Neoarchean suprasubduction zone arc magmatism in southern India: Geochemistry, zircon U-Pb geochronology and Hf isotopes of the Sittampundi Anorthosite Complex

Gondwana Research 23 (2013) 539–557

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Neoarchean suprasubduction zone arc magmatism in southern India: Geochemistry, zircon U-Pb geochronology and Hf isotopes of the Sittampundi Anorthosite Complex M. Ram Mohan a,⁎, M. Satyanarayanan a, M. Santosh b, c, Paul J. Sylvester d, Mike Tubrett e, Rebecca Lam e a

National Geophysical Research Institute (CSIR), Hyderabad, 500007, India Division of Interdisciplinary Science, Faculty of Science, Kochi University, Kochi 780-8520 Japan Journal Center, China University of Geosciences Beijing, 29 Xuehuan Road, Beijing 100083, China d Deptartment of Earth Sciences, Memorial University, St. John's, NL, Canada, A1B3X5 e CREAIT Network, Memorial University, St. John's, NL, Canada, A1C5S7 b c

a r t i c l e

i n f o

Article history: Received 10 February 2012 Received in revised form 29 March 2012 Accepted 9 April 2012 Available online 21 April 2012 Keywords: Geochemistry Zircon U-Pb geochronology Hf isotopes Suprasubduction zone arc Sittampundi Anorthosite Complex Southern India

a b s t r a c t The Sittampundi Anorthosite Complex (SAC) in southern India is one of the well exposed Archean layered anorthosite-gabbro-ultramafic rock associations. Here we present high precision geochemical data for the various units of SAC, coupled with zircon U-Pb geochronology and Hf isotopic data for the anorthosite. The zircon ages define two populations, the older yield a concordia age of 2541± 13 Ma, which is interpreted as the best estimate of the magmatic crystallization age for the Sittampundi anorthosite. A high-grade metamorphic event at 2461± 15 Ma is suggested by the upper intercept age of the younger zircon population. A Neoproterozoic event at 715 ± 180 Ma resulted in Pb loss from some of the metamorphic zircons. The magmatic age of the anorthosite correlates well with the timing of crystallization of the arc-related~ 2530 Ma magmatic charnockites in the adjacent Salem Block, while the metamorphic age is synchronous with the regional metamorphic event. The geochemical data suggest that the rocks were derived from a depleted mantle source. Sub-arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous, aluminous basalt magma. The magma fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. The coherent initial176Hf/177Hf ratios and positive εHf values (1.7 – 4.5) of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source. Our study further confirms that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth. © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Anorthosites are important components of the early Earth's crust and hold critical information on the petrogenetic and geodynamic processes, particularly in relation to magmatism in active convergent margins (Windley and Smith, 1974; Ashwal, 1993; Polat et al., 2009, 2011; Hoffmann et al., 2012). Archean anorthosites are considered to be more calcic than their younger counterparts (Ashwal, 1993), although recent studies (e.g., Windley and Garde, 2009) show that their modern equivalents occur in several Phanerozoic island arcs. Chromite-layered anorthosite complexes occur in the Proterozoic Eastern Ghats belt of Peninsular India and recent studies establish an arc-related origin for these complexes (Dharma Rao and Santosh, 2011; Dharma Rao et al., 2012). The mechanism of formation of such magmatic arcs is closely linked to lithospheric subduction, accretion and subsequent collision

⁎ Corresponding author. E-mail address: [email protected] (M.R. Mohan).

when the arcs crop out within continents (Polat et al., 2009; Santosh et al., 2009; Windley and Garde, 2009; Rollinson et al., 2010). However, there have been only few studies that address the geodynamic and petrogenetic processes associated with the formation of these rocks. The Sittampundi Anorthosite Complex (SAC) in southern India is a well exposed Archean layered anorthosite-gabbro-ultramafic rock association (Subramaniam, 1956; Ramadurai et al., 1975; Bhaskar Rao et al., 1996) that possesses many similarities with other Archean anorthositic complexes such as the Fiskenæsset, Messina, Bushveld and Chimalpahad in terms of tectonic setting, petrogenesis and emplacement mechanism. The SAC occurs within the Palghat-Cauvery Shear Zone which has been considered as the trace of a late Neoproterozoic suture zone developed through the closure of the Mozambique Ocean during the final amalgamation of the Gondwana supercontinent (Collins et al., 2007; Santosh et al., 2009). The SAC is also proximal to the Archean cratonic margin of Dharwar to the north and its boundary with the Proterozoic crustal collage of the Southern Granulite Terrane to the south. A recent study proposes subduction-accretion-collision tectonics in this part of southern India during two critical periods in Earth history – the

1342-937X/$ – see front matter © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2012.04.004

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Archean-Paleoproterozoic boundary and the Precambrian-Cambrian boundary (Santosh et al., 2012). In this paper, we report comprehensive major, trace and REE data from a suite of twenty eight anorthositic rocks, eleven gabbros, four amphibolites and four pyroxenites from the SAC. We also report high precision LA-ICPMS U-Pb and Hf isotope data on zircons from one anorthositic rock. These results provide new insights into the petrogenesis of the anorthositic and associated rocks of this complex and identify the source characteristics and crust-mantle interaction during the evolution of these rocks. Our zircon data offer additional evidence for widespread Neoarchean arc-related magmatism in a subduction setting at the southern margin of the Dharwar craton in southern India. 2. Geological background The broad geological framework of southern India is defined by the granite-greenstone belt of the Archean Dharwar Craton (DC) to the north and the granulite facies collage of the Proterozoic Southern Granulite Terrane (SGT) to the south. Recent studies have largely focused in the region between the Archean-Proterozoic divide along the Palghat-Cauvery Suture Zone (PCSZ) where a network of transpressional shear zones within an area of approximately 70 x 350 km expose features typical of a cryptic suture with remnants of imbricated ocean plate stratigraphy including suprasubduction ophiolitic assemblages (Santosh et al., 2009; Yellappa et al., 2010; Chetty et al., 2011; Sato et al., 2011b; Yellappa et al., 2012). The PCSZ has been variously described in previous studies as: (a) dextral shear zone characterized by the deflection of north-south Archean fabrics to near east-west disposition (Drury et al., 1984; Chetty et al., 2003); (b) Archean-Proterozoic Terrane boundary (Harris et al., 1994); (c) a zone of Paleo- and Neoproterozoic reworking of Archean crust (Bhaskar Rao et al., 1996; Raith et al., 1999); (d) a Neoproterozoic

dextral–ductile transpressive tectonic zone (Meißner et al., 2002; Bhaskar Rao et al., 2003; Chetty et al., 2003) and (e) Neoproterozoic crustal-scale ‘flower structure’ (Chetty and Bhaskar Rao, 2006). Recent studies consider the PCSZ as the trace of the Ediacaran-Cambrian suture extending from Madagascar (Collins et al., 2007; Collins et al., 2008; Raharimahefa and Kusky, 2009). A model involving long-lived Pacifictype Neoproterozoic subduction-accretion history culminating in a Himalayan-style collision during latest Neoproterozoic-Cambrian along this zone, associated with the closure of the Mozambique Ocean and the birth of the Gondwana supercontinent, has also been proposed (Santosh et al., 2009). The magnetotelluric data from this region also support a subduction model with southward polarity (Naganjaneyulu and Santosh, 2010, 2011). The Sittampundi Anorthosite Complex is located about 80 km SSW of the city of Salem (Fig. 1) and is exposed over an area of about 36 km x 2 km. The SAC has been subjected to regional metamorphism and occurs within amphibolites and orthogneisses of the PCSZ (Subramaniam, 1956; Windley and Selvan, 1975; Bhaskar Rao et al., 1996). The lithological units within the SAC largely preserve their original igneous stratigraphy, probably reflecting an isochemical recrystallization of the magmatic rocks. From the base upwards, the Complex consists of magnesite-veined dunite, chromite-layered clinopyroxenite, chromite-layered anorthosite (Fig. 2c), and meta-gabbro (Fig. 2d) that contains clinopyroxenite layers (Dutta et al., 2011; Dharma Rao et al., 2012), capped by banded iron formations and basaltic amphibolites. Although the Complex has been subjected to high-grade metamorphism, the rock units preserve the original igneous stratigraphy. The chromitite layers are of variable thickness; whereas these layers are up to 6 metres thick in anorthosites, they are only up to few tens of centimeters thick in clinopyroxenite (Subramaniam, 1956). Some of the chromitites are associated with metamorphic assemblages such as corundum with plagioclase rims (Koshimoto et al., 2004), and cm-thick schistose layers that contain sapphirine, spinel, corundum, clinozoisite, phlogopite,

Fig. 1. Generalised geological map of Sittampundi Anorthosite Complex (modified after Subramaniam, 1956). Inset showing the location of Southern Granulite Terrain.

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Fig. 2. Representative field photographs of the major lithologies of SAC. (a) Anorthosite with thin chromite layering near Karangalpatti. Note that such anorthositic samples possess high MgO and Cr. (b) Outcrop pattern of massive anorthosite (sample location SA09-16) near Sittampundi. (c) Thick chromite band within anorthosite near Karangalpatti. (d) Outcrop pattern of metapyroxenite.

gedrite and calcic plagioclase. Garnet-bearing retrogressed eclogites (high-pressure granulites), occur as layers and lenses within the anorthosite, showing peak P-T conditions at ~20 kbar and 1020 °C (Sajeev et al., 2009). The SAC is folded into an isoclinal antiform, and weakly refolded on a N–S axial trace (Ramadurai et al., 1975).

3. Petrographic features The anorthosites are mainly composed of calcic plagioclase with subordinate amphiboles, pyroxenes (Fig. 3a and b), opaque and secondary alteration products such as biotite, chlorite and quartz. The plagioclase is mostly coarse-grained, polygonal and typically exhibit triple junction suggesting equilibrium conditions of crystallization (Fig. 3a and b). The less deformed domains of the anorthosite are characterized by untwinned euhedral plagioclase grains, whereas the deformed parts show anhedral grains that display grain boundary migration, lamellar and polysynthetic twinning (Fig. 3c). At places, the plagioclase laths are traversed by fine stringers of mafic minerals (Fig. 3c). The amphiboles and pyroxenes sometimes fill the interstices between the plagioclase grains. Euhedral to sub-hedral hornblende is porphyroblastic and some of the grains carry inclusions of orthopyroxene and quartz which show alteration into biotite and chlorite along the cleavage planes (Fig. 3c). Hornblende is the most dominant mafic mineral in the anorthosites, and the mineral also occurs as inclusions in plagioclase. Both clinopyroxene and orthopyroxene occur, partly altered to hornblende and biotite. In the pyroxenites, coarse grained ortho- and clinopyroxenes show cumulate texture, that are surrounded by fine grained and recrystallized plagioclase and quartz (Fig. 3d). The gabbroic rocks of SAC are generally coarse grained with subhedral to anhedral pyroxenes (clinopyroxene and orthopyroxene) and plagioclase (Fig. 3e). The metagabbro is garnet-bearing at places where they show high degree of deformation and recrystallization. The garnet grains occur as rounded to ovoid porphyroblasts surrounded by

intergrowths of plagioclase and pyroxenes along the grain boundaries (Fig. 3f). In a recent study, Dharma Rao et al. (2012) investigated the mineral chemistry of the chromitites associated with the SAC. The high-aluminium chromites are concentrated in layers between amphibole–rich layers with a dominant mineralogy of amphibolespinel-plagioclase ± sapphirine. The chromite-rich layers contain only amphibole and plagioclase. Mineral compositions illustrated by X- ray composition maps and profiles show subtle chemical differences. The chrome spinels are of refractory grade with Cr2O3 and Al2O3 contents varying between 34 - 40 wt. % and 23 - 28 wt. % respectively. The highly calcic plagioclase in anorthosites and the Fe Al-rich chromite in chromitites are consistent with derivation from a parental magma of hydrous tholeiitic composition (Dharma Rao et al., 2012). The mineralogy and field relations of the Sittampundi chromitites are also similar to anorthosite-hosted chromitites in the Archean Fiskenæsset anorthositic complex in Greenland (Windley, 1973). 4. Sampling and analytical methods Representative samples from the SAC were collected during two field sessions, choosing rocks devoid of any surface alteration or weathering. The anorthosite sample for U-Pb zircon geochronology and Hf isotopes (SA09-16) was collected from a small working pit developed for road building material (N11o.13´.46.9"; E77o.53´.35.6"), very close to the Sittampundi village (Figs. 1c, 2b). The anorthosites here are fresh, massive and plagioclase rich (An85-90) without any visible layering. The interlayered gabbros, pyroxenites and amphibolites were collected from a number of localities depending upon the availability of fresh outcrops. All the samples were initially reduced in a jaw crusher, and then manually fine powdered in agate mortars. The sample suite provides a comprehensive spatial distribution of anorthosites in the SAC, and covers all the major rock units of the complex.

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Fig. 3. Photomicrographs showing the mineral assemblages and textural features of the anorthositic rocks and metagabbros of SAC. Mineral abbreviations: Cpx-clinopyroxene; Opx- orthopyroxene; Plg- plagioclase, Hbl- hornblende; Gt- garnet; Cal-calcite.

4.1. Geochemistry Depending on the relative abundance of plagioclase to mafic minerals, the anorthositic rocks of Archean anorthosite complexes are classified into anorthosites (>90% plagioclase), gabbroic anorthosites (80-90% plagioclase) and anorthositic gabbro (20-35% mafic minerals) (Windley et al., 1973). But this classification is not conclusive in deciphering the petrogenesis of these rocks as the whole rock major element chemistry for cumulate rocks is controlled mainly by the relative abundance of major mineral phases rather than the composition of the melt (Markl and Frost, 1999) such as those of Sittampundi anorthosites where the anorthitic plagioclase content ranges up to 90%. Hence we prefer to term the Sittampundi anorthosites as “anorthositic”, without giving any weightage to the abundance of mafic minerals. The major elements were analyzed by XRF (Phillips ® MAGIX PRO Model 2440), with a relative standard deviation of b 3% (Krishna et al., 2007). For trace elements including rare earth (REE) and high field strength elements (HFSE), the sample powders were dissolved in reagent grade HF: HNO3 acid mixture in Savillex screw top vessels. Solutions were analyzed by ICP-MS (Perkin Elmer SCIEX ELAN DRC II) at the National Geophysical Research Institute (NGRI), Hyderabad. 103Rh was used as an internal standard.

External drift was corrected by repeated analyses of a 1: 5000 solution of UB-N, which was also used as calibration standard. Instrument response was corroborated relative to two independent digestions of AN-G. Precision and accuracy are better than RSD 5% for the majority of trace elements (Balaram and Rao, 2003). Anomalies of HFSE relative to neighboring REE are given as Nb/Nb*, Zr/Zr*, Hf/Hf* and Ti/Ti*. Mg# is calculated as Mg/ (Mg + Fetotal). Chondrite and primitive mantle normalization values are taken from (Sun and McDonough, 1989). 4.2. Zircon separation and U-Pb analysis About 10 kg of a representative sample of the anorthositic rock from the SAC (SA09-16) was crushed and milled, and standard heavy liquid and magnetic techniques were used to separate zircon concentrates that were purified by hand picking under a binocular microscope. The least metamict and least damaged grains were hand-picked for analyses. They were cast into epoxy resin discs and polished to expose the mid-sections of grains. Further assessment of grains and choice of sites for analyses was based on transmitted and reflected light, cathodoluminescence (CL) and back-scattered electron (BSE) images. Representative CL images of the zircon grains from Sittampundi anorthosites are shown in Fig. 4.

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Fig. 4. A-C: Cathodoluminescence images of representative zircon grains for the SAC anorthosite (SA09-16) showing locations of LA-ICPMS spots for geochronology.

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Fig. 4 (continued).

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Fig. 4 (continued).

In situ U–Pb dating of the zircon grains was performed by LA-ICP MS at the Memorial University of Newfoundland using the Element XR and Geolas 193 nm laser, with a 40x40 μm raster that ablated to about 15 μm depth. The laser was set for an energy density of 3–5 J/ cm 2 with a repetition rate of 10 Hz. The U–Pb dating method broadly follows that presented in Kosler et al. (2002), aspirating a 237Np233 U- 209Bi- 205Tl- 203Tl tracer solution to correct for mass discrimination of Pb and U isotopic ratios. The Plesovice and Harvard 91500 reference zircons were run as secondary standards for quality control purposes. The data are presented in Table 2 with 1σ precision that include all sources of known and random uncertainty. Grouped ages discussed in the text are reported with uncertainties of ±2σ. Data were plotted on concordia diagrams using Isoplot/Ex (Ludwig, 2008). 4.3. Hf isotopic analysis Analyses for 176Hf/ 177Hf isotope ratios of zircon were carried out by laser ablation ICP multi-collector mass spectrometry using a Finnigan Neptune at Memorial University. For these analyses, the Geolas193 nm laser ablated either a 40 or 49 μm spot, on top of the U–Pb raster laser spot positions, with an energy density of 5 J/cm 2 and a laser frequency of 10 Hz for 600 pulses per analysis. Plesovice zircon was used as the quality control standard. Present day Hf ratios were back calculated to obtain the initial 176Hf/ 177Hf using the measured Lu/Hf ratios, the Lu decay constant of 1.867x10 - 11 (Soderlund et al., 2004), and 207Pb/ 206Pb age of each grain of zircon. Epsilon Hf values are calculated with 176Hf/ 177Hf and 176Lu/ 177Hf (CHUR) values of 0.282785 and 0.0336, respectively (Bouvier et al., 2008). 5. Results

(Fig. 5). Fe2O3 (0.54-9.00 wt. %) and MgO (0.58-9.79 wt. %) also show marked variation (Table 1). When compared to the massive anorthositic variety, those of layered type and chromite bearing varieties display distinct compositional variations such as high MgO (in the range of 3.67 to 9.79 wt%), lower SiO2 and Fe2O3 (Fig. 5) in addition to having high Ni and Cr values (Table 1). However, the REE compositions are coherent and display slight LREE enriched patterns with moderate fractionation (La/Smcn =1.26-3.91; Gd/Ybcn = 0.78-1.85), together with positive Eu anomalies (Fig. 6a). The extended trace element diagram of the anorthositic rocks exhibit LILE (large ion lithophile element) enriched, HFSE (high field strength element) depleted patterns with visible troughs of Nb, Zr and Ti along with Eu and Sr crests (Fig. 6b). 5.1.2. Gabbros The gabbros from SAC show SiO2 in the range between 49.53 and 63.06 wt. %, and nearly consistent Al2O3 (12.22-18.14 wt. %) (Table 1 and Fig. 5c). The Fe2O3, TiO2 and CaO contents show a decreasing trend with increasing SiO2 (Fig. 5b and d). Trace elements display multiple orders of compositional variations, such as Zr (4.18-26.71 ppm), Nb (0.19-3.32 ppm), Sc (10.66-49.04 ppm), and Th (0.12-0.31 ppm), and the REE compositions are also variable (Table 1). The REE patterns display fractionated patterns, with ten orders of variance in the total REE abundances and either flat or slight to moderately enriched LREE patterns with corresponding HREE depletions (La/Smcn = 0.50-4.46; Gd/Ybcn = 1.09-4.10) (Fig. 6c). These rocks also exhibit mild positive Eu anomalies, albeit much less pronounced as compared to those displayed by anorthositic rocks (average Eu/Eu* 1.38). Apart from the variations as exemplified in the REE plot, the extended trace element diagram of the gabbros exhibit LILE enriched and HFSE patterns with consistent negative anomalies of Nb, Zr and Ti along with the moderately positive Eu and Sr anomalies (Fig. 6d).

5.1. Geochemistry The geochemical data and key elemental ratios for the major rock types of SAC are presented in Table 1. All the rocks show sub-alkalic nature (Nb/Y b0.7), and have broadly tholeiitic affinities with low Zr/Y ratios (Table 1). The salient major, trace and REE features of the SAC rocks are discussed below. 5.1.1. Anorthositic rocks The SAC anorthositic rocks display notable variations in major elements and some trace elements that reflect the cumulate nature of these rocks such as SiO2 (40.78-51.25 wt.%), elevated Al2O3 and CaO (in the range of 18.34-33.39 wt.% and 12.54-20.81 wt.% respectively)

5.1.3. Amphibolites The amphibolites of the SAC display a tight range of SiO2 and Al2O3 (between 49.19 and 53.93 wt.% and 12.53 and 14.50 wt.% respectively), but possess moderate variations of TiO2, Fe2O3 and CaO with a decreasing trend against increasing SiO2 (Fig. 5 and Table 1). Moderate variations are also observed in the trace elements such as Zr (2.25-13.43 ppm), Nb (0.37-3.72 ppm), Sc (11.44-19.99 ppm) and Th (0.11-0.83 ppm). The Chondrite- normalized REE patterns of these rocks are generally LREE depleted (average La/Ybcn 0.83) with mild negative Eu anomalies (Fig. 6e). The extended trace element diagram of the amphibolites display LILE enriched and HFSE depleted patterns (Fig. 6f). Titanium exhibits mild positive and negative

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Table 1 Major and trace elemental compositions for various rock types of SAC. Sample No.

SA-8

SA- 9

SA- 10

SA- 11

SA- 12

SA- 13

SA- 14

SA- 15

SA- 18

SA- 19

SA- 20

SA- 21

Rock Type SiO2 (wt.%) TiO2 Al2O3 Fe2O3 MnO MgO CaO K2O Na2O P2O5 LOI Total Mg# Cr (ppm) Co Ni Rb Sr Cs Ba Sc V Ta Nb Zr Hf Th U Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Cu Zn (La/Yb)cn (La/Sm)cn (Gd/Yb)cn (Eu/Eu*)cn Zr/Hf La/Nb Th/Nb Nb/Ta Th/La Zr/Y Nb/Nb* Zr/Zr* Hf/Hf* Ti/Ti* Y/Ho Nb/Yb Nb/Y

Anorthositic 41.22 0.07 30.62 1.83 0.04 6.53 18.88 0.03 0.79 0.00 1.59 101.59 0.89 24.88 26.13 41.82 0.44 131.49 0.02 31.15 8.40 75.89 0.01 0.10 3.50 0.09 0.02 0.08 1.69 0.41 0.72 0.09 0.44 0.19 0.19 0.22 0.05 0.27 0.07 0.17 0.03 0.16 0.03 9.73 19.41 1.82 1.42 1.11 2.84 41.02 3.98 0.23 7.77 0.06 2.08 0.17 0.85 0.75 0.84 25.52 0.64 0.06

Anorthositic 40.94 0.08 30.87 1.17 0.02 6.79 19.37 0.03 0.74 0.00 1.71 101.71 0.93 153.55 26.40 56.38 0.48 183.39 0.02 30.21 7.71 168.51 0.01 0.06 3.16 0.08 0.02 0.08 1.32 0.50 0.96 0.13 0.62 0.18 0.18 0.19 0.04 0.22 0.05 0.14 0.02 0.12 0.02 11.98 19.34 2.90 1.82 1.25 3.03 40.82 8.52 0.36 8.29 0.04 2.40 0.08 0.66 0.58 1.06 25.86 0.48 0.04

Anorthositic 42.55 0.07 27.99 1.97 0.03 7.93 17.63 0.04 0.79 0.00 1.01 100.01 0.91 129.89 31.84 87.16 0.53 63.15 0.01 25.26 6.87 55.26 0.01 0.07 2.93 0.07 0.02 0.07 1.08 0.33 0.50 0.06 0.27 0.10 0.12 0.15 0.03 0.19 0.04 0.11 0.02 0.12 0.02 14.52 16.33 1.93 2.08 1.00 2.99 40.28 4.62 0.34 8.88 0.07 2.71 0.12 1.22 1.10 1.36 25.50 0.58 0.07

Anorthositic 40.84 0.05 31.39 1.11 0.02 4.14 19.50 0.04 0.91 0.00 2.28 100.28 0.89 35.97 18.87 33.32 0.74 149.99 0.03 31.13 9.49 38.43 0.02 0.09 12.23 0.29 0.04 0.28 1.44 0.62 1.01 0.12 0.46 0.16 0.20 0.19 0.04 0.23 0.05 0.15 0.02 0.15 0.02 12.19 23.75 2.96 2.60 1.04 3.63 42.86 6.64 0.41 6.13 0.06 8.49 0.09 3.18 2.69 0.71 27.63 0.62 0.07

Anorthositic 41.53 0.16 28.33 3.02 0.06 9.79 16.24 0.05 0.81 0.00 0.95 100.95 0.88 6826.34 74.40 162.96 0.71 119.53 0.02 21.26 8.94 223.26 0.02 0.10 3.51 0.09 0.02 0.09 1.59 0.55 1.20 0.16 0.74 0.20 0.25 0.24 0.05 0.27 0.06 0.16 0.03 0.19 0.03 6.82 51.67 2.03 1.79 1.00 3.52 37.40 5.42 0.21 6.73 0.04 2.20 0.15 0.63 0.61 1.77 24.67 0.52 0.06

Anorthositic 43.86 0.08 26.07 1.84 0.03 7.43 19.76 0.05 0.88 0.00 1.48 101.48 0.90 82.99 29.79 51.38 0.42 163.98 0.02 28.38 15.97 66.76 0.01 0.07 4.09 0.12 0.02 0.09 2.52 0.71 1.00 0.14 0.65 0.23 0.33 0.31 0.06 0.44 0.10 0.26 0.04 0.24 0.04 8.54 25.41 2.09 1.94 1.04 3.77 33.00 9.52 0.32 12.17 0.03 1.62 0.06 0.72 0.79 0.72 25.12 0.31 0.03

Anorthositic 41.70 0.08 30.56 1.12 0.01 4.77 20.81 0.04 0.90 0.01 1.21 101.21 0.90 34.28 19.77 33.23 0.45 195.66 0.02 19.29 11.93 76.00 0.02 0.11 3.82 0.13 0.02 0.09 3.23 0.70 1.46 0.20 0.99 0.36 0.49 0.50 0.11 0.60 0.14 0.32 0.04 0.30 0.05 8.60 18.51 1.69 1.27 1.39 3.59 30.19 6.48 0.21 6.69 0.03 1.18 0.12 0.45 0.54 0.46 23.79 0.36 0.03

Anorthositic 40.79 0.08 29.72 1.28 0.02 5.02 19.86 0.04 1.18 0.01 2.17 100.17 0.90 35.25 19.04 36.47 0.44 191.61 0.02 27.59 10.53 84.50 0.01 0.07 5.21 0.15 0.03 0.13 2.25 0.66 1.26 0.16 0.74 0.21 0.31 0.28 0.06 0.37 0.09 0.24 0.04 0.25 0.05 21.25 15.08 1.92 2.01 0.94 3.88 35.42 9.69 0.39 5.58 0.04 2.32 0.07 0.91 0.93 0.79 24.74 0.28 0.03

Anorthositic 41.93 0.06 29.19 0.81 0.01 4.57 20.30 0.04 1.09 0.00 2.37 100.37 0.93 30.56 15.38 29.76 0.33 196.57 0.01 15.73 9.35 70.05 0.00 0.05 2.95 0.07 0.02 0.10 1.52 0.59 1.16 0.14 0.60 0.19 0.28 0.19 0.04 0.25 0.06 0.14 0.02 0.18 0.03 23.57 14.19 2.36 1.97 0.90 4.40 40.62 11.24 0.37 12.75 0.03 1.95 0.07 0.60 0.53 0.76 24.28 0.29 0.03

Anorthositic 41.24 0.06 31.02 1.30 0.02 5.57 19.42 0.04 1.32 0.00 1.47 101.47 0.90 20.75 25.67 37.63 0.42 181.22 0.02 16.92 7.94 52.32 0.06 0.07 3.61 0.12 0.03 0.13 1.12 0.43 0.66 0.08 0.37 0.12 0.16 0.13 0.03 0.21 0.04 0.11 0.02 0.13 0.02 27.34 18.42 2.37 2.29 0.82 3.84 30.90 6.25 0.37 1.08 0.06 3.23 0.09 1.19 1.39 1.17 26.21 0.53 0.06

Anorthositic 40.81 0.09 30.72 1.55 0.02 6.06 19.50 0.04 1.21 0.00 1.68 101.68 0.90 32.89 25.63 41.47 0.40 166.18 0.02 17.50 12.32 127.63 0.01 0.06 3.80 0.12 0.02 0.11 1.79 0.46 0.86 0.11 0.54 0.18 0.21 0.19 0.05 0.30 0.07 0.19 0.03 0.20 0.04 32.96 25.53 1.66 1.67 0.78 3.57 31.97 7.19 0.38 7.88 0.05 2.13 0.10 0.85 0.96 1.19 26.65 0.32 0.04

Anorthositic 40.78 0.10 30.45 1.68 0.03 6.47 19.30 0.04 1.15 0.00 1.98 101.98 0.89 28.65 25.36 38.36 0.45 156.99 0.02 17.70 13.93 260.51 0.01 0.07 3.23 0.11 0.05 0.11 2.77 0.58 1.12 0.16 0.81 0.30 0.36 0.37 0.08 0.47 0.11 0.28 0.04 0.29 0.05 39.32 16.53 1.45 1.26 1.06 3.28 30.78 8.49 0.69 6.70 0.08 1.17 0.08 0.46 0.54 0.73 24.29 0.24 0.02

anomalies (Ti/Ti* 0.64-3.10), probably suggesting inadequate ilmenite fractionation.

The extended trace element diagram of the pyroxenites show LILE enriched and HFSE depleted pattern with distinct negative Nb anomaly and variable Zr and Ti anomalies (Fig. 6h).

5.1.4. Pyroxenites The pyroxenites of the SAC show moderate chemical variation (Table 1). The SiO2 ranges between 45.93 and 51.85 wt. %, the Al2O3, TiO2, Fe2O3 and CaO show mild negative correlation with silica (Fig. 5). The REE patterns exhibit moderately enriched and depleted LREE patterns (La/Smcn = 0.30-2.48) and flat to enriched HREE patterns (Gd/ Ybcn = 0.86-2.35), along with mildly variable Eu anomalies (Fig. 6g). The enrichment of the HREE in some of these rocks as well as in the amphibolites is probably related to the presence of metamorphic garnets.

5.2. Geochronological data Zircons from the anorthositic rocks of Sittampundi (SA09-16) are colorless to light yellowish green. The grains range from long to short prismatic with lengths of 100 - 300 μm and length-width ratios of 2-4. Most grains or grain fragments are subhedral with some showing well rounded terminations or relict cores resulting from partial corrosion through dissolution and recrystallization. The CL images (Fig. 4 a, b and c) display broad oscillatory or striped zoning

M.R. Mohan et al. / Gondwana Research 23 (2013) 539–557

547

SA- 22

SA- 23

SA- 25

SA- 26

SA- 27

SA- 28

SA09-9

SA09-9A

SA09-10A

SA09-10B

SA09-16A

SA09-16B

Anorthositic 40.80 0.07 30.54 1.72 0.03 6.92 18.51 0.05 1.35 0.00 1.39 101.39 0.90 27.67 28.50 43.44 0.41 161.72 0.02 15.19 9.38 135.46 0.01 0.06 3.16 0.09 0.02 0.10 1.25 0.37 0.67 0.10 0.44 0.14 0.18 0.17 0.04 0.21 0.05 0.13 0.02 0.12 0.02 21.11 17.08 2.21 1.69 1.15 3.60 36.24 5.92 0.32 4.77 0.05 2.53 0.11 0.87 0.87 1.09 26.78 0.52 0.05

Anorthositic 40.86 0.07 29.62 1.79 0.03 6.09 19.13 0.05 1.36 0.00 2.41 101.41 0.90 27.30 32.22 41.53 0.48 177.36 0.02 22.95 8.19 204.73 0.01 0.08 4.16 0.12 0.02 0.12 1.10 0.46 0.89 0.12 0.54 0.15 0.25 0.17 0.03 0.18 0.04 0.11 0.02 0.11 0.02 24.60 18.60 3.02 1.99 1.30 4.71 33.61 5.88 0.29 6.91 0.05 3.79 0.12 1.02 1.10 1.07 28.26 0.72 0.07

Anorthositic 41.17 0.09 29.99 1.25 0.02 4.44 19.86 0.04 1.13 0.00 2.10 100.10 0.91 23.85 21.08 36.26 0.49 175.55 0.02 14.02 11.26 71.37 0.09 0.15 4.37 0.11 0.02 0.19 2.26 0.59 0.98 0.13 0.62 0.21 0.29 0.26 0.06 0.33 0.08 0.24 0.03 0.21 0.04 39.77 18.19 2.00 1.82 0.99 3.83 40.41 3.90 0.15 1.66 0.04 1.93 0.16 0.84 0.75 0.94 27.36 0.72 0.07

Anorthositic 42.04 0.07 29.82 0.83 0.01 3.67 20.64 0.04 0.87 0.01 3.33 101.33 0.93 28.39 22.36 33.31 0.45 219.82 0.02 20.23 8.19 327.31 0.01 0.07 4.14 0.11 0.02 0.12 1.36 0.52 1.01 0.12 0.51 0.17 0.28 0.21 0.04 0.24 0.05 0.12 0.02 0.13 0.02 28.20 17.17 2.82 2.01 1.29 4.56 36.43 7.26 0.26 7.78 0.04 3.04 0.10 0.98 0.98 0.92 27.44 0.54 0.05

Anorthositic 41.95 0.06 29.72 0.54 0.01 3.87 20.75 0.04 1.05 0.01 3.02 101.02 0.94 29.07 11.00 26.00 0.34 245.47 0.02 20.53 7.74 61.15 0.01 0.08 4.86 0.11 0.02 0.10 1.99 0.82 1.47 0.18 0.73 0.20 0.24 0.22 0.05 0.31 0.07 0.19 0.03 0.19 0.03 6.61 13.49 3.16 2.58 1.00 3.37 46.24 10.31 0.26 8.56 0.03 2.45 0.06 0.87 0.68 0.68 27.54 0.43 0.04

Anorthositic 41.07 0.07 30.52 0.92 0.01 4.22 19.76 0.04 1.39 0.00 2.96 100.96 0.91 32.57 14.24 30.83 0.38 245.34 0.02 21.93 9.27 140.76 0.02 0.06 5.50 0.13 0.02 0.11 1.88 0.98 2.04 0.27 1.20 0.33 0.33 0.30 0.07 0.29 0.07 0.19 0.03 0.17 0.02 7.97 14.76 4.08 1.91 1.42 3.18 42.71 16.67 0.42 3.35 0.03 2.92 0.05 0.60 0.51 0.55 27.74 0.34 0.03

Anorthositic 46.68 0.09 29.18 3.26 0.05 2.84 16.16 0.04 1.69 0.01 0.42 100.42 0.66 49.51 17.49 67.58 5.53 188.83 0.08 59.69 7.46 32.85 0.02 0.37 11.22 0.28 0.44 0.61 2.74 3.01 5.60 0.62 2.23 0.50 0.33 0.60 0.09 0.48 0.10 0.27 0.05 0.27 0.04 23.00 42.29 8.08 3.91 1.85 1.82 40.32 8.23 1.22 21.41 0.15 4.09 0.08 0.74 0.66 0.39 27.86 1.37 0.13

Anorthositic 45.82 0.08 29.70 2.84 0.04 2.74 17.81 0.05 0.91 0.01 0.56 100.56 0.68 75.69 12.92 61.36 1.13 157.54 0.04 71.54 7.16 40.86 0.11 0.28 5.37 0.14 0.14 0.31 2.08 0.84 1.69 0.23 0.98 0.29 0.27 0.32 0.07 0.36 0.07 0.22 0.04 0.20 0.03 15.58 26.38 3.04 1.90 1.34 2.74 38.41 3.04 0.49 2.42 0.16 2.58 0.25 0.70 0.66 0.63 28.34 1.39 0.13

Anorthositic 45.86 0.08 24.90 6.24 0.08 6.30 14.93 0.04 1.56 0.00 1.34 101.34 0.69 70.59 43.01 143.34 0.91 111.36 0.05 45.29 8.97 36.73 0.14 0.13 3.90 0.13 0.10 0.18 1.41 0.73 1.44 0.20 0.87 0.21 0.23 0.26 0.05 0.25 0.05 0.13 0.03 0.14 0.02 24.63 24.29 3.66 2.21 1.51 2.98 31.03 5.80 0.77 0.92 0.13 2.76 0.13 0.63 0.73 0.81 28.49 0.88 0.09

Anorthositic 46.56 0.10 31.90 1.70 0.02 0.65 17.38 0.01 1.68 0.00 0.65 100.65 0.46 41.76 7.38 43.57 0.88 180.90 0.04 18.54 4.21 33.52 0.55 0.46 3.57 0.08 0.09 0.14 1.55 0.81 1.64 0.19 0.78 0.21 0.27 0.26 0.05 0.27 0.05 0.14 0.02 0.13 0.02 11.06 19.88 4.32 2.46 1.58 3.53 42.26 1.75 0.20 0.84 0.11 2.30 0.43 0.61 0.52 0.92 28.57 3.44 0.30

Anorthositic 48.07 0.06 27.49 3.10 0.04 3.23 16.86 0.04 1.11 0.01 0.99 100.99 0.70 94.59 15.17 60.35 0.83 166.80 0.06 51.46 10.63 44.91 0.03 0.08 2.48 0.08 0.08 0.15 1.73 1.06 1.41 0.17 0.71 0.20 0.25 0.25 0.05 0.30 0.06 0.17 0.03 0.17 0.03 12.19 18.08 4.44 3.42 1.19 3.44 31.86 13.00 1.04 2.53 0.08 1.43 0.04 0.45 0.52 0.65 27.65 0.48 0.05

Anorthositic 49.03 0.07 26.09 3.53 0.05 3.66 16.18 0.02 1.36 0.01 0.18 100.18 0.70 85.11 17.93 52.61 0.57 166.16 0.04 65.02 10.62 38.21 0.07 0.09 2.36 0.07 0.08 0.14 1.69 0.84 1.68 0.23 0.94 0.27 0.25 0.29 0.06 0.30 0.06 0.17 0.03 0.18 0.03 20.06 14.76 3.42 2.02 1.34 2.75 31.84 9.32 0.91 1.30 0.10 1.40 0.08 0.33 0.37 0.60 26.30 0.51 0.05

patterns typical of zircons from high-temperature magmatic intrusions. Most of the grains display thin, bright structure less metamorphic overgrowths. The relatively thin overgrowths in many grains might suggest that the rock was dominantly dry during the subsequent thermal event. In some cases, the internal structures of the dark cores that are surrounded by or variably replaced by CL bright domains have been totally destroyed suggesting metamorphic recrystallization under high temperatures. The discrete grains of metamorphic origin are relatively small in size and show structureless CL bright domains. Fifty-seven U–Pb analyses on 28 grains from sample SA09-16 were performed (Table 2). All the spots show high Th/U in the range of 0.25 and 6.94 (excluding one outlier with Th/U of 33.7). The U-Pb spot ages show a broad spread and the majority of age data fall in the range of ca

2.45 Ga to 2.55 Ga (Fig. 9a). The analysis differentiate zircons with two distinct age populations (older and younger) (Table 2). The combined data from the two zircon populations yield upper and lower intercept ages of 2496± 14 Ma and 999 ±140 Ma respectively on a U-Pb concordia diagram (Fig. 9b). A better approximation of the magmatic age of the SAC is achieved by considering the two populations separately. Accordingly, the older and younger populations are plotted separately on two additional U-Pb concordia plots. The U-Pb data of the older population (n = 14) yields a Concordia age of 2541± 13 Ma (Fig. 9c), which we believe represents the best estimate of the magmatic age of SAC anorthosite. The bulk of the magmatic zircons were reset during highgrade metamorphism at 2461± 15 Ma, reflected by the upper intercept of the younger population (n= 43) (Fig. 9d). Some of these

548

M.R. Mohan et al. / Gondwana Research 23 (2013) 539–557

Table 1 (continued) Sample No.

SA09-18A

SA09-18 C

SA09-22

SA09-29B

SA-1

SA-2

SA-3

SA09-27

SA09-15

SA09-14

SA09-13

SA09-28

Rock Type SiO2 (wt.%) TiO2 Al2O3 Fe2O3 MnO MgO CaO K2O Na2O P2O5 LOI Total Mg# Cr (ppm) Co Ni Rb Sr Cs Ba Sc V Ta Nb Zr Hf Th U Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Cu Zn (La/Yb)cn (La/Sm)cn (Gd/Yb)cn (Eu/Eu*)cn Zr/Hf La/Nb Th/Nb Nb/Ta Th/La Zr/Y Nb/Nb* Zr/Zr* Hf/Hf* Ti/Ti* Y/Ho Nb/Yb Nb/Y

Anorthositic 46.88 0.11 32.01 1.78 0.02 0.58 16.90 0.01 1.69 0.02 0.19 100.19 0.42 33.03 7.17 23.94 0.49 165.59 0.03 58.77 4.87 28.66 0.19 0.41 10.14 0.23 0.09 0.15 3.79 2.73 5.64 0.66 2.46 0.53 0.40 0.64 0.12 0.66 0.13 0.38 0.06 0.35 0.06 16.28 17.34 5.62 3.32 1.53 2.08 44.39 6.67 0.22 2.17 0.03 2.67 0.12 0.61 0.50 0.45 28.25 1.18 0.11

Anorthositic 51.25 0.22 18.34 9.00 0.12 6.50 12.54 0.01 1.99 0.02 0.02 100.02 0.61 191.15 48.34 95.89 0.72 126.96 0.07 66.65 31.79 122.77 0.01 0.07 5.07 0.16 0.10 0.16 3.46 0.77 1.63 0.24 1.14 0.39 0.32 0.45 0.10 0.62 0.13 0.35 0.06 0.34 0.06 33.88 31.85 1.63 1.29 1.09 2.31 31.49 10.56 1.38 6.64 0.13 1.47 0.07 0.53 0.61 1.25 26.02 0.21 0.02

Anorthositic 46.38 0.07 28.51 3.31 0.05 3.04 17.31 0.04 1.28 0.01 0.12 100.12 0.67 116.53 15.80 57.22 0.73 282.18 0.04 53.44 11.45 57.06 0.04 0.11 2.61 0.07 0.10 0.18 2.30 1.03 1.99 0.24 1.00 0.26 0.27 0.33 0.07 0.39 0.09 0.23 0.04 0.22 0.03 21.91 26.57 3.39 2.52 1.27 2.79 38.37 9.59 0.96 2.82 0.10 1.13 0.08 0.35 0.33 0.56 26.76 0.49 0.05

Anorthositic 48.64 0.13 29.65 2.55 0.04 1.23 15.98 0.02 1.75 0.01 0.18 100.18 0.52 29.56 8.91 27.38 0.59 139.62 0.05 53.34 5.42 34.85 0.09 0.16 8.58 0.17 0.06 0.12 2.13 0.70 1.56 0.21 0.95 0.29 0.42 0.35 0.07 0.42 0.08 0.20 0.03 0.19 0.03 17.77 15.93 2.71 1.54 1.56 4.04 50.69 4.40 0.38 1.85 0.09 4.02 0.19 1.13 0.81 0.96 26.63 0.86 0.07

Gabbro 49.53 0.93 12.22 13.23 0.17 7.77 14.26 0.14 1.67 0.08 0.74 100.74 0.56 506.34 59.11 207.66 0.98 158.46 0.03 48.23 47.90 340.05 0.11 2.12 15.42 0.77 0.15 0.12 23.89 3.13 5.21 1.22 7.30 2.44 0.86 2.85 0.48 3.63 0.80 2.16 0.32 1.60 0.37 101.61 225.67 1.40 0.83 1.48 1.00 19.93 1.47 0.07 18.81 0.05 0.65 0.42 0.25 0.46 0.83 29.98 1.33 0.09

Gabbro 49.73 0.91 12.62 13.24 0.20 7.01 13.66 0.44 2.11 0.08 1.11 101.11 0.54 568.41 60.80 225.68 3.53 163.47 0.03 96.62 46.01 323.87 0.20 2.82 21.96 0.97 0.26 0.10 22.33 4.20 6.25 1.37 7.97 2.35 0.80 2.69 0.45 3.44 0.76 2.02 0.30 1.54 0.34 73.61 161.01 1.95 1.15 1.44 0.98 22.70 1.49 0.09 13.87 0.06 0.98 0.37 0.35 0.56 0.86 29.41 1.83 0.13

Gabbro 49.88 0.94 13.51 12.26 0.17 6.66 14.05 0.30 2.13 0.08 0.55 100.55 0.54 502.05 61.83 216.51 0.99 154.88 0.02 69.77 49.04 341.47 0.18 3.19 15.87 0.89 0.13 0.08 23.40 5.43 8.76 1.88 10.24 2.79 0.96 3.09 0.51 3.80 0.79 2.07 0.30 1.55 0.34 128.99 139.96 2.51 1.25 1.65 0.99 17.74 1.70 0.04 17.73 0.02 0.68 0.35 0.21 0.42 0.76 29.49 2.06 0.14

Gabbro 52.94 0.59 13.04 11.35 0.13 8.18 13.04 0.10 0.62 0.02 0.42 100.42 0.61 360.70 68.61 238.37 1.68 102.06 0.04 55.39 47.76 471.15 0.42 1.21 18.77 1.10 0.31 0.05 12.22 5.28 15.29 2.30 13.85 3.32 0.92 2.86 0.35 2.10 0.40 0.98 0.11 0.64 0.14 193.77 85.79 5.90 1.03 3.68 0.89 17.09 4.38 0.26 2.84 0.06 1.54 0.25 0.19 0.41 0.45 30.72 1.88 0.10

Gabbro 60.27 0.69 17.59 8.96 0.11 4.20 5.50 0.86 1.68 0.14 0.29 100.29 0.51 172.24 25.43 64.89 16.91 525.81 0.04 292.66 24.11 177.85 0.40 3.32 14.26 0.75 0.29 0.05 22.90 21.24 41.60 4.90 23.91 4.65 1.55 4.36 0.54 3.45 0.70 1.86 0.22 1.37 0.31 26.73 86.31 11.11 2.95 2.63 1.03 19.14 6.40 0.09 8.37 0.01 0.62 0.11 0.09 0.18 0.36 32.57 2.42 0.14

Gabbro 55.29 0.67 16.11 11.85 0.15 5.12 9.35 0.08 1.27 0.09 1.18 101.18 0.49 155.50 50.18 98.14 1.23 124.57 0.05 47.77 40.78 311.65 0.17 1.78 16.76 0.79 0.16 0.04 26.78 3.45 9.70 1.41 8.58 2.45 0.85 2.58 0.46 3.60 0.81 2.25 0.28 1.80 0.42 104.12 83.27 1.37 0.91 1.19 1.02 21.19 1.93 0.09 10.36 0.05 0.63 0.54 0.25 0.43 0.63 32.95 0.99 0.07

Gabbro 49.95 0.82 16.27 14.71 0.20 7.39 9.82 0.07 0.71 0.06 1.62 101.62 0.53 305.51 67.57 171.66 1.37 67.08 0.06 49.84 48.29 347.70 0.28 2.20 26.71 0.99 0.29 0.08 25.39 1.80 5.32 0.87 5.72 1.98 0.71 2.19 0.45 3.51 0.78 2.10 0.27 1.66 0.38 147.06 106.51 0.78 0.59 1.09 1.04 27.02 0.82 0.13 7.86 0.16 1.05 1.35 0.55 0.74 0.93 32.49 1.33 0.09

Gabbro 63.06 0.50 15.42 9.62 0.08 3.44 4.33 1.55 1.88 0.12 0.72 100.72 0.44 105.51 19.42 47.04 30.96 532.21 0.05 369.65 13.48 133.58 0.35 2.50 17.49 0.62 0.21 0.05 10.20 17.98 30.09 3.21 14.90 2.60 1.17 2.69 0.28 1.61 0.31 0.85 0.10 0.62 0.14 48.00 54.00 20.67 4.46 3.57 1.34 28.42 7.20 0.09 7.23 0.01 1.71 0.09 0.19 0.25 0.44 33.42 4.01 0.24

metamorphic zircons were further affected by Pb loss during a subsequent Neoproterozoic event at 715 ± 180 Ma, as reflected by the lower intercept of the younger zircon population (Fig. 9d). This interpretation is supported by the very good fit (Probability of fit = 0.995) of the discordia line.

5.3. Hf isotopes A total of 39 Lu-Hf isotopic analyses were performed on 22 zircon grains, on domains from where the U–Pb ages were obtained (Table 3), representing all of the age populations identified from different grains (Fig. 4). The initial 176Hf/177Hf isotope ratios for all of the zircons of

SAC anorthosite are clustered between 0.281211 and 0.281301, with calculated εHf units at corresponding crystallization ages ranging between -0.8 and 4.5 (Table 3). The magmatic zircons are clustered even more tightly, between +1.7 and +4.5. In the εHft vs. age plot (Fig. 10), the distribution is essentially confined between CHUR and depleted mantle (DM) for MORB (calculated using the parameters of Griffin et al., 2000; updated by Anderson et al., 2009). This is consistent with derivation of the SAC from depleted mantle, albeit slightly less depleted than that for MORB sources, which is common for island arc magmas (Chauvel et al., 2009). Depleted mantle model ages (TDM) for the zircons (assuming a 176Lu/177Hf=0.022 for mafic crust; Chauvel et al. (2009) range between 2.80 and 3.17 Ga, indicating a possibility of small inputs of older crustal Hf into the mantle sources of the magmatic zircons, and during the

M.R. Mohan et al. / Gondwana Research 23 (2013) 539–557

549

SA09-29

SA09-26A

SA09-26B

GA-29

GA-31

AK-26

AK-37B

GA-2

GA-4

GA-8b

AK1B

KPT-6

Gabbro 55.44 0.25 15.93 10.86 0.13 6.59 9.44 0.04 1.30 0.01 0.46 100.46 0.57 405.42 55.29 157.37 2.87 150.37 0.06 77.21 37.20 258.83 1.80 1.06 4.18 0.17 0.22 0.08 4.76 1.90 3.45 0.40 2.02 0.51 0.34 0.54 0.10 0.71 0.16 0.41 0.05 0.30 0.07 63.88 63.78 4.61 2.43 1.50 1.96 25.22 1.79 0.21 0.59 0.12 0.88 0.38 0.29 0.41 1.14 29.78 3.59 0.22

Gabbro 57.59 0.13 18.14 9.01 0.04 2.50 11.40 0.08 1.10 0.01 0.49 100.49 0.38 127.31 14.09 78.38 0.99 143.90 0.04 15.78 10.66 74.10 0.45 0.19 5.97 0.17 0.15 0.04 3.01 1.05 2.10 0.25 1.34 0.32 0.42 0.37 0.06 0.46 0.09 0.25 0.03 0.16 0.04 14.48 35.13 4.73 2.15 1.91 3.74 35.80 5.67 0.83 0.41 0.15 1.99 0.13 0.63 0.64 0.91 32.51 1.16 0.06

Gabbro 57.74 0.48 18.06 10.05 0.09 3.14 7.00 0.95 2.33 0.16 0.24 100.24 0.41 93.09 19.28 56.48 6.30 632.63 0.03 305.04 12.17 122.59 0.07 1.94 19.68 0.69 0.12 0.03 10.88 19.34 34.90 3.96 18.80 3.15 1.20 2.98 0.31 1.63 0.32 0.87 0.10 0.60 0.14 19.34 126.84 23.09 3.97 4.10 1.18 28.66 9.97 0.06 28.90 0.01 1.81 0.07 0.18 0.22 0.37 34.22 3.23 0.18

Amphibolite 53.77 0.31 12.53 9.33 0.12 11.27 11.12 0.10 1.44 0.01 1.58 101.58 0.73 850.62 59.77 67.77 2.50 13.83 0.08 9.26 11.44 39.10 0.24 0.64 3.42 0.22 0.41 0.15 6.88 1.39 3.21 0.44 2.21 0.69 0.25 0.70 0.15 1.11 0.14 0.47 0.06 0.61 0.10 0.93 16.56 1.64 1.30 0.95 1.10 15.32 2.17 0.63 2.63 0.29 0.50 0.40 0.19 0.45 1.07 48.36 1.05 0.09

Amphibolite 49.19 0.67 14.50 14.12 0.16 7.66 11.45 0.08 2.15 0.01 2.40 102.40 0.54 117.07 73.59 59.67 2.68 12.27 0.15 26.07 18.28 44.58 0.20 0.47 3.23 0.22 0.18 0.10 26.24 0.45 1.13 0.13 0.73 0.44 0.08 0.88 0.30 3.35 0.55 2.21 0.33 3.52 0.59 6.75 18.14 0.09 0.66 0.21 0.39 15.00 0.96 0.39 2.30 0.41 0.12 0.97 0.39 0.95 2.57 47.46 0.13 0.02

Garnetiferous Amph. 53.93 0.75 14.04 11.05 0.20 4.96 13.59 0.17 1.20 0.09 1.75 101.75 0.50 45.67 47.65 21.54 23.80 64.37 0.57 28.95 19.99 54.19 0.48 3.72 13.43 0.83 2.28 0.89 33.53 3.92 10.57 1.55 8.09 2.74 0.51 2.83 0.64 4.95 0.64 2.19 0.31 3.17 0.52 1.16 21.04 0.88 0.92 0.74 0.55 16.10 1.05 0.61 7.79 0.58 0.40 0.96 0.20 0.44 0.64 52.30 1.17 0.11

Garnetiferous Amph. 51.83 0.42 13.66 13.16 0.15 8.11 11.42 0.02 1.22 0.01 1.48 101.48 0.58 56.20 72.97 29.04 1.43 32.59 0.06 10.22 16.53 61.23 0.29 0.37 2.25 0.11 0.17 0.15 5.15 0.55 1.21 0.15 0.79 0.29 0.15 0.35 0.09 0.78 0.11 0.39 0.05 0.55 0.10 8.09 18.12 0.72 1.20 0.52 1.46 20.18 1.50 0.47 1.24 0.31 0.44 0.55 0.32 0.58 3.10 46.09 0.67 0.07

Pyx. 49.23 0.52 6.54 5.36 0.13 19.27 10.30 0.10 0.34 0.01 8.83 100.63 0.89 3045.48 64.77 524.80 1.11 64.91 0.05 126.02 37.54 185.23 0.01 0.02 35.76 0.69 0.08 0.09 6.14 2.90 7.28 1.13 5.27 1.39 0.48 1.55 0.22 1.10 0.24 0.67 0.10 0.54 0.09 94.08 200.10 3.81 1.34 2.35 0.99 52.14 133.00 3.50 2.00 0.03 5.82 0.01 0.91 0.64 0.84 25.64 0.04 0.00

Pyx. 45.93 0.60 7.67 6.97 0.13 18.36 10.21 0.01 0.26 0.01 10.00 100.15 0.85 3046.67 73.61 681.97 0.60 48.13 0.02 117.53 41.74 213.48 0.00 0.02 34.66 0.66 0.08 0.08 6.06 1.01 2.87 0.58 3.40 1.11 0.40 1.28 0.21 1.09 0.24 0.67 0.10 0.55 0.09 260.65 360.59 1.32 0.59 1.92 1.01 52.68 54.12 4.41 4.25 0.08 5.72 0.02 1.23 0.85 1.18 25.05 0.03 0.00

Pyx. 51.85 0.54 6.30 7.20 0.14 17.61 10.02 0.01 0.26 0.01 7.00 100.93 0.84 3089.48 77.45 477.24 0.52 42.87 0.02 117.73 48.05 203.69 0.08 0.04 34.42 0.65 0.13 0.09 6.33 2.87 6.59 1.01 4.72 1.26 0.44 1.38 0.21 1.05 0.26 0.70 0.11 0.59 0.09 50.46 279.70 3.49 1.47 1.94 1.01 53.07 65.37 2.88 0.53 0.04 5.44 0.01 0.98 0.67 0.96 24.63 0.07 0.01

Pyx 48.58 0.48 8.33 8.46 0.16 16.35 11.75 0.02 0.14 0.07 6.27 100.61 0.02 1400.46 70.98 286.43 0.83 51.18 0.02 145.12 32.46 18.89 0.01 0.07 25.71 0.42 0.08 0.08 9.48 2.49 4.33 0.61 2.67 0.65 0.43 0.97 0.16 1.01 0.28 0.86 0.14 0.84 0.13 46.21 144.63 2.12 2.48 0.95 1.67 60.89 33.62 1.05 10.43 0.03 2.71 0.02 1.35 0.81 0.06 34.03 0.09 0.01

Pyx 47.45 0.63 13.98 11.33 0.15 11.10 12.13 0.02 1.11 0.09 2.89 100.89 0.68 1008.33 56.04 137.74 0.74 50.09 0.02 25.62 34.80 253.20 0.03 0.18 8.63 0.29 0.03 0.03 18.55 0.61 2.06 0.50 3.25 1.32 0.50 2.22 0.44 2.87 0.67 2.18 0.37 2.13 0.34 23.15 78.28 0.20 0.30 0.86 0.88 29.83 3.45 0.20 6.47 0.06 0.47 0.37 0.29 0.35 0.87 27.59 0.08 0.01

high-grade regional metamorphic event that formed the metamorphic zircons. 6. Discussion 6.1. Extent of alteration, crustal contamination and tectonic setting As has been well established in previous studies, the various rock types within the PCSZ have all undergone granulite facies metamorphism (e.g., Santosh et al., 2009, and references therein). Some of the Mg-Al and mafic granulites also show ultrahigh-temperature and high pressure metamorphism up to the eclogite facies (Shimpo et al., 2006; Sajeev et al., 2009; Santosh et al., 2009, 2010). Hence it

is imperative to consider the effects and extent of post-magmatic alteration on the geochemical systematics of the SAC rock units. Primary igneous textures are well preserved in the anorthosites and gabbro, but mostly obscured in amphibolites and pyroxenites. Also the signatures of metamorphic recrystallization and alteration are observed at places, from which domains the sampling was avoided. The rocks of SAC had undergone high grade metamorphism and at least two stages of deformation. Although care has been taken during sampling by avoiding visibly altered samples and high strain domains, the geochemical signature is not totally free from the post-magmatic effects. Various geochemical studies have established that immobile elements such as Al2O3, TiO2, Na2O, HFSE, REE (except Ce and Eu), Th and transitional metals are least sensitive to hydrothermal alteration,

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Fig. 5. Harker diagrams to show the behavior of key major elemental abundances of SAC rocks.

even if the rocks are subjected to various metamorphic grades, and therefore these elements and their ratios provide useful insights into the primary petrogenetic processes (Polat and Kerrich, 2000; Piercey et al., 2002; Polat and Hofmann, 2003). Being immobile, zirconium is considered to be a very useful fractionation index based on which other immobile elements/ element ratios can be compared to test for mobility (Polat and Hofmann, 2003). Bivariate plots of Zr/Y, Th, La/Sm, Gd/Yb, V and Nb versus Zr display broad linear trends with few outliers (Fig. 7). Hence it is inferred that these immobile elements are largely unaffected by alteration and metamorphism. Loss on ignition (LOI) values are b3 wt.% for most of the analyzed rocks (except for the pyroxenites), suggesting that the rocks have not been strongly hydrated or carbonated. All the rocks of SAC display strong negative Nb anomalies along with the depletion of other HFSE (Zr, Hf, Ta) relative to REE and LILE (Fig. 6 and Table 1). Such negative anomalies are typically correlated to the primary mantle source characteristic or may reflect crustal contamination (Pearce and Peate, 1995; Polat et al., 2009). In contrast, the Nb and other HFSE depletion has been explained in terms of their high melt-mineral partition coefficient, and hence FeTi phases (e.g. ilmenite, rutile and amphibole) can host bulk of these HFSE, causing the depletion of these elements in the melt during igneous fractionation processes (Foley et al., 2000; Klemme et al., 2002; Hoffmann et al., 2012). The petrographic examination of the pure anorthositic rocks of the SAC does not show the presence of any Fe-Ti phases, hence such igneous fractionation is ruled out. The depletion of HFSE, a consistent feature in all the rock types of SAC could possibly be the outcome of subduction-related mechanism, when these rocks were derived from a depleted mantle source. The coherently positive εHft values for the zircons for the anorthosite of this study (Table 3) further strengthen this observation and the role of crustal contamination is largely excluded. 6.2. Role of fractional crystallisation on cumulate processes and the emplacement of SAC As the role of alteration and crustal contamination are minimal to define the geochemical systematics of SAC rocks, it is necessary to

constrain the role of other extraneous processes such as fractional crystallization. The plots of MgO against CaO, Al2O3, Fe2O3, Ni, Cr and Sc (Fig. 8) reflect fractional crystallization trends to define the fractionation of olivine, pyroxene and plagioclase as observed in Fiskenæsset Complex (Polat et al., 2009). The low abundances of MgO, Ni, Cr (excluding the chromite bearing samples), Co and Sc of anorthositic rocks and gabbros (Table 1) are consistent with the removal of olivine, cpx and/ or opx, and amphibole prior to the accumulation of plagioclase and suggest the derivation of these rocks from fractionated magmas. Such scenario in Fiskenæsset and Naajat Kuuat Complexes of Greenland is attributed to magma chamber processes (Polat et al., 2009; Hoffmann et al., 2012), where the accumulation of these minerals in the early evolution of parental melts could have taken place in magma chambers. Also it is assumed that the plagioclase-pyroxene-olivine layers were derived from a magma chamber(s) that are compositionally stratified; with the late stage crystallization of plagioclase as thick layers on the top and olivinerich dunite layers at the bottom as early crystallizing unit. The accumulation of calcic plagioclase late in the crystallization sequence is consistent with the differentiation of hydrous parental melts, as such scenario is commonly observed in arc related geodynamic settings (Windley, 1995) 6.3. Suprasubduction zone magmatism The data obtained in this study, and those from recent work discussed above suggest that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean – early Paleoproterozoic with the generation and emplacement of suprasubduction zone arc magmas and continental growth. The tectonic setting of Archean layered anorthosites with highly calcic plagioclase and Fe Al-rich chromite has been investigated in various studies of Greenland (Weaver et al., 1981; Dymek and Owens, 2001; Polat et al., 2009, 2010; Rollinson et al., 2010; Polat et al., 2011; Hoffmann et al., 2012). The compositions of plagioclase and chromite in the arc-related Fiskenæsset and Sittampundi Complexes are remarkably similar (Dharma Rao et al., 2012). The field, petrographic and geochemical data also suggest that the Sittampundi Complex is a

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551

Fig. 6. Chrondrite-normalised REE plots and primitive mantle-normalised extended trace element diagrams for the rocks of SAC. Note the negative anomalies of HFSE for all the rock types. Normalisation values are from Sun and McDonough (1989).

fragment of supra-subduction oceanic crust. Similar to the layered complex of Fiskenæsset, the SAC is characterized by a lower ultramafic unit, which passes upwards into gabbro, leucogabbro to anorthosite rocks. The anorthosite and co-genetic pyroxenite interlayered with chromitite seams and the polyphase deformation and upper amphibolite to granulite-facies metamorphism are also comparable features (Windley and Smith, 1974; Windley and Garde, 2009; Rollinson et al., 2010). Both the Fiskenæsset and Sittampundi Complexes are bordered by amphibolites of ocean floor affinity and were subsequently intruded by voluminous protoliths of the regional TTG orthogneisses. A higher geothermal gradient in the Archean probably provided optimal conditions for slab melting that metasomatised the sub-arc mantle wedge by slab-derived melts (Polat et al., 2011) and made it unusually aluminous, and this acted as a source for hydrous aluminous basalts. Dharma Rao et al. (2012) proposed that the Sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a mantle wedge above a subduction zone. The melting process produced hydrous, aluminous basalt, which fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. Comparable subduction-related, high-alumina, hydrous basaltic magma was reported by Eyuboglu et al. (2011) from an Alaskan-type maficultramafic complex in eastern Pontides, N. Turkey.

The metagabbros of the SAC are sporadically garnet-bearing in zones of intense deformation and metamorphism. From a garnetbearing metagabbro (mafic granulite) lens within the anorthosite at Sittampundi, Sajeev et al. (2009) reported peak metamorphic P-T conditions of ~ 20 kbar and 1020 °C. They considered these rocks as retrogressed eclogites and although no age data were presented in their study, the eclogite facies metamorphism was correlated to the Late Neoproterozoic subduction-collision tectonics along the PCSZ associated with the Gondwana assembly. Our present study did not record any Late Neoproterozoic-Cambrian metamorphic zircons from Sittampundi, suggesting that the eclogite facies metamorphism at Sittampundi is unrelated to the Gondwana forming orogeny. The eclogite facies metamorphism in this locality might have been associated with either the early Paleoproterozoic or mid Neoproterozoic subduction tectonics, as recent studies clearly show that the southern cratonic margin of Dharwar was an active convergent zone during both these times (Santosh et al., 2012). 6.4. Crust-mantle interactions The nature of exposed crust in many regions preserve the signature of an interplay of crust-mantle interactions where an initial depleted mantle is variably enriched and leading to a heterogeneous composition through crustal additions (Shirey and Hanson, 1986; Stern, 2011). The

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Fig. 7. Key compatible and incompatible elements and their ratios against Zr (ppm) as an alteration index.

nature, age and extent of these crustal additions are variable from one terrain to the other as seen for example in the Dharwar Craton with more juvenile signature in the Eastern Dharwar region as compared to the older Western Dharwar domain (Jayananda et al., 2000, 2008). The magmatic zircons from the SAC anorthosite exhibit positive εHft values (1.7 to 4.5) that are distributed between CHUR and the depleted mantle (DM) (Fig. 10). Hf model ages are somewhat older than the crystallization ages (Table 3). This suggests that SAC was dominantly derived from a rather homogeneous juvenile magma during Neoarchean subduction in this region, with the possibility of recycling small amounts of older crust into the depleted mantle sources (Santosh et al., 2009; Santosh, 2012). The available whole rock Nd isotopic compositions of the Sittampundi anorthosites with positive εNdt values and Nd model ages between 2.92 and 3.07 Ga further confirms this inference (Bhaskar Rao et al., 1996). 6.5. Age data and tectonic implications Previous geochronological studies from the SAC reported a range of ages partly because of the random sampling and imprecise techniques adopted for dating these rocks, leading to considerable ambiguity in interpreting the timing of emplacement of these rocks and the subsequent thermal events. Bhaskar Rao et al. (1996) reported a wholerock Sm-Nd isochron age of ca. 2935 ± 60 Ma, which they interpreted as the minimum age of the Sittampundi Complex, or more likely to be the timing of early metamorphism. They also reported an εNdt value of +1.85 ± 0.16 for the Sittampundi rocks, which implies that the time difference between the emplacement of the Complex and the

granulite-grade metamorphism was not more than 100 Ma, in which case the Complex must have been emplaced close to 3.0 Ga ago. A whole-rock Rb-Sr isochron age of 2119 ± 198 Ma was also reported from Sittampundi (Bhaskar Rao et al., 1996), which is close to a Sm-Nd mineral (whole-rock-clinopyroxene-garnet) isochron age of 2013 ± 16 Ma (unlocated) of Snow et al. (1986), suggesting reequilibration of parts of the complex at least on a mineral scale by an unknown tectono-thermal event close to 2.0 Ga. Our present study provides the first high precision geochronological data on the SAC from zircons in the anorthosite. The LA-ICP-MS U-Pb data on zircons from the anorthositic rocks from the central part of SAC in our study show two distinct age populations with a broad spread and the majority of age data in the range of ca 2.45 Ga to 2.55 Ga (Fig. 9a). The combined data from these two populations define upper and lower intercept ages of 2496 ± 14 Ma and 999 ± 140 Ma on a concordia diagram (Fig. 9b). When the older and younger populations are plotted separately, the former (n= 14) yields a Concordia age of 2541 ± 13 Ma (Fig. 9c), which we consider as the best estimate of the magmatic age of SAC anorthosite. The bulk of the magmatic zircons were reset during a high-grade metamorphism at 2461 ± 15 Ma, reflected by the upper intercept of the younger population (n= 43) (Fig. 9d). These metamorphic zircons also show Pb loss during a Neoproterozoic event at 715 ± 180 Ma, defined by the lower intercept of younger population (Fig. 9d). The magmatic and metamorphic ages reported here for SAC are synchronous with the regional magmatism and metamorphism as reviewed below. Weighted mean 207Pb–206Pb ages of 2538 ± 6 Ma and 2529± 7 Ma were reported in a recent study by Clark et al. (2009) through SHRIMP

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Table 2 U-Pb data for the zircons of anorthosite sample (SA09-16) from the Sittampundi Anorthosite Complex. Spot SIT2-C1 SIT2-R1 SIT3-R1 SIT3-R2 SIT4-C1 SIT4-R1 (o) SIT4-R2 SIT5-C1 SIT5-R1 (o) SIT6-C1 (o) SIT6-C2 (o) SIT6-R1 SIT6-R2 SIT7-C1 SIT7-R2 SIT9-C1 SIT9-C2 SIT10-C1 SIT10-R1 SIT10-R2 SIT11-C1 SIT11-C2 SIT11-C3 SIT11-R1 SIT12-R1 (o) SIT12-R2 (o) SIT13-C1 SIT13-R1 SIT14-C1 (o) SIT15-R1 SIT15-R2 SIT16-C1 SIT17-C1 SIT17-R1 (o) SIT18-C1 (o) SIT18-C2 (o) SIT20-R1 SIT20-R2 SIT21-R1 SIT21-R2 SIT22-C1 SIT24-C1 SIT27-R1 SIT28-C1 SIT28-C2 SIT29-C1 SIT29-R1 SIT30-C1 SIT30-R1 SIT30-R2 SIT31-R1 (o) SIT31-R2 SIT39-R1 (o) SIT40-R1 (o) SIT41-C1 SIT41-C2 SIT41-C3 (o)

Th (ppm)

U (ppm)

Th/U

41 45 142 5 129 131 214 89 68 62 65 44 71 106 190 330 134 79 124 78 92 84 69 66 1 1 62 65 94 34 41 242 37 105 69 174 356 69 58 40 83 35 177 142 163 120 151 37 36 40 24 52 116 24 42 42 43

130 79 4 3 329 340 439 271 237 204 230 98 245 230 145 536 323 259 308 213 300 227 205 175 3 3 178 180 280 15 17 412 151 227 32 43 606 209 20 13 91 119 44 54 46 45 45 15 17 18 5 8 38 11 18 17 19

0.31 0.57 33.6 1.53 0.39 0.38 0.49 0.33 0.28 0.31 0.28 0.45 0.29 0.46 1.31 0.62 0.42 0.31 0.40 0.37 0.31 0.37 0.34 0.38 0.46 0.47 0.35 0.36 0.34 2.19 2.44 0.59 0.25 0.46 2.17 4.03 0.59 0.33 2.82 3.15 0.92 0.30 3.99 2.62 3.58 2.65 3.36 2.44 2.13 2.23 4.75 6.94 3.05 2.23 2.32 2.46 2.23

207 206

Pb/ Pb

0.1543 0.1616 0.1596 0.1615 0.1592 0.1649 0.1602 0.1601 0.1636 0.1673 0.1663 0.1616 0.1600 0.1563 0.1551 0.1542 0.1439 0.1574 0.1576 0.1544 0.1569 0.1555 0.1567 0.1567 0.1688 0.1657 0.1574 0.1583 0.1640 0.1506 0.1521 0.1557 0.1626 0.1678 0.1666 0.1657 0.1583 0.1617 0.1618 0.1542 0.1530 0.1575 0.1586 0.1609 0.1599 0.1527 0.1541 0.1579 0.1594 0.1585 0.1679 0.1594 0.1660 0.1729 0.1578 0.1562 0.1640

1σ

207 235

0.0010 0.0011 0.0011 0.0013 0.0007 0.0006 0.0006 0.0006 0.0007 0.0007 0.0007 0.0011 0.0007 0.0007 0.0008 0.0006 0.0007 0.0006 0.0007 0.0008 0.0008 0.0008 0.0008 0.0008 0.0014 0.0014 0.0013 0.0011 0.0008 0.0008 0.0007 0.0007 0.0057 0.0007 0.0005 0.0006 0.0005 0.0006 0.0007 0.0008 0.0010 0.0013 0.0005 0.0005 0.0006 0.0007 0.0006 0.0009 0.0008 0.0007 0.0013 0.0009 0.0005 0.0011 0.0007 0.0007 0.0006

Pb/ U

9.3121 10.4845 10.5159 10.3250 10.2668 10.9698 10.3384 10.2554 10.5139 11.5288 11.4471 10.4744 10.1850 9.4383 9.2763 8.7036 6.6973 10.3764 10.1646 9.9595 9.9470 9.2645 9.7289 9.7944 10.8933 11.0327 9.9851 10.4729 10.9275 7.8214 8.2619 9.8075 9.7887 11.3629 11.0764 10.6129 10.0227 10.8055 10.1764 8.5382 7.1550 10.7287 9.7820 10.3439 10.1453 8.0890 8.4479 9.8453 10.0634 10.3840 11.7234 9.5973 11.0969 11.6878 9.9011 9.3419 11.1336

1σ

206 238

0.2259 0.2284 0.3099 0.2174 0.1405 0.2109 0.1469 0.1680 0.2959 0.2476 0.1493 0.3351 0.1381 0.1851 0.1892 0.1729 0.1099 0.1778 0.1666 0.1619 0.1768 0.1645 0.2179 0.2979 0.2076 0.2100 0.2693 0.4192 0.2034 0.1467 0.1174 0.2060 0.1723 0.2736 0.1483 0.3656 0.1929 0.1864 0.2573 0.2085 0.2873 0.3983 0.1519 0.1277 0.2410 0.0971 0.1008 0.1979 0.2969 0.1831 0.1924 0.3083 0.1675 0.1826 0.1538 0.2316 0.1490

Pb/ U

0.4301 0.4678 0.4776 0.4762 0.4638 0.4809 0.4623 0.4595 0.4633 0.4823 0.4979 0.4528 0.4525 0.4298 0.4280 0.4105 0.3300 0.4710 0.4570 0.4582 0.4534 0.4252 0.4448 0.4511 0.4633 0.4713 0.4600 0.4661 0.4815 0.3815 0.3885 0.4424 0.4422 0.4794 0.4818 0.4657 0.4569 0.4792 0.4537 0.3988 0.3343 0.4878 0.4467 0.4662 0.4634 0.3823 0.3917 0.4494 0.4554 0.4750 0.4870 0.4500 0.4803 0.4911 0.4612 0.4413 0.4854

1σ

Rho

207 Pb/206 Pb (Ma)

1σ

207 Pb/235U (Ma)

1σ

206 Pb/238U (Ma)

1σ

Concordance %

0.0102 0.0100 0.0117 0.0068 0.0064 0.0088 0.0071 0.0071 0.0120 0.0122 0.0060 0.0158 0.0047 0.0082 0.0080 0.0072 0.0056 0.0077 0.0077 0.0078 0.0079 0.0081 0.0099 0.0129 0.0087 0.0088 0.0125 0.0203 0.0090 0.0052 0.0061 0.0104 0.0076 0.0109 0.0065 0.0150 0.0083 0.0075 0.0116 0.0099 0.0123 0.0198 0.0067 0.0053 0.0095 0.0051 0.0046 0.0080 0.0139 0.0073 0.0097 0.0112 0.0079 0.0077 0.0064 0.0091 0.0066

0.489 0.492 0.414 0.339 0.504 0.475 0.538 0.470 0.460 0.588 0.461 0.545 0.380 0.485 0.459 0.439 0.515 0.477 0.517 0.523 0.489 0.536 0.497 0.471 0.495 0.491 0.502 0.543 0.500 0.364 0.548 0.558 0.489 0.470 0.505 0.468 0.470 0.453 0.505 0.508 0.457 0.545 0.486 0.460 0.433 0.554 0.493 0.445 0.519 0.435 0.610 0.386 0.542 0.502 0.446 0.417 0.511

2394 2473 2451 2472 2447 2507 2458 2456 2493 2530 2521 2472 2455 2416 2403 2393 2275 2428 2431 2396 2423 2408 2421 2421 2546 2515 2428 2437 2498 2353 2370 2410 2483 2536 2524 2514 2438 2474 2475 2393 2379 2429 2441 2466 2455 2376 2392 2433 2449 2440 2537 2449 2518 2586 2432 2415 2497

11 11 12 14 8 6 6 7 7 7 7 12 7 7 9 7 8 6 7 8 9 9 9 9 14 14 14 12 8 9 8 7 59 7 6 6 5 6 7 9 11 14 6 5 6 7 6 10 8 7 13 10 6 10 8 8 6

2369 2479 2481 2464 2459 2521 2466 2458 2481 2567 2560 2478 2452 2382 2366 2307 2072 2469 2450 2431 2430 2365 2409 2416 2514 2526 2433 2478 2517 2211 2260 2417 2415 2553 2530 2490 2437 2507 2451 2290 2131 2500 2414 2466 2448 2241 2280 2420 2441 2470 2583 2397 2531 2580 2426 2372 2534

22 20 27 19 13 18 13 15 26 20 12 30 13 18 19 18 14 16 15 15 16 16 21 28 18 18 25 37 17 17 13 19 16 22 12 32 18 16 23 22 36 34 14 11 22 11 11 19 27 16 15 30 14 15 14 23 12

2306 2474 2517 2511 2456 2531 2450 2437 2454 2537 2605 2408 2406 2305 2297 2217 1839 2488 2426 2432 2410 2284 2372 2400 2454 2489 2440 2466 2534 2083 2116 2361 2360 2525 2535 2465 2426 2524 2412 2163 1859 2561 2381 2467 2455 2087 2131 2393 2419 2505 2557 2395 2528 2576 2445 2356 2551

46 44 51 30 28 38 31 31 53 53 26 70 21 37 36 33 27 34 34 34 35 37 44 57 39 39 55 89 39 24 28 46 34 47 28 66 37 33 51 46 59 86 30 23 42 24 21 36 62 32 42 50 34 33 28 41 29

96 100 103 102 100 101 100 99 98 100 103 97 98 95 96 93 81 102 100 101 99 95 98 99 96 99 100 101 101 89 89 98 95 100 100 98 100 102 97 90 78 105 98 100 100 88 89 98 99 103 101 98 100 100 101 98 102

(o) = older zircon population (see Fig. 9 for explanation). Concordance (%) = 100 * [206Pb/238U age / 206Pb/207Pb age].

U-Pb analysis of magmatic zircons from arc-related charnockites in the Salem Block, adjacent to the SAC. The oscillatory zoned cores of these zircons and their high Th/U were taken to indicate the crystallization age of the original magmatic protolith to the charnockite. Clark et al. (2009) also reported low Th, Th/U ratio and brightly luminescent overgrowths and complete zircon grains with a weighted mean 207Pb–206Pb ages of 2473 ± 8 Ma and 2482 ± 15 Ma which they correlated with the timing of post-crystallization high-grade metamorphism and partial melting of the magmatic rocks that crystallized at ~ 2530 Ma. The age data from the charnockites of Salem Block are consistent with a model involving accretionary processes on the margin of the Dharwar craton

within a convergent margin. In other recent work, Yellappa et al. (2012) reported petrological and geochemical data from mafic and ultramafic intrusives and zircon U-Pb SHRIMP geochronology of the associated felsic suite from a newly discovered dismembered ophiolitic complex from Devanur within the PCSZ. The rock types in the Devanur complex comprise pyroxenites, gabbros, anorthosites, actinolite-hornblendites, metabasite dykes, dolerites, pyroxene granulites, trondhjemites, pegmatites and thin layers of ferruginous chert. The major and trace element geochemistry of the mafic dykes indicate derivation from a basaltic-andesitic magma with tholeiitic to calcalkaline characteristics. The rocks display negative Nb anomalies with

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Fig. 8. MgO co-variation diagrams for immobile major and trace element data for SAC rocks, exhibiting magmatic fractionation trends.

Fig. 9. LA-ICPMS zircon U-Pb ages for anorthosite sample (SA09-16) from SAC.A: Probability density plot of all measured 207Pb/206Pb ages, showing bimodal populations of "older" and "younger" ages.B: U-Pb Concordia diagram for all ages (n= 57) measured showing upper and lower intercepts for discordia regression.C: U-Pb Concordia diagram for "older" ages (n= 14) measured showing calculated Concordia age.D: U-Pb Concordia diagram for "younger" ages (n = 43) measured showing upper and lower intercepts for discordia regression.

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setting. SHRIMP zircon U-Pb data from the trondhjemite of this complex yield 238U /206Pb ages of 2528 ± 61 and 2545± 56 Ma. In another study, Saitoh et al. (2011) reported SHRIMP zircon U-Pb ages from charnockite and garnet-bearing quartzo-feldspathic garnet gneiss from Kanja Malai, north of Sittampundi, which show 207 Pb/206Pb ages of 2536.1 ± 1.4 Ma and 2532.4± 3.7 Ma from magmatic zircons and 2477.6± 1.8 Ma and 2483.9 ± 2.5 Ma from metamorphic overgrowths. Syntectonic granites with zircon U-Pb ages of 2647 ± 11 Ma for emplacement and 2443 ± 20 Ma for metamorphism were also reported from this locality by Sato et al. (2011a). In previous studies, tonalites, amphibolites, mafic granulites and granites with U-Pb single-crystal zircon ages of 2.53-2.50 Ga, and granites with 2.54 Ga monazites have also been reported from across the PCSZ and adjacent regions (Ghosh, 2004). These ages are also close to those of ~2.5 Ga charnockites and enderbites in the Nilgiri Hills (Raith et al., 1999). A mafic granulite containing intergrown garnets, clinopyroxenes and orthopyroxenes in the Moyar shear zone has yielded a Sm-Nd garnetwhole-rock age of 2355 ± 22 Ma (Meißner et al., 2002). 6.6. Neoproterozoic overprint Fig. 10. εHf vs. 207Pb/206Pb ages for zircons from anorthosite sample (SA09-16) from the SAC.

The metamorphic zircons of SAC document Pb loss and/ or recrystallisation during a Neoproterozoic event at 715 ± 180 Ma, as reflected by the lower intercept of younger population (Fig. 9d). Several studies from the PCSZ and surrounding regions have reported mid-

enrichment of LILE (K, Rb, Ba, Th) and depletion in HFSE (Ti, Nb, Hf, Tb). The tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone Table 3 Lu-Hf isotope data for zircons in anorthosite sample (SA09-16) from the SAC. Sample (present-day ratios) Sample/ Spot

Age (Ma)

± 2σ

176

Hf/177Hf

SIT2-C1 SIT2-C2 SIT2-R1 SIT3-C1 SIT3-R1 SIT4-R1 (o) SIT4-C1 SIT5-C1 SIT5-R1 (o) SIT6-R1 SIT6-R2 SIT6-C1 (o) SIT6-C2 (o) SIT7-C1 SIT7-R1 SIT9-C1 SIT10-C1 SIT10-R1 SIT11-C1 SIT11-C3 SIT13-C1 SIT13-R1 SIT14-C1 (o) SIT15-R1 SIT15-R2 SIT16-C1 SIT17-R1 (o) SIT17-C1 SIT18-C1 (o) SIT18-C2 (o) SIT20-R1 SIT20-R2 SIT21-R1 SIT21-R2 SIT22-C1 SIT24-C1 SIT27-R1 SIT30-C1 SIT39-R1 (o)

2394 2394 2473 2472 2451 2507 2447 2456 2493 2472 2455 2530 2521 2416 2403 2393 2428 2431 2423 2421 2428 2437 2498 2353 2370 2410 2536 2483 2524 2514 2438 2474 2475 2393 2379 2429 2441 2433 2518

11 11 11 14 12 6 8 7 7 12 7 7 7 7 9 7 6 7 9 9 14 12 8 9 8 7 7 59 6 6 5 6 7 9 11 14 6 10 6

0.281355 0.281355 0.281364 0.281274 0.281249 0.281330 0.281344 0.281330 0.281323 0.281293 0.281285 0.281319 0.281341 0.281326 0.281317 0.281330 0.281309 0.281310 0.281331 0.281326 0.281342 0.281299 0.281359 0.281308 0.281283 0.281306 0.281297 0.281335 0.281348 0.281277 0.281368 0.281369 0.281307 0.281293 0.281354 0.281382 0.281399 0.281299 0.281367

Sample (initial ratios)

±2σ

176

Lu/177Hf

0.000036 0.000035 0.000046 0.000037 0.000038 0.000033 0.000034 0.000030 0.000027 0.000028 0.000033 0.000025 0.000028 0.000033 0.000028 0.000041 0.000032 0.000033 0.000025 0.000048 0.000043 0.000036 0.000045 0.000036 0.000034 0.000048 0.000035 0.000041 0.000039 0.000029 0.000041 0.000032 0.000031 0.000032 0.000038 0.000039 0.000032 0.000029 0.000041

0.001554 0.001301 0.001374 0.000034 0.000019 0.001888 0.001987 0.001411 0.001482 0.000568 0.001318 0.001279 0.001752 0.001014 0.001130 0.002046 0.001946 0.001915 0.001676 0.001532 0.002328 0.001888 0.002343 0.000809 0.000896 0.001480 0.000839 0.001055 0.001210 0.001236 0.002102 0.001835 0.000546 0.001074 0.001373 0.001735 0.002333 0.001355 0.002228

±2σ

176

Hf/177Hf(t)

0.000026 0.000024 0.000078 0.000001 0.000004 0.000056 0.000040 0.000070 0.000027 0.000067 0.000052 0.000016 0.000152 0.000023 0.000012 0.000043 0.000046 0.000050 0.000026 0.000019 0.000010 0.000008 0.000097 0.000016 0.000019 0.000041 0.000012 0.000010 0.000014 0.000048 0.000033 0.000032 0.000003 0.000027 0.000049 0.000035 0.000033 0.000056 0.000035

0.281284 0.281295 0.281299 0.281273 0.281248 0.281240 0.281251 0.281264 0.281252 0.281266 0.281223 0.281257 0.281257 0.281279 0.281265 0.281236 0.281219 0.281221 0.281254 0.281255 0.281234 0.281211 0.281247 0.281271 0.281242 0.281238 0.281256 0.281285 0.281290 0.281218 0.281270 0.281282 0.281281 0.281244 0.281291 0.281301 0.281290 0.281236 0.281260

εHf(t)

± 2σ

TDM (Ga)

1.2 1.6 3.6 2.7 1.3 2.3 1.3 2.0 2.4 2.4 0.5 3.5 3.2 1.6 0.8 -0.5 -0.3 -0.2 0.8 0.8 0.3 -0.4 2.3 -0.2 -0.8 0.0 3.6 3.4 4.5 1.7 1.8 3.0 3.0 -0.2 1.2 2.7 2.5 0.4 3.3

1.3 1.3 1.6 1.3 1.4 1.2 1.2 1.1 1.0 1.0 1.2 0.9 1.0 1.2 1.0 1.4 1.1 1.2 0.9 1.7 1.5 1.3 1.6 1.3 1.2 1.7 1.2 1.5 1.4 1.0 1.5 1.1 1.1 1.2 1.4 1.4 1.2 1.0 1.5

3.00 2.97 2.85 2.93 3.04 2.99 3.03 2.98 2.97 2.95 3.11 2.90 2.91 2.99 3.05 3.16 3.16 3.15 3.06 3.06 3.11 3.17 2.98 3.10 3.17 3.13 2.89 2.88 2.80 3.05 2.99 2.90 2.90 3.13 3.00 2.90 2.92 3.10 2.91

Notes: Age is the 207Pb/206Pb date from Table 2. Initial ratios calculated using the 176Lu decay constant (1.867x10- 11/yr) of Soderlund et al. (2004), and the CHUR values of 176Lu/ 177 Hf = 0.0336 and 176Hf/177Hf = 0.282785 of Bouvier et al. (2008). The depleted mantle model age (TDM) is calculated using 176Lu/177Hf = 0.0388 and 176Hf/177Hf = 0.28325 (Griffin et al., 2000; updated by Anderson et al., 2009) and a 176Lu/177Hf = 0.022 for mafic crust (Pietranik et al., 2008); (o) = older (magmatic) population of zircons.

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Neoproterozoic ages. Bhaskar Rao et al. (1996) obtained a Sm-Nd mineral isochron (whole-rock-plagioclase-hornblende-garnet) of 726 ± 9 Ma, which they interpreted as the age of metamorphism (ca. 11.8 kbar and 830 °C). Santosh et al. (2012) presented high precision SIMS zircon U-Pb data from a suprasubduction zone complex at Manamedu in the southern domain of the PCSZ with 206Pb/238U magmatic crystallization ages of 737 ±23 to 782 ± 24 Ma from plagiogranites and 744 ± 11 to 786±7.1 Ma from gabbros. Sato et al. (2011a) reported LA-ICPMS zircon U-Pb ages of 800 ± 14 Ma from the plagiogranites of Manamedu. Their study also reported a younger intercept age 759 ± 41 Ma from zircons in a quartzite and metamorphosed banded iron formation incorporated in the subduction complex. Teale et al. (2011) reported zircon LA-ICPMS 207Pb/206Pb ages of 825 ± 17 Ma from the Kadavur Dome, a gabbro-anorthosite complex in the northern part of Madurai Block, immediately south of the PCSZ. U-Pb ages of 843 ± 23 Ma from metamorphic rims on zircons from the surrounding quartzites reported by them closely compare with Sato et al. (2011b)'s data. In another study, Sato et al. (2011a) reported 206Pb/ 238U ages of 819 ± 26 Ma from arc-related rapakivi granites at Tangalamvaripatti within the southern domain of the PCSZ. All these data suggest a prominent mid-Cryogenian subduction system along the southern margin of the PCSZ (Santosh et al., 2012). All these data confirm the plate tectonic model for the Neoproterozoic history of southern India as proposed by Santosh et al. (2009), prior to the final collisional event in the Ediacaran-Cambrian associated with the assembly of the Gondwana supercontinent. Our data on the Neoarchean suprasubduction zone magmatism at Sittampundi, in conjunction with the recent ages on the Neoproterozoic-Cambrian record in PCSZ and adjacent regions support the proposal of two major episodes of subduction-accretion-collision tectonics in this part of southern India during two critical periods in Earth history – the ArcheanPaleoproterozoic boundary and the Precambrian-Cambrian boundary (Santosh et al., 2012). 7. Conclusions The salient conclusions arising from our geochemical, geochronological and Hf isotopic investigation on the Sittampundi Anorthosite Complex in southern India are as follows. 1. The geochemistry of the SAC rocks suggests that the parent magmas were probably derived from a depleted mantle source. Sub-arc mantle metasomatism and subsequent partial melting resulted in hydrous, aluminous basalt magma, which fractionated at depth to produce a variety of high-alumina basalt compositions, resulting in chromite- and amphibole-layered anorthosites formed within a suprasubduction zone arc magma chamber. 2. The effects of alteration, though evident, did not significantly affect the bulk trace elemental systematics. The geochemical and isotopic signatures do not indicate any significant role for crustal contamination of the juvenile magmas of the SAC, but small amounts of crustal recycling in the mantle source regions are possible. 3. U-Pb analyses of zircons by LA-ICP-MS suggest that the crystallization age of SAC is 2541 ± 13 Ma; the magmatic zircons were reset during high-grade metamorphism at 2461 ± 15 Ma. Some of these metamorphic zircons show further Pb loss during a Neoproterozoic event at 715 ± 180 Ma. The Neoarchean ages obtained in our study are synchronous with the arc-related magmatic suites of this region as well as the regional metamorphic events. 4. Hf isotope data for the zircons show positive εHft values and coherent initial 176Hf/ 177Hf ratios indicating their derivation from a depleted mantle source that produced a homogeneous juvenile magma. 5. The suprasubduction zone arc magmatism indicates an active convergent margin at southern part of the Dharwar Craton with important implications for Neoarchean crustal growth.

Acknowledgements We thank Elis Hoffmann, C.V. Dharma Rao and an anonymous referee from the Journal and Guest Editor Prof. Ali Polat for constructive comments which greatly improved an earlier version of this manuscript. This work was carried out when RM and MSN held DSTsponsored BOYSCAST fellowships at the Department of Earth Sciences, Memorial University. The authors thank the Director, NGRI for permission to publish this work. RM and MSN thank Dr. V. Balaram for his constant support. Geochronological and Hf isotopic work was supported by the funds from the NSERC Discovery Grant of PJS. References Anderson, T., Andersson, U.B., Graham, S., Aberg, G., Simonsen, S.L., 2009. Granitic magmatism by melting of juvenile continental crust: new constraints on the source of Paleoproterozoic granitoids in Fennoscandia from Hf isotopes in zircon. Journal of the Geological Society 166, 233–247. Ashwal, L.D., 1993. Anorthosites. Springer-Verlag, Berlin. Balaram, V., Rao, T.G., 2003. Rapid determination of REEs and other trace elements in geological samples by microwave acid digestion and ICP-MS. Atomic Spectroscopy 24, 206–212. 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. Bhaskar Rao, Y.J., Janardhan, A.S., Kumar, T.V., Narayana, B.L., Dayal, A.M., Taylor, P.N., Chetty, T.R.K., 2003. Sm-Nd model ages and Rb-Sr isotopic systematics of charnockites and gneisses across the Cauvery shear zone, southern India; implications for the Archaean-Neoproterozoic terrane boundary in the Southern Granulite Terrain. Memoir - Geological Society of India 50, 297–317. Bouvier, A., Vervoort, J.D., Patchett, P.J., 2008. The Lu-Hf and Sm-Nd isotopic composition of CHUR; constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth and Planetary Science Letters 273, 48–57. Chauvel, C., Marini, J.-C., Plank, T., Ludden, J.N., 2009. Hf-Nd input flux in the Izu-Mariana subduction zone and recycling of subducted material in the mantle. Geochemistry, Geophysics, Geosystems 10. 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. Chetty, T.R.K., Bhaskar Rao, Y.J., Narayana, B.L., 2003. A structural cross section along Krishnagiri-Palani Corridor, Southern Granulite Terrain of India. Memoir - Geological Society of India 50, 255–277. Chetty, T.R.K., Yellappa, T., Nagesh, P., Mohanty, D.P., Venkatasivappa, V., Santosh, M., Tsunogae, T., 2011. Structural anatomy of a dismembered ophiolite suite from Gondwana: The Manamedu complex, Cauvery suture zone, southern India. Journal of Asian Earth Sciences 42, 176–190. 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., Clark, C., Sajeev, K., Santosh, M., Kelsey, D.E., Hand, M., 2007. Passage through India: the Mozambique Ocean suture, high-pressure granulites and the Palghat-Cauvery shear zone system. Terr. Nova 19, 141–147. Collins, A.S., Clark, C., Chetty, T.R.K., Santosh, M., 2008. Ediacaran-Cambrian tectonic evolution of Southern India. Indian Journal of Geology 80, 23–40. Dharma Rao, C.V., Santosh, M., 2011. Continental arc magmatism in a Mesoproterozoic convergent margin: Petrological and geochemical constraints from the magmatic suite of Kondapalle along the eastern margin of the Indian plate. Tectonophysics 510, 151–171. Dharma Rao, C.V., Santosh, M., Sajeev, K., Windley, B.F., 2012. Chromite–silicate chemistry of the Neoarchean Sittampundi Complex, southern India: Implications for subduction-related arc magmatism. Precambrian Research. http://dx.doi.org/ 10.1016/j.precamres.2011.11.012. Drury, S.A., Harris, N.B.W., Holt, R.W., Reeves-Smith, G.J., Wightman, R.T., 1984. Precambrian tectonics and crustal evolution in South India. Journal of Geology 92, 3–20. Dutta, U., Bhui, U.K., Sengupta, P., Sanyal, S., Mukhopadhyay, D., 2011. Magmatic and metamorphic imprints in 2.9 Ga chromitites from the Sittampundi layered complex, Tamil Nadu, India. Ore Geology Reviews 40, 90–107. Dymek, R.F., Owens, B.E., 2001. Chemical assembly of Archean anorthosites from amphibolite- and granulite-facies terranes, SW Greenland. Contributions to Mineralogy and Petrology 141, 513–528. Eyuboglu, Y., Chung, S.-L., Santosh, M., Dudas, F.O., Akaryali, E., 2011. Transition from shoshonitic to adakitic magmatism in the eastern Pontides, NE Turkey; implications for slab window melting. Gondwana Research 19, 413–429. Foley, S.F., Barth, M.G., Jenner, G.A., 2000. Rutile/melt partition coefficients for trace elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas. Geochimica et Cosmochimica Acta 64, 933–938. Ghosh, J.G., 2004. Age and tectonic evolution of Neoproterozoic ductile shear zones in the Southern Granulite Terrain of India, with implications for Gondwana studies. Tectonics 23.

M.R. Mohan et al. / Gondwana Research 23 (2013) 539–557 Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., Van Achterbergh, E., O’Reilly, S.Y., Shee, S.R., 2000. The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysta in kimberlites. Geochimica et Cosmochimica Acta 64, 133–147. Harris, N.B.W., Santosh, M., Taylor, P.N., 1994. Crustal evolution in South India; constraints from Nd isotopes. Journal of Geology 102, 139–150. Hoffmann, J.E., Svahnberg, H., Piazolo, S., Scherstén, A., Münker, C., 2012. The geodynamic evolution of Mesoarchean anorthosite complexes inferred from the Naajat Kuuat Complex, southern West Greenland. Precambrian Research 196–197, 149–170. Jayananda, M., Moyen, J.F., Martin, H., Peucat, J.J., Auvray, B., Mahabaleswar, B., 2000. Late Archaean (2550-2520 Ma) juvenile magmatism in the Eastern Dharwar craton, southern India: constraints from geochronology, Nd-Sr isotopes and whole rock geochemistry. Precambrian Research 99, 225–254. Jayananda, A., Kano, T., Peucat, J.J., Channabasappa, S., 2008. 3.35 Ga komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-rock geochemistry. Precambrian Research 162, 160–179. Klemme, S., Blundy, J.D., Wood, B.J., 2002. Experimental constraints on major and trace element partitioning during partial melting of eclogite. Geochimica et Cosmochimica Acta 66, 3109–3123. Koshimoto, S., Tsunogae, T., Santosh, M., 2004. Sapphirine and corundum bearing ultrahigh temperature rocks from the Palghat-Cauvery Shear System, southern India. Journal of Mineralogical and Petrological Sciences 99, 298–310. Kosler, J., Fonneland, H., Sylvester, P., Tubrett, M., Pedersen, R.B., 2002. U-Pb dating of detrital zircons for sediment provenance studies - a comparison of laser ablation ICPMS and SIMS techniques. Chemical Geology 182, 605–618. Krishna, A.K., Murthy, N.N., Govil, P.K., 2007. Multielement analysis of soils by wavelength-dispersive X-ray fluorescence spectrometry. Atomic Spectroscopy 28, 202–214. Ludwig, K.R., 2008. User's manual for Isoplot, 3.6, A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Centre Special Publication. Markl, G., Frost, B.R., 1999. The origin of anorthosites and related rocks from the Lofoten Islands, northern Norway: II. Calculation of parental liquid compositions for anorthosites. Journal of Petrology 40, 61–77. Meißner, B., Deters, P., Srikantappa, C., Köhler, H., 2002. Geochronological evolution of the Moyar, Bhavani and Palghat shear zones of southern India: implications for east Gondwana correlations. Precambrian Research 114, 149–175. Naganjaneyulu, K., Santosh, M., 2010. The Central India Tectonic Zone: A geophysical perspective on continental amalgamation along a Mesoproterozoic suture. Gondwana Research 18, 547–564. Naganjaneyulu, K., Santosh, M., 2011. Crustal architecture beneath Madurai Block, southern India deduced from magnetotelluric studies: Implications for subduction-accretion tectonics associated with Gondwana assembly. Journal of Asian Earth Sciences 40, 132–143. Pearce, J.A., Peate, D.W., 1995. Tectonic implications of the composition of volcanic arc magmas. Annual Review of Earth and Planetary Sciences 23, 251–285. Piercey, S.J., Mortensen, J.K., Murphy, D.C., Paradis, S., Creaser, R.A., 2002. Geochemistry and tectonic significance of alkalic mafic magmatism in the Yukon-Tanana terrane, Finlayson Lake region, Yukon. Canadian Journal of Earth Sciences 39, 1729–1744. Pietranik, A.B., Hawkesworth, C.J., Storey, C.D., Kemp, A.I.S., Sircombe, K.N., Whitehouse, M.J., Bleeker, W., 2008. Episodic, mafic crust formation from 4.5 to 2.8 Ga: New evidence from detrital zircons, Slave craton, Canada. Geology 36, 875–878. Polat, A., Hofmann, A.W., 2003. Alteration and geochemical patterns in the 3.7-3.8 Ga Isua greenstone belt, West Greenland. Precambrian Research 126, 197–218. Polat, A., Kerrich, R., 2000. Archean greenstone belt magmatism and the continental growth-mantle evolution connection: constraints from Th-U-Nb-LREE systematics of the 2.7 Ga Wawa subprovince, Superior Province, Canada. Earth and Planetary Science Letters 175, 41–54. Polat, A., Appel, P.W.U., Fryer, B., Windley, B., Frei, R., Samson, I.M., Huang, H., 2009. Trace element systematics of the Neoarchean Fiskenæsset anorthosite complex and associated meta-volcanic rocks, SW Greenland: Evidence for a magmatic arc origin. Precambrian Research 175, 87–115. Polat, A., Frei, R., Scherstén, A., Appel, P.W.U., 2010. New age (ca. 2970Ma), mantle source composition and geodynamic constraints on the Archean Fiskenæsset anorthosite complex, SW Greenland. Chemical Geology 277, 1–20. Polat, A., Fryer, B.J., Appel, P.W.U., Kalvig, P., Kerrich, R., Dilek, Y., Yang, Z., 2011. Geochemistry of anorthositic differentiated sills in the Archean ( 2970Ma) Fiskenæsset Complex, SW Greenland: Implications for parental magma compositions, geodynamic setting, and secular heat flow in arcs. Lithos 123, 50–72. Raharimahefa, T., Kusky, T.M., 2009. Structural and remote sensing analysis of the Betsimisaraka Suture in northeastern Madagascar. Gondwana Research 15, 14–27. Raith, M.M., Srikantappa, C., Buhl, D., Koehler, H., 1999. The Nilgiri enderbites, South India: nature and age constraints on protolith formation, high-grade metamorphism and cooling history. Precambrian Research 98, 129–150. Ramadurai, S., Sankaran, M., Selvan, T.A., Windley, B.F., 1975. The stratigraphy and structure of the Sittampundi complex, Tamilnadu, India. Journal of the Geological Society of India 16, 409–414. Rollinson, H., Reid, C., Windley, B., 2010. Chromitites from the Fiskenaesset anorthositic complex, West Greenland: clues to late Archaean mantle processes. Geological Society, London, Special Publications 338, 197–212.

557

Saitoh, Y., Tsunogae, T., Santosh, M., Chetty, T.R.K., Horie, K., 2011. Neoarchean highpressure metamorphism from the northern margin of the Palghat-Cauvery Suture Zone, southern India: Petrology and zircon SHRIMP geochronology. Journal of Asian Earth Sciences 42, 268–285. Sajeev, K., Windley, B.F., Connolly, J.A.D., Kon, Y., 2009. Retrogressed eclogite (20 kbar, 1020 degrees C) from the Neoproterozoic Palghat-Cauvery suture zone, southern India. Precambrian Research 171, 23–36. Santosh, M., 2012. India's Proterozoic legacy. Geological Society of London Special Publications 365, 259–284. Santosh, M., Maruyama, S., Sato, K., 2009. Anatomy of a Cambrian suture in Gondwana: Pacific-type orogeny in southern India? Gondwana Research 16, 321–341. Santosh, M., Tsunogae, T., Shimizu, H., Dubessy, J., 2010. Fluid characteristics of retrogressed eclogites and mafic granulites from the Cambrian Gondwana suture zone in southern India. Contributions to Mineralogy and Petrology 159, 349–369. Santosh, M., Xiao, W.J., Tsunogae, T., Chetty, T.R.K., Yellappa, T., 2012. The Neoproterozoic subduction complex in southern India: SIMS zircon U–Pb ages and implications for Gondwana assembly. Precambrian Research 192–195, 190–208. Sato, K., Santosh, M., Tsunogae, T., Chetty, T.R.K., Hirata, T., 2011a. Laser ablation ICP mass spectrometry for zircon U-Pb geochronology of metamorphosed granite from the Salem Block: Implication for Neoarchean crustal evolution in southern India. Journal of Mineralogical and Petrological Sciences 106, 1–12. Sato, K., Santosh, M., Tsunogae, T., Chetty, T.R.K., Hirata, T., 2011b. Subduction-accretioncollision history along the Gondwana suture in southern India: A laser ablation ICPMS study of zircon chronology. Journal of Asian Earth Sciences 40, 162–171. Shimpo, M., Tsunogae, T., Santosh, M., 2006. First report of garnet-corundum rocks from southern India; implications for prograde high-pressure (eclogite-facies?) metamorphism. Earth and Planetary Science Letters 242, 111–129. Shirey, S.B., Hanson, G.N., 1986. Mantle heterogeneity and crustal recycling in Archean granite-greenstone belts: evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada. Geochimica et Cosmochimica Acta 50, 2631–2651. Snow, J., Basu, A.R., Tatsumoto, M., 1986. The Sittampundi Complex, S. India; Nd isotopic chronology, lower crustal granulites. Eos, Transactions, American Geophysical Union 67, 400. Soderlund, U., Patchett, J.P., Vervoort, J.D., Isachsen, C.E., 2004. The Lu-176 decay constant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafic intrusions. Earth and Planetary Science Letters 219, 311–324. Stern, C.R., 2011. Subduction erosion: Rates, mechanisms, and its role in arc magmatism and the evolution of the continental crust and mantle. Gondwana Research 20, 284–308. Subramaniam, A.P., 1956. Mineralogy and petrology of the Sittamoundi complex, Salem district, Madras State, India. Geological Society of America Bulletin 67, 317–389. Sun, S.-S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the ocean basins: Geological Society Special Publication, pp. 313–345. Teale, W., Collins, A.S., Foden, J., Payne, J.L., Plavsa, D., Chetty, T.R.K., Santosh, M., Fanning, M., 2011. Cryogenian (similar to 830 Ma) mafic magmatism and metamorphism in the northern Madurai Block, southern India: A magmatic link between Sri Lanka and Madagascar? Journal of Asian Earth Sciences 42, 223–233. Weaver, B.L., Tarney, J., Windley, B., 1981. Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, southern West Greenland; nature of the parent magma. Geochimica et Cosmochimica Acta 45, 711–725. Windley, B.F., 1973. Archaean anorthosites; a review, with the Fiskenaesset Complex, West Greenland as a model for interpretation. Special Publication - Geological Society of South Africa 3, 319–332. Windley, B.F., 1995. The Evoloving Continents. John Wiley and Sons, Chichester, United Kingdom. Windley, B.F., Garde, A.A., 2009. Arc-generated blocks with crustal sections in the North Atlantic craton of West Greenland: Crustal growth in the Archean with modern analogues. Earth-Science Reviews 93, 1–30. Windley, B.F., Smith, J.V., 1974. The Fiskenaesset Complex, west Greenland; Part II; General mineral chemistry from Qeqertarssuatsiaq. Nyt Nordisk Forlag, Copenhagen, Denmark. Windley, B.F., Selvan, T.A., 1975. Anorthosites and associated rocks of Tamil Nadu, southern India. Journal of the Geological Society of India 16, 209–215. Windley, B.F., Herd, R.K., Bowden, A.A., 1973. The Fiskenaesset Complex, West Greenland, Part 1: A preliminary study of the stratigraphy, petrology and whole rock chemistry near Qeqertarssuatsiaq. Bulletin Geological Survey of Greenland 80. Yellappa, T., Chetty, T.R.K., Tsunogae, T., Santosh, M., 2010. The Manamedu Complex: Geochemical constraints on Neoproterozoic suprasubduction zone ophiolite formation within the Gondwana suture in southern India. Journal of Geodynamics 50, 268–285. Yellappa, T., Santosh, M., Chetty, T.R.K., Kwon, S., Park, C., Nagesh, P., Mohanty, D.P., Venkatasivappa, V., 2012. A Neoarchean dismembered ophiolite complex from southern India: Geochemical and geochronological constraints on its suprasubduction origin. Gondwana Research 21, 246–265.