Lu–Hf isotope evidence for Paleoproterozoic metamorphism and deformation of Archean oceanic crust along the Dharwar Craton margin, southern India

Precambrian Research 233 (2013) 206–222

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Lu–Hf isotope evidence for Paleoproterozoic metamorphism and deformation of Archean oceanic crust along the Dharwar Craton margin, southern India Niels M. Noack a,∗ , Reiner Kleinschrodt a , Maria Kirchenbaur a , Raúl O.C. Fonseca b , Carsten Münker a a b

Universität zu Köln, Institut für Geologie und Mineralogie, Zülpicherstrasse 49 b, 50674 Köln, Germany Universität Bonn, Steinmann-Institut, Abteilung für Endogene Prozesse, Poppelsdorfer Schloss, 53115 Bonn, Germany

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 8 January 2013 Received in revised form 20 April 2013 Accepted 23 April 2013 Available online 25 May 2013 Keywords: Lu–Hf garnet geochronology Deformation PCSZ India Archean–Paleoproterozoic boundary Eastern Gondwana amalgamation

The Palghat-Cauvery Shear Zone (PCSZ) marks the southern margin of the Archean Dharwar Craton in southern India. As the shear zone has been inferred to represent the ancient suture of the Proterozoic Mozambique Ocean, the age of the shear zone is crucial for understanding the paleogeographic assemblage of Precambrian crustal blocks in India. Here we present new constraints on the timing of tectonic activity along the Dharwar Craton margin from Lu–Hf garnet geochronology on garnetiferous mafic granulites from the Kanjamalai mafic complex (KMC), located within the Salem Block of the northern PCSZ system. These mafic granulites are intercalated with BIF metasediments. They reveal horizontal, MORBlike REE patterns with a slight depletion of the LREE (La/Ybcn 0.35–0.68) and an absence of Nb depletion, suggesting a protolith of oceanic crust affinity (MORB). An emplacement age of 2536 ± 300 Ma can be inferred from whole rock Lu–Hf geochronology for the mafic suite. Mafic rocks of the KMC suite display positive εHf (2536) values that range between +8.4 and +9.7, indicating a significant mantle source depletion in Neoarchean times. This conclusion is insignificantly affected by the propagated uncertainty of the emplacement age. Lu–Hf dating of ductile deformed high grade garnets forming stretching lineations to regional scale folds yielded a minimum age of 2434 ± 17 Ma for the initial regional deformation. In combination with literature data, a P–T–t path can be compiled for the KMC with peak conditions of 14–16 kbar and 820–860 ◦ C at 2.48 Ga (Anderson et al., 2012) and a retrograde equilibration at 6–7 kbar and 700 ◦ C. Our results indicate that some structural patterns within the PCSZS may represent crustal reworking at a later stage, because the Paleoproterozoic structures are only reworked locally when truncated by regional high strain zones of the northern PCSZ system. Altogether, we propose that subduction–accretion processes in an oceanic setting operated along the southern DC margin at the Neoarchean–Paleoproterozoic boundary. Our results are clearly in contrast to models entirely explaining the PCSZ as a Neoproterozoic–Cambrian suture (500–600 Ma) (Sajeev et al., 2009; Santosh et al., 2009; Yellappa et al., 2012). However, a recently proposed model arguing for a complex, multistage evolution of the PCSZS region (Santosh et al., 2012) is supported by our study © 2013 Elsevier B.V. All rights reserved.

1. Introduction Southern India holds a pivotal position in Gondwana reconstructions of the Neoproterozoic–Cambrian orogenic event (Rogers and Santosh, 2003; Meert, 2003; Collins et al., 2013). Collisional orogenic processes around 600–550 Ma emplaced the southern Indian Peninsula between Antarctica and Madagascar (Collins and Pisarevsky, 2005). Therefore, crustal domains, shear zones and metamorphic belts of southern India, Madagascar, Sri Lanka and Antarctica are commonly correlated (Meert, 2003; Collins et al., 2003). The closure of the Mozambique Ocean is an important event in the amalgamation of eastern Gondwana when southern

∗ Corresponding author. Tel.: +49 2214706113. E-mail address: [email protected] (N.M. Noack). 0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.04.018

India collided with the Madagascar-, Sri Lanka- and Antarctica crustal fragments (Fig. 1) (Meert, 2003; Collins and Pisarevsky, 2005). The Mozambique Ocean suture extends from the East African Orogen into the Lützow-Holm-Belt of Antarctica and is believed to cross through southern India, separating the Archean Dharwar Craton (DC) from the Neoproterozoic mobile belt associated with the Southern Granulite Terrane (SGT) (Collins et al., 2003, 2007; Meert, 2003) (Fig. 1). A crustal domain referred to as the Northern Granulite Terrain or Salem Block represents a transition zone between the DC and the SGT, which was affected by Neoarchean–Paleoproterozoic metamorphism and was only subjected locally to younger metamorphic events (Clark et al., 2009; Collins et al., 2013). The Madurai Block of the SGT south of the Salem Block is characterized by charnockitic gneisses with Neoarcheanand Mesoproterozoic crystallization ages and Neoproterozoic metamorphic overprints (Bartlett et al., 1998; Plavsa et al., 2012;

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Fig. 1. (A) Paleogeographic map of southern peninsular India showing the Indo-Antarctic terrain assemblage during Gondwana formation (modified after Raith et al., 1999). Major crustal shear zones separate the crustal blocks at the southern fringe of the Dharwar Craton (DC). The southernmost of them, the Palghat-Cauvery Shear Zone (PCSZ), has been interpreted as the trace of the Proterozoic Mozambique Ocean (Meert, 2003; Collins et al., 2007; Santosh et al., 2009). Abbreviations: India: BC–Bastar Craton; EGB–Proterozoic Eastern Ghats Belt; GR–Godawari rift; CB–Cuddapah basin; N, B, S–granulite regions of the Nilgiri-, Biligirirangan-, and Shevaroy Hills; SGT–Southern Granulite Terrain; ASZ–Achankovil Shear Zone; KKB–Kerala Khondalite Belt. Madagascar: RSZ–Ranotsara Shear Zone; AC–Androyan Complex. Sri Lanka: WC–Wanni Complex; HC–Highland Complex; VC–Vijayan Complex. Antarctica: NC–Napier Complex; RC–Rayner Complex; POC–Prince Olav Complex; OSG–Ongul & Skallen Groups. FL is the Fermor line south of which granulites occur. (B) Simplified geologic map (modified after the geologic map of Tamil Nadu of the Geol. Survey of India, 1995) with superimposed shear zones as discussed in, e.g., Santosh et al. (2009). The study area KMC–Kanjamalai Mafic Complex is shown together with the mafic bodies of the BLC–Bhavani- and SLC–Sittampundi Layered Complex. The southern part of the PCSZ(S)–Palghat-Cauvery Shear Zone (system) is the CSZ–Cauvery Shear Zone, while the northern part consists of the BSZ–Bhavani Shear Zone; MSZ–Moyar Shear Zone and SASZ–Salem-Attur Shear Zone. The granulite regions of the Nilgiri-, Biligirirangan-, Shevroy- and Kollimalai Hills also belong to the transition region between DC and SGT. This SB–Salem Block (or Northern Granulite Terrain) is characterized by Neoarchean–Paleoproterozoic high grade events with local younger overprints (Collins et al., 2013). Granitic intrusive bodies like Tiruchengodu (T) also occur in the region. The Madurai Block (MB) south of the PCSZ is characterized by a Neoarchean-Mesoproterozoic crustal evolution with Neoproterozoic–Cambrian crustal reworking (Bartlett et al., 1998; Plavsa et al., 2012).

Tomson et al., 2013). The shear zones and lineaments (Drury and Holt, 1980) of the Palghat-Cauvery Shear Zone system (PCSZ(S)) roughly separate the DC from the blocks of the SGT. They are interpreted as representing both traces of the Mozambique suture and crustal scale shear zones belonging to a Neoproterozoic–Cambrian orogen as a result of Gondwana amalgamation (Chetty and Bhaskar Rao, 2006; Collins et al., 2007; Santosh et al., 2009). In recent years, several studies have focused on the PCSZ system as a complex and controversial terrane boundary, promoting the region as a testing ground for paleogeographic models (Meert, 2003; Collins and Pisarevsky, 2005) as well as crustal growth processes in general. These studies include ultra-high temperature metamorphism (Raith et al., 1997; Santosh et al., 2004; Tsunogae

and Santosh, 2006), anatexis (Cenki et al., 2002) and crustal scale fluid flow during high grade metamorphism (Newton et al., 1980; Santosh, 1986; Jackson et al., 1988; Shabeer et al., 2005). The controversy concerning the age and general tectonic and metamorphic style of this complex shear zone system still persists. The data of Meißner et al. (2002), Collins et al. (2007), Clark et al. (2009), Saitoh et al. (2011) and Anderson et al. (2012) hint toward a scenario in which Neoproterozoic orogenesis is focused in the southern part of the PCSZS, near to the Cauvery river, whilst Paleoproterozoic orogenesis is concentrated to the north (Santosh et al., 2012; Collins et al., 2013). Although a growing number of U–Pb zircon ages are available in the literature (Gosh et al., 2004; Clark et al., 2009; Sato

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Fig. 2. The geological map of Kanjamalai shows general field relations of lithological units as well as the localities of analyzed samples. The granulite facies low strain body of the KMC with the mafic sequence, BIF and intrusive granodiorites is surrounded by retrograded lithologies of the SASZ. Comparable mafic sequences with intrusive granodiorites are also present north of the town of Salem. Secondary magnesite deposits formed on outcropping peridotites with an dominant occurrence north of Salem. Kanjamalai mountain is a key outcrop of this lithological association in the whole region. The western flank of Kanjamalai was mapped in this study. Stereographic projections of foliation (f3) and lineation (on f3 planes) for the western flank of Kanjamalai mountain (Stereo 32). The projection of all available structural data shows that f3 foliation plots on a girdle typical for symmetrical folds, while lineation plots roughly around the -pole, indicating that lineation is actually a stretching lineation to the syncline forming d3 deformation.

et al., 2010; Saitoh et al., 2011; Teale et al., 2011; Yellappa et al., 2012; Anderson et al., 2012), the development of a comprehensive tectono-metamorphic model is hampered by the missing correlations of these U–Pb ages with deformational features (e.g. regional folds and shear zones). The proposed structural models for the Palghat-Cauvery region (Chetty and Bhaskar Rao, 2006; Santosh et al., 2009), on the other hand, lack a direct age control on the structural evolution. In order to develop a comprehensive tectono-metamorphic model for the evolution of the PCSZS region, we performed Lu–Hf garnet geochronology on mafic granulite samples from the northern PCSZS region within the Salem Block (Figs. 1 and 2). Garnet is a key mineral for both reconstructing P–T paths and for Lu–Hf geochronology, and therefore it holds a vital clue for combining petrological and geochronological information. A further application of Lu–Hf garnet geochronology to structural problems in high grade assemblages arises from the fact that both the closure temperature of Lu–Hf in garnet and the ductile high temperature deformation of the garnet lattice fall within the same temperature window of ∼700–900 ◦ C (Ji and Martignole, 1994; Scherer et al., 2000). Hence, Lu–Hf garnet geochronology is a powerful tool to tie the age of a metamorphic event to specific P–T conditions and deformational features. Here we present the first Lu–Hf garnet geochronological data for southern peninsular India. The study area, the Kanjamalai Mafic Complex (KMC) (11.62◦ N 78.04◦ E)

(Fig. 2), is a key exposure in the northern PCSZS region (Gosh et al., 2004; Saitoh et al., 2011; Anderson et al., 2012). 1.1. Geologic overview Southern peninsular India is comprised of an Archean granite–greenstone terrain (Dharwar Craton), which forms an Archean consolidated core flanked by high grade terrains to the east (Eastern Ghats Terrain) and south (Southern Granulite Terrain) (Plavsa et al., 2012; Tomson et al., 2013). An amphibolite–granulite facies transition zone, the so called Fermor line (Fermor, 1936) marks the northern boundary of the Southern Granulite Terrain. The SGT consists of a collage of crustal blocks or domains that commonly share granulite facies metamorphic gradients but differ in geochemical properties and timing of granulite facies metamorphism (Bartlett et al., 1998; Gosh et al., 2004; Plavsa et al., 2012; Tomson et al., 2013; Collins et al., 2013). A northern Neoarchean granulite domain known as the Salem Block, is differentiated from the Madurai- and Trivandrum Blocks to the south with predominantly Neoproterozoic high grade metamorphism. The Neoarchean northern domain is dissected and delineated by an anastomosing system of shear zones, the Palghat-Cauvery Shear Zone system (PCSZS). Other authors have referred to this as the Cauvery Shear Zone (CSZ) (Bhaskar Rao et al., 1996; Chetty and Bhaskar Rao, 2006) or the

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Palghat-Cauvery Shear Zone (PCSZ) (Drury and Holt, 1980; Drury et al., 1984; Collins et al., 2007) or the Palghat-Cauvery suture zone (PCSZ) (Santosh et al., 2009 and references therein). Here we refer to a system of localized shear zones between the Fermor line and the Cauvery river, including the Moyar-Bhavani- and Salem-Attur Shear Zones to the north and the Cauvery Shear Zone to the south (Fig. 1). The crustal evolution of the region south of the Fermor line, which is the Southern Granulite Terrain sensu strictu, is complex owing to its polymagmatic and polymetamorphic history. Gosh et al. (2004) reported nine episodes of magmatism and high grade metamorphism between 2.9 Ga and 0.48 Ga based on U–Pb zircon and Sm–Nd whole rock ages from various locations throughout the SGT. Gosh et al. (2004) suggested that the dividing line between crustal domains with a Neoarchean–Paleoproterozoic history similar to that of the DC and domains without crustal relics older than 2.0 Ga lies even south of the PCSZS. The recently proposed two-episodical evolution model for the PCSZS region (Santosh et al., 2012) reflects that subduction–accretion processes on the southern fringe of the DC affected the PCSZS region at the Archean–Proterozoic boundary and also in a Precambrian–Cambrian episode. Raith et al. (1999) proposed paleoproterozoic suture zone processes for the Moyar- and Bhavani crustal segments and suggested an eastward continuation of this differentiable zone (Fig. 1). Multi stage subduction–accretion processes between 2.7 Ga and 2.5 Ga on the eastern fringe of the DC were also suggested by Jayananda et al. (2013). The crustal evolution of the northern part of the SGT south of the Fermor line is in the focus of this study. Within the northern (Salem) Block a belt of elevated charnockite massifs is distinguished from low lands made up of granuliteamphibolite facies assemblages, including felsic hornblendebiotite ortho- and paragneisses with a polymetamorphic history (Gosh et al., 2004) (Fig. 1). These gneisses are divided into ortho-amphibolites (Bhavani gneisses) and supracrustal gneisses (Satyamangalam group) that commonly include calc–silicate marbles, metapelites, quartzites and amphibolites (Chetty and Bhaskar Rao, 2006). The charnockites contain mafic–ultramafic slivers as well as lenses of metapelites, garnet–biotite gneisses, calcsilicates and kyanite bearing meta-quartzites indicating a sediment component (Raith et al., 1999; Rajesh, 2012). Time constrains of the charnockite suite are placed at the Archean–Paleoproterozoic boundary with mainly 2.5 Ga old magmatic zircon cores (3.0–2.9 Ga in the Biligirangan hills) and 2.5–2.4 Ga old metamorphic overprints in the northern block charnockite massifs (Fig. 1) (Peucat et al., 1989; Clark et al., 2009; Rajesh, 2012). In terms of structural geology, the mylonitic shear bands of the PCSZS are mainly found within the Bhavani- and Satyamangalam group gneisses and form an anastomosing pattern with local convergence or divergence along strike (Chetty and Bhaskar Rao, 2006) (Fig. 1). The complex pattern of local shear zones within the PCSZS was analyzed in detail by Chetty and Bhaskar Rao (2006) who reported large-scale north-verging thrusts and related structures accompanied by north–south shortening followed by extensive dextral shearing and migmatisation. Discontinuous mafic and ultramafic bodies intercalated with BIF metasediments of the Bhavani- and Sittampundi layered complexes (BLC and SLC) (Fig. 1) are located within the northern PCSZS region. Those mafic km-scale bodies have been interpreted to mark relics of a paleo-oceanic crust (Bhaskar Rao et al., 1996; Dutta et al., 2011) with important implications for the regional suture zone models (Santosh et al., 2009, 2012; Sajeev et al., 2009). The Kanjamalai mafic complex (KMC) (Figs. 1 and 2) is a further mafic relic body in the northern (Salem) Block located within the Salem-Attur Shear Zone of the PCSZS (Gosh et al., 2004; Saitoh et al., 2011; Anderson et al., 2012). The Kanjamalai mafic complex

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is dismembered from the other mafic complexes of the BLC and SLC. The above-mentioned studies on the KMC have not focused on the mafic suite of KMC and therefore a correlation with the mafic bodies of BLC and SLC in terms of geochemical characteristics and age remains ambiguous. 1.2. Geology of the Kanjamalai mafic complex (KMC) The KMC (11.62◦ N; 78.04◦ E) constitutes a granulite facies low strain enclave located within retrograded and intensely sheared rocks of the Salem-Attur Shear Zone (Fig. 3E), which is the eastward extension of the Bhavani Shear Zone (Fig. 1) (Kumar and Prasannakumar, 2009). The geological map of the western Kanjamalai mountain range (Fig. 2) shows field relations in addition to those suggested by Saitoh et al. (2011). The Kanjamalai mountain range extends as an 8 km long and 1–2 km wide structure with large continuous outcrops of lithological units. These units comprise banded (garnetiferous) mafic granulites with remnants of ultramafic cumulate boudins and associated banded magnetite–quartzites (BMQ) (Fig. 3A). These units were intruded by granodiorites that were deformed and show late stage migmatisation and intrusions of pegmatoids. Within the KMC low strain body primary deformational structures are preserved from shearing and retrogression of the shear zone. The tectonic structure of the KMC is a closed northward dipping syncline with steep limbs and a doubly plunging fold axis that extends along an E–W direction (Fig. 2). Multiphase deformation is a typical feature of mafic low strain bodies in the region (Mukhopadhyay et al., 2003). The mafic–ultramafic formation of KMC exhibits a layered structure from hand specimen up to mapping scale (Fig. 3A and B). Banding of the mafic rocks of the KMC suite is due to a primary compositional zonation as it is typical of layered intrusions. An intercalation of fine grained gabbroic and coarse grained garnet–pyroxene layers (probably former cumulates) was developed during the magmatic intrusive history of the suite. The layered structure was preserved during metamorphism and successive deformation. The oldest visible foliation in the mafic rocks (s1) is the result of overprinting of the primary magmatic layering by flattening and shearing. Rootless relic folds, eye folds and fold structure interferences mainly observed in BMQ outcrops indicate that the deformational evolution most probably included even more stages of deformation than observed in the mafic rocks. S1 structures were subject to regional scale folding as observed in the doubly plunging syncline of KMC. Outcrops on the limbs of the regional scale KMC syncline show tight parasitic s- or z-folds with their fold axes parallel to the regional fold axis. In the hinge region of the regional fold parasitic m-folds with a corresponding, steeply dipping axial plane schistosity (s2) can be observed. A distinct stretching lineation is developed parallel to the fold axes (Fig. 3C). This lineation is defined by elongated garnets, garnet aggregates and pyroxenes which suggests a ductile, high temperature deformation of the high grade assemblage (Ji and Martignole, 1994; Wang and Ji, 1999). 2. Sample description Four mafic granulite samples (Sa6, Sa15, Sa18, and Sa22) were selected for a detailed petrological study including electronmicroprobe (EMPA) element mapping of mineral phases, P–T calculations and Lu–Hf garnet geochronology. Mineral assemblages of the mafic samples are given in Table 1. The sample localities are indicated in the geological map (Fig. 2). The mafic granulite sample Sa15 originates from a transition zone between the northern fold limb and the western hinge zone of

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Fig. 3. Field relations for mafic granulites. (A) View of the western face of KMC doubly plunging syncline with intercalation of garnetiferous mafic granulite (gmg), metagranodiorite (mgd) and BIF (BMQ). (B) Interlayering of fine grained gabbroic and coarse grained garnet–pyroxene layers (Photo by M. Raith). (C) Lineation of elongated garnets on an s1 foliation plane. (D) Megacrystic garnet aggregate and melt segregation in a strongly garnetiferous mafic granulite. (E) High strain fabric in retrograded granulites of the Salem–Attur Shear zone (Photo by M. Raith).

the KMC syncline (Fig. 2). Sa15 is characterized by an assemblage of plagioclase + garnet + clinopyroxene + quartz + amphibole + opaque phase with elongated garnet–pyroxene aggregates related to stretching lineation parallel to the regional KMC fold axis. Garnet domains within those aggregates are anhedral with diameters of 2–4 mm. The general fabric of those aggregates is best developed in sample Sa4. Sample Sa22 displays an assemblage of plagioclase + garnet + clinopyroxene + orthopyroxene + quartz + amphibole + opaque

phase, showing coronal growth of garnet around plagioclase. Sample Sa22 originates from a similar position as Sa15 relative to the fold structure (Fig. 3). Samples Sa6 and Sa18 represent the garnet-free mafic lithotype. Whereas sample Sa18 contains strikingly big hornblende crystals (up to 1 cm in length), Sa6 is a fine grained (mm-sized crystals) rock. Both samples were included in the Lu–Hf whole rock isochron in order to cover the full range of the gabbroic suite of KMC.

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Table 1 Mineral assemblages of representative samples from Kanjamalai. Sample no.

Rock type

Sa2 (11.62497 N; 78.04527 E) Sa4 (11.62132 N; 78.04236 E) Sa5 (11.62215 N; 78.0428 E) Sa6 (11.61665 N; 78.03178 E) Sa11 (11.61664 N; 78.02316 E) Sa15 (11.62113 N; 78.02930 E) Sa18 (11.61874 N; 78.02887 E) Sa22 (11.61094 N; 78.05152 E)

hpc gmg ow mg rg gmg mg gmg

Grt +++

+++b +++a

Cpx

Opx

+++ +++ +++ +++

+++

+++ +++ ++

Pl

Qtz

++c +++ +++ ++ +++ +

+

+++ +++ +++c +++ +++c

++

++

Am +++ +++ ++ ++ +++ ++ +++ ++

Ilm

Pyr

+

+

Ol

Bt

Ap

Ep

Zrn

++

+

+

+

++

+

+

+

+++ abundant, ++ moderate, + rare. Rock types: mg–mafic granulite; gmg–garnetiferous mafic granulite; hpc–hornblende pyroxene meta–cumulates; ow–olivine Websterite; rg–retrograded granulite. a Coronal grt. b Remnants of coronal growth. c pl also as inclusion in grt or cpx.

3. Analytical techniques The four mafic granulite samples were screened by microscope following subsequent major element bulk rock analysis by XRF and major element analysis of mineral phases by EMPA. Whole rock major element analyses were obtained using a Phillips PW 2400 at Cologne at 50 kV. The reproducibility of the in-house standard was 1% for major elements. The JEOL EPMA JXA 8900RL microprobe at Universität zu Köln was used in WDS mode employing an acceleration voltage of 15 kV, and beam current of 15 nA. Bulk rocks were also investigated for their REE contents using a Perkin Elmers Quadrupole ICPMS at Universität zu Köln. The analysis of standard materials (BIR-1 and BHVO-2) was in good agreement with values reported by Dulski (2001). Mafic granulite samples that were used for Lu–Hf isotope analysis were screened for a minimum amount of alteration following the approach of Polat and Hofmann (2003) where a Ce/Ce* anomaly of 0.9–1.1, loss on ignition (LOI) of <6% and an absence of carbonate and quartz veining of the samples was taken as mirroring small degrees of alteration. A total of four mafic granulite whole rock samples (Sa4, Sa15, SA18, and SA22), from the KMC exposure were further prepared for Lu–Hf analysis. Additionally, we analyzed three garnet fractions as well as one pyroxene fraction from a 3 kg rock slab of sample SA15 for their Lu–Hf composition. Thin sections of the same sample slab were closer investigated by microscope and electron microprobe for thermodynamic modeling. After the removal of a thin weathering crust the samples were crushed in a steel mortar. Whole rock powders (grain size <32 ␮m) were prepared from rock chips using an agate mill. Garnet and pyroxene mineral separates were purified from a sieved fraction using a Frantz® magnetic separator. Four mineral separates of 100–200 mg were handpicked under a binocular. Grains of one garnet separate were visibly free of impurities (separate grt2) whereas the second and third garnet fractions (grt 1 and 3) contained some relics of pyroxene, amphibole and plagioclase representing the marginal parts of garnet aggregates. Grains of the pyroxene separate (d) were visibly inclusion-free. Owing to the presence of micro inclusions, such as ␮m scale rutile needles or zircons, a tabletop digestion technique was employed. This method includes the selective digestion of the silicate matrix of the mineral separates in HF: HNO3 :HClO4 in closed Teflon vials on a hotplate at 120 ◦ C as described in Lagos et al. (2007). Mineral separates and whole rock powders were spiked with a mixed 176 Lu-180 Hf tracer prior to their digestion. Whole rock powders (100 mg) were digested in a 1:1 mix of conc. HF and conc. HNO3 in ® Parr Teflon bombs at 180 ◦ C for four days. Complete digestion was achieved by adding HClO4 during drydown. The separation of Lu and Hf from the rock matrix on a single column using Eichrom® Ln-Spec resin followed the method of Münker et al. (2001) and included an additional clean-up step for the Hf cut (Lagos et al., 2007).

Lutetium and Hf isotopic compositions were measured in static mode on a Thermo-Finnigan® Neptune MC–ICPMS at the SteimannInstitut in Bonn. A 179 Hf/177 Hf of 0.7325 was applied for mass bias correction using the exponential law. For interference corrections 173 Yb, 175 Lu, 181 Ta, and 182 W signals were monitored. Details of the measurement protocol for Lu and Hf are described in Münker et al. (2001); Blichert-Toft and Frei (2001), Albarède and Beard (2004), and Vervoort et al. (2004). Measured ratios of 176 Hf/177 Hf are reported relative to the 176 Hf/177 Hf of 0.282160 of the Münster AMES standard, which is isotopically indistinguishable from the JMC-475 standard. The external reproducibility for 176 Lu/177 Hf is ∼ ±0.2% (2) and ±40 ppm (2) for 176 Hf/177 Hf. For the isochron calculations the external reproducibility was estimated by the empirical relationship 2 external reproducibility ∼4 m (where  m is the standard error of a single analysis). Error magnification for non-ideal sample spiking is incorporated in the 2-error for 176 Lu/177 Hf. The procedural blanks were less than 40 pg for Hf and less than 33 pg for Lu. Several garnet grains of samples Sa15 and Sa22 were also analyzed in situ by laser ablation mass spectrometry along line profiles, in order to measure their Hf and Lu abundances. Laser ablation of these garnet grains was carried out using a Resonetics M50-E ATL Excimer 193 nm laser system coupled to a ThermoFinnigan X-series 2 quadrupole ICP-MS (Steinmann Institut, Universität Bonn). Spot sizes were set between 55 ␮m and 75 ␮m, depending on the size of the garnets analyzed, as well as the amount of mineral inclusions found in the individual grains. Laser fluence at the sample surface was measured at 7 J/cm−2 , and the laser repetition rate was set to 10 Hz. Count rates were normalized using 29 Si as the internal standard, and one external standard; the NIST-612 glass reference standard (as described by Jochum et al., 2011). The isotopes 29 Si, 43 Ca, 47 Ti, 89 Y, 139 La, 146 Nd, 147 Sm, 172 Yb, 173 Yb, 175 Lu, 177 Hf, 178 Hf, and 180 Hf were monitored. In addition, external reproducibility was checked by evaluating another glass standard (NIST-610; Jochum et al., 2011), and treating it as an unknown sample every ten spots performed on our garnets. Trace element abundances for the NIST 610 glass were well within error of previously measured values (Jochum et al., 2011), and external reproducibility was typically better than 6% (2) for all relevant trace element nuclides monitored over the measurement time. Data reduction and evaluation was carried out following the procedure laid out by Longerich et al. (1996). 4. Petrology 4.1. Petrography Mineral assemblages of representative samples are given in Table 1. In garnetiferous mafic granulites of KMC a paragenesis of almandine garnet + clinopyroxene + plagioclase + amphibole + quartz ± orthopyroxene ± ilmenite is found. The modal abundance

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of minerals varies largely with the dm-scale banding. Gabbroic bands containing cpx/opx + amp + pl ± grt alternate with up to 10 cm thick garnet rich layers made up of megacrystic grt + cpx aggregates (with crystals of 2–3 cm) (Fig. 3D). Those aggregates can contain remnants of pl and also qtz. In samples lacking those aggregates, garnet coronae occur around plagioclase crystals. Amphibole is either intergrown with grt + cpx aggregates as constituent of the high grade assemblage or forms by retrograde alteration of pyroxenes. Plagioclase is involved in the grt forming reaction:

Apart from the diffusional alteration along mineral interfaces, garnets show a homogenous distribution of major elements (including Mn) that lacks any indication of prograde zonation. The line profiles for MnO within the same garnets (Figs. 4 and 5) show no distinct enrichment of Mn in either cores or rims. In contrast, laser ablation ICPMS profiles of Lu and Hf performed on garnets from samples Sa15 and Sa22 identify typical prograde zonation patterns with distinct Lu peaks in the core region (Figs. 4 and 5).

pl + opx(+cpx) = grt + cpx + qtz.

5. Results

(1)

Garnet-free leucocratic layers contain up to 60% of plagioclase. Anorthite-rich plagioclase crystals show Huttenlocher intergrowth, formed during cooling from high temperatures. Chalcopyrite is the main ore phase and is concentrated in mafic Fe-rich layers along with garnet. Ilmenite may also occur as an additional Fe-ore. 4.2. Mineral chemistry Mineral compositions that were determined by EMPA are given in Table 2. Garnets display compositions of Alm 50–60%, Pyr 24–33% and And 15–18%. Core to rim garnet profiles show flat patterns for FeO, MgO, MnO and CaO in the core region with typical retrograde equilibration rims. Plagioclase composition varies with the amount of garnet in the sample. Garnetiferous samples contain intermediate plagioclase with An 30–40% while plagioclase in garnet-free samples has An 55–74%. Compositional zoning of plagioclase varies, and an enrichment of An-component may be found either in cores or rims. Anorthite content and zonation of plagioclases depends on neighboring mineral species and grain size with CaO enrichment of pl next to grt and cores of larger grains unaffected by retrograde equilibration. Clinopyroxenes display rather uniform compositions with En 40–44%, Wo 46–50% and Fs of 8–14% in the mafic granulites. Orthopyroxenes occur in sample Sa22 and are Mg-rich with En 70% and Fs 30%. Owing to complex grt-cpx-pl networks in the garnetiferous domains the interpretation of element profiles may be difficult. In such a case element distribution maps give a clearer picture.

5.1. P–T calculations The grt-pl-cpx assemblage is present in all garnetiferous mafic granulites, and Sa22 additionally contains moderate amounts of orthopyroxene. Therefore, the results of the grt-cpx-pl geobarometer of Newton and Perkins (1982) and the grt-cpx geothermometer of Ellis and Green (1979) can be compared in all samples (Table 3). The exchange of Ca, Fe and Mg between garnet and clinopyroxene (Ellis and Green, 1979) suggests temperatures of ∼700 ◦ C (Fe–Mg KD = 6.93) for all samples. At 700 ◦ C the geobarometer of Newton and Perkins (1982), which is based on the Ca-(Mg) exchange between grt-pl-cpx, yields a range in pressures from 6 kbar to 7 kbar in our samples. Sample Sa22 with arrested coronal growth of garnet and orthopyroxene relics yields higher estimates of 8–11 kbar (Sen and Bhattacharya, 1984; Moecher et al., 1988; Powell and Holland, 1988; Bhattacharya et al., 1991). Pseudosections were calculated with the PERPLEX program (Connolly, 2005) using the standard protocol and the Holland and Powell (1998) dataset. Fluid- and melt-phases were excluded in the model. Calculations were done for bulk compositions of mafic granulite samples Sa15 and Sa22 in order to define the position of mineral assemblage fields and mineral isogrades within the P–T field. For those bulk rock chemistries garnet is stable above 4–6 kbar at temperatures of 600–800 ◦ C with a moderately positive slope of the grt-in-isograde. Assemblages devoid of orthopyroxene consisting of plagioclase + garnet + clinopyroxene + quartz are stable above 10–13 kbar at temperatures of 600–800 ◦ C. 5.2. Bulk composition of major- and trace elements

4.3. Distribution of major and trace elements in garnet assemblages We performed X-ray element mapping of various minerals in mafic samples. In Fig. 4 is an example of a grt-cpx-pl network in Sa15 while Fig. 5 shows garnet coronal growth observed in sample Sa22. EMPA element maps of grt-cpx-pl networks and coronal garnets for major elements (MgO, FeO, MnO, and CaO) reveal the nature of element distribution within garnets and the element diffusion between mineral phases in a given assemblage. These observations are combined with LA–ICPMS profiles for trace elements (HREE + Hf) in garnets of the analyzed assemblages. Distribution maps of MgO in garnets within grt-cpx-pl networks in sample Sa15 show a slight depletion of MgO in peripheral garnet zones, especially when clinopyroxene is the neighboring phase (Fig. 7). In the case of coronal garnet growth (Fig. 5), any internal variations of MgO, FeO and MnO content are absent. Element maps for CaO show clear indication of element diffusion within minerals as well as between mineral species. The CaO contents of garnets are clearly higher around clinopyroxene inclusions and less so around plagioclase (Fig. 4). A different phenomenon is inverse Ca zoning of plagioclase in the domains of coronal garnet around plagioclase in Sa22 (Fig. 5). Calcium distribution maps of those plagioclase crystals consistently show that plagioclase rims have higher Ca content than plagioclase cores.

Table 4 shows XRF and Quadrupole ICPMS element concentration data for the mafic units of the KMC suite. Mafic granulites exhibit SiO2 contents of 47–50 wt% and Al2 O3 contents ranging from 12 wt% to 15 wt%. Contents of MgO (7–14 wt%) and FeO* (11.5–16 wt%) of this compositionally banded unit are quite variable. Garnetiferous samples exhibit lower Mg# (31–45) in comparison to samples devoid of garnet (53–58). Contents of MgO in relation to various major and trace elements like FeO*, Al2 O3 , CaO, TiO2 , MnO, Ni, Cr, Zr, and Gd show both positive and negative co-variations. Zirconium contents (18–26 ppm) are comparably low in the studied samples. A considerable difference in the contents of Cr between garnetiferous mafic samples (47–354 ppm) and the garnet free sample Sa6 (999 ppm) is observed. The mafic suite of KMC follows typical tholeiitic fractionation trends (e.g., Fig. 6). Mafic granulites and cumulate rocks of KMC exhibit parallel and flat REE- and incompatible trace element-patterns, resembling typical present day N-MORB patterns (Fig. 6) with up to 10 times chondritic abundances. The light REE are depleted (La/Ybcn 0.35–0.68) whereas MREE remain flat. Field relations suggest that the reworking in the SASZ affected the mafic suite as well as the felsic intrusives. Samples SA11 (mafic layer) and (felsic layer) (Table 4) were chosen as examples to represent the heterogeneity of the shear zone association. The reworked

Table 2 Representative electron microprobe analysis of garnet (O = 12), plagioclase (O = 6) and pyroxene (O = 6). Analytical errors are 1–5%. Rock type Sample no.

gmg Sa22 grt rim

gmg Sa15 grt rim

gmg Sa15 grt core

gmg Sa4 grt rim

gmg Sa22 pl core, incl. in grt

gmg Sa22 pl rim, incl. in grt

gmg Sa15 pl incl. in grt

gmg Sa15 pl core, matrix

gmg Sa4 pl incl. in grt

gmg Sa22 cpx core

gmg Sa22 opx core

gmg Sa15 cpx incl. in grt

gmg Sa15 cpx core

gmg Sa4 cpx incl. in grt

mg Sa6 opx matrix

SiO2 Al2 O3 TiO2 Cr2 O3 MgO FeO CaO MnO Na2 O K2 O Total

39.3 22.2 0 0.05 8.43 22.9 7.16 0.54 0.03 0 101

39.1 22.5 0.02 0 8.44 22.8 7.19 0.68 0 0 101

38.7 22.2 0.07 0 6.14 27.3 6.59 0.67 0.02 0 102

38.4 21.8 0.11 0.01 6.52 26.6 6.67 0.75 0.04 0 101

38.7 22.1 0.03 0.08 6.95 25.3 6.81 0.89 0 0.02 101

48.4 31.8 0 0 0 0.03 15.1 0.01 3.16 0.02 98.5

48.2 32.2 0.01 0.02 0.01 0.12 15.7 0.01 2.79 0.02 99.1

59.0 24.8 0.07 0.01 0.40 0.28 6.93 0.01 7.92 0.08 99.5

59.0 25.0 0.01 0 0.01 0.02 7.15 0.00 8.36 0.07 99.6

54.4 28.6 0 0 0 0.08 10.8 0.02 5.65 0.14 99.7

52.4 3.01 0.33 0.11 14.6 6.15 23.1 0.01 0.67 0 100

53.4 2.21 0.06 0.06 25.8 18.9 0.29 0.15 0 0.01 101

51.0 3.2 0.35 0 13.2 8.96 21.3 0.05 1.27 0.01 99.3

51.8 3.45 0.32 0.03 12.6 9.61 21.6 0.01 1.53 0 101

51.6 3.72 0.42 0 13.2 7.40 22.4 0.12 1.08 0 99.9

52.3 2.91 0.03 0.06 25.5 19.3 0.15 0.01 0.01 0 100

Si Al Ti Mg Fe* Ca Mn

2.98 2.00 0 0.96 1.46 0.58 0.04

2.97 2.02 0 0.96 1.45 0.59 0.04

2.97 2.01 0 0.70 1.75 0.54 0.04

2.97 1.99 0.01 0.75 1.72 0.55 0.05

2.97 2.00 0 0.80 1.62 0.56 0.06

Total

8.01

8.02

8.02

8.03

8.02

Si Al Ti Mg Fe* Ca Na K Total

2.25 1.74 0 0 0 0.75 0.29 0 5.03

2.23 1.76 0 0 0.01 0.78 0.25 0 5.03

2.65 1.31 0 0.03 0.01 0.33 0.69 0 5.02

2.65 1.33 0 0 0 0.34 0.73 0 5.05

2.46 1.53 0 0 0 0.53 0.5 0.01 5.03

Si Al Ti Cr Mg Fe* Ca Na Total

1.93 0.13 0.01 0 0.80 0.19 0.91 0.05 4.02

1.94 0.10 0 0 1.39 0.57 0.01 0.00 4.02

1.92 0.14 0.01 0 0.74 0.28 0.86 0.09 4.05

1.92 0.15 0.01 0 0.70 0.30 0.86 0.11 4.05

1.92 0.16 0.01 0 0.73 0.23 0.89 0.08 4.03

1.91 0.13 0 0 1.39 0.59 0.01 0.00 4.03

Mg# Alm Prp Grs Sps

0.40 0.47 0.32 0.18 0.01

0.40 0.47 0.32 0.17 0.01

0.29 0.57 0.24 0.14 0.01

0.30 0.54 0.25 0.14 0.02

0.33 0.53 0.27 0.16 0.02

An Ab Or

0.73 0.27 0

0.76 0.24 0

0.32 0.67 0.01

0.32 0.68 0

0.51 0.48 0.01

Mg# En Fs Wo

0.81 0.42 0.10 0.48

0.71 0.7 0.29 0.01

0.60 0.39 0.15 0.46

0.70 0.41 0.09 0.50

0.76 0.40 0.12 0.48

0.70 0.70 0.30 0

*

N.M. Noack et al. / Precambrian Research 233 (2013) 206–222

Remark

gmg Sa22 grt core

Total Fe as FeO.

213

214

N.M. Noack et al. / Precambrian Research 233 (2013) 206–222

Fig. 4. The orthopyroxene-free assemblage of Sa15 is characterized by the absence of prograde zonation for major 2+ cations as it is seen in the MnO profile and the CaO and MgO element maps. Zones of retrograde diffusive exchange between mineral species are observed in the MgO and CaO distribution maps. Undisturbed bell shaped Lu profiles argue for slow diffusion of 3+ cations which should be even slower for Hf4+ (Ganguly et al., 2010). Prograde Lu profiles in garnets from sample Sa15 suggest that Lu–Hf garnet ages obtained for this sample are likely to constrain garnet growth (Cheng et al., 2008; Kirchenbaur et al., 2012).

mafic rocks in the shear zone have a considerably higher L.O.I. of 2.42% and striking enrichments in elements like K2 O (1.61 wt%), Sr (396 ppm), Ba (322 ppm), Zr (96 ppm) and Ce (40.2 ppm) as well as other LREE, when compared to non-retrogressed mafic granulites. These heterogeneous, retrograded granulites associated with the SASZ are characterized by steep LREE enriched patterns (La/Ybcn = 40.25) that strongly differ from the flat patterns of the mafic suite (Fig. 6). 5.3. Lu–Hf geochronology Table 5 illustrates Lu–Hf isotope data for the four analyzed whole rock samples Sa6, Sa15, Sa18, and Sa22 as well as for the mineral separates (garnet and pyroxene) from Sa15. The 176 Lu/177 Hf

for the whole rocks ranges from 0.04189 to 0.05777, while garnet separates show strongly elevated 176 Lu/177 Hf ratios of 0.7053 for an impure garnet separate and 0.9378 for pure garnet. The 176 Lu/177 Hf ratio of the pyroxene separate is significantly lower than that of the corresponding whole rock sample (0.0008411). Whole rocks display 176 Hf/177 Hf ratios of 0.283460–0.284213 while garnet separates range from 0.314084 to 0.325092 and the pyroxene separate yields a 176 Hf/177 Hf ratio of 0.281451. Isochron diagrams are shown in Fig. 7. Isochron regressions were calculated using ISOPLOT v3.0 (Ludwig, 2001) and using a decay constant  176 Lu of 1.867 × 10−11 year−1 (Scherer et al., 2001; Söderlund et al., 2004). The four whole rock samples define a regression line (MSWD = 7.9) with an apparent age of 2536 ± 300 Ma. A 5-point regression line (MSWD = 23) including garnet and

Table 3 Selected thermo- and barometers for representative garnetiferous samples (grt-pl-cpx-(opx)-qtz assemblage) of KMC. Core compositions yield insignificantly higher conditions than rim compositions for respective samples. See text for discussion. Sample

Sa15

Sa4

Sa22

Remark Newton and Perkins (1982)

Rim and core compositions 6.6 kbar at 700 ◦ C 6.9 kbar at 700 ◦ C – – 674 ◦ C – –

Rim compositions 6 kbar at 700 ◦ C

Core compositions 6.4 kbar at 700 ◦ C

– – 680 ◦ C – –

10.9 kbar (opx) 7.9 kbar (opx) 700 ◦ C 700 ◦ C (opx) 735 ◦ C (opx)

Moecher et al. (1988) Powell and Holland (1988) Ellis and Green (1979) Sen and Bhattacharya (1984) Bhattacharya et al. (1991)

N.M. Noack et al. / Precambrian Research 233 (2013) 206–222

215

Table 4 Major and trace element data for mafic samples of Kanjamalai including ultramafic cumulates, mafic granulites and retrogressed granulites. LOI signifies the loss of ignition of each sample. Error levels are 1–5% for major elements. Due to a detection limit of 0.01 wt% for Na and Mg, the data were reduced accordingly. Sample Rock type

Sa5 ow

Sa2 hpc

Sa6 mg

Sa18 mg

Sa15 gmg

Sa22 gmg

Sa4 gmg

Sa11(m)* rg

Sa11(f)* rg

wt% SiO2 TiO2 Al2 O3 FeO* MnO MgO CaO K2 O Na2 O P2 O5

51.9 0.170 3.41 10.3 0.180 23.8 8.24 0.120 0.19 0.010

47.8 0.305 9.73 11.5 0.180 17.9 9.70 0.100 1.08 0.020

48.6 0.485 11.5 11.4 0.180 14.2 11.4 0.195 1.47 0.040

50.4 0.440 15.1 10.6 0.150 11.1 10.6 0.210 1.92 0.050

47.2 1.19 14.0 16.2 0.270 6.72 11.2 0.030 1.90 0.100

48.5 0.575 15.3 12.5 0.180 9.35 12.4 0.035 1.57 0.040

46.9 0.760 14.6 14.3 0.220 8.39 11.7 0.130 2.00 0.050

49.7 1.16 14.4 12.2 0.150 5.48 8.11 1.61 3.89 0.135

57.7 0.730 16.3 6.09 0.090 3.22 5.64 2.54 4.50 0.490

218 53.1 999 367 86.5 91.2 0.770 78.2 15.8 22.0 1.23 0.0275 11.3 1.31 4.30 0.753 4.06 1.39 0.522 2.11 0.389 2.60 0.588 1.76 0.267 1.77 0.274 0.916 0.130 1.78 0.0430 0.0420

190 35.2 187 281 60.1 67.2 1.18 64.1 11.9 26.0 0.794 0.0260 16.8 1.32 3.48 0.541 2.92 1.01 0.447 1.59 0.287 1.93 0.439 1.29 0.195 1.31 0.202 0.751 0.0577 1.31 0.106 0.0671

378 49.1 47.0 72.6 83.9 112 0.075 118 18.3 26.7 2.94 – 4.09 1.30 5.15 0.987 5.88 2.06 0.837 2.97 0.515 3.43 0.726 2.05 0.294 1.87 0.279 1.06 0.155 0.630 0.0076 0.0010

264 45.5 354 148 238 74.4 0.451 59.3 15.2 18.3 0.591 0.0150 3.68 0.545 2.21 0.473 2.93 1.23 0.489 2.01 0.378 2.60 0.583 1.72 0.262 1.76 0.264 0.717 0.0380 1.12 0.0040 –

341 46.9 231 150 12.6 154 0.076 87.6 19.2 22.8 1.06 0.0100 29.6 1.18 4.10 0.752 4.40 1.66 0.645 2.58 0.473 3.35 0.747 2.24 0.337 2.27 0.343 0.898 0.0506 3.11 – 0.0011

168 21.7 66.0 69.4 7.56 118 14.6 396 19.5 96.0 6.31 0.0269 322 18.7 40.2 4.86 20.0 4.34 1.54 4.30 0.650 3.91 0.766 2.04 0.290 1.83 0.268 2.43 0.338 6.36 1.23 0.149

123 10.6 37.0 26.5 3.01 85.5 17.1 1307 20.9 233 6.67 0.0305 1157 90.6 185 22.3 88.7 14.9 3.51 9.88 1.07 4.73 0.769 1.82 0.231 1.52 0.211 5.13 0.222 13.1 3.54 0.144

0.476 100 58

0.198 101 54

−0.078 98.8 32

0.222 101 45

0.068 99.2 39

2.42 99.4 33

1.03 98.7 37

ppm V Sc Cr Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

117 173 35.5 35.6 1834 1526 478 462 74.6 16.5 49.4 78.2 2.36 0.260 15.1 15.2 3.33 9.33 6.77 13.3 0.599 0.477 0.0140 0.0110 4.50 8.41 0.739 0.611 1.71 1.90 0.223 0.341 1.03 1.80 0.328 0.671 0.115 0.288 0.496 1.12 0.0883 0.213 0.621 1.52 0.135 0.353 0.398 1.09 0.0577 0.162 0.394 1.12 0.0615 0.176 0.210 0.447 0.0296 0.0300 1.82 1.83 0.119 – 0.0575 0.0020

LOI % Total (wt%) Mg#

0.113 98.7 72

0.710 99.3 63

BHVO2 standard

BIR1 standard

31.3 243 97.9 110 109 7.57 317 23.6 178 18.6 0.0850 111 15.0 37.9 5.26 24.6 5.93 2.01 6.18 0.902 5.27 0.966 2.45 0.322 1.97 0.280 4.82 1.31 1.15 1.22 0.323

42.7 419 161 97.3 67.2 0.151 91.2 14.0 15.2 0.582 – 5.72 0.615 1.96 0.369 2.41 1.06 0.510 1.80 0.344 2.45 0.553 1.63 0.240 1.58 0.240 0.677 0.0930 0.420 0.0596 0.0083

Elements in cursive measured by ICPMS, all others by XRF. *(m) signifies the mafic and *(f) the felsic sample of the retrograded rocks of the SASZ.

Table 5 Lu–Hf isotope data for whole rocks, mineral separates and standard materials. 2-errors correspond to the last significant digits. For calculations a decay constant for 176 Lu of 1.867 × 10−11 a−1 (Scherer et al., 2001; Söderlund et al., 2004) and the CHUR values of Blichert-Toft and Albarède (1997) were used. 176

Lu/177 Hf ± 2

176

Hf/177 Hf ± 2

Sample

Lu (ppm)

Hf (ppm)

Whole rocks Sa6 Sa15 Sa18 Sa22

0.235 0.273 0.206 0.270

0.632 0.906 0.699 0.663

0.05282 0.04279 0.04189 0.05777

± ± ± ±

11 9 8 12

0.283974 0.283467 0.283460 0.284213

± ± ± ±

7 9 7 7

Mineral separates Sa15 grt1 Sa15 grt2 Sa15 grt3 Sa15 cpx

0.872 0.882 0.901 0.0109

0.143 0.135 0.182 1.83

0.8726 0.9378 0.7053 0.0008411

± ± ± ±

17 19 14 17

0.322073 0.325092 0.314084 0.281451

± ± ± ±

12 18 11 5

Standards BIR-1 DR-N Ja-1

0.243 0.376 0.438

0.576 2.93 2.47

0.05982 ± 12 0.01823 ± 3 0.02517 ± 5

0.283272 ± 10 0.282777 ± 14 0.283277 ± 4

εHf (2536 ± 300 Ma) 8.4 9.1 9.1 9.7

± ± ± ±

2.0 5.2 4.2 1.8

216

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Fig. 5. The preserved reaction assemblage of sample Sa22 with pl + opx + cpx → grt + qtz + cpx shows retrograde diffusion of the Ca2+ cations between garnet and plagioclase while MgO is distributed homogenously in the garnet. Garnets are unzoned for major elements with a typical flat MnO EPMA profile (4) whereas three LA-ICPMS profiles (1–3) consistently show decreasing Lu concentrations from the outer rim toward the plagioclase inclusions. While major two valent cations like Mg2+ and Fe2+ reached a diffusive equilibrium throughout the P–T path (Fig. 8) resulting in retrograde P–T estimates, Lu3+ remained immobile due to lower diffusion rates and larger distances (Tirone et al., 2004). The characteristic decreasing outer-inner rim Lu profiles suggest inward garnet growth toward plagioclase cores.

clinopyroxene separates as well as the whole rock Sa15 yields an age of 2434 ± 17 Ma. A two point isochron including the whole rock of Sa15 and the most radiogenic garnet separate reproduces this age. The ages obtained from the isochron relationships show the

same age range (Archean–Proterozoic boundary) within the errors of the respective regression lines. 6. Discussion 6.1. Constraints on the magmatic protolith The Lu–Hf whole rock isochron (Fig. 7A) is an approach toward determining a primary magmatic age for the mafic protoliths (2536 ± 300 Ma). Despite the considerably large error, this age is in agreement with Neoarchean emplacement ages of 2.8–2.9 Ga reported for mafic domains elsewhere in the PCSZS region (Bhaskar Rao et al., 1996; Gosh et al., 2004). Beyond these age constraints, the following arguments support that the mafic rocks from KMC actually represent Neoarchean oceanic crust:

Fig. 6. Primitive mantle-normalized (Palme and O’Neill, 2003) incompatible trace element diagrams of mafic granulites from KMC include garnetiferous and nongarnetiferous granulites, as well as meta-ultramafic samples as well as retrogressed lithologies associated with the SASZ. The patterns for non-retrogressed mafic rocks of KMC resemble typical N-MORB patterns devoid of Nb depletion (Hofmann, 1988), while retrogressed samples show enrichment in less compatible elements up to Y. Note the strong depletion in U and Th for mafic samples from KMC. The AFM plot for the same sample suite shows a general tholeiitic trend for the magmatic evolution of the mafic suite while the retrogressed samples are enriched in Na + K. Note the general mobility of Na and K during fluid–rock interaction.

1) Mafic rock units in the KMC are closely intercalated with BMQ/BIF rocks that are generally believed to exclusively form on the ocean floor (e.g. Garrels et al., 1973; Gnaneschwar Rao and Naqvi, 1995; Polat and Frei, 2005). The Neoarchean–Proterozoic boundary is a globally recognized period of enhanced BIF formation (Klein, 2005). 2) Major element compositions and trace element patterns normalized to the primitive mantle (Palme and O’Neill, 2003) of the KMC mafic suite resemble both modern- and Archean MORB type rocks (Table 4 and Fig. 6). The parallel N-MORB like incompatible trace element patterns (Fig. 6) suggest that the mafic assemblages are ocean crust relics. Because addition of subduction zone fluids

N.M. Noack et al. / Precambrian Research 233 (2013) 206–222

0.2844

176

Hf/177Hf

0.2842

A

WR-Isochron Sa 22

B

0.33

217

WR-Cpx-Grt-Isochron (Sa 15) grt 2 grt 1

0.2840

Sa 6 0.2838 0.2836 0.2834

Sa18

Age = 2536 ± 300 Ma 176 177 Initial Hf/ Hf =0.28141 ± 29 MSWD = 7.9

Sa15

0.2832 0.038 0.042 0.046 0.050 0.054 0.058 0.062 176

Lu/177Hf

grt 3

0.31

0.29

cpx

WR

Initial

Age = 2434 ± 17 Ma 176 177 Hf/ Hf =0.28143 ± 21 MSWD = 23

0.27 0.0

0.2

0.4

0.6 176

0.8

1.0

1.2

Lu/177Hf

Fig. 7. Lu–Hf isochrons for KMC samples. (A) Whole rock isochron for 4 whole rock samples (Sa6, Sa15, Sa18, and Sa22) constrains the magmatic formation of the KMC mafic suite. (B) The 5-point isochron cpx-WR-grt for sample Sa15 is controlled by the isotopic values of the whole rock and the most radiogenic garnet separate, constraining the period of garnet growth. Isochron plots were generated with ISOPLOT 2.49 (Ludwig, 2001).

typically generates negative Nb anomalies that are not observed here, it is unlikely that the studied samples formed in an Arc or Back-arc setting. However, striking similarities of geochemical characteristics (major- and trace element abundances) exist between our samples and other Archean MORB-like lithologies, for example amphibolites from Fiskenaesset (Polat et al., 2009) and 2.5 Ga mafic assemblages from the Wutai-belt in China (Wang et al., 2004). 3) Positive initial εHf values for t = 2.5–2.9 Ga for KMC mafic rocks argue for melting of the protoliths from a mantle source depleted as early as in the Archean. Using the age obtained from the whole rock isochron (2536 Ma), a rather strong depletion of the mantle source of the KMC mafic suite becomes evident, with εHf (t) values for the four whole rock samples ranging between +8.4 and +9.7. For an age of 2900 Ma obtained from Sm–Nd ages of mafic bodies within the PCSZS (Bhaskar Rao et al., 1996) the εHf (t) values range from +3.1 to +7.7, still indicating a highly depleted mantle source of the KMC mafics. Hence, the conclusion that the mantle sources of KMC mafic rocks are depleted is insignificantly affected from the uncertainty in the emplacement age.

6.2. Constraints on the relative timing of garnet formation and deformation Minimum estimates for P–T conditions (Table 3) for the clinopyroxene–garnet–plagioclase assemblage in samples Sa15 and Sa4 (Ellis and Green, 1979; Newton and Perkins, 1982) suggest temperatures of 700 ◦ C and pressures of 6–7 kbar whereas sample Sa22 with relics of orthopyroxene yields considerably higher pressure estimates of 8–11 kbar (Sen and Bhattacharya, 1984; Moecher et al., 1988; Powell and Holland, 1988; Bhattacharya et al., 1991). In slowly exhumed granulite terranes such geothermobarometers are likely to reflect post peak conditions (Newton and Perkins, 1982) since major elements used in the P–T calculations might remain in diffusive equilibrium throughout the rock’s P–T path. This is reflected by the report of considerably higher P–T estimates of 14–16 kbar and 800–860 ◦ C by pseudosection modeling derived from a kyanite bearing felsic granulite from the same outcrop (Anderson et al., 2012). Metamorphic garnets in our samples were formed by net-transfer-reactions of plagioclase + clinopyroxene + orthopyroxene (+Fe-ore phases), producing variable amounts of garnet + quartz due to bulk chemistry of the local whole rocks. Mineral reactions with coronal garnet growth

around plagioclase relics are the dominant features in our samples. A lower Mg-number of the ambient bulk composition facilitated garnet growth, so that pristine garnet coronas and relics of orthopyroxene are only seen in the high-Mg sample Sa22. Primary prograde major element zoning was overprinted by internal diffusion. EMPA element maps (Figs. 4 and 5) reveal that garnets are homogenized with respect to major elements, a typical feature of high temperature garnets that experienced slow cooling. Only for MgO and CaO marginal diffusive exchange with neighboring minerals can be observed, resulting in a depletion of these elements in garnet rims (<100 ␮m). The observed major element distributions in garnets and co-existing minerals suggest that garnets remained at higher temperatures long enough to equilibrate prograde zoning of 2+ cations, resulting in post-peak P–T estimates, while opx-free grt-cpx-pl-qtz assemblages suggest initially higher P–T conditions of 14–16 kbar above 800 ◦ C (Fig. 8). Modeling of major element profiles in almandine rich garnets by Caddick et al. (2010) showed that high peak temperatures (∼800 ◦ C) and long time intervals (∼30 Ma) at elevated temperatures are needed to erase prograde zoning of major elements. In contrast to 2+ cations, laser ablation ICPMS profiles of trivalent Lu, Yb and Y in garnets from samples Sa15 and Sa22 reflect typical prograde zonation patterns with distinct Lu (Yb, Y) enrichments in the core region (Figs. 4 and 5). These bell-shaped patterns of HREE in our garnets were thus not erased under prevailing metamorphic conditions (cf. Skora et al., 2006). However, downward sloping Lu profiles from the outer region of garnet coronas toward the plagioclase inclusions in sample Sa22 (Fig. 5), likely reflect inward garnet growth. The preservation of Lu zonation in KMC garnets is likely reflecting that the intra-crystalline diffusion of tri- or tetravalent trace elements in garnet is significantly lower than for divalent major elements, such as Mn or Mg (e.g., Tirone et al., 2004). Such decoupled zonation-equilibration systematics in garnet between major elements and REE were previously reported for garnets from a garnetiferous peraluminous granite with a polymetamorphic history (Dutch and Hand, 2010) and also in high grade garnetiferous mylonites from the red river shear zone in N-Vietnam (Anckiewicz et al., 2012). The regional deformation that led to the formation of the KMC synform, as well as the ductile elongation of garnets found parallel to the fold axes, suggest that high temperature equilibration is clearly post tectonic. Grain boundary configurations and internal deformation features were annealed due to the prevalent high ambient temperatures (i.e. ∼700 ◦ C). Therefore, the Lu–Hf garnet ages measured here, represent minimum ages for the regional deformation.

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Ur–Pb zircon ages of 2477.6 ± 1.8 Ma in a charnockite and of 2483.9 ± 2.5 Ma in a garnetiferous quartzo-feldspathic gneiss from the same outcrop (Saitoh et al., 2011; Anderson et al., 2012) predate our 2434 ± 17 Ma constraint for the period of garnet growth. Generally, it is believed that spot analysis of metamorphic growth zones of zircons reveal the timing of zircon crystallization during metamorphism. The latter is restricted by the availability of Zr due to the breakdown of an associated mineral phase or melt formation (Whitehouse and Platt, 2003 and references therein). Our data therefore shows that garnet growth in the mafic granulites of the KMC postdates the formation of metamorphic zircon in the felsic suite. The high MSWD of 23 for the 5 point regression line (2434 ± 17 Ma) might indicate ployphase garnet growth or, most likely, the presence of older, unequilibrated inclusions. 6.4. Sampling bias issues for garnet and the effect of inclusions

Fig. 8. Proposed P–T–t path for KMC taking in account peak metamorphic conditions of 14–16 kbar and 800–860 ◦ C (1) published by Anderson et al. (2012) from a kyanite bearing felsic gneiss associated with the mafic granulites. Thermobarometry of the garnetiferous mafic granulites (2 and 3) (Table 3) yields only minimum P–T estimates due to slow cooling of the former high grade (pl + grt + cpx + qtz) assemblage observed in Sa15 and Sa4. The results are consistent with Saitoh et al. (2011). The grt-in and opx-out isogrades were modeled via Perplex pseudosection using the bulk composition spread of Sa15 and Sa22. The orthopyroxene relicts in sample Sa22 (Fig. 8) mirror the asymptotical curvature of the P–T path relative to the opx-out isograde. A time line plot summarizes the magmatic formation of the KMC mafic suite, the growth of zircon rims (Anderson et al., 2012) and the period of garnet growth.

6.3. Metamorphic age of garnet In order to interpret the Lu–Hf ages obtained for KMC samples within a comprehensive tectonometamorphic model, it is important to distinguish garnet growth ages from cooling ages. The peak metamorphic temperature at the KMC was eventually as high as 860 ◦ C (Saitoh et al., 2011; Anderson et al., 2012). The closure temperature (Tc ) of the Lu–Hf system in garnetiferous rocks is likely to be as high as 1000 ◦ C (Shu et al., 2012) and is generally believed to be in the range of 650–900 ◦ C, higher than the Tc of the Sm–Nd isotopic system (Scherer et al., 2000; Anczkiewicz et al., 2007; Kelly et al., 2011). Temperatures during metamorphism did not exceed the Tc of the Lu–Hf isotopic system and therefore our data likely reflects garnet growth. This is further corroborated by the trace element distribution in the garnets, especially those of Lu–Hf, which can provide vital information about diffusive re-distribution of these elements or the preservation of their original growth zonation patterns. The bell shaped, prograde Lu (Yb and Y) zonation in the analyzed garnets of sample Sa15 (Fig. 4) suggests that the 2434 ± 17 Ma Lu–Hf whole rock–garnet–clinopyroxene age indeed constrains garnet growth. Retrograde resorption of garnet and possible effects on the Lu–Hf isochrons as studied in detail by Kelly et al. (2011) are not observed in our samples. Therefore, the distribution of Lu and Hf were unaffected by intracrystalline-diffusion during metamorphism above 700 ◦ C and throughout the cooling history of the KMC.

Garnet crystals (2–4 mm) in sample Sa15 occur predominantly as angular shaped intergrowths with pyroxene, and relics of coronal garnet around plagioclase can also be observed. In addition, inclusions in garnet may include quartz and also ilmenite/rutile as well as pyrite. A separation of fragments from garnet cores and rims (cf. Herwartz et al., 2011) in this sample under a binocular microscope is not possible, because of the homogenous composition of the garnets throughout the garnet–clinopyroxene–plagioclase networks (Fig. 4). However the robust zonation of Lu and the complex garnet growth geometries in sample Sa15 combined with a rather arbitrary sampling of garnet by handpicking most likely also resulted in arbitrarily mixed core (high Lu/Hf ratios) and rim fractions (lower Lu/Hf) in the analyzed separates. Putative core fractions would represent an initial garnet growth episode, and rim fractions would be related to a later stage of garnet growth. In this respect, the large MSWD of 23 for the whole rock–garnet–cpx isochron discussed above (Fig. 7B) is likely to also reflect an incomplete isotopic equilibrium between garnets and the matrix. Kohn (2009) predicted high MSWD values for garnet-whole rock isochrons as a consequence of Lu zonation and contrasting intra-crystalline diffusion systematics of Lu and Hf in natural garnets. Even microscopic inclusions of zircon and rutile in garnet could severely affect the Hf isotopic composition of garnet (Scherer et al., 2000). The table top digestion method employed here prevents the dissolution of nonsilicates and the Hf isotopic contamination of garnet. This method has been successfully used in Lu–Hf garnet geochronology (Lagos et al., 2007; Herwartz et al., 2008, 2011; Kirchenbaur et al., 2012). Importantly, garnets in sample Sa15 have 176 Lu/177 Hf ratios higher than 0.7, indicating a low level of Hf contamination by zircon impurities (cf. Scherer et al., 2000). 6.5. Dating the deformation Deformational features like foliation and lineation in the garnetiferous mafic granulites are closely connected to the plastic deformation of garnets as can be seen in elongated, flattened aggregates of the high grade garnet + pyroxene assemblage forming the lineation related to the synclinal structure of KMC (Figs. 2 and 3C). Plastic deformation of garnet is constrained to high temperatures above 800 ◦ C (Ji and Martignole, 1994). A deformation of high grade minerals at significantly lower temperatures than the 650 ◦ C minimum closure temperature proposed for the Lu–Hf system (Scherer et al., 2000; Bearpaw mountains and Huiznopala samples) would have resulted in a stronger retrogression of the assemblages, which was not observed in any of our samples. In the case of a significantly higher closure temperature than 650 ◦ C, as discussed above (Shu et al., 2012), we have constrained the timing of an early high temperature deformational phase in the PCSZS region. This deformation is connected to a regional orogenic event imprinting large

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Fig. 9. Proposed model for the tectonic evolution of the southern Dharwar Craton margin involving an accretionary wedge of Neoarchean–Paleoproterozoic subduction–accretion assemblages including TTG intrusive complexes in mafic-oceanic crust and ocean floor sediments. The association of mafic sea floor relics (3.0–2.8 Ga) (Bhaskar Rao et al., 1996; Gosh et al., 2004) within later intrusive TTG rocks (2.9–2.5 Ga) (Rajesh, 2012) north of the Cauvery Shear zone actually represents a collisional margin related to crustal growth along the Dharwar Craton. A possibly sedimentary origin of the Nilgiri unit and Neoarchean-Paleoproterozoic suturing for the Moyar- and Bhavani segments was first proposed by Raith et al. (1999). This Neoarchean crustal segment experienced high grade metamorphism and synchronous regional deformation as well as migmatisation around 2.48–2.43 Ga and is not part of the Neoproterozoic PCSZ collisional orogen as proposed earlier (Sajeev et al., 2009; Santosh et al., 2009). Abbreviations: N, B, S–granulite regions of the Nilgiri-, Biligirirangan- and Shevaroy Hills.

scale folding in at least three stages on the mafic units of the Bhavani–Salem–Namakkal sector at the boundary of Dharwar Craton and Southern Granulite Terrane (Mukhopadhyay et al., 2003; Gosh et al., 2004). The timing of the initial deformation resulting in regional scale folding of the mafic units can then be constrained to a minimum age of 2434 ± 17 Ma.

crystallization of the dated garnet population began some time after the event that resulted in growth rims of zircon. If a migmatic event triggered both garnet and zircon growth the overlapping ages are likely to constrain the timing of migmatism within the KMC suite.

6.6. Constraining a P–T–t path

6.7. Implications for the tectonics of the southern Dharwar Craton margin

The suggested P–T–t path for the KMC is shown in Fig. 8, with peak conditions of 14–16 kbar at temperatures of 800–860 ◦ C (Anderson et al., 2012). This takes into account that slow cooling might enable relatively fast diffusion of 2+ cations to maintain their equilibrium throughout the cooling path, resulting in lower P–T estimates (Table 3). Slow cooling is also suggested by the ∼80 Ma discrepancy between the Lu–Hf and the reported Sm–Nd isochron ages (Meißner et al., 2002). Meißner et al. (2002) argued for a crystallization age of 2355 ± 22 Ma in a comparable relict body in the Moyar Shear Zone. The Sm and Nd trace element profiles in our garnets from the KMC are however flat (equilibrated) and therefore Sm–Nd ages might indeed be interpreted as cooling ages. The observed orthopyroxene-free assemblages of most of the analyzed garnetiferous mafic granulites indicate that P–T conditions may have exceeded the opx-out isograde. A P–T path that asymptotically runs along the opx-out isograde could explain the opx relicts in sample Sa22. The garnet stability field in the KMC mafic granulites on the other hand lies above 4–6 kbar at temperatures of 600–800 ◦ C and exceeds the opx-out isograde. Garnet growth and high temperature (700–900 ◦ C) deformation of garnet are constrained to 2434 ± 17 Ma by our Lu–Hf garnet isochron age as we have discussed earlier. The fact that U–Pb metamorphic zircon ages (Saitoh et al., 2011; Anderson et al., 2012) in intercalated felsic rocks from KMC predate the Lu–Hf age of 2434 Ma suggests that the

Based on the Paleoproterozoic ages of garnets in ocean floor relic bodies within the PCSZS region we propose a tectonic model with a collisional wedge including Neoarchean ocean floor (Bhaskar Rao et al., 1996) along the southern margin of the Archean Dharwar Craton (Fig. 9). Direct dating of the mafic relics (Bhaskar Rao et al., 1996; this study) in the PCSZS region clearly does not support a model exclusively explaining these units as an entirely Neoproterozoic suture (Sajeev et al., 2009; Santosh et al., 2009). The revision of these former models has been initiated by Santosh et al. (2012) who suggested that two subduction–accretion events affected the region between Dharwar Craton and Madurai Block; one at the Neoarchean–Paleoproterozoic boundary, and the other at the Precambrian–Cambrian boundary. Further detailed investigation is needed to discriminate these two episodes and their related geologic processes. The association of mafic sea floor relics (3.0–2.8 Ga) (Bhaskar Rao et al., 1996; Gosh et al., 2004) and later intrusive TTG rocks (2.9–2.5 Ga) (Rajesh, 2012) north of the Cauvery Shear zone is interpreted here to represent a collisional margin related to the evolution of the Dharwar Craton. This Neoarchean crustal segment experienced high grade metamorphism and synchronous regional deformation as well as migmatisation around 2.48–2.43 Ga and should not be mistaken as the Neoproterozoic PCSZ collisional orogen as proposed earlier (Sajeev et al., 2009; Santosh et al., 2009).

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A Sm–Nd garnet age of 2355Ma ± 22 Ma (Meißner et al., 2002) from an enderbite relict body from the Moyar Shear Zone might constrain the timing of exhumation of the northern domain of the Salem Block. The northern shear zones of the Palghat Cauvery system (SASZ, MSZ) are hosted within Neoarchean crust that was reworked along local shear zones around 600 Ma (Meißner et al., 2002; Chetty and Bhaskar Rao, 2006). Our proposed model (Fig. 9) defines an accretionary wedge formed during Neoarchean–Paleoproterozoic subduction–accretion processes including TTG intrusive complexes in mafic-oceanic crust and ocean floor sediments on the southern margin of the DC. Present day exposures in the northern part of the PCSZS region represent deep seated, deformed thrust sheets of this collisional wedge characterized in the field as large scale fold structures (Mettupalayam–Salem–Namakkal) (Figs. 1 and 9). North-South crustal shortening and the formation of large scale folds and northward thrusting (Chetty and Bhaskar Rao, 2006) is a possible scenario for the Neoarchean–Paleoproterozoic crustal growth episode recorded in the Salem Block. Primary structures as well as mineral assemblages are likely to have experienced reworking at later stages of the crustal evolution. Within the KMC, late stage crustal reworking is observed in minor mylonitic shear bands and in hydrated high strain zones (Figs. 2 and 3E). The high strain (shear) bands at the margins of KMC related to crustal reworking in the Salem-Attur Shear Zone are developed sub-parallel to the fold limbs of the regional Kanjamalai fold and show intensively reworked primary structures with a steep lineation whereas the fold hinge region with a comparably flat lineation remained unaffected by this reworking. 7. Conclusions Lutetium-Hf garnet geochronology combined with petrological information on mafic granulites from the Kanjamalai mafic complex in southern India allows the following conclusions on the tectonomagmatic evolution of the southern Dharwar Craton margin, which overlaps with the northern PCSZS region: 1) The mafic protoliths of the Kanjamalai mafic complex originate from a depleted mantle source. Trace element systematics and whole rock Lu–Hf systematics suggest protolith formation in a Neoarchean MORB setting, in the absence of subduction zone components. 2) The magmatic, metamorphic and deformational history of the PCSZS region is complex and comprises several tectonic episodes. Neoarchean formation of oceanic crust is followed by later stage (2434 ± 17 Ma) high grade metamorphism and regional deformation in a subduction–accretion event on the southern Dharwar Craton margin, which is not related to regional Neoproterozoic suture zone models. 3) Large scale folds in the northern PCSZS region like the KMC syncline are related to this Neoarchean–Paleoproterozoic orogenic event. Those primary structures are truncated and reworked at a yet later stage as indicated by shear zone fabrics. Primary fold hinge regions remained unaffected by reworking. 4) The KMC suite experienced peak metamorphic conditions of 14–16 kbar at temperatures of 800–860 ◦ C (Anderson et al., 2012) and an equilibration of the grt-cpx-pl assemblage at 6–7 kbar at temperatures of ∼700 ◦ C. 5) Plastically deformed almandine garnets of KMC mafic granulites associated with the Neoarchean–Paleoproterozoic event retained bell-shaped prograde concentration profiles for Lu and other trivalent HREE cations, whereas major element profiles equilibrated along the regional P–T path. Therefore, a 2434 ± 17 Ma Lu–Hf garnet age obtained here for KMC

garnetiferous granulites constrains metamorphic garnet growth and subsequent garnet deformation rather than a cooling below the Lu–Hf closure temperature. Given P–T information, the closure temperature for Lu–Hf in garnet in our sample might be as high as 860 ◦ C. 6) Decoupling of intracrystalline diffusion in garnet for 2+ (mobile) major and 3+ (immobile) trace elements in natural samples hampers a direct correlation of Lu–Hf (Sm–Nd) garnet ages and P–T conditions derived from garnet–geothermobarometry. Acknowledgements N.N. is grateful for the financial support of DAAD (thesis abroad grant) during field work in India in 2009. The authors are very grateful to H.U. Kasper of Cologne University for ICPMS trace element measurements. N.N. wants to thank P. Sengupta, M. Raith and J.E. Hoffmann for helpful discussion. M. Raith and P. Sengupta kindly provided initial ideas and resources for this project. Dr. Thirukumaran, Dr. Sridahan and Karthik K. from Salem college provided local guidance and valuable support during field work. The constructive comments of two anonymous reviewers are highly appreciated and significantly contributed to the improvement of the manuscript. Guochun Zhao is thanked for editorial handling. References Albarède, F., Beard, B., 2004. Analytical methods for non-traditional isotopes. Reviews in Mineralogy and Geochemistry 55, 113–152. ´ Anczkiewicz, R., Szczepanski, J., Mazur, S., Storey, C., Crowley, Q., Villa, I.M., Thirlwall, M.F., Jeffries, T.E., 2007. Lu–Hf geochronology and trace element distribution in garnet: Implications for uplift and exhumation of ultra-high pressure granulites in the Sudetes, SW Poland. Lithos 95, 363–380. Anckiewicz, R., Thirlwall, M., Alard, O., Rogers, N.W., Clark, C., 2012. Diffusional homogenization of light REE in garnet from the Day Nui Con Voi Massif in NVietnam: Implications for Sm–Nd geochronology and timing of metamorphism in the Red River shear zone. Chemical Geology 318-319, 16–30. Anderson, J.R., Payne, J.L., Kelsey, D.E., Hand, M., Collins, A.S., Santosh, M., 2012. High-pressure granulites at the dawn of the Proterozoic. Geology 40, 431–434. Bartlett, J.M., Dougherty-Page Nigel, J.S., Harris, B.W., Hawkesworth, C.J., Santosh, M., 1998. The application of single zircon evaporation and model Nd ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India. Contributions to Mineralogy and Petrology 131, 181–195. Bhaskar Rao, Y.J., Chetty, T.R.K., Janardhan, A.S., Gopalan, K., 1996. Sm–Nd and Rb-Sr ages and P–T history of the Archean Sittampundi and Bhavani layered meta-anorthosite complexes in Cauvery Shear Zone, South India: evidence for Neoproterozoic reworking of Archean crust. Contributions to Mineralogy and Petrology 125, 237–250, http://dx.doi.org/10.1007/s004100050219. Bhattacharya, A., Krishnakumar, K., Raith, M., Sen, S.K., 1991. An improved set of a–X parameters in Fe–Mg–Ca garnets and refinement of the orthopyroxenegarnet thermometer and the garnet–orthopyroxeneplagioclase–quartz barometer. Journal of Petrology 32, 629–656. Blichert-Toft, J., Albarède, F., 1997. The Lu–Hf isotope geochemistry of chondrites and the evolution of the mantle–crust system. Earth and Planetary Science Letters 154, 349. Blichert-Toft, J., Frei, R., 2001. Complex Sm–Nd and Lu–Hf systematics in metamorphic garnets from the isua supracrustal belt, West Greenland. Geochimica et Cosmochimica Acta 65, 3177–3187. Caddick, M.J., Konopásek, J., Thompson, A.B., 2010. Preservation of garnet growth zoning and the duration of prograde metamorphism. Journal of Petrology 51, 2327–2347, http://dx.doi.org/10.1093/petrology/egq059. Cenki, B., Kriegsman, L.M., Braun, I., 2002. Melt-producing and melt-consuming reactions in the Achankovil cordierite gneisses, South India. Journal of Metamorphic Geology 20, 543–561. Cheng, H., King, R.L., Nakamura, E., Vervoort, J.D., Zhou, Z., 2008. Coupled Lu–Hf and Sm–Nd geochronology constrains garnet growth in ultra-high-pressure eclogites from the Dabie orogen. Journal of Metamorphic Geology 26, 741–758, http://dx.doi.org/10.1111/j.1525-1314.2008.00785.x. Chetty, T.R.K., Bhaskar Rao, Y.J., 2006. The Cauvery Shear Zone, Southern Granulite Terrain, India: a crustal-scale flower structure. Gondwana Research 10, 77–85. Clark, C., Collins, A.S., Timms, N.E., Kinny, P.D., Chetty, T.R.K., Santosh, M., 2009. SHRIMP U–Pb age constraints on magmatism and high-grade metamorphism in the Salem Block, southern India. Gondwana Research 16, 27–36. Collins, A.S., Kröner, A., Fitzsimons, I.C.W., Razakamanana, T., 2003. Detrital footprint of the Mozambique ocean: U–Pb SHRIMP and Pb evaporation zircon geochronology of metasedimentary gneisses in eastern Madagascar. Tectonophysics 375, 77–99.

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