs time diagram for the metatonalites and metadiorites, with their bounding Nd evolution fields shown in blue and pink, respectively, compared with ‘Newer Granite’ data from southern Scotland (Distinkhorn, Loch Doon, Criffel, Fleet; Stephens & Halliday, 1984) and xenoliths from Fidra and Hawk’s Nib as in (a) (Downes et al., 2001 and 2007, respectively). Uncertainties are smaller than the plotting symbols in both diagrams. Fig. 8 Open in new tabDownload slide (a) Present-day 143Nd/144Nd vs 87Sr/86Sr diagram for the metatonalite and metadiorite xenoliths compared with mafic lower crustal xenoliths from Fidra (Downes et al., 2001, including data from Halliday et al., 1993) and gabbroic and anorthositic xenoliths from Hawk’s Nib (Downes et al., 2007, including data from Halliday et al., 1993). (b) εNdt vs time diagram for the metatonalites and metadiorites, with their bounding Nd evolution fields shown in blue and pink, respectively, compared with ‘Newer Granite’ data from southern Scotland (Distinkhorn, Loch Doon, Criffel, Fleet; Stephens & Halliday, 1984) and xenoliths from Fidra and Hawk’s Nib as in (a) (Downes et al., 2001 and 2007, respectively). Uncertainties are smaller than the plotting symbols in both diagrams. The East Lothian metadiorites have similar Nd isotopic compositions to the metatonalites but exhibit a wider range of 87Sr/86Sr. They overlap the main Fidra and Hawk’s Nib values (Fig. 8a). Metadiorite HoA-3 from Ayrshire has more radiogenic Nd than any of the East Lothian xenoliths, but also has a very high Sm/Nd ratio. It has a similar Sr isotopic composition to the metatonalites. Apart from the two Fidra outliers (Fig. 8a), which fall within the group interpreted as contaminated by Downes et al. (2001), the East Lothian xenoliths (metadiorites and metatonalites) are generally indistinguishable from those from Fidra. However, they have less radiogenic Nd than about half of the Hawk’s Nib xenoliths. This is also evident on a plot of initial Nd versus time (Fig. 8b), which shows that about half of the Hawk’s Nib xenoliths are more radiogenic than the East Lothian xenoliths at c. 274 Ma [a minimum age for the Hawk’s Nib host intrusion as reported by Downes et al. (2007)]. By contrast, all but the three least radiogenic Fidra xenoliths fall within the field of the Midland Valley meta-igneous xenoliths from this study. About half of the ‘Newer Granite’ plutons (SSS and TSZ; Stephens & Halliday, 1984) also overlap with the Nd evolution trend of the metadiorite xenoliths, although both groups extend to more extreme values (Fig. 8b). Geothermobarometry Temperature Metamorphic temperatures were calculated from a single metadiorite xenolith, HSH-48, with well-developed antiperthite (Fig. 4b), that yielded a temperature of c. 793–816 ± 40°C (Fig. 9) using the Solvcalc 2.0 program (Wen & Nekvasil, 1994) and the two-feldspar thermometer of Fuhrman & Lindsley (1988). This temperature estimate represents the closure temperature for feldspar exsolution and hence provides a minimum temperature for the metamorphism. No pressure calculation was possible owing to the high degree of alteration, which has replaced all of the original clinopyroxene and quartz in this xenolith. Fig. 9 Open in new tabDownload slide P–T estimates for East Lothian and Ayrshire metadiorite xenoliths calculated using the McCarthy & Patiño Douce (1998) clinopyroxene–plagioclase–quartz geobarometer and the Fuhrman & Lindsley (1988) two-feldspar geothermometer. Apart from HSH-48, average pressure estimates are shown for each rock, calculated over a range of temperatures between 700 and 900°C (Supplementary Data Electronic Appendix 5). Temperature estimates only are available for HSH-48, which lacks fresh clinopyroxene and no other geobarometer is available. Fig. 9 Open in new tabDownload slide P–T estimates for East Lothian and Ayrshire metadiorite xenoliths calculated using the McCarthy & Patiño Douce (1998) clinopyroxene–plagioclase–quartz geobarometer and the Fuhrman & Lindsley (1988) two-feldspar geothermometer. Apart from HSH-48, average pressure estimates are shown for each rock, calculated over a range of temperatures between 700 and 900°C (Supplementary Data Electronic Appendix 5). Temperature estimates only are available for HSH-48, which lacks fresh clinopyroxene and no other geobarometer is available. Pressure The clinopyroxene–quartz–plagioclase geobarometer of McCarthy & Patiño Douce (1998) was developed for rocks that lack garnet and therefore can be used on the metadiorite xenoliths. The barometer was applied to five metadiorite xenoliths (Fig. 9), four from East Lothian (PCW-00, PCW-13, PCW-97 and HSHe-27) and one from Ayrshire (HoA-3). Details of the P–T results are presented in Supplementary Data Electronic Appendix 5. Because these samples do not exhibit feldspar exsolution, two-feldspar thermometry was not possible. However, their granular texture suggests equilibration at high temperature and hence a temperature range of 700–900°C was assumed for the pressure calculations, to match the conditions of the McCarthy & Patiño Douce (1998) experiments and consistent with the minimum temperature estimate (c. 800°C) from HSH-48. For the assumed temperature range of 700–900°C, the xenoliths yield an apparent pressure range of 5·7–10·1 kbar, for which the individual pressure calculations carry an uncertainty of ±1·0 kbar (McCarthy & Patiño Douce, 1998). However, using the minimum temperature of c. 800°C from HSH-48, the resulting range is c. 6·5–9·0 kbar. Pressure estimates calculated for four out of five of the metadiorite xenoliths differ by less than 1 kbar. However, HSHe-27 exhibits weak Al zoning in clinopyroxene, where Al decreases from core to rim (Supplementary Data Electronic Appendix 3), and hence yields rim pressures c. 1·5–2 kbar lower than in the cores. U–Pb zircon geochronology Zircons from both the metadiorite and metatonalite xenoliths have been analysed for U–Th–Pb isotopes by SIMS and for U–Pb by LA-MC-ICP-MS. Representative scanning electron CL images of dated zircons are presented in Figs 10 and 11. Isotopic data and calculated ages are listed in Supplementary Data Electronic Appendix 6 and are shown in Tera–Wasserburg diagrams in Figs 12 and 13. A detailed account of the zircon petrography and U–Pb geochronology of each xenolith is presented in Supplementary Data Electronic Appendix 2. Zircon grains from nine metadiorite xenoliths (Fig. 12a–l) have been analysed (Table 1): six from East Lothian (PCW-70, PCW-54, PC-355, HSH-1, HSH-48 and BC-7), one from Tollis Hill (TH-25) and two from Ayrshire (BdHe-16 and HoA-3). Zircons have been analysed from four metatonalite xenoliths (Fig. 13a–e), all from East Lothian (BC-9, PC-378, PC-502 and PC-503). Geochronology results are summarized in Table 4. Fig. 10 Open in new tabDownload slide Representative zircon cathodoluminescence images from metadiorite xenoliths, labelled by analysis number and 206Pb/238U age (in Ma, with 2σ errors, Supplementary Data Electronic Appendix 6). Analyses: white circles, LA-MC-ICP-MS U–Pb; white dashed ellipses, SIMS; yellow circles, Lu–Hf, together with εHft values (Supplementary Data Electronic Appendix 7). Scale bars represent 100 µm. Zircon 15 of xenolith PCW-54 in (b) is also illustrated in transmitted light to show that the inclusion-rich core is clearly distinguishable from the inclusion-free rim. Fig. 10 Open in new tabDownload slide Representative zircon cathodoluminescence images from metadiorite xenoliths, labelled by analysis number and 206Pb/238U age (in Ma, with 2σ errors, Supplementary Data Electronic Appendix 6). Analyses: white circles, LA-MC-ICP-MS U–Pb; white dashed ellipses, SIMS; yellow circles, Lu–Hf, together with εHft values (Supplementary Data Electronic Appendix 7). Scale bars represent 100 µm. Zircon 15 of xenolith PCW-54 in (b) is also illustrated in transmitted light to show that the inclusion-rich core is clearly distinguishable from the inclusion-free rim. Fig. 11 Open in new tabDownload slide Representative zircon cathodoluminescence images from metatonalite xenoliths. Zircon 8 of xenolith PC-378 in (b) is also illustrated in transmitted light to show the inclusion-rich core, which is clearly distinguishable from the inclusion-free rim. Scale bar represents 100 µm. Data sources, symbols and labels as in Fig. 10, except that 207Pb/206U ages are shown for some grains that have been affected by lead loss (see text). Fig. 11 Open in new tabDownload slide Representative zircon cathodoluminescence images from metatonalite xenoliths. Zircon 8 of xenolith PC-378 in (b) is also illustrated in transmitted light to show the inclusion-rich core, which is clearly distinguishable from the inclusion-free rim. Scale bar represents 100 µm. Data sources, symbols and labels as in Fig. 10, except that 207Pb/206U ages are shown for some grains that have been affected by lead loss (see text). Fig. 12 Open in new tabDownload slide Tera–Wasserburg diagrams for metadiorite xenoliths showing U–Pb data (Supplementary Data Electronic Appendix 6) as 2σ error ellipses. Thick ellipses, SIMS analyses; thin ellipses, LA-MC-ICP-MS analyses; dark grey filled ellipses, magmatic zircon cores interpreted as inherited grains; light grey filled ellipses, magmatic zircon analyses interpreted as defining the protolith age; white ellipses, analyses obtained from metamorphic zircon domains (cores, rims and interiors); diagonally shaded ellipse, analysis of CL-bright rim. Unless otherwise labelled, ages displayed in the plots are concordia ages. Insets in (c) and (f), and (l), show the weighted average of 207Pb/206Pb ages colour-coded to correspond to the individual error ellipses. Inset in (j) shows the U content of the analyses. Fig. 12 Open in new tabDownload slide Tera–Wasserburg diagrams for metadiorite xenoliths showing U–Pb data (Supplementary Data Electronic Appendix 6) as 2σ error ellipses. Thick ellipses, SIMS analyses; thin ellipses, LA-MC-ICP-MS analyses; dark grey filled ellipses, magmatic zircon cores interpreted as inherited grains; light grey filled ellipses, magmatic zircon analyses interpreted as defining the protolith age; white ellipses, analyses obtained from metamorphic zircon domains (cores, rims and interiors); diagonally shaded ellipse, analysis of CL-bright rim. Unless otherwise labelled, ages displayed in the plots are concordia ages. Insets in (c) and (f), and (l), show the weighted average of 207Pb/206Pb ages colour-coded to correspond to the individual error ellipses. Inset in (j) shows the U content of the analyses. Fig. 12 Open in new tabDownload slide Continued. Fig. 12 Open in new tabDownload slide Continued. Fig. 13 Open in new tabDownload slide Tera–Wasserburg diagrams for metatonalite xenoliths showing zircon U–Pb data (Supplementary Data Electronic Appendix 6) as 2σ error ellipses. Thick ellipses, SIMS analyses; thin ellipses, LA-MC-ICP-MS analyses; dark grey filled ellipses, magmatic zircon cores interpreted as inherited grains; light grey filled ellipses, magmatic zircon analyses interpreted as defining the protolith age; white ellipses, analyses obtained from metamorphic zircon domains (cores, rims and interiors); diagonally shaded ellipses, CL-bright rim analyses. Inset within (c) shows the weighted average of 207Pb/206Pb ages, colour-coded to correspond to the individual error ellipses. Fig. 13 Open in new tabDownload slide Tera–Wasserburg diagrams for metatonalite xenoliths showing zircon U–Pb data (Supplementary Data Electronic Appendix 6) as 2σ error ellipses. Thick ellipses, SIMS analyses; thin ellipses, LA-MC-ICP-MS analyses; dark grey filled ellipses, magmatic zircon cores interpreted as inherited grains; light grey filled ellipses, magmatic zircon analyses interpreted as defining the protolith age; white ellipses, analyses obtained from metamorphic zircon domains (cores, rims and interiors); diagonally shaded ellipses, CL-bright rim analyses. Inset within (c) shows the weighted average of 207Pb/206Pb ages, colour-coded to correspond to the individual error ellipses. Table 4 Summary of U–Pb zircon geochronology of meta-igneous xenoliths from southern Scotland Xenolith Locality* Region† Inheritance Xenolith protolith Metamorphism Inherited cores (Ma) Error (2σ) n Magmatic cores (Ma) Error (2σ) n CL-grey cores and rims (Ma) Error (2σ) n CL-bright rims (Ma) Error (2σ) n Metadiorites PCW-70 PC EL 449‡ 9 6 417 4 8 402 3 16 — — — PCW-54 PC EL 446 10§ 4 414 4 4 401 3 8 — — — PC-355 PC EL — — — 418‡ 15 6 395 3 9 383 7 1 HSH-1 HSH EL 453‡ 20 3 418 5 4 396 4 6 — — — HSH-48 HSH EL — — — — — — 401 3 7 — — — BC-7 BC EL — — — 419 3 7 397 6 2 — — — TH-25 TH CB — — — 413 4 5 — — — — — — HoA-3 HoA Ay 480 9 1 411 3 6 370 8 1 — — — 451‡ 11 6 BdHe-16 BdH Ay 463 21 1 — — — 391 3 4 — — — Metatonalites BC-9 BC EL — — — 446¶ 14 1 411 4 4 391 3 5 PC-378 PC EL — — — — — — 395 4 4 — — — PC-502 PC EL — — — 459 6 2 387 4 3 — — — PC-503 PC EL — — — 430 5 3 404 3 7 382 4 1 Xenolith Locality* Region† Inheritance Xenolith protolith Metamorphism Inherited cores (Ma) Error (2σ) n Magmatic cores (Ma) Error (2σ) n CL-grey cores and rims (Ma) Error (2σ) n CL-bright rims (Ma) Error (2σ) n Metadiorites PCW-70 PC EL 449‡ 9 6 417 4 8 402 3 16 — — — PCW-54 PC EL 446 10§ 4 414 4 4 401 3 8 — — — PC-355 PC EL — — — 418‡ 15 6 395 3 9 383 7 1 HSH-1 HSH EL 453‡ 20 3 418 5 4 396 4 6 — — — HSH-48 HSH EL — — — — — — 401 3 7 — — — BC-7 BC EL — — — 419 3 7 397 6 2 — — — TH-25 TH CB — — — 413 4 5 — — — — — — HoA-3 HoA Ay 480 9 1 411 3 6 370 8 1 — — — 451‡ 11 6 BdHe-16 BdH Ay 463 21 1 — — — 391 3 4 — — — Metatonalites BC-9 BC EL — — — 446¶ 14 1 411 4 4 391 3 5 PC-378 PC EL — — — — — — 395 4 4 — — — PC-502 PC EL — — — 459 6 2 387 4 3 — — — PC-503 PC EL — — — 430 5 3 404 3 7 382 4 1 Ages with 2σ errors are concordia ages (n > 1) or 206Pb/238U ages (n = 1), unless otherwise stated. * Localities: PC, Partan Craig; HSH, Horseshoe Vent; BC, Beggar’s Cap; BdH, Baidland Hill; HoA, Heads of Ayr; TH, Tollis Hill. † Regions: EL, East Lothian; Ay, Ayrshire; CB, Central Belt of the Southern Uplands. ‡ Weighted average of 207Pb/206Pb ages. § Error is given at 95% confidence level. ¶ 207Pb/206Pb age (n = 1). Open in new tab Table 4 Summary of U–Pb zircon geochronology of meta-igneous xenoliths from southern Scotland Xenolith Locality* Region† Inheritance Xenolith protolith Metamorphism Inherited cores (Ma) Error (2σ) n Magmatic cores (Ma) Error (2σ) n CL-grey cores and rims (Ma) Error (2σ) n CL-bright rims (Ma) Error (2σ) n Metadiorites PCW-70 PC EL 449‡ 9 6 417 4 8 402 3 16 — — — PCW-54 PC EL 446 10§ 4 414 4 4 401 3 8 — — — PC-355 PC EL — — — 418‡ 15 6 395 3 9 383 7 1 HSH-1 HSH EL 453‡ 20 3 418 5 4 396 4 6 — — — HSH-48 HSH EL — — — — — — 401 3 7 — — — BC-7 BC EL — — — 419 3 7 397 6 2 — — — TH-25 TH CB — — — 413 4 5 — — — — — — HoA-3 HoA Ay 480 9 1 411 3 6 370 8 1 — — — 451‡ 11 6 BdHe-16 BdH Ay 463 21 1 — — — 391 3 4 — — — Metatonalites BC-9 BC EL — — — 446¶ 14 1 411 4 4 391 3 5 PC-378 PC EL — — — — — — 395 4 4 — — — PC-502 PC EL — — — 459 6 2 387 4 3 — — — PC-503 PC EL — — — 430 5 3 404 3 7 382 4 1 Xenolith Locality* Region† Inheritance Xenolith protolith Metamorphism Inherited cores (Ma) Error (2σ) n Magmatic cores (Ma) Error (2σ) n CL-grey cores and rims (Ma) Error (2σ) n CL-bright rims (Ma) Error (2σ) n Metadiorites PCW-70 PC EL 449‡ 9 6 417 4 8 402 3 16 — — — PCW-54 PC EL 446 10§ 4 414 4 4 401 3 8 — — — PC-355 PC EL — — — 418‡ 15 6 395 3 9 383 7 1 HSH-1 HSH EL 453‡ 20 3 418 5 4 396 4 6 — — — HSH-48 HSH EL — — — — — — 401 3 7 — — — BC-7 BC EL — — — 419 3 7 397 6 2 — — — TH-25 TH CB — — — 413 4 5 — — — — — — HoA-3 HoA Ay 480 9 1 411 3 6 370 8 1 — — — 451‡ 11 6 BdHe-16 BdH Ay 463 21 1 — — — 391 3 4 — — — Metatonalites BC-9 BC EL — — — 446¶ 14 1 411 4 4 391 3 5 PC-378 PC EL — — — — — — 395 4 4 — — — PC-502 PC EL — — — 459 6 2 387 4 3 — — — PC-503 PC EL — — — 430 5 3 404 3 7 382 4 1 Ages with 2σ errors are concordia ages (n > 1) or 206Pb/238U ages (n = 1), unless otherwise stated. * Localities: PC, Partan Craig; HSH, Horseshoe Vent; BC, Beggar’s Cap; BdH, Baidland Hill; HoA, Heads of Ayr; TH, Tollis Hill. † Regions: EL, East Lothian; Ay, Ayrshire; CB, Central Belt of the Southern Uplands. ‡ Weighted average of 207Pb/206Pb ages. § Error is given at 95% confidence level. ¶ 207Pb/206Pb age (n = 1). Open in new tab Zircon petrography Zircons in the metadiorite xenoliths typically contain resorbed magmatic cores that are oscillatory-zoned (Fig. 10a–d), idiomorphically zoned (Fig. 10g) or CL-dark (Fig. 10f, h and i). These cores are readily apparent under CL, but also, owing to the presence of mineral inclusions such as quartz, clinopyroxene, feldspar and other unidentified minerals, they are commonly visible even under transmitted light (Fig. 10b). Zircon cores are surrounded by one or more metamorphic rims. Inner rims, when present, are usually CL-grey, of variable thickness (up to 150 µm) and either homogeneous or weakly zoned and inclusion-free (Fig. 10b). Outer rims, when present, are usually thinner (up to c. 20 µm), CL-bright and also inclusion-free (Fig. 10a, b and i). Some zircons exhibit only a CL-grey homogeneous or weakly zoned interior (no distinguishable core and rim are visible) or a core surrounded rarely by a CL-bright rim (Fig. 10c, grain 11), both of which are inclusion-free. In these examples, the magmatic cores are usually present below the polished surface and are visible in transmitted light. Sometimes, however, the inclusion-rich interior is not detectable and hence these whole grains are interpreted as metamorphic (e.g. Fig. 10b, grain 10), especially in xenoliths BdHe-16 (Fig. 10h) and HSH-48 (Fig. 10e), in which the zircons are mainly or exclusively of this type. BdHe-16 is unusual in that several zircon grains display small U-richer CL-dark cores and U-poor CL-grey rims, both of which are interpreted as metamorphic (see below). Metatonalite xenoliths contain zircons that usually have oscillatory-zoned (Fig. 11a) or idiomorphically zoned (Fig. 11c and d) cores interpreted as magmatic. As with the metadiorite zircons, the metatonalite zircon cores often contain abundant mineral inclusions (Fig. 11b) and when not exposed are also visible within the grain using transmitted light. The cores are usually surrounded by two sets of inclusion-free rims that are considered to have a metamorphic origin. The inner rims have a variable thickness (up to 60 µm) with a grey, homogeneous or weakly zoned CL response (Fig. 11a–c). The outer rims tend to be CL-bright and are generally thin (<20 µm, Fig. 11a–d). However, xenolith BC-9 exhibits several exceptionally thick (up to c. 80 µm) CL-bright outer rims (Fig. 11a) probably owing to shallower polishing, which provided a fortuitous opportunity to date the otherwise challenging outer rims (Fig. 11a, and see results of dating below, also shown in Fig. 13a). As for the metadiorite xenoliths, the magmatic cores are sometimes not present and thus the entire zircon grains have a metamorphic origin. U–Pb zircon geochronology Magmatic zircons in metadiorite xenoliths. U–Pb data from magmatic zircon cores in the metadiorite xenoliths tend to be concordant with an overall spread of 206Pb/238U ages, ranging from 480 to 383 Ma (Fig. 12; Supplementary Data Electronic Appendices 2 and 6). This wide range is partly explained by Pb loss and is not present in all samples. U–Pb data exhibiting this broad age range tend to be bimodal, c. 415 and c. 453 Ma (Supplementary Data Electronic Appendix 6), where the younger age group is interpreted as dating the metadiorite protoliths, whereas the older zircon cores are taken to be inherited. Magmatic cores in two metadiorites, BC-7 and TH-25, yielded U–Pb concordia ages of 419 ± 3 Ma and 413 ± 4 Ma respectively, interpreted as their protolith ages (Fig. 12h and i). Magmatic cores from PC-355 yielded concordant analyses (Fig. 12e) but, owing to lead loss, a concordia age cannot be calculated. Instead, they provide an indistinguishable protolith age of 418 ± 15 Ma, based on the weighted average of their 207Pb/206Pb ages (Fig. 12e). Zircon cores in PCW-70, PCW-54, HSH-1 and HoA-3 tend to fall into two groups (Fig. 12a–d, f and k, respectively; Table 4), the younger of which has 206Pb/238U ages between 418 and 411 Ma. Individually, these define concordia ages of 417 ± 4 Ma (PCW-70, Fig. 12c), 414 ± 4 Ma (PCW-54, Fig. 12d), 418 ± 5 Ma (HSH-1, Fig. 12f) and 411 ± 3 Ma (HoA-3, Fig. 12k), all interpreted as protolith ages. Overall, the weighted average of the protolith ages of the seven metadiorite xenoliths is 415 ± 3 Ma (MSWD = 3). Inherited zircons in the metadiorite xenoliths. Of the metadiorite samples containing older cores (i.e. PCW-70, PCW-54, HSH-1, HoA-3 and BdHe-16) only those from PCW-54 yielded a concordia age (446 ± 10 Ma). The older cores in PCW-70, HSH-1, HoA-3 and BdHe-16 do not yield concordia ages and their spread of 206Pb/238U ages (from 411 to 480 Ma, Supplementary Data Electronic Appendix 6; Fig. 12a, b, f, j and k) is interpreted as the result of Pb loss. In support of this interpretation, these data individually display tightly grouped 207Pb/206Pb ages, whose weighted averages (Table 4) are 449 ± 9 Ma (PCW-70), 453 ± 20 Ma (HSH-1) and 451 ± 11 Ma (HoA-3, Fig. 12l). The latter excludes one older inherited grain (grain 27) from HoA-3 with a 206Pb/238U age of 480 ± 9 Ma (Fig. 12k). In addition, BdHe-16 also has a single old zircon core (53-02, Fig. 10j) with a 206Pb/238U age of 463 ± 21 Ma. Twenty older cores from all five metadiorite xenoliths yielded a weighted mean 207Pb/206Pb age of 449 ± 7 Ma (MSWD = 0·81) and are interpreted as inherited zircons. Magmatic zircons in metatonalite xenoliths. Zircons from three of the four East Lothian metatonalites (BC-9, PC-502, PC-503) have yielded protolith ages. These ages were obtained from analyses of oscillatory-zoned, idiomorphically zoned or CL-dark cores, which are considered to offer the best prospect of obtaining (magmatic) protolith ages. However, many of these appear to have undergone some degree of Pb loss, based on their disposition on the Tera–Wasserburg diagram. In PC-502, seven concordant zircon cores define a bimodal distribution (Fig. 13c). The two oldest grains yield a concordia age of 459 ± 6 Ma (Fig. 13c) and are interpreted as dating the protolith. The younger group of five analyses have 206Pb/238U ages ranging from 434 to 417 Ma. All seven analyses are interpreted to represent a single population disturbed by lead loss. The five analyses defining the younger age range have a weighted average 207Pb/206Pb age of 456 ± 23 Ma (Fig. 13c, inset) consistent with Pb loss after 459 Ma. In PC-503, eight idiomorphically zoned cores yield a bimodal distribution of 206Pb/238U ages ranging from 503 to 424 Ma. The three youngest cores yield a concordia age of 430 ± 5 Ma (Fig. 13d), which is interpreted as the crystallization age of the xenolith protolith. The 206Pb/238U ages of the older group of four cores range from 503 to 462 Ma, and they are interpreted as inherited grains. They have a weighted mean 207Pb/206Pb age of 467 ± 32 Ma (Table 4; Fig. 13d, inset). A single oscillatory-zoned core from a third metatonalite, BC-9, is discordant (Fig. 13a;Supplementary Data Electronic Appendix 6), owing to Pb loss. This core has a 207Pb/206Pb age of 446 ± 14 Ma (Fig. 13a), which is interpreted as a protolith age. The similarity in ages of two of the metatonalite protoliths (BC-9 and PC-502) and the inherited zircons in the metadiorites (Table 4) suggests that these zircon populations have a related origin. Pooling the calculated protolith ages from BC-9 and PC-502 together with the ages of the inherited components in the metadiorite xenoliths PCW-70, PCW-54, HSH-1 and HoA-3 (Table 4) yields a weighted average age of 453 ± 8 Ma (MSWD = 0·4). It is suggested that this is the best available estimate for the age of the earlier group of metatonalite protoliths. The somewhat younger age of 430 ± 5 Ma from tonalite PC-503 is interpreted as dating a later magmatic episode. U–Pb zircon geochronology of metamorphic zircon. Zircon grains in both types of xenolith commonly display two sets of rims, a thicker (up to 150 µm) CL-grey inner rim and a thin (up to 20 µm) CL-bright outer rim. The CL-bright outer rims are generally better developed in the metatonalites, but are commonly too thin, or have U contents (and hence radiogenic Pb contents) too low to provide useful age information. The rims are interpreted to have a metamorphic origin because of their weakly zoned CL texture, lack of mineral inclusions, and generally lower U contents compared with the magmatic zircons. In this study, a total of 90 analyses have been acquired from metamorphic zircon rims in 12 xenoliths, 83 of which are from CL-grey inner rims. In addition, seven CL-bright outer rims were successfully analysed in three xenoliths (PC-355, BC-9 and PC-503). The Tollis Hill xenolith (Southern Uplands) also displays CL-bright metamorphic rims (Fig. 10g), but these proved too thin to analyse. Altogether, the 90 analyses from metamorphic zircons yielded 206Pb/238U ages ranging from 415 to 370 Ma (Supplementary Data Electronic Appendix 6). The metadiorites and metatonalites have similar distributions of 206Pb/238U ages (Fig. 14a), with one major peak at c. 405–390 Ma (Fig. 14a). Fig. 14 Open in new tabDownload slide (a) Histogram of metamorphic zircon 206Pb/238U ages (Supplementary Data Electronic Appendix 6) from metadiorite and metatonalite xenoliths. (b) Concordia ages calculated for metamorphic zircon domains in individual xenoliths (Table 4), including the single 206Pb/238U age of the metamorphic rim in HoA-3. Error bars and quoted errors are 2σ. Fig. 14 Open in new tabDownload slide (a) Histogram of metamorphic zircon 206Pb/238U ages (Supplementary Data Electronic Appendix 6) from metadiorite and metatonalite xenoliths. (b) Concordia ages calculated for metamorphic zircon domains in individual xenoliths (Table 4), including the single 206Pb/238U age of the metamorphic rim in HoA-3. Error bars and quoted errors are 2σ. In the metadiorites, metamorphic zircon rims in PCW-70 (Fig. 12b) gave a concordia age of 402 ± 3 Ma (n = 16), excluding the two youngest analyses, which appear to have been affected by Pb loss (analyses 56b and 65b; Supplementary Data Electronic Appendix 6). Eight analyses of metamorphic zircon rims in PCW-54 (Fig. 12d) yielded a similar concordia age of 401 ± 3 Ma (n = 8). Analyses of metamorphic zircon interiors in HSH-48 (Fig. 12g) result in a concordia age of 399 ± 4 Ma (n = 8) or 401 ± 3 Ma (n = 7) if the youngest analysis (grain 7a; Supplementary Data Electronic Appendix 6) is excluded on the basis of Pb loss. In addition, nine analyses of inner rims in PC-355 (Fig. 12e) and six analyses of inner rims in HSH-1 (Fig. 12f) yielded concordia ages of 395 ± 3 Ma and 396 ± 4 Ma, respectively, and two zircon rim analyses from BC-7 gave a concordia age of 397 ± 6 Ma (Fig. 12h). In the Ayrshire xenoliths, four CL-dark core analyses from BdHe-16 define a concordia age of 391 ± 3 Ma (Fig. 12j) and a single rim analysis in HoA-3 yielded a 206Pb/238U age of 370 ± 8 Ma (Fig. 12k). In the metatonalites, four metamorphic inner rims in PC-378 (Fig. 13b) yielded a concordia age of 395 ± 4 Ma. Analyses of two CL-grey sector-zoned interiors and one weakly zoned zircon rim from metatonalite PC-502 define a concordia age of 387 ± 4 Ma (Fig. 13c). Eleven concordant analyses from CL-grey zircon rims in PC-503 have 206Pb/238U ages ranging from 411 to 381 Ma and a weighted average 207Pb/206Pb age of 397 ± 7 Ma. The younger analyses are probably due to lead loss, whereas the seven oldest analyses define a concordia age of 404 ± 3 Ma (Fig. 13e). Twelve CL-grey rims in BC-9 have a range of 206Pb/238U ages between 415 and 372 Ma. The four oldest of these yield a concordia age of 411 ± 4 Ma (Fig. 13a). This is within error of the weighted average 207Pb/206Pb age of 414 ± 13 Ma for all 12, consistent with lead loss in the younger analyses. Importantly, these grains exhibit CL-bright outer rims, five of which yield a concordia age of 391 ± 3 Ma (MSWD = 0·15, Fig. 13a). Analyses of CL-bright outer rims in BC-9 can be combined with analyses of CL-bright outer rims in two other xenoliths (one each in metadiorite PC-355 and metatonalite PC-503) to yield a concordia age of 389 ± 3 Ma (MSWD = 0·1; Fig. 14b). The single dated outer rim from HoA-3 yields an even younger 206Pb/238U age of 370 ± 8 Ma (Fig. 12k). In summary, U–Pb concordia ages from CL-grey inner rims calculated using multiple analyses of individual xenoliths (i.e. excluding the single analysis from HoA-3), range from 411 ± 4 to 387 ± 4 Ma (Fig. 14b). These data can be divided into three age groups: 411 ± 4 Ma, 400 ± 3 Ma and 391 ± 4 Ma (Fig. 14b), each of which may be interpreted as a metamorphic event. The petrographically distinct CL-bright outer rims yield a concordia age of 389 ± 3 Ma, which is indistinguishable from the youngest metamorphic event recorded by the CL-grey inner rims. Lu–Hf zircon isotopic composition Zircons from six metadiorite and two metatonalite xenoliths have been analysed for their Lu–Hf isotopic compositions (Fig. 15) by LA-MC-ICP-MS on previously dated domains. Data are presented in Supplementary Data Electronic Appendix 7. Fig. 15 Open in new tabDownload slide Hf isotopic evolution of magmatic (including inherited) and metamorphic zircon from meta-igneous xenoliths from the Midland Valley (East Lothian and Ayrshire) and Southern Uplands (Tollis Hill), compared with the Hf isotopic evolution of new crust and depleted mantle (DM) after Dhuime et al. (2011), calculated using the 176Lu decay constant from Söderlund et al. (2004) relative to CHUR (Bouvier et al., 2008). Fig. 15 Open in new tabDownload slide Hf isotopic evolution of magmatic (including inherited) and metamorphic zircon from meta-igneous xenoliths from the Midland Valley (East Lothian and Ayrshire) and Southern Uplands (Tollis Hill), compared with the Hf isotopic evolution of new crust and depleted mantle (DM) after Dhuime et al. (2011), calculated using the 176Lu decay constant from Söderlund et al. (2004) relative to CHUR (Bouvier et al., 2008). In the metadiorites, zircons defining the protolith ages have εHft=415 values ranging from +0·1 to +11·1. Of these, HoA-3 from Ayrshire and the single xenolith from Tollis Hill, TH-25, yield the highest values, with very narrow ranges (i.e. +8·7 to +11·1 and +8·9 to +9·6, respectively). Inherited zircons from the metadiorites have similarly high εHf values (εHft=453 = +1·6 to +10·8). Zircons that define the protolith age in the metatonalite xenoliths exhibit a narrow range of positive εHft values; in PC-502 εHft=453 = +7·8 to +9·0 and in PC-503 εHft=430 = +8·1 to +8·3. The εHft values in the metatonalites tend to be higher than in the metadiorites, which also indicates their relatively juvenile origin (Fig. 15). Metamorphic zircon rims in the metadiorites and the metatonalites have similarly high εHft values compared with the magmatic zircon cores (εHft=400 of –1·1 to +8·6 and +7·1 to +7·9, respectively), consistent with closed-system metamorphism (i.e. recycling of Hf from resorbed zircon cores into the metamorphic rims). In the BdHe-16 xenolith both CL-bright and CL-dark zircon domains, including CL-dark core 52-3 which is considered as inherited in origin, show a narrow range of overall εHft=400 values from +3·7 to +4·7. However, CL-dark cores tend to have somewhat higher εHft=400 values (+4·1 to +4·7) compared with the CL-bright rims. DISCUSSION Caledonian plutonic events recorded in meta-igneous xenoliths Metadiorite xenoliths from East Lothian, Ayrshire and Tollis Hill and metatonalites from East Lothian provide evidence for at least three early Palaeozoic plutonic events. The metadiorites have yielded protolith ages ranging from c. 419 to 411 Ma (n = 7, Fig. 12, Table 4), the weighted average of which is 415 ± 3 Ma (Fig. 12, Table 4). The metatonalites are interpreted to record two magmatic episodes. The earlier one (453 Ma) is based on U–Pb concordia ages from two metatonalites, together with the ages of inherited zircons from the metadiorites (Table 4). In addition, one of the metatonalites is interpreted as the product of magmatism at c. 430 Ma. Late Caledonian plutonic event—xenolith evidence for unexposed ‘Newer Granite’ bodies The East Lothian and Ayrshire xenolith localities are situated in the region occupied by the SSS of ‘Newer Granite’ plutons (Fig. 2; Stephens & Halliday, 1984). Seven metadiorite xenoliths have yielded protolith crystallization ages of c. 415 Ma (Fig. 16), which correspond well to the age range of SSS granites south of the Highland Boundary Fault (e.g. Distinkhorn, Loch Doon). These plutons are located closest to the xenolith localities and show a narrow age range from 413 to 408 Ma (Halliday et al., 1980a; Thirlwall, 1988; Fig. 16), within error of the protolith age of the metadiorites. Fig. 16 Open in new tabDownload slide Geochronological chart showing published isotopic ages of Cambrian (Camb) to Devonian magmatic rocks (including clasts) in selected Caledonian terranes in Britain and Ireland compared with U–Pb zircon ages from deep crustal metadiorite and metatonalite xenoliths (this study). Protolith ages of the metadiorite xenoliths coincide well with the ‘Newer Granite’ magmatism. Metatonalite protolith ages and inherited zircon cores in the metadiorites overlap with Ordovician arc-related magmatism, which is strongly diachronous in the different terranes. Ages from Britain (mainly Scotland) are from Longman et al. (1979), Compston et al. (1982), Stephens & Halliday (1984), Dunham & Wilson (1985; in Oliver et al., 2008), Kneller & Aftalion (1987), Thirlwall (1988), Rogers et al. (1989), Haughton & Halliday (1991), Rogers & Dunning (1991), Hughes et al. (1996), Oliver et al. (2000, 2008), Carty (2001, in Oliver et al. 2008), Dempster et al. (2002), J. Mendum (personal communication in Strachan et al. (2002), Millward & Evans (2003), Fraser et al. (2004; in Oliver et al., 2008), Morris et al. (2005), Neilson et al. (2009), Appleby et al. (2010), Kimbell et al. (2010) and Miles et al. (2014). Ages of Irish rocks are from Halliday et al. (1980b), O’Connor et al. (1982, 1984, 1987, 1989), Long et al. (1984), Aftalion & Max (1987), Thirlwall (1988), Chew & Schaltegger (2005,), Flowerdew et al. (2005), Chew et al. (2008), Cooper et al. (2008, 2011, 2013), Kirkland et al. (2008, 2013), Draut et al. (2009), Hollis et al. (2012), Anderson et al. (2016) and Fritschle et al. (2018a, 2018b). Geological timescale from Cohen et al. (2013, updated 2018). Fig. 16 Open in new tabDownload slide Geochronological chart showing published isotopic ages of Cambrian (Camb) to Devonian magmatic rocks (including clasts) in selected Caledonian terranes in Britain and Ireland compared with U–Pb zircon ages from deep crustal metadiorite and metatonalite xenoliths (this study). Protolith ages of the metadiorite xenoliths coincide well with the ‘Newer Granite’ magmatism. Metatonalite protolith ages and inherited zircon cores in the metadiorites overlap with Ordovician arc-related magmatism, which is strongly diachronous in the different terranes. Ages from Britain (mainly Scotland) are from Longman et al. (1979), Compston et al. (1982), Stephens & Halliday (1984), Dunham & Wilson (1985; in Oliver et al., 2008), Kneller & Aftalion (1987), Thirlwall (1988), Rogers et al. (1989), Haughton & Halliday (1991), Rogers & Dunning (1991), Hughes et al. (1996), Oliver et al. (2000, 2008), Carty (2001, in Oliver et al. 2008), Dempster et al. (2002), J. Mendum (personal communication in Strachan et al. (2002), Millward & Evans (2003), Fraser et al. (2004; in Oliver et al., 2008), Morris et al. (2005), Neilson et al. (2009), Appleby et al. (2010), Kimbell et al. (2010) and Miles et al. (2014). Ages of Irish rocks are from Halliday et al. (1980b), O’Connor et al. (1982, 1984, 1987, 1989), Long et al. (1984), Aftalion & Max (1987), Thirlwall (1988), Chew & Schaltegger (2005,), Flowerdew et al. (2005), Chew et al. (2008), Cooper et al. (2008, 2011, 2013), Kirkland et al. (2008, 2013), Draut et al. (2009), Hollis et al. (2012), Anderson et al. (2016) and Fritschle et al. (2018a, 2018b). Geological timescale from Cohen et al. (2013, updated 2018). Stephens & Halliday (1984) described the granitoids of the SSS and TSZ suites as commonly having dioritic or granodioritic compositions, with pyroxene as the major mafic phase. This corresponds rather well to the metadiorite xenoliths. Mantle-normalized whole-rock trace element patterns of the metadiorites closely resemble those of the SSS plutons (Fig. 7). The εNdt=400 values for the SSS and TSZ plutons (+1·5 to –3·0) indicate a relatively juvenile origin (Halliday, 1984), similar to the whole-rock εNdt=415 values of two metadiorites from East Lothian and one from Ayrshire (+2·9 to –0·1 and +5·4, respectively, Table 3). The εHft=415 values for zircons from the East Lothian (–0·1 to +7·18) and Ayrshire (+8·7 to +11·1) metadiorite xenoliths are broadly comparable with these whole-rock Sm–Nd data. Hence, we interpret the metadiorite xenoliths as samples of unexposed ‘Newer Granite’ pluton(s). The TH-25 metadiorite xenolith from Tollis Hill has a protolith age of 413 ± 4 Ma, similar to the metadiorites from East Lothian and Ayrshire. The Tollis Hill locality lies within the region of the Trans-Suture Zone plutons (Fig. 2; Brown et al., 2008), which have ages ranging from 414 to 384 Ma (Halliday et al., 1980a; Thirlwall, 1988; Oliver et al., 2008; Miles et al., 2014). Hence the Tollis Hill xenolith is considered to have been derived from an unexposed TSZ pluton. However, the zircon εHft values of +3·8 to –0·3 (Miles et al., 2014) for three of the largest plutons of the TSZ suite (Criffel, Fleet and Shap; Fig. 2) are somewhat lower compared with the weighted average of the Tollis Hill zircon sample (εHft=415 = 9·2 ± 0·6, n = 4). Evidence from the xenoliths for unexposed ‘Newer Granite’ plutons within both the Midland Valley and the Southern Uplands shows that the volume of late Caledonian plutonic bodies in southern Scotland must be greater than can be estimated from surface exposures (see Miles et al., 2016 for comparison). These new results support the suggestion of Halliday et al. (1993) that late Caledonian magmatism is responsible for the crystallization of the parent body of at least some of the Scottish deep crustal xenoliths. The presence of intense ‘Newer Granite’ plutonism in the Midland Valley is also supported by the c. 420 Ma Rb–Sr whole-rock–mineral ages of granite boulders from the Lower Old Red Sandstone sediments of the Strathmore Syncline (NW Midland Valley), which were derived from a source in the north, argued to have been from within the Midland Valley (Haughton & Halliday, 1991). Late Ordovician to mid-Silurian arc magmatism in the Midland Valley Based on the surface geology (i.e. the surface trace of a splay of the SUF) the East Lothian xenolith localities are part of the Southern Uplands terrane. However, several lines of evidence (see above) suggest that the xenoliths are derived from the underlying Midland Valley terrane. Magmatic rocks of Late Ordovician age are not exposed in the Midland Valley, which has thwarted attempts to validate the Midland Valley arc model of Bluck (1983), at least on a local scale in this region of Scotland. Although the Ballantrae Ophiolite (Stone, 2014) contains plagiogranites (Bloxam, 1981) that provide a plausible source for the metatonalite xenoliths, these rocks have yielded older ages (U–Pb zircon, 483 ± 4 Ma; Bluck et al., 1980). An origin of the metatonalite xenoliths within the Midland Valley terrane provides critical support for the model proposed by Bluck (1983) that introduced the arc concept. Zircons that define the c. 453 Ma magmatic episode in two metatonalites (PC-502 and BC-9) and four metadiorites (PCW-54, PCW-70, HSH-1 and HoA-3, as inherited grains) have generally positive εHft zircon values (Fig. 15). Hence, these are interpreted to originate from a related juvenile source and support evidence for Late Ordovician arc magmatism. The whole-rock εNdt=453 values of +1·6 to +3·8 (Table 3) and trace element patterns (Fig. 7) also support a relatively juvenile arc origin for the metatonalites. Granitoids of similar age (c. 460–448 Ma), occur in the Grampian Highland terrane, but these are S-type, two-mica granites with negative εNdt values (c. –10 to –12), high initial 87Sr/86Sr ratios (>0·712; summarized by Oliver et al., 2008) and inherited zircons with a similar age distribution to that of their Dalradian country rocks (Appleby et al., 2010). These features are very different from those that characterize the metatonalite xenoliths. The Lakesman terrane of northern England also preserves evidence for contemporaneous Late Ordovician magmatism; that is, the c. 450 Ma Borrowdale and Eycott volcanic groups (Millward & Evans, 2003), which have been related to southward subduction of the Iapetus Ocean beneath a peri-Gondwanan terrane, now recognized as Ganderia (Waldron et al., 2014). Deep seismic profiles across the Iapetus Suture (Hall et al., 1984; Beamish & Smythe, 1986; Freeman et al., 1988; Fig. 1) suggest that the northern margin of Ganderia underlies the Southern Uplands, which means that it can be a potential source for the c. 450 Ma crustal domain. Anderson & Oliver (1996) argued that mylonitic xenoliths from the Longford Down terrane of northern Ireland, equivalent to the Southern Uplands terrane, are samples from a continental wedge that tectonically underlies the accretionary prism complex of the Southern Uplands. Miles et al. (2014) argued using whole-rock Pb isotopic data (from Thirlwall, 1986, 1989), zircon Hf and oxygen isotopic data, that the Trans-Suture Zone ‘Newer Granites’, all situated within or south of the Central Belt of the Southern Uplands, were influenced by this underlying basement. However, the evidence supporting the presence of Ganderian crust beneath southern Scotland all comes from the southern region of the Southern Uplands and its Irish equivalent (i.e. south of the Orlock Bridge Fault; Fig. 1). Moreover, none of the seismic profiles indicate that underlying exotic crust extends as far north as the Southern Uplands Fault. Hence, it is considered unlikely that Ganderia is a source for the c. 453 Ma crustal domain; that is, the early metatonalite xenoliths are not samples of the deep crust of the Southern Uplands or of a peri-Gondwanan arc. The preferred, alternative, explanation is that the metatonalite xenoliths represent a buried Ordovician Midland Valley magmatic arc. Even though it is unexposed, the presence of a buried Ordovician magmatic arc within the Scottish Midland Valley has long been assumed and it has played a key role in geotectonic models and palaeogeographical reconstructions (e.g. Bluck, 1983, 1984, 2013; Chew & Strachan, 2014). Until now, the evidence for an Ordovician Midland Valley arc has been indirect and was largely based on the provenance of clasts in Middle Ordovician to Silurian (Fig. 2) sediments in the Midland Valley terrane (e.g. Bluck, 1983, 1984, 2002; Phillips et al., 2004, 2009). The geochemistry and isotopic characteristics of the metatonalites (negative Nb anomaly, positive whole-rock εNdt and strongly positive zircon εHft values) strongly support a magmatic arc origin. The relatively young (Late Ordovician) age for arc magmatism implied by the c. 453 Ma age for the metatonalite xenoliths is consistent with continuing northward subduction of the Iapetus Ocean and the continuing development of the Southern Uplands accretionary prism complex at this time (Floyd, 2001). Critical support for the Midland Valley arc model can be found along-strike to the SW, in the equivalent to the Midland Valley terrane in Ireland. Here, the Tyrone Igneous Complex provides one of the most extensive exposures of ophiolitic and arc-related rocks within the British and Irish Caledonides (Cooper et al., 2011). The Tyrone Igneous Complex comprises the Tyrone Plutonic Group, representing a suprasubduction-zone ophiolite dated between 480 Ma (Cooper et al., 2011) and 484 Ma (Hollis et al., 2013) and the structurally overlying c. 475–469 Ma Tyrone Volcanic Group. Hollis et al. (2012) proposed that the Tyrone Volcanic Group represents a peri-Laurentian island arc–back-arc sequence that underwent several episodes of intra-arc rifting prior to its accretion at c. 470 Ma to an outboard peri-Laurentian microcontinental block, the Tyrone Central Inlier. Post-accretion calc-alkaline intrusions, with ages ranging from 470·3 ± 1·9 to 464·3 ± 1·5 Ma, cut both the Tyrone Igneous Complex and the Tyrone Central Inlier and have been associated with the continuing closure of the Iapetus Ocean (Cooper et al., 2011). The structurally lowermost Tyrone Central Inlier contains mostly paragneisses with a sillimanite-bearing metamorphic assemblage (c. 670°C and 6·8 kbar, 40Ar–39Ar biotite cooling age of 468 ± 1·4 Ma) and leucosomes (207Pb/206Pb zircon age of 467 ± 12 Ma), both of which are cut by granite pegmatites with Rb–Sr muscovite–feldspar ages of c. 457 ± 7 Ma (Chew et al., 2008). The timing of leucosome and pegmatite formation provides further evidence for post-collision magmatic activity within the Midland Valley terrane. The youngest three magmatic zircon cores in metatonalite xenolith PC-503 define a protolith age of c. 430 Ma for this sample and have strongly positive εHft values. This age is significantly younger than that for the inherited zircons in this sample and the protolith ages of the other metatonalites. This sample is interpreted as a later pulse of magmatic activity within the same arc system. This is supported by the fact that the rock’s mineralogy and whole-rock chemistry is similar to the other (c. 453 Ma) metatonalites. We note that all exposed examples of ‘Newer Granite’ related magmatism (both intrusive and extrusive) in southern Scotland are younger than c. 424 Ma (Fig. 2). Moreover, the continuation of subduction at this time is supported by continuing accumulation of sediment and deformation in the Southern Uplands accretionary prism (Floyd, 2001), which did not cease until later in Wenlock times. Subduction-related magmatism may have been continuous between c. 450 and 430 Ma, consistent with the occurrence of metabentonite layers (Batchelor, 1999) of Late Llandovery to Early Wenlock age (Batchelor & Clarkson, 1993) in the Silurian inliers along the southern margin of the Midland Valley. The oldest inherited zircons, present in a metatonalite and two metadiorites, have 206Pb/238U ages ranging from 503 to 462 Ma (n = 6). They do not define a single concordia age and because of their small number and large age interval, it is challenging to define the number and age of their source(s). We note, however, that Late Cambrian to Early Ordovician zircons are present in deep crustal metasedimentary xenoliths from East Lothian and have probably been derived from them by assimilation (Badenszki, 2014, in preparation). In addition, zircons within this age range are known from the Highland Border Ophiolite, which crops out along the northern margin of the Midland Valley, and from a metasedimentary xenolith within it (Chew et al., 2010) and also from the Ballantrae Ophiolite (Bluck et al., 1980). Links between the various deep crustal xenolith types in the Midland Valley The c. 453 Ma inherited zircon population within the c. 415 Ma metadiorite xenoliths suggests that the metadiorites intruded through or within the Late Ordovician Midland Valley arc rocks represented by the metatonalites. Metatonalite xenoliths have not been found in the Ayrshire region in the west of the Midland Valley (Fig. 2), but inherited zircons in metadiorite HoA-3 suggest their presence in the deep crust of Ayrshire. Based on their whole-rock trace element contents (Fig. 7) and Sm–Nd and Rb–Sr isotopic compositions (Fig. 4), the Fidra mafic granulites, which are not dated but have an assumed Middle Permian protolith age (Downes et al., 2001), are indistinguishable from the meta-igneous xenoliths discussed in this study, especially the metadiorites. This might suggest that at least some of the Fidra xenoliths come from a crustal domain that was previously affected by Late Ordovician and ‘Newer Granite’ magmatism. By contrast, based on trace element and isotopic comparisons (Figs 7 and 8), it is less likely that the meta-igneous xenoliths share a common origin (and age) with those from Hawk’s Nib. Metamorphic P–T conditions and depth of origin of the meta-igneous xenoliths Geothermobarometry yielded only a broad range of pressures (c. 5–10 kbar) for the metadiorites from both East Lothian and Ayrshire and a minimum temperature of c. 793–816°C for one of the East Lothian metadiorites. Using this temperature estimate a narrower pressure range of c. 6·5–9 kbar can be calculated. These results may correspond to the metamorphic P–T conditions of the Acadian event and/or to the depth of origin of the samples in Carboniferous times, in which case the xenoliths would originate from middle to lower crustal depths. Pressure and temperature estimates cannot be obtained from the metatonalites. Using their densities, Upton et al. (1984) suggested that these samples represent the middle crust of the Midland Valley on the LISPB seismic profile (Bamford et al., 1978; Barton, 1992). However, a composite trondhjemitic–pyroxene granulite xenolith from Fidra (Hunter et al., 1984) raises the possibility that quartz- and plagioclase-rich lithologies can also be present in the lower crust and are not exclusively indicative of an upper or middle crustal position. Calculated pressures from the xenoliths correspond to a c. 18–38 km depth of origin (i.e. middle to lower crust) using an average crustal density from the LISPB seismic refraction profile (2·8 g cm–3; Barton, 1992), which runs close to the East Lothian and Tollis Hill xenolith localities (Fig. 1). These results are similar to the P–T estimates for rare orthopyroxene- and clinopyroxene-bearing (± plagioclase ± magnetite, no quartz) meta-igneous xenoliths from Fidra (Hunter et al., 1984), which were interpreted to have sampled the lower crust, at depths of 18–35 km (Hunter et al., 1984; Upton et al., 1984). The compositional zoning in clinopyroxene in HSHe-27 and the associated minor drop in calculated pressure is challenging to interpret. Taken at face value, it might reflect a real pressure decrease as the stress regime switched from transpressional to transtensional following the closure of the Iapetus Ocean (c. 420–400 Ma; Brown et al., 2008). However, because of the intense alteration resulting in widespread loss of clinopyroxene, it is impossible to determine how widespread and in what other localities this effect might have been recorded. Calculated pressure and temperature values of the metadiorite xenoliths could correspond either to the depth of crystallization of their protolith or to the metamorphic conditions of the younger, Acadian event. It is not possible to constrain the P–T conditions of the older (c. 415 Ma), metamorphic event. Timing of metamorphism Two main types of metamorphic zircon rims (CL-grey inner and CL-bright outer rims) are commonly present in both metatonalites and metadiorites, suggesting two phases of zircon growth. A concordia age of 389 ± 3 Ma (MSWD = 0·1) from seven CL-bright outer rims is interpreted to date the youngest well-established metamorphic event. This is recorded by three East Lothian xenoliths (PC-355, PC-503 and BC-9). A single analysis from a xenolith from Heads of Ayr (Ayrshire, Fig. 2) may indicate a younger age (c. 370 Ma). The significance of this age is unknown, but it could indicate that a younger event has affected the western Midland Valley. The oldest age obtained from CL-grey inner rims was from an East Lothian metatonalite (BC-9; Table 4) and is considered to date an earlier episode of metamorphism at c. 411 Ma, which would be roughly coeval with the crystallization of the metadiorite xenoliths at c. 415 Ma. This older metamorphic phase is broadly contemporaneous with the overthrusting of the Southern Uplands onto the southern margin of the Midland Valley (e.g. Bluck, 2013) and the ‘Newer Granite’ magmatism (and associated heating). This older metamorphic event is also present in East Lothian metasedimentary xenoliths (Badenszki, 2014). Most of the dated CL-grey inner rims fall within the time range from 402 ± 3 to 387 ± 4 Ma (Table 4) and can be assigned to two statistically defined groups (400 ± 3 and 391 ± 4 Ma, both weighted averages of concordia ages from individual xenoliths; Fig. 14b). This age range may be interpreted as either a continuous zircon growth event or two distinct metamorphic episodes (400 ± 3 and 391 ± 4 Ma), the younger of which is coeval with the formation of the CL-bright zircon rims. Metasedimentary xenoliths from East Lothian (Badenszki, 2014) yielded similar metamorphic zircon U–Pb ages of c. 404 Ma, suggesting that the major phase of metamorphism was the same for deep crustal xenoliths throughout the Midland Valley. Metamorphism in the time interval from c. 400 to 391 Ma corresponds to the age range of the Acadian Orogeny, best known in NW England and Wales (summarized by Soper & Woodcock, 2003; Woodcock et al., 2007). The Acadian Orogeny and the accompanying deformation were the result of the northward subduction (beneath Laurussia) and consequent closure of the Rheic Ocean, which was followed by an episode of flat-slab subduction (Woodcock et al., 2007). Deformation and/or metamorphic effects related to the Acadian Orogeny have not been widely reported from Scotland (i.e. north of the Iapetus Suture zone). However, they have been recognized in the Moniaive Shear Zone in the Southern Uplands, in the Strathmore Syncline in the Midland Valley (Soper & Woodcock, 2003; Figs 1 and 2) and in the Rosemarkie Inlier in the Northern Highlands (Mendum & Noble, 2010). Our dating of the Midland Valley xenoliths provides further evidence that the Acadian metamorphism has affected southern Scotland. Because of their overlapping age range, both the CL-grey inner and CL-bright outer zircon rim formation events are attributed to the Acadian Orogeny. SUMMARY AND CONCLUSIONS Two felsic meta-igneous xenolith types, metadiorite and metatonalite, have been identified from the classic Scottish East Lothian and Ayrshire xenolith localities, based on their mineralogy, whole-rock geochemistry and U–Pb zircon geochronology. This study has tested four hypotheses regarding the origin of these xenoliths: (1) Precambrian basement; (2) Permo-Carboniferous underplating; (3) ‘Newer Granite’ magmatism; (4) Ordovician arc magmatism. In the light of the new in situ U–Pb zircon geochronology results presented in this study, a Precambrian basement origin for both the metadiorites and metatonalites can be ruled out. The results also show that, unlike the Hawk’s Nib and Fidra mafic xenoliths, the metadiorites and metatonalites are not related to Permo-Carboniferous underplating. Instead, the metadiorites have a protolith age of c. 415 Ma, with zircon εHft=415 values ranging from +0·1 to +11·1, interpreted as the product of ‘Newer Granite’ magmatism (South of Scotland Suite), supporting the suggestion of Halliday et al. (1993) for the origin of some of the Midland Valley xenoliths. A link between the metadiorite xenoliths and the ‘Newer Granite’ magmatism is supported by their similar mineralogy and trace element patterns, and their comparable range of whole-rock εNdt values (–0·1 to +5·4 and –3 to +1·5, respectively; Stephens & Halliday, 1984; Halliday et al., 1993). Two of the metatonalites and most of the inherited zircons found in the metadiorites have similar zircon U–Pb ages of c. 453 Ma and εHft=453 zircon values ranging from +7·8 to +9·0 and +1·5 to +10·8, respectively. The relatively juvenile isotopic composition of the metatonalites, together with their negative Nb anomalies, suggests a subduction-related (magmatic arc) origin for these samples. The presence of the c. 453 Ma inherited grains within the metadiorites also implies that the ‘Newer Granite’ related pluton(s), in most cases, intruded into or through an older, Late Ordovician crustal domain represented by the metatonalites, which possibly underlies much of the Midland Valley terrane. This arc may have been active until Mid-Silurian times, because one of the metatonalite xenoliths has a somewhat younger protolith age of c. 430 Ma, with strongly positive zircon εHft=430 values of +8·1 to +8·3. Scarce inherited zircons with 207Pb/206Pb ages ranging from 531 to 469 Ma from both metatonalites and metadiorites may imply the presence of other sources within the crust. Meta-igneous xenoliths from both East Lothian and Ayrshire (i.e. at both ends of the Midland Valley) were all affected by the same metamorphic zircon-forming events between c. 400 and 391 Ma. The youngest and petrographically distinct zircon-forming event (growth of CL-bright rims) took place at 389 ± 3 Ma. These events probably correspond to the Acadian Orogeny, which is otherwise poorly defined in Scotland. Possibly an older phase of metamorphism affected the metatonalites at c. 411 Ma, perhaps associated with the emplacement of the ‘Newer Granite’ plutons and overthrusting of the Southern Uplands onto the southern margin of the Midland Valley. Geothermobarometry calculations for the metadiorites indicate a minimum temperature of c. 793–816°C and a pressure range from c. 6·5 to 9 kbar. These results may reflect the metamorphic P–T conditions of the Acadian event and/or suggest middle to lower crustal depths of origin for the xenoliths. ACKNOWLEDGEMENTS We thank Roy Fakes (BGS) for facilitating access to the British Geological Survey xenolith collection in Edinburgh. We also thank Tom Culligan (UCD School of Earth Sciences) for skilfully making countless excellent thin sections, Nick Marsh (University of Leicester) for help with XRF analyses, Lev Ilyinsky and Kerstin Lindén (NORDSIM) for assistance with SIMS analyses and zircon imaging, Michael Murphy (UCD School of Earth Sciences) for support with Rb–Sr and Sm–Nd analyses, and Penelope Lanchaster for her help with the Lu–Hf analyses at UCD. Constructive and detailed reviews by Hilary Downes, Richard Wysoczanski and an anonymous reviewer have led to significant improvements and are gratefully acknowledged. FUNDING This research project was initially funded by an IRCSET PhD research grant awarded to E.B., a ‘SYNTHESYS’ grant (number SE-TAF-758) and a 2009 student travel bursary from the Mineralogical Society of Great Britain and Ireland, all awarded to E.B., and SFI RFP grant (11/RFP.1/GEO/3079) awarded to J.S.D. 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TI - Age and Origin of Deep Crustal Meta-igneous Xenoliths from the Scottish Midland Valley: Vestiges of an Early Palaeozoic Arc and ‘Newer Granite’ Magmatism
JF - Journal of Petrology
DO - 10.1093/petrology/egz039
DA - 2019-08-01
UR - https://www.deepdyve.com/lp/oxford-university-press/age-and-origin-of-deep-crustal-meta-igneous-xenoliths-from-the-kSUz5UVbNb
SP - 1543
VL - 60
IS - 8
DP - DeepDyve
ER -