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

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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 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes 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 À11.1 and À12.7, 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.
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