Shonkinites from Salem, Southern India: Implications for Cryogenian Alkaline Magmatism in Rift-Related Setting ⇑ Xiao-Fang He A, M

Journal of Asian Earth Sciences 113 (2015) 812–825

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Journal of Asian Earth Sciences

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Shonkinites from Salem, southern India: Implications for Cryogenian alkaline magmatism in rift-related setting ⇑ Xiao-Fang He a, M. Santosh a, , Ze-Ming Zhang b, Toshiaki Tsunogae c, T.R.K. Chetty d, M. Ram Mohan d, S. Anbazhagan e a School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, China b Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China c Graduate School of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba, Ibaraki 305-8572, Japan d National Geophysical Research Institute, Hyderabad 500 007, Andhra Pradesh, India e Department of Geology, Periyar University, Salem, Tamil Nadu, India article info abstract

Article history: Alkaline and potassic igneous rocks, although minor components of the continental crust, provide impor- Received 15 April 2015 tant constraints on magma processes and tectonic settings. Here we report petrology, mineral chemistry, Received in revised form 17 June 2015 whole rock geochemistry, zircon U–Pb and Lu–Hf data on shonkinites from the Salem Block in the Accepted 3 July 2015 Southern Granulite Terrane of India. The shonkinite is composed mainly of porphyritic clinopyroxene, Available online 3 July 2015 K-feldspar and olivine, and intrudes into the surrounding ultramaﬁc rocks composed of wehrlite (clinopyroxene and olivine) and dunite (mostly olivine with minor spinel). The geochemical data on Keywords: shonkinites from Salem show alkaline and ultrapotassic features with marked enrichment in incompat- Petrology ible elements, and high K O and K O/Na O ratios suggesting magma derivation from metasomatized Zircon U–Pb geochronology and Lu–Hf 2 2 2 isotopes lithospheric mantle. Their HFSE depletion relative to LILE compositions, coupled with positive Nb anoma- Rift tectonics lies suggests continental rift setting. The zircon grains in shonkinite show typical magmatic crystalliza- Shonkinite tion features and their LA-ICP-MS U–Pb data yield weighted mean age of 818 ± 6.3 Ma (MSWD = 0.32).

Southern Granulite Terrane The zircon eHf values range between 11.1 and 12.7, consistent with magma derivation from an

enriched mantle, with depleted mantle model ages (TDM) in the range of between 2406 and 2513 Ma, indicating the involvement of Neoarchean–Paleoproterozoic subduction-related components. Together with similar-aged alkaline plutons with comparable geochemical features, we envisage a major alkaline magmatic event associated with rifting during Cryogenian in the northern part of the Southern Granulite Terrane. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction settings of continental rift systems or subduction zone magmatic suites in convergent margins (Fitton and Upton, 1987). Volatile-rich, ultramaﬁc to maﬁc alkaline and potassic igneous The Southern Granulite Terrane (SGT) in India witnessed sub- rocks are minor constituents of the continental crust. However, duction of oceanic lithosphere associated with the closure of the their petrogenesis has been the focus of several studies in under- Mozambique Ocean during Neoproterozoic in relation to the tec- standing mantle dynamics and crust-mantle interaction since the tonics associated with the global assembly of the Gondwana super- primitive melts from which these rocks crystallized are often continent (Collins et al., 2007; Santosh et al., 2009). While strongly enriched in many incompatible elements and therefore convergent margin tectonics operated along the Palghat-Cauvery many workers have correlated their genesis with enriched/metaso- Suture Zone with a southward subduction polarity, the slab pull matized continental mantle (e.g. Bailey, 1987; Fraser et al., 1985; force also generated an aborted rift to the north extending roughly Hawkesworth et al., 1985; Menzies et al., 1987). Alkaline rocks E–W and swinging NE in the eastern domain (Santosh et al., 2014). occur in different tectonic environments including intraplate This aborted rift is garlanded with a series of alkaline plutons including carbonatites, pyroxenites, alkali syenites and granites, among other rock types (Renjith et al., 2014; Santosh et al., 2009, ⇑ Corresponding author. 2014). In the domain of this rift at the southern periphery of the E-mail address: [email protected] (M. Santosh). http://dx.doi.org/10.1016/j.jseaes.2015.07.002 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved. X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825 813

Fig. 1. Generalized geological map of the Southern Granulite Terrane in India showing the major alkaline plutons and the study area. After Santosh et al. (2014, 2013), Collins et al. (2014), and Plavsa et al. (2012).

Salem Block (Fig. 1) shonkinite in association with other ultramafic of crustal blocks carrying protoliths ranging in age from rocks have been reported (Reddy et al., 1995). Eoarchean to latest Neoproterozoic (Santosh et al., 2009, 2015; In this study, we investigated the shonkinites and associated Collins et al., 2014). The crustal blocks are dissected by major ultramafic rocks in terms of petrology, geochemistry and zircon lithospheric-scale transpressional zones (Chetty and Bhaskar Rao, U–Pb geochronology and Lu–Hf isotopes. The data are used to eval- 2006), many of which are identified as the traces of oceanic sutures uate the petrogenesis of the alkaline rocks and their implications (Collins et al., 2007) ranging in age from Neoarchean to latest on the Cryogenian plate tectonic processes in the region. Neoproterozoic-Cambrian (Santosh et al., 2009). The Salem Block represents one of the major blocks in the 2. Geological background northern domain of the SGT, immediately to the south of the Archean Dharwar Craton. The basement of the block is considered The Southern Granulite Terrane (SGT) at the southernmost to have formed during Neoarchean and was metamorphosed dur- domain of Peninsular India (Fig. 1) is characterized by a collage ing early Paleoproterozoic at P–T conditions of 14–16 kbar, and 814 X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825

850 °C) (Anderson et al., 2012; Clark et al., 2009). The major rock distilled HF + HNO3 in Teflon screw-cap capsules at 200 °C for types in the block are orthogneisses, metasedimentary rocks (mig- 5 days, dried, and then digested with HNO3 at 150 °C for 1 day. matites), charnockites and granitoids, and stretches from the The final step was repeated once. Dissolved samples were diluted southern boundary of the Dharwar Craton to the Palghat-Cauvery to 49 ml with 1% HNO3 and 1 ml 500 ppb indium was added to Suture Zone (PCSZ) (Santosh et al., 2009; Clark et al., 2009; the solution as an internal standard. Trace element abundances Plavsa et al., 2012). The younger intrusions within this block were determined by inductively coupled plasma mass spectrome- include the Cryogenian (ca. 820–800 Ma) alkaline complexes and try (ICP-MS) using a Finnigan MAT Element spectrometer, which the Ediacaran (ca. 559 Ma) Sankari–Tiruchengode granitoid has analytical uncertainties within 5% for most elements. complex. The alkaline shonkinitic rocks and ultramafic association occur 3.3. Zircon U–Pb and Lu–Hf analysis northwest of Salem town (Long. 78°050–78°110 and Lat. 11°400–1 1°460), close to the E–W trending aborted rift zone along which Zircon grains were separated by gravimetric and magnetic sep- several alkaline intrusive suites of similar ages have been identified aration from crushed rock samples, and then purified by hand pick- (e.g., Santosh et al., 2014). The outcrop pattern of the alkaline and ing under a binocular microscope at the Yu’neng Geological and ultramafic intrusions in Salem show discordant relationship with Mineral Separation Survey Centre, Langfang City, Hebei Province, the structural features of the country rocks (Fig. 2). The ultramafics China. The grains were mounted in epoxy resin discs and polished are represented by dunite and occur as elongated intrusions trend- to reveal mid-sections, followed by gold sputter coating. Zircons ing ENE–WSW. Although variably serpentinized, fresh and unal- were imaged under both transmitted and reflected light, and were tered rocks are also present. These rocks do not carry chromite imaged using cathodoluminescence (CL) to identify internal struc- and are therefore different from the ultramafic rocks in layered tures and choose potential target sites for U–Pb analyses. The CL complexes. Rare titanoclinohumite occurs as secondary veins in imaging was carried out at the Beijing Geoanalysis Centre. the serpentinized dunites (Reddy et al., 1988). The dunites are Individual grains were mounted along with the standard extensively veined by secondary magnesite and there are several TEMORA 1, with 206Pb/238U age of 417 Ma (Black et al., 2003), onto mines in the Salem area which exploit this ore. double-sided adhesive tape and enclosed in epoxy resin discs. The The alkaline rocks occur as plugs and dykes within and also discs were polished to a mid depth and gold coated for cathodolu- along the margins of the ultramafics (Fig. 2). Based on the mode minescence (CL) imaging and U–Pb isotope analysis. Zircon mor- of occurrence, Reddy et al. (1995) grouped them into (1) major phology, inner structure and texture were examined by using a and minor plug-like intrusives, (2) minor dykes and (3) lensoid JSM-6510 Scanning Electron Microscope (SEM) equipped with a bodies occurring along the periphery of the dunite bodies. The backscatter probe and Chroma CL probe. The zircon grains were alkaline rocks show intrusive relation with the ultramafic rocks also examined under transmitted and reflected light images using and carry xenoliths of dunite. Preferred orientation of pyroxene a petrological microscope. and feldspar crystals and textural variation from coarse to Zircon U–Pb dating and trace elements analysis were simulta- medium-fine grained from the center toward the periphery of neously conducted by LA-ICP-MS at the State Key Laboratory of the intrusions have also been noticed (Fig. 3). Geological Processes and Mineral Resources, China University of In this study, we collected four representative samples of the Geosciences, Wuhan. Detailed operating conditions for the laser shonkinites and surrounding ultramafic rocks (wehrlite and ablation system and the ICP-MS instrument are as described by dunite) and the representative samples were used for petrology, Liu et al. (2008, 2010). Laser sampling was conducted using an mineral chemistry, bulk geochemistry, and zircon U–Pb and Excimer 193 nm GeoLas 2005 System with a spot size of 32 lm. Lu–Hf analysis. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. Nitrogen was introduced into the central gas flow (ArtHe) of the Ar plasma in the LA-ICP-MS analysis, which 3. Analytical methods increases the sensitivity for most elements by a factor of 2 compared with the results without adding nitrogen (Hu et al., 2008). To 3.1. Mineral chemistry keep time-dependent elemental fractionation at a low level, a laser frequency of 4 Hz and a laser energy of 60 mJ were applied. The Chemical analyses of all minerals were carried out using an uncertainty of analysis is within 1% for the zircon standard. electron microprobe analyzer (JEOL JXA8530F) at the Chemical Zircon 91500 was used as external standard for U–Pb dating, Analysis Division of the Research Facility Center for Science and and was analyzed twice every five analyses. Time-dependent drifts Technology, the University of Tsukuba. The analyses were per- of U–Th–Pb isotopic ratios were corrected using a linear interpola- formed under conditions of 15 kV accelerating voltage and 10 nÅ tion (with time) for every five analyses according to the variations sample current, and the data were regressed using an oxide-ZAF of 91500 (i.e. two zircon 91500 + five samples + two zircon 91500). correction program supplied by JEOL. Below we summarize the Preferred U–Th–Pb isotopic ratios used for 91500 are from salient results from mineral chemistry data from the analyzed Wiedenbeck et al. (1995). Uncertainty in the preferred values for rocks. Representative compositions of minerals are given in the external standard 91500 was propagated to the ultimate Table 1.Fe3+ of olivine, clinopyroxene, orthopyroxene, and spinel results for the samples. Concordia diagrams and weighted mean was calculated based on stoichiometry. calculations were made using Isoplot 4.15 (Ludwig, 2003). Trace element compositions of zircons were calibrated against 3.2. Geochemistry multiple-reference materials (BCR-2G and BIR-1G) combined with internal standardization (Liu et al., 2010). The preferred values of Major and trace elements were analyzed in National Research element concentrations for the USGS reference glasses used are Centre of Geoanalyses, Beijing (China). For major element analyses, from the GeoReM database (http://georem.mpch-mainz.gwdg.de/). mixtures of whole rock powder (0.5 g) and Li2B4O7 + LiBO2 (5 g) In situ zircon Hf isotopic analyses were conducted on the same were made into glass discs and analyzed by X-ray fluorescence spots or in the adjacent domains where U–Pb dating was done. The spectroscopy (XRF) with an AXIOS Minerals spectrometer. The ana- analytical procedures followed those described by Yuan et al. lytical uncertainties were generally within 0.1–1% (RSD). For trace (2008). The energy density of 15–20 J/cm2 and a spot size of element analyses, whole rock powders (40 mg) were dissolved in 45 lm were used. The flattest, most stable portions of the signal X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825 815

Fig. 2. (a) The tectonic frame work of Mettur-Dharmapuri shear zone, at the contact between the northern frontal fold thrust belt of the SGT and the Dharwar craton. (b) Detailed geological map of the study area showing the alkaline association in Salem Block and the sample localities (modiﬁed after Reddy et al., 1995). 816 X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825

Fig. 3. Representative field photographs. (a) Exposure of shonkinite in Salem. (b) and (c) Coarse-grained shonkinite showing porphyritic pyroxene and plagioclase. (c) Fine- grained shonkinite. were selected for analysis. Adjustment for the isobaric interference 4.1.2. Wehrlite of 176Yb on 176Hf was performed in ‘real time’ as advocated by The rock is composed of clinopyroxene (50–60%) and olivine Woodhead et al. (2004), which involved measuring the (40–50%) (Fig. 4c). Clinopyroxene occurs as medium-grained sub- interference-free 172Yb and 173Yb during the analysis, calculating hedral mineral ranging in size from 0.8 to 1.5 mm. It also occurs mean bYb values from 172Yb and 173Yb and using the 176Yb/172Yb as coarse-grained (3.6 mm) phenocrysts in the matrix of ratio of 0.5886 (Chu et al., 2002). Zircon 91500 was used as the ref- medium-grained clinopyroxene and olivine. Olivine (0.2–2.6 mm) erence standard with a 176Hf/177Hf ratio of 0.282306 ± 10 is rounded and occurs as aggregates forming olivine-rich domains (Woodhead et al., 2004). All the Lu–Hf isotope analysis results in the rock. Rare K-feldspar (0.16 mm) occurs along the grain were reported with an error of 1r. The decay constant of 176Lu of boundary of clinopyroxene and olivine. 1.865 1011 year1 was adopted (Scherer et al., 2001). Initial 176Hf/177Hf ratios Hf(t) were calculated with reference to the chon- 4.1.3. Dunite dritic reservoir (CHUR) of Blichert-Toft and Albarède (1997) at the The rock comprises olivine (>99%) and accessory spinel (<1%) time of zircon growth from the magma. Single-stage Hf model age (Fig. 4d). The olivine contains fine-grained inclusions of both fluids

(TDM) was calculated with respect to the depleted mantle with and carbonate minerals. The olivine is heavily cracked suggesting present-day 176Lu/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 deformation after crystallization. Spinel is dark reddish in color C (Griffin et al., 2000). Two-stage Hf model age (TDM) was calculated and occurs as fine-grained mineral along the grain boundary of with respect to the average continental crust with a 176Lu/177Hf olivine. ratio of 0.015 (Griffin et al., 2002). 4.2. Mineral chemistry

4. Results 4.2.1. Olivine Olivine in the shonkinite from Salem (sample SLMS 3) shows Fo 2+ 4.1. Petrography (Mg/(Fe + Mg) = XMg) values ranging from Fo0.76 to Fo0.77 with NiO contents from 0.19 to 0.21 wt.%, which is nearly consistent 4.1.1. Shonkinite with the composition of the olivine in wehrlite from this locality

Shonkinite from Salem is a dark-grayish and coarse-grained (Fo0.70–0.72, NiO = 0.30–0.39 wt.%) (Fig. 5a). In contrast, the olivine rock with typical magmatic texture. The rock is composed mainly in dunite is more forsteritic as Fo90–91 with NiO content between of clinopyroxene (35–45%), K-feldspar (40–50%), and olivine (10– 0.17% and 0.19%. There is no significant difference in composition 20%) (Fig. 4a). Pale greenish clinopyroxene occurs as very between core and rim of the minerals. coarse-grained euhedral to subhedral mineral (often as phenocrysts) ranging in size from 0.7 to 4.3 mm. The mineral contains 4.2.2. Clinopyroxene fine-grained (<0.1 mm) inclusions of olivine, biotite, and apatite, Clinopyroxene in shonkinite is augite in composition particularly in the core domain (Fig. 4b). Olivine (0.3–1.0 mm) also (XMg = 0.84–0.88, Ca = 0.77–0.80) and is characterized by high occurs as rounded subhedral mineral around clinopyroxene. acmite component of 8.2 to 12.3 mol.% (Fig. 5b) due to the high K-feldspar (0.4–2.8 mm) is anhedral and fills the matrix of clinopy- Fe3+/(Fe2+ +Fe3) ratio of 0.35–0.52. Core of the porphyritic clinopyroxene and olivine, suggesting that it crystallized at the last stage. roxene is slightly enriched in Si and depleted in Al (1.91–1.92 and Olivine contains internal cracks which were probably formed by 0.13–0.14 pfu, respectively) than that of rim (1.87–1.89 and 0.17– deformation after the crystallization. Wavy extinction of 0.18 pfu). Clinopyroxene in the wehrlite is slightly Fe-rich as

K-feldspar also supports the deformation event. XMg = 0.81–0.85. Similar compositional zoning of decreasing Si Table 1 Representative electron microprobe analyses of minerals in shonkinite (SK), wehrlite (WH), and dunite (DN). Sample number: SK: SLMS 3, WH: MS32-11, DN: MD32-10A.

Mineral Olivine Clinopyroxene Orthopyroxene Biotite K-feldspar Spinel Sample No. SLMS 3 SLMS 3 SLMS 3 MD 32-11 MD 32-11 MD3 2-10A MD3 2-10A SLMS 3 SLMS 3 MD 32-11 MD 32-11 SLMS 3 SLMS 3 MD32-11 SLMS 3 MD 32-11 MD 32-10A Rock type SK SK SK WH WH DN DN SK SK WH WH SK SK WH SK WH DN Remarks Core Rim In Cpx Core Rim Core Rim Core Rim Core Rim In Cpx In Cpx

SiO2 39.21 38.81 38.47 38.05 37.86 40.95 40.72 52.63 51.26 52.62 50.88 55.11 39.16 38.05 64.17 64.66 0.05 Al2O3 0.01 0.00 0.00 0.03 0.00 0.02 0.04 3.00 4.12 2.31 4.30 1.52 14.20 14.26 18.84 18.34 7.42 812–825 (2015) 113 Sciences Earth Asian of Journal / al. et He X.-F. TiO2 0.02 0.02 0.00 0.00 0.00 0.03 0.00 0.41 0.63 0.25 0.57 0.03 2.56 2.22 0.05 0.06 0.43 Cr2O3 0.00 0.01 0.00 0.00 0.00 0.03 0.00 0.30 0.40 0.41 0.27 0.05 0.30 0.29 0.00 0.03 48.70 Fe2O3 0.00 0.99 0.91 0.99 0.90 0.64 0.13 3.00 2.96 1.22 3.83 1.15 13.15 FeO 21.96 21.18 22.13 25.24 25.68 9.38 9.41 4.52 5.01 5.80 5.46 12.38 5.18 14.22 0.09 0.15 26.38 MnO 0.26 0.30 0.25 0.30 0.38 0.17 0.19 0.18 0.19 0.20 0.22 0.27 0.00 0.12 0.00 0.04 0.70 NiO 0.21 0.19 0.18 0.13 0.10 0.29 0.18 0.00 0.00 0.02 0.01 0.00 0.13 0.06 0.05 0.00 0.01 MgO 39.88 40.15 39.17 36.86 36.30 49.65 49.24 15.46 15.05 15.00 14.71 29.24 22.25 16.31 0.00 0.03 4.12 CaO 0.00 0.03 0.04 0.00 0.04 0.00 0.00 20.46 19.45 19.89 18.32 0.73 0.11 0.18 0.25 0.06 0.00 Na2O 0.00 0.01 0.00 0.02 0.00 0.00 0.00 1.05 1.07 1.07 1.27 0.05 0.30 0.08 2.08 0.89 0.02 K2O 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.01 9.88 8.78 13.42 14.96 0.02 ZnO 0.16 0.01 0.00 0.00 0.06 0.05 0.01 0.00 0.00 0.00 0.11 0.02 0.00 0.00 0.03 0.00 0.54 Total 101.72 101.69 101.17 101.62 101.35 101.21 99.92 101.01 100.14 98.79 99.95 100.56 94.05 94.55 98.99 99.21 101.52 Si 1.001 0.990 0.991 0.990 0.991 0.993 0.998 1.916 1.885 1.958 1.879 1.953 5.672 5.688 2.973 2.998 0.002 Al 0.000 0.000 0.000 0.001 0.000 0.000 0.001 0.129 0.178 0.101 0.187 0.063 2.423 2.511 1.028 1.002 0.302 Ti 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.011 0.017 0.007 0.016 0.001 0.278 0.249 0.002 0.002 0.011 Cr 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.009 0.012 0.012 0.008 0.001 0.034 0.035 0.000 0.001 1.331 3+ Fe 0.000 0.019 0.018 0.019 0.018 0.012 0.002 0.082 0.082 0.034 0.106 0.031 0.342 Fe2+ 0.469 0.452 0.477 0.549 0.562 0.190 0.193 0.138 0.154 0.180 0.169 0.367 0.627 1.778 0.003 0.006 0.763 Mn 0.006 0.007 0.006 0.007 0.008 0.004 0.004 0.006 0.006 0.006 0.007 0.008 0.000 0.015 0.000 0.001 0.020 Ni 0.004 0.004 0.004 0.003 0.002 0.006 0.004 0.000 0.000 0.000 0.000 0.000 0.015 0.007 0.002 0.000 0.000 Mg 1.516 1.526 1.504 1.429 1.416 1.793 1.798 0.838 0.824 0.831 0.809 1.544 4.800 3.632 0.000 0.002 0.212 Ca 0.000 0.001 0.001 0.000 0.001 0.000 0.000 0.798 0.766 0.793 0.725 0.028 0.016 0.028 0.013 0.003 0.000 Na 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.074 0.076 0.077 0.091 0.003 0.083 0.022 0.187 0.080 0.001 K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 1.825 1.674 0.793 0.885 0.001 Zn 0.003 0.000 0.000 0.000 0.001 0.001 0.000 0.000 0.000 0.000 0.003 0.001 0.000 0.000 0.001 0.000 0.014 Total 2.999 3.000 3.000 3.000 3.000 3.000 3.000 4.000 4.000 4.000 4.000 4.000 15.775 15.638 5.001 4.980 3.000 Mg/(Fe + Mg) 0.76 0.77 0.76 0.72 0.72 0.90 0.90 0.86 0.84 0.82 0.83 0.81 0.88 0.67 0.22 K/(Ca + Na + K) 0.80 0.91

Abbreviations: K: shonkinite; WH: wehrlite; DN: dunite. 817 818 X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825

Fig. 4. Photomicrograph of shonkinite and related rocks from Salem. (a) Coarse-grained subhedral to subhedral clinopyroxene in shonkinite (sample SLMS 3). Rounded olivine coexists with the clinopyroxene. K-feldspar occurs in the matrix of the ferro-magnesian minerals. (b) Fine-grained inclusions of olivine and biotite within clinopyroxene in shonkinite. (c) Euhedral clinopyroxene in the matrix of subhedral clinopyroxene and olivine in wehrlite (sample MD32-11). (d) Euhedral to subhedral olivine and associated ﬁne-grained spinel in dunite (sample MD32-10A).

Fig. 5. Compositional diagram showing Fo and NiO content of olivine (a); a compositional diagram showing XMg and acmite content of clinopyroxene (b).

(1.93–1.96 to 1.88–1.89 pfu) and increasing Al (0.10–0.11 to 0.18– although its TiO2 content (2.2–2.3 wt.%) is similar to that in 0.19 pfu) from core to rim is also observed in clinopyroxene in the shonkinite. wehrlite. 4.2.5. Feldspars

4.2.3. Spinel K-feldspar in wehrlite (Or91–92) is close to the end-member Spinels in the Salem dunite are Fe rich (XMg = 0.20 and 0.22) and composition of orthoclase, whereas that in shonkinite contains characterized by high Cr# (=Cr/(Al + Cr)) of 0.81–0.84 as chromian up to 30 mol.% albite component as Or69–73. spinel. Their Fe3+/(Fe2++Fe3) ratio is 0.31–0.33. ZnO content for the spinel is low, 0.40–0.66 wt.%. Table 1 shows the relationship 4.3. Major and trace elements between Fo content of olivine and Cr# in coexisting spinel in the dunite, suggesting that the compositions are close to the ﬁeld of 4.3.1. Shonkinites subduction-related peridotite. In order to characterize the bulk composition of the shonkinites from Salem, we analyzed two samples in this study (Table 2). The 4.2.4. Biotite data from these samples are integrated with those reported from Biotite within clinopyroxene in shonkinite is characterized by the same locality in a previous study (Reddy et al., 1995). The older high XMg (0.88–0.89) and moderate TiO2 content (2.5–2.6 wt.%). The data base has limitations on analytical methods and therefore we biotite inclusion in wehrlite is also magnesian (XMg = 0.67–0.68), rely more on the compositional trends for our interpretations. X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825 819

Anomalies of HFSE relative to neighboring REE are given as Nb/Nb⁄, Salem are indicative of mantle involvement in the derivation of ⁄ ⁄ ⁄ Zr/Zr , Hf/Hf and Ti/Ti . Mg# is calculated as Mg/(Mg + Fetotal). these rocks. Their chondrite normalized REE plots (Sun and The shonkinites of Salem show SiO2 contents in the range of 51.29 McDonough, 1989) show fractionated trend in the absence of any and 56.50 wt.%, K2O>3.0wt.%(Fig. 6a). Compared to the shonkinites conspicuous Eu anomaly (Fig. 7a), suggesting that there has been from some other localities such as Svidnya and Elchuru, the Salem no fractionation of plagioclase. A similar trend has also been shonkinites have the lowest Al2O3 (5.77–6.74 wt.%) and highest observed in earlier studies (Reddy et al., 1995). The total REE con- CaO (9.74–13.03 wt.%) (Fig. 6b). Their MgO is high in the range of tents are generally high (56.4–110.2, Table 2) suggesting REE

13.40 and 16.90 wt.%, and the K2O/Na2O values are >2 (Fig. 6cand enrichment in the alkaline magma. The primitive mantle normal- Table 2). All these features are in compliance with the ultrapotassic ized multi element diagram (Sun and McDonough, 1989) shows nature of these rocks and their distinction from lamproites and other distinct negative troughs of HFSE (high ﬁeld strength elements) alkaline rocks (Foley et al., 1987). such as Nb, Zr and Ti, similar to the trace element systematics The positive correlation between MgO and Cr (Fig. 6d), higher reported from the Salem shonkinites earlier (Fig. 7b). The HFSE Mg#, elevated Ni and Cr compositions of the shonkinites from depletions are considered to be a common feature of arc magma, those which interacted with continental crust, or from which Table 2 Ti-rich phases crystallized (Foley et al., 2000). Whole-rock chemistry of representative shonkinites.

Rock type Shonkinite Shonkinite 4.4. Zircon geochronology Sample No. SLMS 3 SLMS 6B

SiO2 53.50 51.29 We selected a representative sample of shonkinite from Salem TiO 0.20 0.32 2 (sample SLMS-3) for zircon geochronology using the LA-ICP-MS Al2O3 5.77 6.74 FeO 4.99 7.42 method. The analytical results are presented in Table 3 and repre-

Fe2O3 1.37 1.46 sentative zircon CL images are shown in Fig. 8. The age data are MnO 0.14 0.16 plotted in concordia diagrams (Fig. 9a) and these, together with MgO 13.40 16.90 the age data histograms are shown in Fig. 9b. REE data on the zir- CaO 13.03 9.74 con grains are listed in Table 4 and plotted in Fig. 9c. Na2O 1.05 1.18

K2O 3.23 3.25 Most zircon grains from sample SLMS-3 are brownish, transpar- P2O5 0.07 0.09 ent to translucent, and show ellipsoidal or rounded morphology + H2O 0.54 0.28 with length varying from 200 to 500 lm and a length to width LOI 0.85 0.05 Total 98.14 98.88 ratio of 2:1–5:1 (Fig. 8). The grains display clear banded type of K + Na 4.28 4.43 magmatic crystallization. A total of 16 spots were analyzed. K+Na Ca 8.75 5.31 Excluding 1 spot (spot number 1 which is highly discordant and K/Na 3.08 2.75 considered to be a statistical outlier), 15 of the sixteen spots yield Mg# 79.34 77.53 a weighted mean age of 818 ± 6.3 Ma (MSWD = 0.32). A/CNK 0.33 0.48 A/NK 1.35 1.52 In terms of zircon trace elements, the Th contents show a range

TFe2O3 6.91 9.70 of 21–214 ppm and U contents range from 57 to 216 ppm, with TFeO 6.22 8.73 Th/U ratios in a range of 0.3 to 1.0, mostly around 0.5. In the ⁄ ⁄ Fe /(Fe + Mg) 0.32 0.34 chondrite-normalized REE diagram (Fig. 9c), the zircon data show Sc 45.3 36.5 V 116.0 123.0 obvious positive Ce anomalies, slightly negative or no Eu anomalies Co 45.3 65.8 and predominant enrichment in HREEs, which suggest dominantly Ni 215.0 392.0 magmatic origin of the analyzed zircons. Cr 663.0 1096.0 Cu 2.4 13.6 Zn 47.6 74.8 4.5. Hf isotopes Ga 9.6 9.2 Rb 2.2 107.0 A total of 7 Lu–Hf isotopic analyses were performed on the zircon Sr 252.0 624.0 grains, representing all of the age populations identified from differ- Ba 168.0 2263.0 La 21.6 6.3 ent grains, on domains from where the U–Pb ages were obtained 176 177 Ce 45.3 17.7 (Table 5; Fig. 10). The initial Hf/ Hf isotope ratios of zircons in Pr 5.4 2.8 the Salem shonkinites are tightly clustered between 0.281900 and Nd 22.3 14.4 0.281951, with calculated eHf units at corresponding crystallization Sm 4.6 3.8 Eu 1.3 1.2 ages ranging between 11.1 and 12.7 (Table 5). In the eHf(t) vs. age Gd 3.9 3.9 plot (Fig. 10), the distribution is essentially confined between the Tb 0.5 0.5 CHUR and 2.5 Ga mantle evolution line. This is consistent with Dy 2.5 2.5 derivation of the Salem shonkinites from an enriched mantle, where Ho 0.4 0.5 the evolved signature might represent interaction of the mantle Er 1.1 1.4 Tm 0.1 0.2 with inputs from the older recycled crust. Depleted mantle model 176 177 Yb 0.9 1.1 ages (TDM) for the zircons (assuming a Lu/ Hf = 0.0322 for mafic Lu 0.1 0.2 crust; Chauvel et al. (2009) range between 2398 and 2513 Ma, fur- Y 11.1 11.9 ther confirming the older crustal inputs. Zr 11.9 23.9 Hf 0.6 0.9 Nb 1.6 0.3 5. Discussion Ta 0.1 0.1 Pb 3.4 3.6 Sn 0.3 0.7 5.1. Petrogenesis and tectonic setting Th 0.5 0.6 U 0.2 0.1 ‘Ultrapotassic’ and ‘highly potassic’ are terms used to describe RREE 110.2 56.4 rocks which have high contents of K2O and other incompatible 820 X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825

Fig. 6. Major elemental compositions of the Salem shonkinites to demonstrate the alkaline nature and the distinction of these rocks from lamproites and previous studies of shonkinite from different areas.

elements together with a high K2O/Na2O ratio (Foley et al., 1987). Some of these rocks also possess other features such as high Mg-number (100 Mg/(Mg + Fe), Ni and Cr which are characteristic of relatively primitive basaltic magmas (Cross, 1897; Holmes and Harwood, 1932; Jaques et al., 1984; Smith and Skinner, 1984; Mitchell, 1985; Bergman, 1986). The salient geochemical features of these rocks include high content of alkalis and especially of K,

high K/Na ratio, enrichment of Ba and Sr, depletion of Al2O3, among other characteristics (Vladykin et al., 2001). Their magma evolution

trends include the decrease of MgO, CaO, Cr2O3, NiO content, K/Na, increase of the agpaitic coefficient Ka, and those of Th, U, Zr and Mg# at the last stages. These rocks also display enrichment in REE, with the light REE prevailing over the heavy REE, with a low negative anomaly and similar REE patterns (Vladykin et al., 2001). Alkaline rocks have been reported from different tectonic settings including continental rift systems and subduction zone magmatic suites in convergent margins (Fitton and Upton, 1987). Rift-related (Taylor et al., 1980; Payette and Martin, 1987; Fitton, 1987), and subduction-related process (Pearce, 1983; Halliday et al., 1987; Druecker and Gay, 1987) have different geochemical signatures (Wilson, 1989). However, complex settings such as a collisional suture when overprinted by rifting lead to difficulties in identifying the precise tectonic framework. Such scenario is typical for ultrapotassic rocks including shonkinites, and therefore the role of subducted slab in the genesis of these rocks remain debated (e.g., Nelson et al., 1986; Thompson et al., 1990; Foley and Peccerillo, 1992). Examples for alkaline rocks in rift-related include those reported by Lustrinio et al. (2005), Attoh et al. (2007) and Heaman et al. (2007). From Peninsular India, Biswal et al. (2004) and Upadhyay et al. (2006) reported rift-related alkaline rocks from Khariar and Elchuru complexes, respectively. A similar model was earlier presented for the Elagiri complex by Miyazaki et al. (2003). In a recent study, Santosh et al. (2014) also identified Fig. 7. Chondrite normalized REE diagram to demonstrate the REE enrichment in rift-related settings for alkali granites and syenites of Cryogenian alkaline rocks. Normalisation values are from Sun and McDonough (1989). age from the SGT. X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825 821

Models on the petrogenesis of alkaline rocks invoke different sources for the magmas, including metasomatized mantle sources enriched in LILE and LREE (Lynch et al., 1993), or low degree partial melting of asthenospheric mantle followed by crystal fractionation (%) (Fitton, 1987). Alkali basaltic magmas (Brown and Becker, 1986), melting of lower crustal rocks with volatile enriched mantle mag- Th mas (Lubala et al., 1994), and partial melting of a phlogopite or 232

Pb/ amphibole bearing lithospheric mantle (Bonin, 2004) are alternate

208 models. The interaction between an asthenosphere-derived melt and the overlying lithospheric mantle has also been proposed

U (Menzies, 1987). 238 The occurrence of shonkinites in alkaline complexes as in the Pb/

206 present study from Salem can be compared with similar associations reported from elsewhere including the Elchuru alkaline com-

U plex in the Prakasam igneous province in Peninsular India

235 (Upadhyay et al., 2006), the Kola peninsula alkaline province in Pb/ Russia (Petrovskii et al., 2014), the Palaborwa complex in South 207 Africa (Russell et al., 1954), the alkaline-ultramaﬁc, potassic and ) Concordance r carbonatitic association in West Greenland (Larsen and Rex, Pb 1992), and the Svidnya magmatic potassic-alkaline association in 206

Pb/ the Aldan Shied in Russia (Vladykin et al., 2001), among various 207 Age (Ma ± 1 other examples. The shonkinite associations in various parts of the world have several common geochemical and petrogenetic features. The Elchuru shonkinite for which a rift-related model was proposed

Th shows major and trace elements coupled with Sr, Nd and Pb iso- 232 topic data suggesting multi-stage magmatic history. The parent Pb/

208 magma was generated from sub-continental lithospheric mantle enriched in the LILEs, HFSEs and underwent fractionation of amphibole and clinopyroxene (Upadhyay et al., 2006). In Kola alkaline province, the potassic peridotite–shonkinite magmatic series were derived from sub-lithospheric mantle sources (Larsen and Rex, 1992). In contrast, the shonkinites from West Greenland are characterized by enrichment in volatiles and incompatible ele-

U Rho ments such as Ba, Rb, K, REE, P, Zr and Nb, consistent with deriva- 238 tion through small degrees of melting of an enriched mantle Pb/ source. They correlated to subduction-related setting, particularly 206 due to their occurrence along former continental collision zone, away from rift settings. Wyman and Kerrich (1989) described Late Archean shoshonitic lamprophyre dykes from the Superior

U Province in Canada which are related to tectonic processes along

235 terrane boundaries during accretion. Pb/ The geochemical features of shonkinites from Salem are charac- 207 terized by alkaline and ultrapotassic composition according to the classiﬁcation of Foley et al. (1987). The extreme enrichment in

incompatible elements, and the high K2O and K2O/Na2O ratios of these rocks suggest an enriched source, generally considered to Pb

206 reﬂect mantle metasomatism as mentioned in the examples above. ) r Pb/ The negative Nb and other HFSEs in the primitive 207 Isotope ratios (±1 mantle-normalized extended variation diagram for the shonkinites of Salem also indicates that older subduction-derived components were involved in the evolution of these rocks (Fig. 7). They also display HFSE depletions relative to LILE compositions, which coupled with their positive Nb anomalies suggest continental rift setting. U Th/U

238 The zircon Hf isotopic signature with eHf values showing a wide range from 11.1 and 12.7 are also in accordance with melts gen-

Th erated from an enriched mantle source (Hollanda et al., 2006).

232 Similar features together with ultrapotassic chemistry and enrichment in LREE and LILE in Cryogenian alkaline plutons from the SGT

T have been considered to reﬂect ﬂuid-related metasomatized man- Pb concentration(ppm) tle source (Santosh et al., 2014). Shoshonitic and ultrapotassic rocks derived from the lower crust that was underplated by magma derived from an enriched lithospheric mantle have also been shown to possess these features (Lu et al., 2013). The distribution of the alkaline plutons and pyroxenites along a SLMS-3-02SLMS-3-03SLMS-3-04SLMS-3-05 9.96 36.1SLMS-3-06 16.09SLMS-3-07 21.1 16.64 154SLMS-3-08 22.02 44.7SLMS-3-09 63.3 16.23 82.7SLMS-3-10 206 16.10 73.5SLMS-3-11 98.2 0.333 51.7SLMS-3-12 94.2 9.17 133 0.752 20.64 0.455 53.8SLMS-3-13 0.0676 15.31 0.878SLMS-3-14 97.4 24.4 0.0638 0.0681 24.15 0.0029 76.7 0.552SLMS-3-15 94.2 0.0658 40.0 0.531 57.5SLMS-3-16 0.0017 0.0023 57.0 1.2521 16.79 124 0.0610 76.5 0.570 0.0024 0.0673 12.81 214 1.2096 92.0 1.2898 0.429 0.0535 0.0022 149 0.0666 13.03 57.4 0.621 1.2185 0.0022 0.0332 0.625 37.3 0.0429 0.0693 0.1351 216 1.1503 0.0022 102 0.0654 39.5 0.0436 0.513 1.2785 0.1369 0.0675 0.1373 0.0025 0.0017 79.1 0.0384 1.2792 0.0019 0.991 0.1340 0.0676 0.0410 0.561 81.8 0.0014 0.0022 0.0015 1.3085 0.3013 0.472 0.1369 0.0396 1.2100 0.0688 0.0015 0.0021 0.1373 0.0664 0.483 0.3669 1.2379 0.3374 0.0493 0.0416 0.0705 0.0015 0.1396 0.0335 0.0017 0.3082 1.2399 0.0015 0.0022 0.0681 0.0408 0.0389 0.0419 0.1362 0.0019 0.0026 0.3325 0.0015 0.1350 1.2862 0.0395 0.0389 0.3399 1.2224 0.0024 0.0009 0.1335 0.0013 0.0016 1.2931 0.0415 0.3475 857 0.0017 0.0334 0.0009 0.1332 0.0411 0.0436 1.2233 0.0017 0.3020 744 0.0453 0.0011 872 0.0418 91 0.4507 0.1356 1200 0.0016 0.0012 0.1331 0.0428 0.4015 0.0397 57 0.1339 71 0.0012 824 639 0.0398 0.0018 76 0.3823 0.0018 0.1303 850 0.0397 0.0014 805 0.0018 841 24 81 833 0.0010 0.4969 809 0.0394 0.3773 0.0015 63 0.0010 15 0.3917 19 909 817 777 69 0.0400 20 0.0010 0.0404 787 0.3375 836 827 854 0.0393 829 75 10 18 836 0.0009 811 0.0011 66 0.0404 857 18 67 0.0011 849 825 8 827 9 18 805 0.0012 900 8 65 830 820 818 22 36 808 944 830 842 9 15 52 783 819 75 872 8 18 823 99 17 76 26 822 8 817 840 18 18 811 73 814 808 97 843 98 9 22 827 10 15 99 806 20 811 23 10 20 786 93 22 789 820 806 20 9 99 787 810 27 99 19 10 10 790 782 19 10 96 98 792 801 20 98 9 780 18 21 98 800 22 97 99 24 96 97 Sample spots Element Table 3 LA-ICP-MS age data of zircons from shonkinite of Salem. major cratonic lineament in southern India had prompted earlier 822 X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825

isotopic data from this study suggest that the source magmas involved components derived from the melting of an older sub- ducting slab, possibly related to the Neoarchean convergent tectonics identified in this region. Similar recurrent alkaline magmatic events ranging in age between Archean and Mesozoic-Tertiary have been identified in west Greenland (Larsen and Rex, 1992). The shonkinites and lamproites of west Greenland that show subduction-related signatures such as negative Nb and other HFSE depletions (Larsen and Rex, 1992) are similar to that of Salem shonkinites which occur along former collisional suture. The rift-related alkaline rocks such as those of Elchuru (Madhavan et al., 1992; Upadhyay et al., 2006) possess Fig. 8. Representative Cathodoluminescence (CL) images of zircon grains from the trace elemental signatures that are distinct from Salem shonk- shonkinite (sample SLMS 3). Zircon U–Pb ages (Ma) are also shown by Ma. inites, including the markedly negative anomalies of HFSEs (Fig. 7b and Table 2). Such anomalies could also result from high workers to propose an enriched mantle source and a continental melt-mineral partition coefficient, leading to the depletion of these rift environment (Madhavan et al., 1992; Saravanan and elements in the melt during igneous fractionation processes (Foley Ramasamy, 1995). A genetic link has also been proposed between et al., 2000; Hoffmann et al., 2012; Klemme et al., 2002). Although the dunites and the alkaline rocks of Salem (Murthy, 1982), with a positive correlation between the Nb and Ti compositions within a partial melting of an enriched mantle or clinopyroxene- melt system might suggest fractionation of the Nb–Ti bearing min- phlogopite rich source (Foley, 1992). Reddy et al. (1995) noted that erals such as magnetite, ilmenite, and rutile, the negative correla- the elevated REE compositions with fractionated trends, enrich- tion between the Nb anomalies and TiO2 for the Salem ment of incompatible elements, high Mg#, Cr and Ni contents shonkinites excludes this possibility. The eHf anomalies (as low as and Sr initial ratios of the Salem shonkinites correspond to an 12.7) and the older Hf model ages are suggestive of the involve- enriched mantle source, followed by crustal contamination during ment of older crustal components (Fig. 10 and Table 5), possibly magma ascent. those generated during the Neoarchean–Paleoproterozoic subduc- Although relatively minor in volume, the shonkinites and asso- tion event. Melts derived at high P–T conditions from the ca. 2.5 Ga ciated alkaline rocks in the SGT show a clear disposition along old delaminated/eclogitised slab must have enriched the mantle, crustal scale shear/suture zones. The geochemical and zircon Hf which subsequently acted as the source for the alkaline rocks.

Fig. 9. U–Pb concordia plots, mean age data histograms for shonkinite from Salem block (SLMS 3) and chondrite-normalized zircon REE distribution patterns. X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825 823

Table 4 REE data on zircon grains from shonkinite of Salem.

Sample spots La (ppm) Ce (ppm) Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Tb (ppm) Dy (ppm) Ho (ppm) Er (ppm) Tm (ppm) Yb (ppm) Lu (ppm) SLMS-3-02 0.022 15.1 0.046 1.09 1.95 1.20 10.1 3.42 45.5 19.4 99.2 24.2 262 59.7 SLMS-3-03 0.013 88.5 0.096 2.00 5.31 3.78 41.4 14.5 184 77.8 376 83.1 800 163 SLMS-3-04 0.0025 44.7 0.039 1.00 2.52 1.85 18.4 7.27 96.4 41.7 208 47.2 473 98.3 SLMS-3-05 0.12 21.2 0.28 4.11 5.09 3.01 22.9 6.61 73.7 27.8 130 28.7 294 64.2 SLMS-3-06 0.046 40.3 0.044 0.49 2.25 1.76 19.5 6.81 92.6 40.5 206 48.0 486 103 SLMS-3-07 0.016 32.8 0.028 0.72 2.85 1.83 18.6 6.29 84.7 36.5 186 42.3 434 93.7 SLMS-3-08 0.0086 35.4 0.065 1.24 2.50 1.98 20.5 7.00 92.5 40.2 202 46.0 475 99.4 SLMS-3-09 0.022 17.6 0.032 0.67 1.32 0.99 11.3 3.80 51.4 21.9 113 27.3 286 62.5 SLMS-3-10 0.0090 45.7 0.015 1.44 3.34 1.99 22.8 8.16 105 45.9 230 52.1 529 110 SLMS-3-11 0.11 35.6 0.085 1.28 2.67 1.98 21.3 7.24 99.6 42.4 211 47.6 483 103 SLMS-3-12 0.023 38.6 0.064 0.55 1.94 1.76 20.7 7.11 93.5 42.3 213 49.8 514 105 SLMS-3-13 0.11 115 0.73 11.1 16.8 10.0 90.1 27.3 312 116 529 110 1047 204 SLMS-3-14 0.0000 37.8 0.047 0.80 2.87 1.91 20.8 7.21 95.5 41.1 208 47.8 491 104 SLMS-3-15 0.0000 22.1 0.042 0.55 1.72 1.47 15.0 5.06 64.5 27.8 143 34.0 355 78.0 SLMS-3-16 0.0000 27.5 0.021 0.81 2.00 1.35 14.5 5.25 70.1 30.9 159 36.7 386 80.8

Table 5 LA-ICP-MS Lu–Hf isotope data of zircons from the shonkinite of Salem.

176 177 176 177 176 177 C No. Age (Ma) Lu/ Hf Hf/ Hf 2s Hf/ Hfi eHf(0) eHf(t) TDM (Ma) TDM (Ma) fLu/Hf SLMS-3-04 829 0.001919 0.28193 0.000012 0.281900 29.8 12.6 1906 2508 0.94 SLMS-3-05 811 0.001272 0.281971 0.00001 0.281952 28.3 11.1 1816 2406 0.96 SLMS-3-06 827 0.002011 0.28194 0.000009 0.281909 29.4 12.3 1896 2490 0.94 SLMS-3-07 830 0.002066 0.281961 0.000011 0.281929 28.7 11.5 1869 2444 0.94 SLMS-3-08 842 0.00219 0.28195 0.000011 0.281915 29.1 11.7 1891 2465 0.93 SLMS-3-09 823 0.001839 0.281928 0.00001 0.281900 29.8 12.7 1905 2513 0.94 SLMS-3-10 817 0.0023 0.281955 0.000012 0.281920 28.9 12.1 1890 2472 0.93

Chandrasekaran, 1994), the Elagiri alkali syenite-pyroxenite complex (Miyazaki et al., 2003) and the alkali syenite of Angadimogar and ultrapotassic granite of Peralimala (Santosh et al., 2014), among other suites, and those precisely dated from these suites all show broadly Cryogenian ages. Models on the Neoproterozoic tectonics in the SGT proposed southward subduction of the Mozambique Ocean lithosphere associated with the assembly of the Gondwana supercontinent (e.g., Collins et al., 2014; Santosh et al., 2009, 2012; Yellappa et al., 2010). Santosh et al. (2014) reported ca. 780–800 Ma U–Pb ages from zircons in the alkaline rocks in Angadimogar and Peralimala and proposed that these rocks formed in an aborted rift. The Rb-Sr isotopic ages of 808 Ma given in Reddy et al. (1995) and the 770 Ma in Kumar and Gopalan (1991) from Sevattur are also consistent with the zircon U–Pb ages reported from Salem Fig. 10. Zircon Hf isotopic evolution diagram for shonkinite from Salem block shonkinite in our study. Renjith et al. (unpublished data) obtained (SLMS 3). CHUR, chondritic uniform reservoir. Depleted mantle evolution is ca. 832 Ma age from zircons in the Sundamalai peralkaline pluton calculated by using eHf(t) = 16.9 at t = 0 Ma and eHf(t) = 6.4 at t = 3.0 Ga. The corresponding lines of crustal extraction are calculated by using the 176Lu/177Hf emplaced in the eastern extension region of the belt. The 770– ratio of 0.015 for the average continental crust (Griffin et al., 2002). 832 Ma ages from alkaline plutons and carbonatites in the region confirm that the broadly Cryogenian alkaline magmatism occurred in a rift-related setting along a major transcrustal zone in the SGT. The reactivated paleo-suture served as the weak zone for magma ascent and emplacement. The hydrous metasomatism during 6. Conclusions magma ascent might explain the incompatible element enrichment. There is also evidence for extensive CO2 metasomatism in The shonkinite from Salem is characterized by alkaline and the form of magnesite veins that traverse the ultramafic rocks, ultrapotassic features with marked enrichment in incompatible which is consistent with the finding of Murthy (1982). elements, and high K2O and K2O/Na2O ratios suggesting magma derivation from metasomatized lithospheric mantle. Their HFSE depletion relative to LILE compositions, coupled 5.2. Emplacement age and regional geodynamics with positive Nb anomalies are correlated to magma tectonics in continental rift setting. In addition to the shonkinites investigated in this study, several Zircon U–Pb data indicate crystallization age of 818 ± 6.3 Ma for alkaline complexes occur in the northern part of the SGT including the shonkinite. The age is comparable with those of the other the Sevattur alkaline-carbonatite complex (Kumar et al., 1998), the alkaline plutons of various compositional varieties reported Pakkanadu carbonatite complex (Navaneethakrishnan and from the same belt. 824 X.-F. He et al. / Journal of Asian Earth Sciences 113 (2015) 812–825

The zircon eHf values between 11.1 and 12.7 suggest magma Foley, S.F., Barth, M.G., Jenner, G.A., 2000. Rutile/melt partition coefficients for trace derivation from an enriched mantle. The depleted mantle model elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas. Geochim. Cosmochim. Acta 64, ages (TDM) are in the range of between 2406 and 2513 Ma 933–938. suggesting melting of Neoarchean–Paleoproterozoic subduction- Fraser, K.J., Hawkesworth, C.J., Erlank, A.J., Mitchell, R.H., Scott Smith, B.H., 1985. Sr, related components. Nd and Pb isotope and minor element geochemistry of lamproites and kimberlites. Earth Planet. Sci. Lett. 76, 57–70. The Cryogenian alkaline magmatism occurred along an aborted Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterbergh, E., O’Reilly, rift in the northern part of the Southern Granulite Terrane. S.Y., Shee, S.R., 2000. 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