High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean

GR-00832; No of Pages 13 Gondwana Research xxx (2012) xxx–xxx

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High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean K. Sajeev a,⁎, B.F. Windley b, E. Hegner c, T. Komiya d, e a

Centre for Earth Sciences, Indian Institute of Science, Bangalore 560012, India Department of Geology, The University of Leicester, Leicester LE1 7RH, UK Universität München (LMU), Dept. für Geo‐und Umweltwissenschaften, Theresienstr. 41, D-80333 München, Germany d Department of Earth Science & Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo, Japan e Research Center for the Evolving Earth and Planets, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152‐8551, Japan b c

a r t i c l e

i n f o

Article history: Received 30 September 2011 Received in revised form 27 April 2012 Accepted 4 May 2012 Available online xxxx Keywords: High-P–T eclogites Archaean subduction P–T path Geochemistry Sm–Nd garnet geochronology

a b s t r a c t Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at > 10–11 kbar and 450–650 °C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900 °C and at 14 to 30 kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22 kbar and 1060– 1040 °C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330 Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite– granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a >90 km× 40 km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70 km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/ retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean. © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction to Archaean eclogites The eclogite facies defines the temperature–pressure conditions of basaltic rocks that have been subjected to high-pressure (HP) metamorphism. The petrogenetic grid of Oh and Liou (1998) demonstrates that most eclogites (garnet + clinopyroxene, but no plagioclase) formed under low-temperature/high-pressure conditions at ca. 450 ⁎ Corresponding author. Tel.: + 91 80 229 3404. E-mail addresses: [email protected] (K. Sajeev), [email protected] (BF. Windley), [email protected] (E. Hegner), [email protected] (T. Komiya).

to 650 °C and above 10 kbar. Such eclogites result from subduction of low-temperature oceanic crust, and are known to be largely of Proterozoic and Phanerozoic age. It is interesting that, according to Snoeyenbos et al. (1995), the oldest reliably dated HP metamorphic rocks at that time were the 1100 Ma eclogitic rocks at Glenelg in NW Scotland (see later). However, there are a number of recently reported and reliably dated Proterozoic, partly retrogressed eclogites or HP granulites (O'Brien and Rötzler, 2003) in deep crustal gneisses that formed above 10 kbar: The Sittampundi Complex, southern India (2541± 138 Ma magmatic age and 2487–2461 Ma metamorphic age; Bhaskar Rao et al., 1996; Ram Mohan et al., 2012) at 20 kbar, 1020 °C (Sajeev et al., 2009); eclogites in the Usagaran belt, Tanzania at ca. 2000 Ma,

1342-937X/$ – see front matter © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2012.05.002

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

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18 kbar and 750 °C (Möller et al., 1995; Collins et al., 2004); mafic granulites, Snowbird zone, Canada at 1900 Ma, 19–13 kbar and 960–890 °C (Baldwin et al., 2003); garnet websterites in Siberia at 2480–2410 Ma, 12 kbar and 540 °C (Ota et al., 2004); eclogites in the Belomoride belt of Russia at 2120–1860 Ma, 10.0± 0.5 kbar and 700 ±40 °C (Kozlovskii and Aranovich, 2010), and at 2450–2120 Ma 13.5–15 kbar, 805 °C (Travin and Kozlova, 2009); eclogites, Aldan Shield, Siberia at b2400 Ma, 10–15 kbar, uncertain temperature (Smelov and Beryozkin, 1993); eclogites in the North China Craton at 1860–1820 Ma (or 2500 Ma or 2000–1900 Ma), 14–12 kbar, and ca. 800 °C (O'Brien et al., 2005; Kröner et al., 2006; Zhai et al., 2010; Zhai and Santosh, 2011); eclogites, Nagssugtoqidian, E. Greenland at ca. 1850 Ma, 5.2 ±1 kbar and 600±70 °C (Messiga et al., 1990); and eclogites at Glenelg in NW Scotland at ca. 1100 Ma, 25–18 kbar and 1000–850 °C (Brewer et al., 2003; Storey et al., 2005; Sajeev et al., 2010b). However, a minimum pressure of 10 kbar for HP metamorphism may be deemed as surprisingly low, because many Precambrian granulite terrains have peak pressures of ~10 kbar, in which case HP metamorphism should be regarded as the norm for lower (or middle) crustal rocks, a concept that has as yet not been considered. Archaean eclogites (with no matrix plagioclase) and HP granulites include: granulites in the Central Limpopo belt, Zimbabwe at 3100–2700 Ma, ca. 11 kbar and 840 ± 50 °C (Droop, 1989); eclogites, Russian Belomorides at 2870 Ma, 13–16 kbar and 700–750 °C (Mints et al., 2010; Volodichev et al., 2004); mafic granulites in East Siberia at 3000–2700 Ma, 12–9 kbar and 950–750 °C (Moskovchenko et al., 1993); mafic granulites, Barberton, South Africa at 3200 Ma, 15–12 kbar and 650–600 °C (Moyen et al., 2006); garnet clinopyroxenites and garnet websterites, SW Norway at 3330±190 Ma, 63 kbar (UHP poorly documented) and 870±50 °C (Spengler et al., 2009); and mafic

granulites at Athabasca, Canada at 3200–2600 Ma, >15 kbar and 1000 °C (Snoeyenbos et al., 1995). The age and PT conditions of the above Proterozoic and Archaean HP rocks are summarized in Table 1. Problems of understanding the occurrence of HP rocks include uncertainties in earlier literature of precise isotopic ages, which often led to misinterpreting Phanerozoic eclogitic rocks as Precambrian in age, and the fact that only a limited range of individual geothermobarometers was earlier available in contrast to current, more comprehensive quantitative phase diagrams, which probably give higher pressure values. The aim of this paper is to report evidence from the central granulite/ retrogressed eclogite facies region of Scotland of widespread formation in the Neoarchaean of HP–HT mafic–ultramafic granulites that retain evidence of previous eclogitic mineralogy. 2. Geology of the central granulite facies region of the Lewisian Complex The Lewisian Complex in NW Scotland includes an Archaean, central granulite facies region (formerly known as Scourian) that was weakly reworked in the Palaeoproterozoic. This central region is flanked to the north and south by Archaean regions that were strongly reworked by Palaeoproterozoic Laxfordian deformation and metamorphism (e.g., Kinny et al., 2005; Park, 2009; Goodenough et al., 2010; Wheeler et al., 2010). The central granulite facies region (Fig. 1) (the Assynt and Gruinard terranes of Kinny et al., 2005) comprises predominant (75–80%) pyroxene and/or hornblende tonalite–trondhjemite– granodiorite (TTG) orthogneisses (Rollinson and Windley, 1980; Rollinson, 1996; Park, 2009) with U–Pb zircon protolith ages that range from 3030 to 2960 Ma (Friend and Kinny, 1995, 2001; Kinny and Friend, 1997; Kinny et al., 2005). However, based on SHRIMP

Table 1 Representative analyses of all major mineral phases from Achiltibuie (sample G36) and Scouriemore (sample G21). Sample no

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total O Si Ti Al Cr Fe Mn Mg Ca Na K Total cation Fe3 + Fe2 + XMg* XMg Alm Spe Pyr Grs Adr

Garnet

Omphacite-rich

Porphyroblasts

Inclusions in garnet

Clinopyroxene Porphyroblasts

G36

G36

G21

G21

G36

G36

G36

G36

G36

G36

G21

G21

38.7 0.0 21.9 0.0 23.6 0.5 7.4 7.0 0.0 0.0 99.1 12 2.999 0.000 2.001 0.001 1.530 0.033 0.853 0.584 0.000 0.000 8.000 0.000 1.529 0.358 0.358 0.510 0.011 0.284 0.194 0.000

39.5 0.0 22.3 0.0 19.6 0.0 9.5 8.4 0.0 0.0 99.4 12 3.003 0.000 1.993 0.002 1.244 0.001 1.074 0.681 0.000 0.001 8.000 0.000 1.244 0.463 0.463 0.415 0.000 0.358 0.227 0.000

38.0 0.0 21.4 0.1 28.5 0.9 4.3 6.5 0.0 0.0 99.7 12 3.000 0.001 1.993 0.005 1.880 0.061 0.511 0.547 0.006 0.001 8.005 0.014 1.866 0.215 0.214 0.625 0.020 0.171 0.176 0.007

38.2 0.0 21.6 0.0 27.4 0.8 5.4 6.0 0.0 0.0 99.5 12 2.999 0.002 1.997 0.001 1.800 0.055 0.637 0.507 0.000 0.004 8.002 0.006 1.794 0.262 0.261 0.599 0.018 0.213 0.165 0.004

52.3 0.1 7.7 0.1 8.3 0.1 9.9 17.8 3.0 0.0 99.3 6 1.933 0.002 0.336 0.003 0.257 0.004 0.548 0.704 0.214 0.002 4.003 0.009 0.248 0.689 0.681 0.009 0.000 0.067 0.201 0.570 0.111

52.1 0.1 8.0 0.0 8.8 0.1 9.2 18.3 3.0 0.0 99.6 6 1.928 0.002 0.348 0.001 0.272 0.004 0.507 0.726 0.213 0.001 4.003 0.008 0.264 0.658 0.651 0.008 0.001 0.072 0.200 0.581 0.093

52.1 0.0 7.1 0.1 9.5 0.1 9.4 18.8 2.6 0.0 99.8 6 1.934 0.001 0.312 0.001 0.295 0.003 0.518 0.749 0.189 0.001 4.004 0.013 0.283 0.647 0.637 0.013 0.003 0.066 0.175 0.615 0.093

51.9 0.0 5.4 0.1 9.0 0.1 11.9 19.2 1.6 0.0 99.2 6 1.935 0.001 0.236 0.002 0.280 0.004 0.662 0.767 0.114 0.001 4.002 0.007 0.274 0.708 0.702 0.007 0.000 0.065 0.105 0.636 0.149

51.8 0.5 3.0 0.1 8.3 0.1 13.1 22.0 0.6 0.0 99.7 6 1.936 0.015 0.134 0.002 0.260 0.004 0.727 0.882 0.047 0.001 4.006 0.017 0.243 0.749 0.736 0.017 0.004 0.064 0.001 0.752 0.102

51.9 0.6 3.2 0.1 8.0 0.0 13.1 22.2 0.7 0.0 99.8 6 1.933 0.017 0.140 0.003 0.249 0.001 0.727 0.885 0.048 0.001 4.003 0.009 0.240 0.752 0.745 0.009 0.000 0.067 0.004 0.750 0.100

52.9 0.1 3.9 0.0 11.3 0.0 12.5 16.9 1.9 0.0 99.6 6 1.973 0.001 0.173 0.000 0.353 0.000 0.694 0.676 0.138 0.000 4.008 0.024 0.330 0.678 0.662 0.024 0.008 0.027 0.111 0.617 0.202

52.7 0.6 3.1 0.0 12.7 0.1 12.5 16.6 1.7 0.0 99.9 6 1.969 0.018 0.138 0.000 0.396 0.003 0.694 0.664 0.121 0.000 4.004 0.012 0.384 0.644 0.637 0.012 0.004 0.031 0.073 0.601 0.229

Ac Ca-Eskola CaTs Jd Aug Opx cont.

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

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Fig. 1. Map of the major part of the Neoarchaean central Scourian region of the Lewisian, NW Scotland south of the Laxford Shear Zone. The main sites of the gabbro–ultramafic complexes at Achiltibuie (sample G36), Scouriemore (sample G21), and Drumbeg, mentioned in the text, are indicated, together with minor localities. Their host orthogneisses have different protolith and metamorphic zircon ages in the two regions. Detailed geological maps and sample locations are provided as insets.

U–Pb zircon ages and Lu–Hf isotopic signatures Whitehouse and Kemp (2010) argued for a single magmatic event at ca. 2850 Ma with older grains being inherited, and for a ca. 3.05–3.2 Ga mantle source for the granulites. There is isotopic evidence for two episodes of metamorphism. A late Archaean event is suggested by a U–Pb zircon age of ca. 2720 Ma for a leucotonalite sheet at Scourie (Corfu et al., 1998). In addition, Corfu et al. (1994) reported a U–Pb zircon age for 2760–2710 Ma metamorphism, which is in agreement with Zhu et al. (1997a,b) based on monazite Pb–Pb ages. A second (Badcallian) granulite facies metamorphism at ca. 2480–2490 Ma was proposed by Kinny et al. (2005), and supported by Rb–Sr and Pb–Pb whole-rock datasets of Humphries and Cliff (1982). However, this metamorphic event was interpreted by Corfu et al. (1998) as a period of rehydration and magmatism. Recently, Love et al. (2010) pointed out that the ca. 2480–2490 Ma event was missing in the Gruinard terrane (locality of our sample G-36) and they took this as evidence for a different metamorphic history compared with that in the Assynt terrane (locality of our sample G-21). The Central Region was intruded by a major swarm of basaltic Scourie dykes at ca. 2000–2400 Ma (Evans and Tarney, 1964; Heaman and Tarney, 1989; Waters et al., 1990). The above synopsis shows that in spite of several decades of isotopic research there is still much uncertainty and controversy about the age, significance and correlations of particular events within the Central Region and between those in the Central Region and the Northern and Southern Regions, as exemplified by the discussions of Whitehouse (1989) and Whitehouse and Kemp (2010).

More than forty major and hundreds of minor lenses and layers of relict, high-grade, mafic–ultramafic complexes (Fig. 1a) occur throughout the gneisses in the central granulite facies region (O'Hara, 1961, 1977; Bowes et al., 1964; Davies, 1974; Windley et al., 1981). They range from about a meter to ca. 100 m thick and up to ca. 1 km long. Ultramafic layers consist of dunite, websterite, wehrlite, lherzolite, orthopyroxenite, clinopyroxenite, and intercalated mafic lithologies are predominantly gabbro and leucogabbro. HP metamorphism has converted the igneous assemblages to metamorphic equivalents usually with abundant garnet in the mafic rocks; garnet metagabbro (garnet, clinopyroxene and plagioclase) is the most common lithology. Similar Neoarchaean layered mafic–ultramafic complexes occur in the Northern and Southern Regions (Fig. 1), which underwent major amphibolite-facies reworking in the Palaeoproterozoic (Davies and Watson, 1977; Park, 2009). The Sm–Nd whole-rock analyses of mafic–ultramafic rocks by Whitehouse (1989) yielded 2850 ± 95 Ma (at Achilitbuie), 2670 ± 110 Ma (at Scouriemore), and 2910 ± 55 Ma (at Drumbeg). A more precise Sm–Nd whole-rock age of 2710 ± 50 Ma was reported for 9 samples from Scourie Bay (Cohen et al., 1991), which agrees within error with the date of Whitehouse (1989). These ages obtained with the whole-rock Sm–Nd isochron method need to be treated with caution especially for interpreting and assigning metamorphic events. At Achiltibuie, where surrounding TTG gneisses give U–Pb ages of 2820 to 2730 Ma, Whitehouse (1989) obtained a Sm–Nd whole-rock isochron of 2850 ± 95 Ma from mafic rocks. Thus, the Sm–Nd whole

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

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rock age does not preclude emplacement of the mafic complex during late generation of granitic crust as can be inferred with more confidence at Scouriemore. Whitehouse et al. (1996) obtained an errochron age of 2940 ± 90 Ma for amphibolites, and an isochron age of 2850± 70 Ma for a hornblende metagabbro, which overlaps the U–Pb zircon ages of TTGs from the Gruinard Bay terrane. The mafic–ultramafic complexes at Achiltibuie, Scouriemore, Scourie (First Inlet), and Drumbeg (Fig. 1) have Sm–Nd whole-rock ages of 2849 ± 91 Ma, 2675 ± 250 Ma, 2836± 110 Ma, and 2923 ± 54 Ma, respectively (Whitehouse, 1989 updated in Whitehouse et al., 1996). Equivalent amphibolitic lenses in the Southern Region have ages of 2943 ± 91 Ma and 2846 ± 73 Ma (Whitehouse et al., 1996). Rare metasediments at kyanite/sillimanite-grade occur in the gneisses of the central region (Barnicoat and O'Hara, 1979; Barnicoat, 1983; Okeke et al., 1983; Cartwright et al., 1985). These rocks have been useful for determination of peak temperature–pressure conditions at 1000 °C and 10 kbar (Barnicoat, 1987) or 990–920 °C at >11 kbar (Cartwright and Barnicoat, 1987); these values compare with ca. 1000 °C and 17–12 kbar from mafic–ultramafic rocks (O'Hara, 1977; Savage and Sills, 1980; Barnicoat, 1983; Cartwright and Barnicoat, 1989), and 820 ± 50 °C and 11 kbar for a range of compositions from trondhjemitic to ultramafic (Rollinson, 1981). Tectonic crustal thickening, largely by thrusting, has been commonly considered, but vaguely defined or explained, to be the most likely mechanism for this high-grade metamorphism (Barnicoat, 1987; Cartwright and Barnicoat, 1989). Fluid inclusions enriched in CO2 are ubiquitous in the granulite-facies assemblages, and thus Andersen et al. (1997) concluded that the granulite facies metamorphism was triggered by an influx of CO2-rich fluids. This paper presents data from two layered complexes with retrogressed eclogitic assemblages: (1) At Achiltibuie (sample G36; 58° 00′56″N, 5° 20′8″W, UK grid reference NB 03260805—Fig. 1a,b) the layered gabbro–ultramafic body (Sills et al., 1982) is the best

preserved in the central granulite facies region; its garnets typically lack plagioclase rims (Bowes et al., 1964) (Fig. 2a). Sm–Nd wholerock dating of four mafic rocks from Achiltibuie (Whitehouse, 1989) gave an isochron age of 2850 ± 90 Ma. Other amphibolites produced an errochron age of 2940 ± 90 Ma, and a hornblende-metagabbro an isochron age of 2850 Ma ± 70 Ma (Whitehouse et al., 1996). Within errors, these ages overlap the U–Pb zircon ages of 2820 to 2730 Ma (Whitehouse et al., 1996) for TTGs from the Gruinard Bay terrane, but the large uncertainty in the Sm–Nd ages does not constrain when the mafic rocks were emplaced relative to the granitoid magmatism. The garnet-rich gabbro–ultramafic body consists predominantly of layers of garnet meta-gabbro up to several meters thick and layers up to 15 cm thick of garnet websterite, olivine-bearing meta-peridotite± garnet (garnet lherzolite), meta-anorthosite, garnetite, and amphibolite± garnet that contain relicts of retrogressed eclogite (Fig. 2a). Our sample G36 is from a 30 cm-thick garnet-bearing ultramafic lens with an eclogitic mineral assemblage. (2) At Scouriemore (sample G21; 58° 21′.02″N, 05° 10′.39″ W, UK grid reference NC 14104462—Fig. 1a,c) the Camas nam Buth gabbro– ultramafic body (Sills et al., 1982) consists of alternating layers of garnet-rich ultramafic rocks that are up to several meters thick, garnet-bearing meta-gabbro that contains retrogressed eclogitic layers up to 30–40 cm thick (such as sample G21), and amphibolite ± garnet that also contains relicts of retrogressed eclogite (Fig. 2b). Sm–Nd dating of the mafic rocks at Scourie Bay yielded errochron ages of 2670 ± 110 Ma (Whitehouse, 1989) and 2720 ± 50 Ma (Cohen et al., 1991); initial εNd values were 1.0 and 1.8, respectively. Whitehouse (1989) also speculated that the older mafic rocks were cumulates of former oceanic crust from which the younger TTG gneissic protoliths were derived. However, Cohen et al. (1991) proposed a contradictory (in our opinion unlikely) mechanism, involving pre-existing oceanic crust and magma crystallization to form the TTGs.

Fig. 2. Field and textural relations of the retrogressed eclogite samples. a, b) Outcrops showing eclogite relicts within amphibolitic retrogressed eclogites from Achiltibuie and Scouriemore. c) Thin section of Achiltibuie retrogressed eclogite sample G36 showing the textural relations between peak garnet (Grt) and clinopyroxene (Cpx) porphyroblasts. Late retrogressive amphibole (Amph) and ilmenite (Ilm) occur along grain boundaries and cracks. d) X-ray elemental map of Na in sample G36 that shows an omphacite (Omp) inclusion within a garnet core. The clinopyroxene (Cpx) in association with garnet (Grt) was altered to augite during retrogression and overprinted by an amphibole (Amph)–plagioclase (Pl) assemblage. e) Calculated XMg (Mg/(Fe + Mg) elemental variation map of the same area as in d), showing that both garnet and clinopyroxene are mostly homogeneous. f) Calculated XMg X-ray elemental map for sample G36, showing late amphibole overgrowth along cleavage planes of a clinopyroxene porphyroblast.

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

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3. Analytical methods 3.1. Microprobe analysis Electron microprobe analyses of the eclogite samples were carried out using a JEOL JXA-8900R microprobe at the Okayama University of Science, operating with an accelerating voltage of 15 kV, and a beam current of 12 nA. Natural and synthetic silicates and oxides were used for calibration. The data were reduced using ZAF correction procedures. 3.2. Sm–Nd isotopic and chemical analyses The retrogressed eclogite samples were ground and sieved into several size fractions and a whole-rock powder was prepared in a tungsten–carbide milling pot. Garnet (Grt)- and clinopyroxene (Cpx)-rich size fractions were separated using a Frantz magnetic separator. Clean mineral separates were handpicked under a binocular microscope. In addition, the enriched garnet separates (Grt-c) obtained from the magnetic separator and a whole-rock powder (WR) were analyzed. The garnet-leaching method (Hegner et al., 2010) is modified from that of Anczkiewicz and Thirlwall (2003). The acid-leached garnet residue and whole-rock samples were decomposed in a mixture of 2 ml concentrated HF and ~50 μl HClO4 in steel-lined PTFE-bombs (PTFE‐ crucible) at ~180 °C for one week. The fluorides/perchlorates were dissolved in 6 N HCl in the same bomb at ~180 °C for one day. The clinopyroxene separate was washed in acetone and 6 N HCl for 1 h at ~120 °C and thoroughly rinsed in ultrapure water. It was decomposed in a mixture of HF–HClO4 at ~100 °C in a PFA beaker over a period of one week. The samples were spiked with a 149Sm– 150Nd tracer solution. Chromatographic element separation followed the procedure described in Hegner et al. (1995). Nd and Sm were loaded on Re-filaments using diluted phosphoric acid and measured as metals in a double filament configuration. Total procedural blanks of b100 pg for Nd and Sm are not significant for the analyzed concentration levels. Nd isotopic abundance ratios were analyzed on a Thermo Scientific Triton mass spectrometer in static data collection mode. 143Nd/144Nd ratios of the samples are relative to 146Nd/144Nd= 0.7219 using an exponential mass-fractionation law. The La Jolla Nd reference material yielded 143Nd/ 144Nd = 0.511847 ± 8 (2 s.d., n = 10). Data regression analysis was performed with ISOPLOT 2.7 (Ludwig, 1993) using input errors of 0.5% (2σ) for 147Sm/144Nd and 0.002% (2σ) for the143Nd/ 144 Nd ratios. 3.3. Major and trace element analysis The major and trace elements were determined by XRF and ICP-MS at the University of Kwa Zulu, Durban South Africa. The methods and data quality were described in Wilson (2003). The rare earth element (REE) compositions of garnet, clinopyroxene, amphibole and plagioclase were determined by laser ablation, inductively-coupled plasma mass spectrometry (LA-ICP-MS) at the Tokyo Institute of Technology, Japan. The analytical procedures strictly followed the method outlined by Iizuka and Hirata (2004) and Komiya et al. (2008). A non-matrixmatched calibration standard, NIST612 silicate glass, was employed to determine V, Sr, Y, Ba, REE, Hf and Pb contents using 43Ca or Si as an internal standard. The compositions were obtained from ablation craters of different size (30–120 μm) depending on their contents, with an integration time of 30 s, laser repetition rate of 10 Hz, and emission power of 3 mJ. The ICPMS was tuned to maximum sensitivity for 139La by adjusting the ICP torch position, ion lenses and carrier gas flow-rate. The precisions of abundance measurements for La in garnet, and for heavy REEs of plagioclase are only 30% (2σ) or worse because of the extremely low contents in the minerals (Iizuka and Hirata,

5

2004). But, the precisions of abundance measurements for REEs except for La in garnet, all the REEs in clinopyroxene and amphibole, and light REEs and Eu in plagioclase are better than 20% (2σ) (Iizuka and Hirata, 2004).

4. Petrology and thermodynamic modeling The Achiltibuie retrogressed eclogite sample G36 consists of granoblastic garnet and clinopyroxene in an approximate 1:1 proportion that are separated by a corona of minor amphibole (Fig. 2c). Rutile occurs as thin lamellae in garnet and clinopyroxene, and as rare grains in the matrix. Ilmenite always occurs as a minor phase along with retrograde amphibole. Minor melt pockets in the host gneisses indicate partial melting and dehydration. In the matrix plagioclase is rare and locally forms symplectites with amphibole surrounding garnet and clinopyroxene (Fig. 2d,e). The symplectites probably developed during decompression and hydration of a garnet–clinopyroxene–rutile–melt assemblage after peak metamorphism. Amphibole and minor plagioclase also grew extensively along cleavage planes of clinopyroxene in a textural form that is similar to exsolution lamellae (Fig. 2f). A few inclusions of clinopyroxene in garnet and of garnet in clinopyroxene have survived late amphibole–plagioclase overgrowths. Garnets are mostly homogeneous, enriched in almandine-pyrope with a small grossular component (pyrope [Mg/(Mg + Fe + Ca)]34.5–30.2, almandine [Fe/(Mg + Fe + Ca)]50.3–45.5, grossular [Ca/(Mg + Fe + Ca)]17.3–11.8). Clinopyroxene inclusions in garnet are enriched in an omphacite component (XJd up to 20.1 [Na–Fe 3 +–2Ti] ∗ 100); the clinopyroxene end-member calculation in this study followed the method of Katayama et al. (2002). The granoblastic clinopyroxene is augite with a low sodium content (augite73.7–66.1[Ca–(2Al [4])–(0.5(Al–2Al[4]– K–(Na–Fe3 +–2Ti)]). The lack of sodium in clinopyroxene is attributed to the late overprinting of amphibole (Na2O up to 2.91 wt.%) and plagioclase (albite86.4 anorthite13.2). In the symplectites plagioclase grains are richer in anorthite component (albite32.1.4 anorthite65.5), and amphiboles have a similar composition as those in the lamellae. Details of the chemical compositions of the minerals are presented in Tables 1 and 2. In order to quantify the metamorphic conditions of the peak mineral assemblages, the phase relations for a bulk composition in the chemical system CaO–Na2O–K2O–FeO–MgO–TiO2–Al2O3–SiO2–H2O, as estimated from the mineral modes and compositions of garnet + clinopyroxene + rutile with minor amphibole–plagioclase, were computed as a function of pressure and temperature (Fig. 3a). This calculation uses free energy minimization (Connolly, 2005) with the thermodynamic data of Holland and Powell (1998); the solution models are given in Table 3. The XPrp [Mg/(Fe + Mg + Ca)] and XGrs [Ca/(Fe + Mg + Ca)] isopleths for garnet and XMg [Mg/(Fe + Mg)] and XJd [Na/(Ca + Na)] for clinopyroxene (Fig. 3a) in the phase diagram provide a basis for establishing the peak-metamorphic conditions. The conditions, at which Na-rich clinopyroxene inclusions within garnets are in equilibrium with the garnet cores that are situated within the plagioclase-absent field for a clinopyroxene– garnet–rutile–melt, are ca. 1050 °C and ca. 23 kbar (Fig. 3a). This finding suggests that the calculated minimum temperature–pressure condition for the peak assemblage falls well within the eclogite field. The cores of garnets in the retrogressed eclogite sample G21 at Scouriemore contain orthopyroxene inclusions that possibly formed during the prograde metamorphic stage. Because of intense overprinting of amphibole and minor plagioclase, clinopyroxene in this sample has a low Na2O content. Nevertheless, the thermodynamic modeling data, based on a bulk chemical composition calculated for a domain with a maximum peak mineral assemblage, indicate a peak temperature– pressure condition similar to that of the Achiltibuie retrogressed eclogite sample. After peak metamorphism the Scouriemore sample followed an isothermal decompression (Fig. 3b) P–T segment.

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

6

K. Sajeev et al. / Gondwana Research xxx (2012) xxx–xxx

Table 2 Representative analyses of major retrograde mineral phases from Achiltibuie (sample G36) and Scouriemore (sample G21). Sample no

Plagioclase

Amphibole

Lamellae

Symplectites

Symplectites

lamellae

G36

G36

G36

G21

G21

G21

G21

G21

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O H2O Total O Si Ti Al Cr Fe Mn Mg Ca Na K Total cation

66.1 0.0 19.5 0.0 0.1 0.0 0.1 2.9 10.6 0.1 – 99.3 8 2.933 0.000 1.018 0.000 0.003 0.000 0.006 0.140 0.913 0.003 5.017

50.0 0.0 30.0 0.0 0.9 0.1 0.1 14.0 3.8 0.4 – 99.4 8 2.313 0.000 1.635 0.000 0.034 0.005 0.005 0.694 0.340 0.025 5.052

48.9 0.0 30.5 0.0 1.2 0.2 0.5 1.0 1.3 9.8 – 93.5 8 2.408 0.001 1.774 0.000 0.050 0.006 0.039 0.055 0.126 0.617 5.076

51.2 0.7 3.3 0.1 7.9 0.1 13.6 13.3 0.6 0.0 – 90.7 8 2.712 0.028 0.207 0.003 0.348 0.003 1.073 0.753 0.058 0.001 5.185

56.5 0.0 26.6 0.0 0.0 0.0 0.0 10.0 6.0 0.2 – 99.5 8 2.556 0.000 1.420 0.000 0.002 0.000 0.000 0.486 0.529 0.010 5.003

56.4 0.0 26.2 0.0 0.2 0.0 0.0 10.1 6.1 0.1 – 99.2 8 2.561 0.001 1.404 0.000 0.007 0.001 0.000 0.490 0.534 0.009 5.007

An Ab Or

0.132 0.864 0.003

0.655 0.321 0.023

0.068 0.158 0.774

0.928 0.071 0.001

0.474 0.516 0.010

0.474 0.517 0.008

41.5 1.0 16.6 0.0 10.3 0.0 12.5 11.7 2.6 1.0 2.1 99.6 23 6.049 0.112 1.951 0.909 0.001 0.219 1.037 0.000 2.721 1.830 0.748 0.189 2.000 17.767

41.5 1.1 16.5 0.0 10.2 0.0 12.7 11.9 2.8 0.1 2.1 99.1 23 6.050 0.116 1.950 0.879 0.000 0.333 0.911 0.002 2.759 1.852 0.791 0.011 2.000 17.654

2.000 0.170 0.767 0.724

2.000 0.148 0.654 0.752

Si Ti AlIV AlVI Cr Fe3 + Fe2 + Mn Mg Ca Na K OH* Total cation (Ca + Na) Na (B) (Na + K) Mg/(Mg + Fe2 +)

Fig. 3. Isochemical phase diagrams modeled with thermodynamics for a) Scouriemore retrogressed eclogite sample G21 and b) Achiltibuie retrogressed eclogite sample G36. The thick arrows represent their temperature–pressure evolution after attaining their peak-metamorphic condition (filled circle). The thin-numbered lines are compositional isopleths used to determine the precise peak-metamorphic conditions. Abbreviations: Cpx—clinopyroxene, Grt—garnet, Rt—rutile, Qtz—quartz, Coe—coesite, Pl—plagioclase, Bt—biotite, Ilm— ilmenite, Tit—titanite, Amph—amphibole, Opx—orthopyroxene.

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

K. Sajeev et al. / Gondwana Research xxx (2012) xxx–xxx Table 3 Solution notation, formulae and model sources for phase diagram calculation. Symbol Solution Cpx Grt Opx Melt Amph Pl Bt

Formula

Clinopyroxene Garnet

Na1 − yCa2yMgxyFe(1 − X)yAlySi2O6 Fe3xCa3yMg3(1 − x + y + z / 3)Al2 − 2zSi3 + zO12, x+y≤1 Orthopyroxene [MgxFe1 − x]4 − 2yAl4(1 − y)Si4O12 Melt Na–Mg–Al–Si–K–Ca–Fe hydrous silicate melt Amphibole Ca2–2wNaz + 2w[MgxFe1 − x]3 + 2y + zAl3 − 3y − wSi7 + w + yO22(OH)2, w + y + z ≤ 1 Feldspar KyNaxCa1 − x − yAl2 − x − ySi2 + x + yO8, x + y ≤ 1 Biotite K[MgxFeyMn1 − x − y]3 − wAl1 + 2wSi3 − wO10(OH)2, x + y ≤ 1

Source 1 2 1 3, 4 5 6 7

Unless otherwise noted, the compositional variables x, y, and z may vary between zero and unity and are determined as a function of the computational variables by free-energy minimization. Sources: 1. Holland, and Powell (1996). 2. Holland, and Powell (1998). 3. Holland and Powell (2001). 4. White, et al. (2001). 5. Dale et al. (2005). 6. Fuhrman and Lindsley (1998). 7. Powell and Holland (1999).

5. Rock composition and Sm–Nd mineral dating The major- and trace-element data of the Scouriemore granulite sample G21 and those of Achiltibuie granulite sample G36 are listed in Table 4 and the normalized trace-element data are shown in Fig. 4a and b. Rock alteration, burial at depth, and metamorphism probably modified the concentrations of the fluid-mobile elements so that primary concentrations of these elements are not preserved. In addition, these formerly gabbroic samples from layered mafic–ultramafic bodies may represent in part cumulate compositions as can be inferred from the high Cr-content in sample G36 (Table 4), so that the trace element patterns do not reflect precisely the original melt compositions. The low SiO2 and high Fe2O3, MgO and Cr concentrations in both samples support an origin as mafic–ultramafic members of layered intrusions. The trace element patterns (Fig. 4a,b) of the Scouriemore sample G21 are similar to those of rocks from subduction zones and/ or with mantle-derived magmas contaminated by felsic crust (see Horodyskyj et al., 2007). Negative Nb- and Eu-anomalies support this inference. A negative Eu-anomaly is also consistent with melt fractionation at mid-crustal levels. Initial εNd values calculated for an age range from 2900 to 2700 Ma, overlapping that suggested by whole-rock Sm– Nd dating (Whitehouse, 1989; Cohen et al., 1991), indicate low initial ε values ranging from +0.4 to −0.9 (Table 5). There is a large difference from the values for a model-depleted upper mantle of +3.1 at 2900 Ma and +3.6 at 2700 Ma (Jacobsen et al., 1984). This strongly supports the idea of a large proportion of older crust in the parental magma of the Scouriemore sample. Old continental margins or intra-plate settings with crustal underplating of mantle-derived melts are likely, but an oceanic environment or an origin from oceanic crust is not supported. The sample G36 from Achiltibuie has flat REE patterns and overall low REE abundances (Fig. 4a). The magma of this sample had accumulated clinopyroxene as indicated by the high Cr-concentration of 620 ppm so that the trace element pattern maybe biased to that of clinopyroxene. The distinct negative Nb-anomaly in this sample, however, is not consistent with accumulated clinopyroxene (Baier et al., 2008), but rather suggests an origin of the parental magma from a mantle source affected by subduction processes and/or contamination by continental crust. The whole-rock sample G36 gives, for an age range of 2900 to 2700 Ma, initial εNd value of 1.8, indicating a higher proportion of depleted mantle material than in sample G21. It is clearly

7

Table 4 Chemical composition of retrogressed eclogite samples from Souriemore and Achilitbuie. Sample

G21

G36

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Total LOI Sr Ba Hf Zr Ta Nb Y U Th Pb Cr Ni La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

42.51 0.49 13.62 24.24 0.54 7.39 10.18 0.47 0.07 0.18 99.69 1.33 34.3 33.7 0.569 23.2 0.528 1.56 58.1 0.129 0.711 1.03 296 34 15.9 42.5 5.84 24.7 5.89 1.05 7.60 1.40 9.61 2.11 6.22 0.968 6.41 0.993

47.27 0.55 13.21 13.41 0.23 10.23 13.65 1.39 0.27 0.02 100.23 0.14 66.2 32.4 0.456 14.3 0.283 0.824 12.7 0.0130 0.109 1.08 623 149 2.27 6.11 0.890 4.27 1.35 0.493 1.83 0.33 2.25 0.492 1.44 0.219 1.45 0.226

Major-element concentrations reported as anhydrous values in wt.%, trace-element concentrations in μg/g.

lower than values expected from oceanic lithosphere with an assumed range of +3.1 to +3.6. Given the evidence for subduction-related processes and/or input of older crust in sample G36, an origin of the mafic– ultramafic rocks from “pre-existing oceanic crust” is not supported (Table 5). The HP-mineral assemblage and whole-rock powder of the two retrogressed eclogite samples studied for PT conditions were also dated by the Sm–Nd method; the data are listed in Table 5 and plotted in Fig. 5a,b. The isotopic compositions of garnet, whole-rock and clinopyroxene fractions of sample G21 (Scouriemore) produce an isochron with a cooling age of 2334± 10 Ma (MSWD = 0.75; Fig. 5a), indicating that all minerals were in isotopic equilibrium during the HP event. The whole-rock powder and handpicked garnet sample produce an indistinguishable age of 2341 ± 15 Ma. Our Sm–Nd cooling age is considerably younger than previous Sm–Nd garnet ages, U–Pb zircon and Pb–Pb monazite ages of ca. 2480 Ma for the Badcallian high-grade event in the Lewisian complex. We can only speculate about the geological meaning of these data. There is a possibility that this sample records a long-lasting open-system behavior of the Sm–Nd system due to heating of the crust by Scourie dyke magma emplacement or localized magma emplacement in the lower crust preceding the initial dyke phase at ca. 2420 Ma (Heaman and Tarney, 1989; Zhu et al., 1997a, 1997b). Note that this was after the Inverian amphibolite facies metamorphic event at 2490–2480 Ma (Corfu et al., 1998), and after the intrusion of the Scourie dykes at 2420–2400 Ma (Heaman and Tarney, 1989; Corfu et al., 1994).

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

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K. Sajeev et al. / Gondwana Research xxx (2012) xxx–xxx

Fig. 5. Sm–Nd isochron diagrams for peak pressure mineral assemblages in the retrogressed eclogite samples G21 (a, Scouriemore) and G36 (b, Achiltibuie). WR = whole-rock powder; Grt-c = garnet concentrate, GRT = garnet hand-picked.

Fig. 4. CI-normalized trace-element patterns for retrogressed eclogite samples G21 (Scouriemore) and G36 (Achiltibuie). Normalizing values are from McDonough and Sun (1995).

Alternatively, because the sample shows strong evidence of retrogression from a peak metamorphic mineral assemblage, the unusually young age of G21 may also be interpreted as reflecting a partial isotopic resetting of the Sm–Nd system during the Scourian dyke event. This finding is new for the metamorphic history of the Assynt Terrane and needs further examination. The well-preserved peak metamorphic mineral assemblage in sample G36 (Achiltibuie) yielded a distinctly older cooling age of 2483 ± 13 Ma (MSWD 1.4; Fig. 5b). The data for the whole-rock Table 5 Sm–Nd isotopic data for whole rock and HP mineral samples of Scourian granulites G21 (Scourie Bay) and G36 (Achiltibuie). Sample G21—whole rock

Omphacite Garnet hand-picked Garnet concentrate G36—whole rock

Omphacite Garnet hand-picked Garnet concentrate

Nd [μg/g]

147

3.679

15.13

0.1470

10.44 4.111 4.175 1.149

57.50 3.333 4.294 3.564

0.1098 0.7455 0.5897 0.1950

Sm [μg/g]

1.582 0.7747 0.8117

4.959 0.4179 1.279

Sm/144Nd

0.1928 1.121 0.3837

143

Nd/144Nd (m)

0.511705 ± 5 εNd (2.9 Ga) = 0.4a εNd (2.7 Ga) = − 0.9 0.511135 ± 6 0.520937 ± 6 0.518496 ± 4 0.512700 ± 6 εNd (2.9 Ga) = 1.8 εNd (2.7 Ga) = 1.8 0.512646 ± 5 0.527869 ± 6 0.515765 ± 4

143 Nd/144Nd normalized to 146Nd/144Nd = 0.7219. m = measured ratio. External precision for 143Nd/144Nd is ~ 1.0 × 10− 5 (2 s.d.). Error of 147Sm/144Nd ~ 0.5% (2 s.d.). The 143Nd/144Nd ratios are relative to 143Nd/144Nd = 0.511847 ± 8 (2 s.d., n = 10) in the La Jolla Nd standard. m. = measured ratio. a Listed εNd values for assumed age brackets of emplacement.

and handpicked garnet sample alone produce an indistinguishable age of 2483 ± 15 Ma. Previous Sm–Nd dating of similar lithologies from Scouriemore produced errochrons of 2490 Ma (Humphries and Cliff, 1982) similar to the precise Sm–Nd isochron age obtained for the Achiltibuie sample G36 in this study. A high-grade event at ca. 2480 Ma has also been inferred from a U–Pb zircon age of a pegmatite dyke at Scouriemore and a U–Pb age of titanite in metasediments (Corfu et al., 1994). These authors considered that the 2480 Ma age reflects the amphibolite facies metamorphism related to the Inverian event (later dated as 2490–2480 Ma, Corfu et al., 1998) that produced transpressional block movements and uplift along an Inverian shear zone. We also interpret the HP Sm–Nd mineral age of 2483 Ma as recording the cooling history of the Inverian event, which retrogressed the earlier 2760–2710 Ma regional granulite facies metamorphism. It has been suggested that an important criterion for the definition of the Assynt and Gruinard terranes is the lack of a Bacallian 2480 Ma event in the Gruinard terrane (e.g., Love et al., 2004). The 2480 Ma Sm–Nd cooling age of the Achiltibuie sample, however, is good evidence for a “Badcallian” event in the Gruinard terrane, implying a common thermal history with the Assynt terrane, and possibly invalidating the terrane subdivision suggested by Love et al. (2004). 6. Rare earth element contents of minerals Rare earth element (REE) contents of four rock-forming minerals from retrogressed sample (G21) (garnet, clinopyroxene, amphibole and plagioclase, Table 6) were determined by laser ablation, inductively-coupled plasma mass spectrometry (LA-ICPMS). This relatively retrogressed sample was chosen because of the presence of domains with late stage amphibole and plagioclase. Garnet shows high concentrations of heavy REE (HREE) (Yb from 125 to 67 ppm) and is depleted in the light REE (LREE; Fig. 6). Average CI chondrite-normalized

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

K. Sajeev et al. / Gondwana Research xxx (2012) xxx–xxx

9

Table 6 Trace element composition of the major elements in sample G21. ppm

Garnet

P Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

103.965 0.752 429.299 33.679 0.008 0.719 0.078 0.719 0.479 5.947 3.857 1.493 11.562 4.288 47.325 15.309 56.439 9.630 67.275 10.257

Clinopyroxene 92.381 BD 448.943 17.209 0.030 BD 0.084 0.693 0.381 4.750 3.018 1.550 10.827 4.263 44.047 16.060 64.090 12.258 89.621 14.613

132.468 2.260 495.301 55.546 BD 4.186 0.153 0.790 0.500 7.685 8.453 2.741 18.407 5.431 54.180 18.126 63.757 11.647 80.943 11.702

116.683 BD 472.702 62.845 0.024 BD 0.125 0.709 0.342 8.267 5.629 2.009 12.233 4.171 48.366 16.499 71.945 14.879 125.680 23.613

137.791 0.179 483.045 68.778 BD BD BD 0.425 0.362 6.455 7.204 2.331 13.842 4.081 49.516 17.885 70.492 14.992 128.184 25.419

BD 6.212 40.785 102.778 0.027 BD 15.583 58.410 10.236 49.741 15.678 2.457 14.822 2.458 12.362 1.846 3.907 0.352 1.724 0.146

24.427 5.310 25.222 60.173 0.050 BD 7.541 29.047 5.053 26.118 7.760 1.317 8.015 1.266 6.417 1.041 1.840 0.198 0.758 0.072

Amphibole 21.684 5.287 23.505 70.863 0.073 0.679 8.390 32.654 5.675 27.715 8.060 1.478 8.135 1.230 6.439 0.863 1.813 0.201 0.666 0.069

22.315 4.437 30.534 71.855 0.072 BD 7.349 30.077 5.303 27.936 8.672 1.325 9.336 1.489 7.572 1.291 2.320 0.254 1.027 0.097

486.862 6.480 125.597 87.502 0.042 BD 1.715 8.477 1.984 9.405 4.952 1.136 11.180 3.530 21.950 4.638 12.606 1.855 11.237 1.477

Plagioclase 513.396 1.397 119.275 70.098 BD BD 2.493 8.574 1.610 12.319 3.827 0.568 10.534 2.060 19.998 4.551 13.180 1.358 12.969 1.867

556.103 30.356 237.418 168.242 0.187 22.018 2.146 8.666 2.226 10.489 7.925 1.527 19.818 4.295 38.985 9.799 21.521 3.119 14.457 3.277

105.326 384.845 0.758 0.313 0.023 531.139 19.328 24.220 1.801 5.461 0.574 2.613 0.406 0.047 0.218 0.033 0.070 BD BD 0.003

91.960 327.386 0.495 0.279 BD 338.388 21.674 27.138 1.998 6.231 0.659 2.712 0.292 0.026 0.174 0.004 0.022 0.001 BD BD

114.465 296.896 0.773 4.880 0.013 296.870 19.743 25.513 2.040 6.258 0.584 2.389 0.556 0.051 0.266 0.025 0.078 0.014 0.050 BD

BD: below detection limit.

(McDonough and Sun, 1995) La (LaN) and Yb (YbN) values range from below the detection limit to 0.647, and 417.9–780.6 respectively (Fig. 6a). All garnet grains have a slight depletion in Eu and a high Sm concentration. The Sr content in garnet is very low (0.2–2.6 ppm) and in some cases below the detection limit. It has high Zr (17–69 ppm), P (92–138 ppm) and Y contents (430–495 ppm). Clinopyroxene grains show clear LREE-enrichment (LaN: 31.0–65.8, YbN: 4.1–10.7) (Fig. 6b). Eu is depleted relative to the neighboring REE. The negative Euanomalies in the peak metamorphic minerals garnet, clinopyroxene, and amphibole as well as in the whole-rock (Fig. 4a) represent the vestige of plagioclase fractionation.

Amphibole in the retrograde rims shows a relatively flat REE profile (LaN 7.2–10.5, YbN 69.8–89.8), which plots between those of the reactants garnet and clinopyroxene (Fig. 6c). Amphiboles also have a clear negative Eu anomaly (EuN 10.1–27.1; Fig. 6b). Plagioclase crystals contain very low REE contents with LREE enrichment relative to HREE, and they show the typical positive Eu anomaly (EuN 42.4–46.4; Fig. 6d), while other minerals and the whole-rock have a negative Eu anomaly. The mineral compositional data are very similar to the REE compositions of retrogressed eclogites from the Sittumpundi Complex, southern India (Sajeev et al., 2009). The REE compositions suggest

Fig. 6. CI-normalized trace-element patterns for the mineral assemblages in retrogressed eclogite sample G21. a) Garnet, b) Clinopyroxene, c) Amphibole and d) Plagioclase.

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

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equilibrium between the primary and retrograde assemblages, which is in agreement with our petrographic observations. 7. Tectonic implications for Archaean subduction processes TTG protoliths formed at Scouriemore located in the Assynt terrane have been dated at ca. 3030 to 2960 Ma (Friend and Kinny, 1995), although a more restricted window of ca. 2850 Ma, proposed by Whitehouse and Kemp (2010), suggests that the older ages were due to inheritance. The age of the gabbro–ultramafic complex at Scouriemore, which is not well constrained, probably occurred sometime between ca. 2900 and 2700 Ma (Whitehouse, 1989; Cohen et al., 1991). Granulite-facies metamorphic pulses were recorded during two events at ca. 2700 Ma (Corfu et al., 1994) and 2480 Ma (Kinny et al., 2005). At Achiltibuie located in the Gruinard Terrane (Friend et al., 2007) U–Pb SHRIMP zircon ages of 2.82 to 2.73 Ga contain evidence of a high-grade event confined to ca. 2.73 Ga (Whitehouse, 1989). The Achiltibuie mafic–ultramafic complex was dated by the wholerock Sm–Nd method at 2850 to 2940 Ma (Whitehouse, 1989). The gabbro–ultramafic complexes at Achiltibuie and Scouriemore have igneous lithologies that are very similar, and in many cases identical, to those of other layered complexes and relicts throughout the 90 kmwide central granulite/retrogressed eclogite facies region (Fig. 1). Moreover, the metamorphic assemblages based on variations of olivine, clinopyroxene, orthopyroxene, garnet, and plagioclase are similar throughout all the best-preserved gabbro–ultramafic complexes (Bowes et al., 1964 later confirmed by our observations). For example, at Drumbeg (Fig. 1) the complex contains layers of spinel lherzolite, wehrlite, and garnet gabbro that contain lenses up to 30 × 40 cm of garnet peridotite, garnet websterite, and eclogite (Bowes et al., 1964). The trace element signatures of some of these layered mafic–ultramafic complexes are most similar to those of layered complexes from modern mid-oceanic ridges (Rollinson, 1987). In the context of metamorphism, magma crystallization and trace-element geochemical data, the U–Pb zircon ages of the protoliths of the tonalitic gneisses were interpreted as evidence of derivation from magmatic arcs (Tarney and Weaver, 1987; Corfu et al., 1994). The temperature–pressure history of the retrogressed eclogite samples from the Achtilbuie and Scouriemore localities indicates that they were subducted to depths of ca. 70 km and subsequently exhumed to mid-crustal levels. The fact that many other layered complexes in the 90 × 40 km central granulite/retrogressed eclogite facies region (e.g., at Drumbeg, Fig. 1) have similar or identical lithologies and eclogitic mineral assemblages as represented by samples G21 and G36 strongly suggests they were likewise subducted to HP depths. However, the host TTG gneisses retain no evidence of having undergone HP–HT metamorphism except that meta-ironstones at Scouriemore retain evidence of equilibration at ca. 1000 °C (Barnicoat and O'Hara, 1979; Savage and Sills, 1980), and feldspar compositions of granitic rocks indicate crystallization at more than 1000 °C prior to granulite facies metamorphism (Rollinson, 1982). The Sm–Nd mineral isochron ages of the samples of this study of 2334 Ma and 2483 Ma likely represent the decompression stage long after the peak eclogite conditions. The 2483 Ma age from Achiltibuie is in agreement with a large number of ca. 2500 Ga metamorphic ages commonly from zircon rims and Sm–Nd mineral isochrons from across the central region, which have variously been interpreted as the result of amphibolite-grade retrogression. There is some evidence based on Sm–Nd garnet ages suggesting a possible high-grade event at 2490 Ma. This view leads to the speculation that we are looking at a singular case of prolonged open Sm–Nd-system behavior in our sample. Alternatively, the estimated age could be the representation of a retrogression event. The major reason for interpreting this age-span as the decompression interval from the highpressure granulite stage is supported by the observation that in the

studied samples the peak phase of omphacite crystallization is present only as inclusions within garnet cores. Most of the minerals now present in the rocks represent the retrograde decompression stage. Thus we infer that the HP metamorphism most likely occurred before the regional granulite-grade overprint at 2760–2710 Ma (see below) and before the invariant/amphibolite facies retrogression at 2490–2480 Ma. Zhu et al. (1997a, 1997b) carried out Pb–Pb dating of monazite and inferred a thermal history with events at ca. 2720 Ma, 2540 Ma and 2410 Ma coinciding with the onset of Scourian dyke intrusion. Thus one could also speculate that the Sm–Nd system in our sample may have remained open due to crustal heating by underplated Scourie magmas that erupted as early as 2420 Ma (Heaman and Tarney, 1989). Heating of the lower crust due to upwelling mantle must have occurred earlier, so that the high-grade event was “extended” beyond 2480 Ma. The 2334 Ma results from Scouriemore are much younger and require further comment and discussion. It must also be noted that in thrust-stacked terranes crustal sections may well have undergone different cooling histories, in particular when invaded by basaltic magmas. We see evidence for subduction and/or input of old crust—this precludes a model suggesting an oceanic lithosphere origin or a mid-ocean ridge origin. Clearly the rocks come from the vicinity of older (Scourie) continental crust with emplacement/underplating of mantle-derived melts. From the present estimated and published data we suggest two tectonic models to explain the evolution of the central granulite/ retrogressed eclogite facies region: (1) The mafic–ultramafic complexes were subducted to eclogite facies depths, and then exhumed to a midcrustal level where they were intruded by voluminous TTG protoliths. All these lithologies then underwent granulite facies metamorphism and deformation, after which they were further isoclinally folded and thrusted during wedge extrusion to attain their current sub-horizontal structure (Coward, 1974; Maruyama, 1997). (2). Alternatively, the crust of the central granulite–amphibolite/retrogressed eclogite facies region consisting of the mafic–ultramafic complexes and the intruded TTG protoliths was subducted to HP depths, and during exhumation all the rocks were re-equilibrated in the granulite facies. During this and further exhumation, the mafic–ultramafic rocks lagged behind in their retrogression, and thus are preserved today as granulite facies rocks with eclogitic relicts within the TTG granulite facies gneisses. Considering the common understanding these days of comparable situations in many parts of the world (see later), we favor the second model. Similar relationships of HP/UHP mafic–ultramafic relicts in lowpressure host rocks that retain little or no evidence of their having been at HP/UHP are well known in many Phanerozoic orogens. For example, in Norway quartzo-feldspathic gneisses with a current amphibolite facies mineral assemblage, which previously underwent HP metamorphism, occupy an area of >60,000 km 2, and yet contain only a few volume percent of relict HP eclogite blocks (Kylander-Clark et al., 2007). Other examples are in, the Palaeozoic Caledonides of East Greenland (Elvevold and Gilotti, 2000), the late Palaeozoic–early Mesozoic suture zone in northern Vietnam (Nakano et al., 2010), the Dabie Shan-Qinling orogen in China (Carswell et al., 2000) that extends to South Korea (e.g., Kwon et al., 2009; Sajeev et al., 2010a), the kmthick Cretaceous Sanbagawa belt in Japan that has been so thoroughly retrogressed to amphibolite facies that only a few cm-thick eclogitic relicts have been preserved (Terabayashi et al., 2005), and the Cambrian Kokchetav belt in Kazakhstan (Parkinson et al., 2002). We also note that in the Variscan orogen of Sardinia, Italy 450 Ma amphibolites contain lenses of amphibolitized eclogites, but the evidence for the early eclogite stage of metamorphism is only provided by omphacite relics in garnet porphyroblasts (Franceschelli et al., 2007), reminiscent of the Lewisian central granulite/retrogressed eclogite facies region. Other examples of Phanerozoic amphibolitized eclogite boudins in lower grade gneisses include: the Palaeozoic Massif Central of France (Nicollet and Leyreloup, 1978; Bodinier et al., 1988), and the Bohemian Massif in Poland (Steltenpohl et al., 1993); the

Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002

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northern Tianshan of Kyrgyzstan (Kröner et al., 2012); and Beishan in N. China (Qu et al., in press). The presence of highly comparable analogs permits us to make a comparable interpretation i.e., eclogite facies assemblages, of Phanerozoic to Archaean age, necessitate subduction of crustal rocks to provide the necessary PT conditions and fluid activity for the prerequisite HP mineralogy. Therefore, irrespective of the preferred model for the central granulite/retrogressed eclogite facies region, our discovery of high-pressure/high-temperature metamorphic assemblages strongly suggests that lithospheric subduction was transporting crustal rocks to eclogitic depths as early as ~2700 Ma, as also concluded, inter alia, by Mints et al. (2010) in Russia. If our interpretation is correct, it implies that the granulite facies metamorphism of the central (Scourian) region of the Lewisian was not a prograde event, as commonly supposed, but was retrogressive after earlier eclogite facies metamorphism. Moreover, it would provide a plausible explanation for the long-vexed question of why the dominant structure of the central region of the Lewisian is sub-horizontal (Coward, 1974): “over most of the area between Scourie and Achiltibuie the foliation is remarkably consistent and (except where affected by later folding) is broadly sub-horizontal” (Sheraton et al., 1973). The fate of most crustal slabs that have been subducted to high-pressure depths and exhumed to a high crustal level is to end-up with a sub-horizontal attitude (Maruyama, 1997; Agard et al., 2009). For example, the dominant structure of the exhumed high-grade rocks and mafic–ultramafic complexes in the Japanese Islands is subhorizontal (Terabayashi et al., 2005), as also is the ca. 40 km-long slab of exhumed Cambrian UHP rocks at Kokchetav in Kazakhstan (Yamamoto et al., 2002), and the exhumed mid-crustal Central Crystalline slab or Great Himalayan Sequence of the Himalayan orogen, which is underlain by geophysically-constrained, sub-horizontal to shallowdipping granulite and eclogite facies lower crust of the Indian Shield (Kaneko, 1997; Searle et al., 2011). Finally, another important factor, which bears on the problem of Archaean crustal eclogites in TTG gneisses, is the presence of xenoliths of Archaean eclogites in younger kimberlites and basalts (e.g., Pearson et al., 1991; Ireland et al., 1994). From the trace-element and isotope chemistry of low-MgO eclogites in West Africa, Barth et al. (2001) concluded that they are remnants of oceanic crust that was partially melted during subduction to high-pressure to produce a TTG magma and a residue of such eclogite at a convergent Archaean plate margin (see also Rollinson, 1997). However, we should also consider the fact that high-MgO Archean eclogite xenoliths in kimberlites have similar major and trace-element compositions and high Cr contents as the high MgO eclogite cumulates in Phanerozoic continental arcs like the Sierra Nevada of California (and the eclogitic rocks in the Lewisian described here) and that a link between Archaean high MgO eclogites and TTG protoliths is plausible (Horodyskyj et al., 2007). This provides an elegant model to explain the common association between lenses of eclogite and the protoliths of engulfing TTG gneisses, which could be Archaean (like the central granulite/ retrogressed eclogite facies region, and many other examples listed above) or younger (e.g., Paleozoic in Kyrgyzstan, Kröner et al., 2012) in age. This seems to have been a common mechanism of continental crust formation through time, but it is a model that is not well appreciated in the literature.

Acknowledgments We are grateful to Martin Whitehouse, Ali Polat and M. Santosh for critical and creative comments. We are also thankful to T. Itaya, S. Maruyama and S. Kwon for facilities and encouragement. A. Hofmann, University of Johannesburg, kindly provided geochemical data, and L.M. Iaccheri helped with sample preparation and isotopic analysis at LMU. This manuscript is a contribution to the Ministry of Earth Science, Government of India, project MoES/ATMOS/PP-IX/09.

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Please cite this article as: Sajeev, K., et al., High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean, Gondwana Res. (2012), doi:10.1016/j.gr.2012.05.002