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Chapter 20

Provenance, timing of sedimentation and of metasedimentary suites from the Southern Granulite Terrane,

PULAK SENGUPTA1*, MICHAEL M. RAITH2, ELLEN KOOIJMAN3,4, MOUMITA TALUKDAR1, PRIYADARSHI CHOWDHURY1, SANJOY SANYAL1, KLAUS MEZGER5 & DHRUBA MUKHOPADHYAY6 1Department of Geological Sciences, Jadavpur University, Kolkata-700032, India 2Steinmann Institut, Universita¨t Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany 3Institut fu¨r Mineralogie, Universita¨tMu¨nster, Corrensstrasse 24, 48145 Mu¨nster, Germany 4Department of Geosciences, Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden 5Institut fu¨r Geologie, Universita¨t Bern, Baltzerstrasse 1 þ 3, 3012 Bern, Switzerland 6Raman Centre for Applied and Interdisciplinary Sciences, 16A Jheel Road, Kolkata 700075, India *Corresponding author (e-mail: [email protected])

Abstract: The Southern Granulite Terrane of India exposes remnants of an interbanded sequence of orthoquartzite–metapelite– calcareous rocks across the enigmatic Palghat–Cauvery Shear Zone (PCSZ), which has been interpreted as a Pan-African terrane bound- ary representing the eastward extension of the Betsimisaraka Suture Zone of . Zircon U–Pb geochronology of metasedimen- tary rocks from both sides of the PCSZ shows that the precursor sediments of these rocks were sourced from the Dharwar and the adjoining parts of the Indian shield. The similarity of the and the vestiges of Grenvillian-age orogenesis in some metasedi- mentary rocks contradict an interpretation that the PCSZ is a Pan-African terrane boundary. The lithological association and the likely basin formation age of the metasedimentary rocks of the Southern Granulite Terrane show remarkable similarity to the rock assemblage and timing of sedimentation of the Palaeoproterozoic to Neoproterozoic shallow-marine deposits of the Purana basins lying several hundred kilometres north of this terrane. Integrating the existing geological information, it is postulated that the shallow-marine sedi- ments were deposited on a unified land-mass consisting of a large part of Madagascar and the Indian shield that existed before Neopro- terozoic time, part of which was later involved in the Pan-African orogeny.

Supplementary material: Details of the zircon U–Pb LA-MC-ICP-MS analyses of samples are available at http://www.geolsoc.org. uk/SUP18793.

The Proterozoic sedimentary sequences consisting of , amalgamated, is a case in point. Based on contrasting sedimentary and that cover a vast expanse of the crystalline sources of detrital zircon populations in a suite of high-grade meta- basement of peninsular India are known as the Purana sedimentary pelitic rocks, Collins et al. (2003) argued that the Neoarchaean rocks (Fig. 20.1, reviewed in Malone et al. 2008). Detailed facies Antananarivo block and the Meso- to Neoarchaean Antongil analyses suggest that the Purana sediments were deposited in an block of Madagascar represent opposite sides of the Mozambique intracontinental shallow-marine environment (Basu et al. 2008; Ocean that closed during the Pan-African orogeny. The junction Malone et al. 2008; Bickford et al. 2011a, b). U–Pb isotope analy- between the two blocks, the Betsimisaraka Suture Zone (BTSZ), sis of detrital zircon grains in the clastic sediments suggests that the has been extended to merge with the Palghat Cauvery Shear sediments of the Purana basins were sourced from the exhumed Zone (PCSZ) of the SGT (Collins et al. 2003, 2007a, b; see metamorphic rocks of the Indian shield (reviewed in Basu et al. Fig. 20.1). One implicit assumption in this correlation is that the 2008; Malone et al. 2008). The Southern Granulite Terrane (SGT) crustal blocks that lie to the north (the Salem Block, Clark et al. that lies several hundred kilometres south of the present-day 2009a) and to the south (Madurai Block, Ghosh et al. 2004) of exposure limit of the Purana rocks shows detached outcrops of the PCSZ represent disparate crustal terranes that were juxtaposed interbanded quartzite, metapelite (sensu stricto) and marble þ only during the Late Neoproterozoic orogeny. Extant geological calc-silicate rocks (Fig. 20.2). The temporal and genetic relations and geochronological studies present conflicting views on the of this metasedimentary rock ensemble, which is generally con- status of the PCSZ. Vestiges of mafic–ultramafic rock associations sidered to be the metamorphosed equivalent of the Purana sand- were interpreted to represent slices of Neoproterozoic ocean crust stone–shale–limestone sequence (reviewed in Sengupta et al. (Sajeev et al. 2009; Yellappa et al. 2012). Inferred high-pressure 2009), are not understood. As metamorphic overprints obliterated to ‘eclogite’ facies metamorphism of late Neoproterozoic age most of the sedimentary features, U–Pb isotope analyses of detrital (Sajeev et al. 2009) and crustal-scale shear deformation along zircon grains and their in situ overgrowths can provide valuable the PCSZ (Chetty 1996) have been cited as supportive evidence information on the provenance, timing of sedimentation and of the geodynamic model proposed by Collins et al. (2003). On metamorphism of the sedimentary sequences of the SGT. This the other hand, continuation of 2.5 Ga old felsic crust across the information may also help to establish whether these metasedi- so-called PCSZ (Braun & Kriegsman 2003; Ghosh et al. 2004; mentary rocks are temporally and genetically related to the for review of references up to 2003; Bhaskar Rao et al. 2003; Purana sediments exposed further north. U–Pb isotope data from Brandt et al. 2014), lack of evidence in favour of crustal-scale detrital zircon grains also help in tracing the loci of trans- shear deformation in many parts of the PCSZ (Mukhopadhyay continental orogenic belts and sutures that stitched together dispa- et al. 2003) and the occurrence of middle Neoproterozoic pluton- rate continental fragments to form supercontinents (e.g. Collins ism and metamorphism on both sides of the PCSZ are in gross et al. 2003; de Waele et al. 2011). The Mozambique suture, disagreement with proposed continuation of the BTSZ of along which the supercontinent Gondwanaland may have been Madagascar along the PCSZ. With this backdrop we present

From:Mazumder,R.&Eriksson, P. G. (eds) 2015. Precambrian Basins of India: Stratigraphic and Tectonic Context. Geological Society, London, Memoirs, 43, 297–308, http://dx.doi.org/10.1144/M43.20 # 2015 The Geological Society of London. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

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Fig. 20.1. Reconstructed east Gondwanaland showing the positions and broad tectonic , Madagascar, Sri Lanka and Antarctica (modified after Ghosh et al. 2004; de Waele et al. 2011; Turner et al. 2014). Neoproterozoic metamorphic episodes include one or both of Grenvillian and Pan-African metamorphism. MG, Marwar Group; ADFB, Aravalli–Delhi Fold Belt; VB, Vindhyan Basin; DB, Deccan Basalt; BuC, Craton; ChB, Basin; SC, Singhbhum Craton; BaC, Bastar Craton; EDC, Eastern Dharwar Craton; WDC, Western Dharwar Craton; CG, Closepet ; EGP, Eastern Ghats Province; CB, Cuddapah Basin; KP, Krishna Province; SB, Salem Block; CSS, Cauvery Shear System; MB, Madurai Block; TB, Trivandrum Block; BM, Bemarivo domain; An, Antongil Craton; Ant, Antananarivo Craton; Ms, Masora Craton; Ik, Ikalamavony subdomain; A-M, Anaboriana– Manapotsy; (a) Itremo Group; (b) Sahantaha Group; (c) Maha Group; Vo, Vohibory domain; And, Androyen domain; Ano, Anoysen domain; WC, Wanni Complex; HC, Highland Complex; VC, Vijayan Complex; NC, Napier Complex; RC, Rayner Complex; LHC, Lu¨tzow–Holm-Bay Complex. (1) Moyar– Shear Zone (MSZ); (2) Bhavani Shear Zone (BSZ); (3) Moyar–Bhavani Shear Zone (MBSZ); (4) Palghat–Cauvery Shear Zone (PCSZ); (5) Karur–Kambam–Painavu–Trichur Shear Zone (KKPTSZ); (6) Achankovil Shear Zone (ACSZ); (7) Central Indian Shear Zone (CITZ); (8) Son–Narmada South Fault; (9) Son– Narmada North Fault; (10) Betsimisaraka Suture Zone (BTSZ); (11) Ranotsara–Bongolava Shear Zone (RBSZ). laser ablation-inductively coupled plasma mass spectrometry Geological background (LA-ICPMS) U–Pb isotope dates of zircon populations from three key rock samples from the SGT. Two of them are quart- The Precambrian shield of peninsular India consists of Palaeo- to zose–metasedimentary rocks collected from the northern part of Neoarchaean nuclei that are girdled by Proterozoic fold belts/ the PCSZ (Fig. 20.2). The third sample is a porphyritic granite granulite terranes (Fig. 20.1; reviewed in Naqvi & Rogers 1987; that intruded and was subsequently deformed and metamorphosed Ramakrishnan & Vaidyanathan 2008). A large part of the meta- together with an intercalated sequence of orthoquartzite–metape- morphosed basements is covered by Meso- to Neoproterozoic lite–metacarbonate lying to the south of the PCSZ (i.e. on the unmetamorphosed sedimentary sequences consisting of sand- Madurai Block, Fig. 20.2). The geochronological data of the stone–shale–carbonate associations which, in the Indian litera- zircon populations from the three key rock samples are compared ture, are known as the rocks of Purana basins (Holland 1909). with zircon U–Pb ages reported from south of the PCSZ, the The Vindhyan, Chhattisgarh and Cuddapah basins are examples adjoining parts of the Indian shield and northern Madagascar. of the most extensive Purana basins (Fig. 20.1). Recent studies The motivation for this is (a) to test if the metasedimentary on several Purana basins have revealed that, barring a few parts rocks within the SGT are temporally and genetically related to where sedimentation continued until late Neoproterozoic time, the Purana sediments lying further north, (b) to put constraints sedimentation in these basins ended before 900 Ma (reviewed on the provenance of the sedimentary protoliths, the timing of for- in Basu et al. 2008; Malone et al. 2008; Bickford et al. 2011a; mation and inversion of the sedimentary basins that developed Turner et al. 2014). U–Pb ages of detrital zircon grains in the across the PCSZ, (c) to test the hypothesis that the PCSZ is a Neo- clastic sediments of the Vindhyan and Chhattisgarh basins suggest proterozoic terrane boundary and represents the eastward con- that sediments were sourced from the adjoining metamorphic ter- tinuation of the BTSZ and (d) to compare the crustal architecture ranes of the Indian shield (Patranabis-Deb et al. 2007; Malone and age provinces of northern Madagascar with those within the et al. 2008; Bickford et al. 2011a; Turner et al. 2014). Sedimento- SGT with the goal of understanding the disposition of different logical attributes of the Purana sediments suggest their deposition crustal domains in the Indo-Madagascar landmass prior to the in a shallow-marine environment (reviewed in Patranabis-Deb Pan-African orogeny. et al. 2007; Malone et al. 2008). The southern margin of the Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

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Fig. 20.2. General geological map of southern peninsular India showing the inferred shear zones. Also shown in the inset map are sample locations of this study as well as those of Collins et al. (2007b) and Kooijman et al. (2011). This map is modified after Ghosh et al. (2004). (1) Nagercoil Hills; (2) ; (3) Biligirirangan Hills; (4) Nilgiri Hills; (5) ; (6) Kollimalai Hills; (7) Palni Hills; (8) Coorg Hills. All other acronyms are as in Figure 20.1.

Cuddapah Basin defines the southernmost limit of the preserved Shear Zone (Fig. 20.2; reviewed in Braun & Kriegsman 2003). Purana basins (Fig. 20.1). The SGT girdles the southern edge of In contrast to the Salem Block, the Madurai Block and the Trivan- the Palaeo- to the Mesoarchaean Western and Neoarchaean drum Block show abundant metasedimentary rocks that were Eastern Dharwar Craton (Fig. 20.1). The geological history of affected by high-grade metamorphism during Late Neoproterozoic the SGT is complex and is succinctly reviewed in a number of time (Ediacaran–, reviewed in Braun & Kriegsman publications (Braun & Kriegsman 2003; Ghosh et al. 2004; 2003; Ghosh et al. 2004; Collins et al. 2007b among others). A Plavsa et al. 2012; Brandt et al. 2014). The major magmatic and c. 300 km-long and 70 km-wide zone that separates the Salem metamorphic episodes in the SGT and the adjoining crustal Block from the Madurai Block is composed of a plethora of domains of the Indian shield are graphically presented in rocks that are akin to both the Salem Block and the Madurai Figure 20.3. On the basis of the extant geological information, Block and have a protracted magmatic, deformation and meta- the SGT can be divided into three roughly east–west elongated morphic history ranging from c. 2900 to c. 500 Ma (reviewed in blocks (Fig. 20.2). In this communication the term ‘block’ is Chetty 1996; Mukhopadhyay et al. 2003; Ghosh et al. 2004; used in a loose sense to imply a segment of continental crust Dutta et al. 2011; Plavsa et al. 2012; Brandt et al. 2014). It has having characteristic geological features. These ‘blocks’ may been suggested in many studies that a number of crustal-scale or may not be bound by terrane boundaries or shear zones. The shear zones (collectively termed the Cauvery Shear System, northernmost part, the Salem Block, is composed dominantly of CSS) have dissected the rocks of this narrow belt (Fig. 20.2; Neoarchaean felsic orthogneisses (/enderbite/horn- reviewed in Chetty 1996; Dutta et al. 2011; Plavsa et al. 2012; blende–biotite ) and contains enclaves of mafic–ultramafic Brandt et al. 2014). The PCSZ, which marks the southern boundary rocks and minor banded iron formation. The rocks of the Salem of the CSS, is perceived to be a terrane boundary along which Block underwent high-grade metamorphism during the earliest the Madurai and Trivandrum blocks were amalgamated with Palaeoproterozoic time (Clark et al. 2009a; Peucat et al. 2013). the Salem Block, and remaining part of the CSS during the The southernmost part is composite in nature and can be subdi- Pan-African orogeny (reviewed in Collins et al. 2007a, b; Yellappa vided into the northern Madurai Block and the southern Trivan- et al. 2012). An interlayered suite of metasedimentary rocks con- drum Block that are presumed to be separated by the Achankovil sisting of orthoquartzite–metapelite–marble–calc-silicate rocks Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

300 P. SENGUPTA ET AL.

Fig. 20.3. Timing of magmatic and metamorphic events in the Cauvery Shear System, Madurai Block, Western Dharwar Craton, Eastern Dharwar Craton, Krishna Province and Eastern Ghats Province. References: (1) Ghosh et al. (2004); (2) Bhaskar Rao et al. (1996); (3) Clark et al. (2009b); (4) Upadhyay et al. (2006); (5) Kooijman et al. (2011); (6) Mohan et al. (2013); (7) Plavsa et al. (2012); (8) Raith et al. (1999); (9) Saitoh et al. (2011); (10) Santosh et al. (2012); (11) Teale et al. (2011); (12) Yellappa et al. (2012); (13) Sato et al. (2012); (14) Sato et al. (2011a); (15) Sato et al. (2011b); (16) Kovach et al. (1998); (17) Ramakrishnan & Vaidyanathan (2008); (18) Dobmeier & Raith (2003); (19) Bose et al. (2011); (20) Rekha et al. (2013).

occurs as detached outcrops in a country of Neoarchaean to Palaeo- Collins et al. (2007b) postulated that the PCSZ represents the east- proterozoic felsic orthogneisses (reviewed in Meissner et al. 2002; ward continuation of the BTSZ (Fig. 20.1; reviewed in Collins et al. Ghosh et al. 2004; Collins et al. 2007a, b; Sengupta et al. 2009; 2003). The idea proposed by Collins et al. (2007a) has been Kooijman et al. 2011; Plavsa et al. 2012; Brandt et al. 2014). supported (Chetty 1996; Sajeev et al. 2009; Yellappa et al. 2012) The metasedimentary rocks occur on both sides of the PCSZ and contradicted (Ghosh et al. 2004; Plavsa et al. 2012; Brandt (Fig. 20.2). U–Pb ages of zircon in a few metasedimentary et al. 2014) by subsequent studies. Lack of convincing field evi- rocks suggest that the latest deformation and metamorphism dence in favour of crustal-scale shear zones in the eastern part of of the metasedimentary rocks occurred during Pan-African time the PCSZ (Mukhopadhyay et al. 2003) also contradicts the model (c. 650–520 Ma, Raith et al. 2010; Kooijman et al. 2011). Petrolo- of Collins et al. (2007a). gical studies on the metasedimentary rocks, albeit from only two localities near the PCSZ, demonstrate that the metasedimentary packets were buried to great depth (corresponding to .9 kbar) Analytical methods, description of samples and were metamorphosed between 700 and 800 8C (Sengupta et al. 2009; Raith et al. 2010). The northernmost outcrop of the Zircon grains were separated from the studied rock samples metasedimentary rocks of the SGT lies several hundred kilometres employing standard procedures (crushing, sieving, magnetic and south of the unmetamorphosed sedimentary sequence of the heavy liquid separation). Representative zircon grains were then Cuddapah Basin (Figs 20.1 & 20.2). In a reconstruction of Gond- hand-picked using a binocular microscope. The separated zircon wanaland (Fig. 20.1), the Western Dharwar Craton and the SGT fractions included all sizes and morphologies and for each are juxtaposed against the Mesoarchaean Antongil–Masora and sample over 100 grains were selected and mounted on one-inch the Neoarchaean Antananarivo blocks of Madagascar, respect- epoxy discs. The internal structures of the grains were documented ively. On the basis of contrasting sources of protoliths of the meta- by cathodoluminescence and back-scattered electron imaging sedimentary rocks overlying the Meso- and Neoarchaean blocks using a Jeol 8900 JXA Superprobe. The U–Pb isotope ratios of of Madagascar, it is assumed that a suture zone (BTSZ) exists the grains were measured using a Thermo-Fisher Element 2 between these two Archaean blocks (reviewed in Collins et al. sector field ICP-MS coupled to a New Wave UP193HE ArF 2003). The BTSZ is considered as a late Neoproterozoic suture excimer laser system at the Institut fu¨r Mineralogie, Westfa¨lische zone along which the Mozambique Ocean might have been con- Wilhelms-Universita¨t, Mu¨nster, Germany. Details of the analytical sumed (Collins et al. 2003, 2007a). Based on a few U–Pb ages of techniques are given in Kooijman et al. (2012). Repeat in-run detrital zircon grains in metasedimentary rocks of the SGT, measurements of the 91500 standard zircon (Wiedenbeck et al. Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

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1995) yielded an external reproducibility (2s, n ¼ 20) of 1.5% for Morphology and zoning patterns of zircon. The zircon grains have 206Pb/238U and 2.8% for 207Pb/206Pb. All uncertainties are rounded to irregular outlines but a few elongated grains (width– reported at the 2s level. length ratio c. 1:2) show a subhedral to euhedral shape. The oscil- Three rocks, two metapelites and one porphyritic granite, were latory zoning in most grains shows planar truncations, suggesting chosen for this study. The metapelite samples were collected that they are detrital grains (Fig. 20.5a). Overgrowths on oscil- close to the northern margin of the PCSZ (Fig. 20.2), whereas latory zoned detrital grains show higher luminescence and a the porphyritic granite that intruded and co-folded with the ortho- ‘patchy’ internal structure (Fig. 20.5a). They are very thin along quartzite–pelite–carbonate unit of Alambadi was collected south the prisms but rather broad at the crystal ends. At the immediate of the PCSZ (Fig. 20.2). In the following section we present a brief interface with the detrital grains, very thin high and low lumines- description of the field features and highlight the morphology and cence zones are developed. Ragged interfaces indicate minor zoning patterns of the zircon grains recovered from the three rocks. resorption of the detrital grains through dissolution by pore fluids. The patchy zoning of the rims argues against their crystal- lization from a melt for which the rock lacks evidence. Metapelite UNG64 (N11827.9600E77827.9600)

The sample was collected from a metasedimentary enclave within Psammitic paragneiss N58 (N1185.9490E7888.9640) the virtually undeformed Pan-African (c. 560 Ma, U–Pb zircon date, Brandt et al. 2010) alkaline granite complex of This metasedimentary rock sample was collected from a roadside (Fig. 20.4a). This is a garnet–biotite–kyanite that is profu- exposure near the northern bank of the Cauvery River (Fig. 20.2). sely veined by quartz (Fig. 20.4b). Intercalated marble and calc- A compositional banding is manifested by centimetre- to deci- silicate rocks were deformed and metamorphosed together with metre-thick quartz-rich (psammitic) bands that are intercalated the metapelite. Petrological study demonstrated that the metasedi- with quartz-poor pelitic schist (Fig. 20.4c, d). In the psammitic mentary sequence was buried to a depth corresponding to 9 kbar layers, a prominent schistosity subparallel with the composi- (700–750 8C), followed by uplift along a steeply decompres- tional banding is defined by preferred orientation of biotite. The sive pressure–temperature path (Sengupta et al. 2009). Contact schistosity and the compositional banding show outcrop-scale metamorphism by the pegmatitic granite caused replacement of folds (Fig. 20.4c). Porphyroblasts of kyanite and garnet are devel- bladed kyanite by fibrous sillimanite (Sengupta et al. 2009). oped within the metapelitic layers. The pelitic bands contain very

Fig. 20.4. Field features of the studied samples. (a) Enclave of metapelite (UNG64) within the pegmatoidal granite from Sankagiri. (b) Syn-metamorphic quartz veins in the kyanite–biotite–garnet-bearing metapelite enclave in sample UNG64. (c) Quartz-rich bands

intercalated with pelitic schist showing F2 folds (sample N58). (d) Schistosity defined by biotite-rich layers in the pelitic layers of sample N58. Note the compositional banding parallel to schistosity. (e) Porphyritic granite with euhedral to subhedral megacrysts of K-feldspar set in a fine- grained matrix of quartz–biotite–plagioclase (A72). Note a raft of calc-silicate rock within the porphyritic granite. (f) Stretched K-feldspar augen defining a

foliation that is folded by F2 fold in A72. Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

302 P. SENGUPTA ET AL.

Fig. 20.5. Cathodoluminescence (CL) images of zircon grains from the studied samples: (a) UNG64 (metapelite), (b) N58 (psammitic paragneiss) and (c) A72 (porphyritic granite). Numbers to the upper left of grains indicate grain number. Ages younger than 1000 Ma are cited as 206Pb/238U ages, whereas for older zircon domains, the 207Pb/206Pb ages are given.

thin (,1 cm-thick) leucosomes that never exceed more than country rocks were subsequently deformed and metamorphosed. 15 vol% of the rock. A distinct L-S fabric and augen-structure was developed in the granite (Fig. 20.4e, f), although in low-strain domains a magmatic flow structure defined by euhedral K-feldspar phenocrysts in a Morphology and zoning patterns of zircon. Zircon grains show finer-grained matrix of plagioclase, quartz and biotite may still diverse morphologies and internal structures (Fig. 20.5b). Most be preserved. The L-S fabric is folded and the geometry of the of the grains are rounded to anhedral with a few grains showing folds is similar to the regional F folds defined by the adjoining euhedral prismatic shape. Core regions of both euhedral and anhe- 2 metasedimentary package (Mukhopadhyay et al. 2003; our data). dral grains show fine oscillatory zoning that is truncated by grain outline as well as highly luminescent, structureless, discontinuous rims (Fig. 20.5b). In a few euhedral grains, the oscillatory zoned Morphology and zoning patterns of zircon. Zircon grains exhibit a cores are truncated and overgrown by thin low-luminescence variety of morphology with most grains showing euhedral pris- mantles and the latter are truncated by high-luminescence rims matic shapes (Fig. 20.5c). Aspect ratios of these grains are c. 3:1. (Fig. 20.5b). Featureless low-luminescence domains on high- The morphology of euhedral grains is consistent with their crystal- luminescence cores (with or without oscillatory zoning) are lization from a melt phase, presumably during magmatic crystalli- present in a few grains. The morphology of the oscillatory and zation of the protolith of the rock. The euhedral shape is less high-luminescence cores and their truncation by high-lumines- distinct and the aspect ratio varies considerably in other grains cence rims suggests that the cores are detrital grains on which (Fig. 20.5c). Distinct oscillatory zoning patterns largely preserve the luminescent rims grew in situ during metamorphism. the magmatic crystal shape and growth structures. A few grains show featureless low-luminescence cores. Highly luminescent and featureless overgrowths are restricted to the ends of the crys- Porphyritic granite A72 (N11845.2020E7883.4280) tals and rarely truncate the oscillatory zoning (Fig. 20.5c).

The sample was collected from a porphyritic granite which intruded the folded metasedimentary (orthoquartzite–shale– Results carbonate) unit near Alambadi, south of the supposed PCSZ (Fig. 20.2). Rafts of calc-silicate rocks and quartzite in the In this section 206Pb/238U and 207Pb/206Pb dates of the different granite further corroborate the intrusive relation with the metasedi- textural domains of the representative zircon grains from each mentary unit (Fig. 20.4a). The granite and the metasedimentary sample are presented. Details of the analyses are represented Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

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Fig. 20.6. (a, b) Concordia diagrams showing the results of U–Pb analyses of zircon grains from metasedimentary rocks. Histograms plus probable peaks of the age cluster of the detrital zircon grains of ,10% discordance are shown in the inset: (a) sample UNG64; (b) sample N58. (c) Concordia diagram showing the results of U–Pb analyses of zircon grains from the A-type granite sample A72. Histogram plus probable peaks of the age cluster of the crystallization age of zircon grains showing ,10% discordance are shown in the inset. (d) The age histograms plus probability curve for all Pan-African metamorphic zircon rims from all the three samples UNG64, N58 and A72. Two distinct age populations with peaks at 572+2 and 639 + 4 Ma are evident. in Figure 20.6. The U–Pb data of zircon grains in each sample are for discussion. The oscillatory zoned domains give ages tightly plotted in concordia diagrams. Only concordant to near-concor- clustering between 790 and 760 Ma with an average of dant ages (,10% discordance) were considered for provenance 768 + 3 Ma. The rare dark luminescent cores give slightly older analysis and bracketing the age of sedimentation. Furthermore, dates (803–815 Ma). The three overgrowths on the oscillatory 207Pb/206Pb ages are used for spots that yielded ages older than zoned domains give an average age of 570 + 19 Ma. 1000 Ma whereas 206Pb/238U ages are used for spots that yielded ages younger than 1000 Ma. The program Isoplot-3.00 (Ludwig 2003) was used for statistical analysis of the age data and con- Discussion struction of Concordia plots. Timing of sedimentary basin formation on the SGT and the Metapelite (sample UNG 64) temporal relation with the Purana basins of the Indian shield

A total of 61 spots were analysed on 23 zircon grains; 40 out of 61 Constraining the time of sedimentation of the metamorphosed analyses show ,10% discordance. The truncated oscillatory detri- orthoquartzite–shale–carbonate sequence is not straightforward. tal grains show an age spectrum ranging from 2509 to 836 Ma Nevertheless, the age of the oldest metamorphic event that affected (Fig. 20.5a). Near-concordant ages of detrital zircons cluster in the sediments and the youngest age of detrital zircon grains two distinct populations: 2448+7 and 897 + 20 Ma (Fig. 20.6a). provide the temporal window within which the sedimentation and The ages of thin overgrowths that presumably date the in situ hence the basin formation took place (Collins et al. 2003, 2012; growth of metamorphic zircon fall within 633–487 Ma. Kooijman et al. 2011; Sato et al. 2011a, b, 2012). Figure 20.7 pre- sents the concordant to near-concordant ages of detrital and meta- Psammitic paragneiss (sample N58) morphic zircons from two metasedimentary samples (UNG64 and N58) from the CSS. Also included in Figures 20.7 and 20.8 are published ages of metamorphic and detrital zircon grains from In sample N58 69 spots were analysed on 34 grains. Out of 69 metasedimentary sequences of northern Madagascar, the CSS, analyses, 46 dates show ,10% discordance while the remain- the Madurai Block, the Trivandrum Block and the Vindhyan ing analyses show 30–89% discordance. The detrital cores basin – the largest Purana basins in the Indian shield. The detrital yield a spectrum of ages (3030–2230 Ma for ,10% discordant and metamorphic zircon populations in sample UNG 64 indicate grains; Figs 20.6 & 20.5b). Compared with the metapelite UNG deposition of the sedimentary suite in the narrow time span 64, the detrital zircon population of this sample does not contain between 836 and 633 Ma (Fig. 20.6a). This age bracket of sedi- Neoproterozoic components and shows a tight cluster around mentary basin formation is in good agreement with the age 2395 + 6 Ma (81% of the analyses, Fig. 20.6b). Three-spot ana- 206 238 bracket of sedimentation (689–513 Ma) of the protoliths of lyses of the highly luminescent rims yielded Pb/ U ages in some metapelites from the Madurai and Trivandrum blocks the range of 567–525 Ma (two reversely concordant and one (Fig. 20.7; Collins et al. 2007b). Owing to the paucity of ,10% discordant). younger detrital zircon grains, a wide time bracket for sedimentary basin formation is obtained from the zircon data of two samples Porphyritic granite (sample A72) that lie close to the PCSZ (N58: 2280–567 Ma; Ayy-1, Raith et al. 2010: 1992–615 Ma). The four metasedimentary samples A total of 81 spots were analysed on 27 grains out of which only from the Madurai Block analysed by Kooijman et al. (2011) eight analyses show .10% discordance and are not considered suggest that sedimentation in these areas occurred within a broad Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

304 P. SENGUPTA ET AL.

Fig. 20.7. Zircon U–Pb geochronology of detrital zircon and the metamorphic overgrowths in the SGT, Vindhyan (Purana) basins and central and northern Madagascar. References: (1) Raith et al. (2010); (2) Kooijman et al. (2011); (3) Collins et al. (2007b); (4) Brandt et al. (2011); (5) Turner et al. (2014); (6) Cox et al. (2004); (7) Cox et al. (1998); (8) de Waele et al. (2011); (9) Tucker et al. (2007, 2011); (10) Collins et al. (2003); (11) Fitzsimons & Hulscher (2005).

Fig. 20.8. Geological and geochronological attributes of the crystalline basement and sedimentary cover in the Indo-Madagascar land mass. Note that the area bound by thick dashed lines in the Indo-Madagascar land mass shows remarkable similarity in terms of geological evolution at least from 2.5 Ga. Because of lack of information on rocks lying south of that line the sign ‘??’ is used. All the abbreviations are the same as Figure 20.1. References: (1) de Waele et al. (2011); (2) Cox et al. (2004); (3) Collins et al. (2007a, b); (4) Kooijman et al. (2011); (5) this study; (6) Plavsa et al. (2012); (7) Ghosh et al. (2004); (8) Raith et al. (1999); (9) Braun & Kriegsman (2003); (10) Collins et al. (2003); (11) Sato et al. (2011a, b); (12) Teale et al. (2011). Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

SOURCE AND TIME OF SEDIMENTATION 305 time span of c. 1750–990 Ma (Fig. 20.7). A similar wide time felsites; Malone et al. 2008), evidence for 2400–2200 Ma old bracket for sedimentary basin formation (1900–515 Ma) is also igneous or partial melting events that led to zircon growth reported by Collins et al. (2007b) from the Trivandrum Block are hitherto not reported from the Indian shield. Intriguingly, (Fig. 20.7). All of these observations support the view that sedi- 2400–2200 Ma-old detrital zircons are abundant in the Vindhyan mentation of the protoliths of the metasedimentary rocks that sedimentary sequence that covers the northern Indian shield developed on the Archaean/Palaeoproterozoic gneissic basement (Turner et al. 2014; Figs 20.1 & 20.6). This observation raises of the CSS and Madurai Block occurred over a protracted period three possibilities: (1) the zircon grains in the Vindhyan sequence of time. The intercalated sequence of quartzite–metapelite–meta- are derived from the Aravalli Craton; (2) rocks of this age range carbonate rocks that are exposed in the CSS and Madurai Block is have not yet been dated from the Indian shield; or (3) rocks of likely to represent the metamorphosed equivalents of platformal this age range are lying beneath the Phanerozoic volcanic and sedi- shallow-marine sandstone–shale–carbonate suites. The precursor mentary rock cover (see Fig. 20.2). In recent communications, de sediments of these sequences, therefore, bear a resemblance to the Waele et al. (2011) and Tucker et al. (2011) argued that c. 2300– Purana basin fills in terms of broad depositional environment, the 2200 Ma-old detrital zircon grains that are present in the meta- age populations of detrital zircons and the timing of sedimentary sedimentary rocks of northern Madagascar were sourced from basin formation (see Figs 20.6 & 20.7). The southernmost the Aravalli Craton. It is, therefore, possible that the detrital exposure limit of the Purana basins, the Cuddapah Basin and its zircon grains from the third cluster could have been sourced equivalents occurs several hundred kilometres north of the north- from the Aravalli Craton, although recycling of Purana sedimen- ernmost exposure limits of the Proterozoic metasedimentary tary components remains an alternative possibility. Evidently, a rocks of the SGT (Fig. 20.1). It seems, therefore, likely that the more detailed study is warranted to support or reject the other Purana basins extended further south, at least up to Tirunelveli, two possibilities. The Neoproterozoic detrital zircon population where exposures of interbanded quartzite, metapelite and marble defining the fourth age cluster (1092–836 Ma) could have been are widespread (Fig. 20.7). In this scenario, the southern part of derived from several areas within India and adjoining Sri Lanka, the vast shallow-marine intracontinental deposits of the extended namely the Madurai Block, the Wanni and Vijayan Complexes Purana basins were deformed and metamorphosed during the of Sri Lanka, and the Eastern Ghats Belt, where crustal growth Pan-African orogenesis (cf. Sengupta et al. 2009; Raith et al. during this period has been documented (Fig. 20.3 and the refer- 2010; Kooijman et al. 2011; Sato et al. 2011a; Brandt et al. 2014). ences cited therein). It is, however, not known if all of these ter- ranes were exhumed at this time to provide detrital components of the sedimentary precursor of sample UNG64. Provenance of precursor sediments of the studied metasedimentary rocks Timing of metamorphism Although the inferred precursors of the metasedimentary units that occur across the PCSZ resemble the stable shelf deposits of the Concordant to near-concordant (,10% discordance) ages of Purana basin fills, superimposed deformation and metamorphism zircon rims that developed on detrital (in samples UNG64 and have obliterated most sedimentary structures (e.g. palaeoslope N58) or magmatic zircon grains (sample A72) are likely to date markers) of the former rocks. Nevertheless an attempt has been the metamorphism that affected these rocks. In view of overlap- 206 238 made to identify the likely hinterlands of the metasedimentary ping Pb/ U ratios in zircon rims in all the three samples, the assemblage integrating the U–Pb/Pb–Pb ages of the detrital data are combined for statistical analysis. The data define two dis- zircon grains and geochronological information of the crystalline tinct age populations that fall within tight clusters at 639 + 4 and basement rocks from the adjoining parts of the Indian shield and 527 + 2 Ma (Fig. 20.6d), supporting the view that the metasedi- from the that were once contiguous within the recon- mentary rocks and the porphyritic granitoids underwent two dis- structed Gondwanaland (Fig. 20.3). Concordant and nearly tinct metamorphic episodes. Two phases of metamorphism in the concordant 206Pb/238U and 207Pb/206Pb ages of detrital zircon SGT across the Ediacaran–Cambrian boundary are also reported populations in the spatially adjoining samples UNG64 and N58 by Raith et al. (2010; 615 + 11 and 529 + 9 Ma) and Kooijman can be divided into four age clusters: 3030–2600, 2550–2450, et al. (2011; clustering at 590 and 520 Ma). The older phase of 2400–2200 and 1092–836 Ma. Zircon grains of the oldest popu- metamorphism is also supported by the occurrence of the metape- lation, although few in number and restricted to sample N58, are lite (sample UNG64) as enclave within c. 560 Ma-old alkaline likely to have their source in the Western Dharwar Craton and granite in the Sankagiri area, and the thermal effects caused by the CSS where rocks of this age are reported (Fig. 20.3). The the granite host (Brandt et al. 2010). In view of this observation, Vindhyan sedimentary sequence is considered to be representative the 639 + 4 Ma age is interpreted to date the metamorphism and of the Purana basins and it contains detrital zircons that exhibit coeval deformation of the garnet þ kyanite-bearing metapelite ages ranging from c. 3.3 to 1.0 Ga. Therefore, the possibility enclave (sample UNG64, Sengupta et al. 2009). Effects of Edia- cannot be completely ruled out that these oldest zircon popu- caran or Cambrian metamorphic events are also widespread lations, at least part of them, could also have been derived from throughout the SGT (reviewed in Braun & Kriegsman 2003; recycling of the Purana sedimentary sequences. It is intriguing Ghosh et al. 2004; Santosh et al. 2006; Collins et al. 2007a; that sample UNG64, which marks the northernmost outcrop of Clark et al. 2009b; Kooijman et al. 2011; Brandt et al. 2014). the quartzite–metapelite–marble ensemble in the SGT, does not contain any contribution from the Western Dharwar Craton (or Can the PCSZ be a Neoproterozoic terrane boundary? from Purana sedimentary rocks), whereas the quartzite sample MR-48 of Kooijman et al. (2011) that comes from an area south Several recent studies have argued that the PCSZ represents a Neo- of the PCSZ has this older component. Sampling bias and varied proterozoic terrane boundary along which the southern Madurai drainage patterns may explain this diversity of detrital zircon Block was welded to the northern Salem Block during the population. Detrital zircons of the 2550–2450 Ma age cluster are Pan-African orogenesis (e.g. Collins et al. 2003; Sajeev et al. most likely derived from the Eastern Dharwar Craton and the 2009; Yellappa et al. 2012). In the context of this suggestion the CSS where rocks of this age range are reported (Fig. 20.3). Identi- following observations are relevant: fication of the source of zircon grains of the 2400–2200 Ma age cluster is not straightforward. Extant geochronological infor- 1. Orthoquartzite–shale–carbonate sequences of shallow- mation, although sparse, suggests that, barring one locality from marine origin occur as deformed and metamorphosed keels , where 2240 Ma old felsic rocks are found (Khairmalia in the orthogneiss basement on both sides of the PCSZ. 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2. Age populations of detrital zircons in quartzites and metape- basement of the Antananarivo Block, the CSS (samples lites indicate a similar provenance of the clastic material, UNG64 and N58) and the Madurai Block (Fig. 20.7). with a substantial contribution from the Western Dharwar 4. c. 800–700 Ma rift-related magmatism (A-type granitoids, Craton (and/or Purana sedimentary sequence). alkaline rocks and massif-type anorthosite) and associated 3. The c. 955 Ma metamorphism of the metasedimentary thermal metamorphism are common to all of the blocks packages exposed south of the supposed PCSZ suggests that (Collins et al. 2003; Ghosh et al. 2004; de Waele et al. sedimentation was completed before this time. This then sup- 2011; Kooijman et al. 2011; Brandt et al. 2014; and this ports the interpretation that the Madurai Block, which hosts study). these metasedimentary rocks, was connected to the Dharwar 5. The Proterozoic basement, the sedimentary cover rocks and Craton (the source of .3.3 Ga old detrital zircon grains) the c. 700–800 Ma magmatic rocks of the Antananarivo before c. 955 Ma (Fig. 20.8). Block, the CSS and the Madurai Block became intensely 4. The 800–700 Ma old magmatic rocks (A-type porphyritic deformed and metamorphosed during the c. 600–500 Ma granite, massif-type anorthosite, carbonatite–alkaline rocks, Ediacaran/Cambrian tectonothermal events (Figs 20.7 & reviewed in Braun & Kriegsman 2003; Ghosh et al. 2004; 20.8). Brandt et al. 2014) that represent magmatism in an exten- sional setting are present on both sides of the PCSZ In view of the marked similarity in geological histories of northern (Fig. 20.8). Taken together these observations contradict the Madagascar and the SGT, it seems likely that the Precambrian interpretation that the PCSZ is a Neoproterozoic terrane rocks of the Antananarivo Block, the Salem Block, the CSS and boundary. Our study, therefore, supports the view of Mukho- Madurai Block formed a unified landmass (Greater Dharwar padhyay et al. (2003), Ghosh et al. (2004) and Plavsa et al. Craton) at least before 800 Ma. If the c. 955 Ma metamorphic (2012) that no significant break in terms of geological and impressions on some metasedimentary units of the Madurai geochronological characters exists across the PCSZ. Block are considered, this unified Indo-Madagascar landmass seems to have existed before early Neoproterozoic time. The pres- ence of c. 2300–1800 and .3000 Ma-old detrital zircon popu- lations in the metasedimentary units of the SGT as well as Implications of this study in the context of the northern Madagascar supports the proposition of Tucker et al. Indo-Madagascar connection (2011, 2014) and de Waele et al. (2011) that the Western Dharwar Craton and adjoining parts of the Indian shield provided In the reconstructed Gondwanaland the Indian shield is juxtaposed detritus for the Proterozoic sedimentary basins developed on the against Madagascar (Fig. 20.8). In this reconstruction the Palaeo- unified Indo-Madagascar landmass. If this model remains valid, to Mesoarchaean Western Dharwar Craton faces the Antongil– a large part of the Neo- and Meso-Archaean crusts of central and Masora Block of the same age, whereas the Salem Block, the northern Madagascar (at least up to the Ranotsara–Bongolava CSS and the Madurai Block face the Neoarchaean Antananarivo Shear Zone, Fig. 20.8) and the SGT of India would represent one Block of east Madagascar (Fig. 20.8, reviewed in Rekha continuous strand line of the perceived Mozambique Ocean. The et al. 2013). Highlighting contrasted age populations of detrital southern limit of this shallow-marine sedimentary sequence zircons in metasedimentary units lying across the BTSZ should be marked by metasedimentary rocks where the ‘detrital (.3300 Ma ages in the eastern part and 2300–1800 Ma ages in footprint’ of Mesoarchaean rocks would be recognized the western part of the BSTZ), Collins et al. (2003) considered (Fig. 20.8). At present this signature is traceable up to the Siruma- the BTSZ to represent a ‘suture zone’ along which the lai hills in the Madurai Block (Fig. 20.8). Geological evidence for Mozambique Ocean closed during the Pan-African orogeny. The the closure of this ocean and, hence, the location of the late Neo- implication of this model is that the western Antananarivo Block proterozoic suture should be searched in areas lying further and eastern Mesoarchaean Antongil–Masora Block represent south of this unified crustal block. Detailed geological and geo- two sides of the Mozambique Ocean and the two blocks were jux- chronological studies on rocks from the southern part of the taposed only during the late Neoproterozoic time. The model of Madurai Block and the Trivandrum Block are required to ascertain Collins et al. (2003), however, has been recently challenged by if the Mozambique Ocean suture passed through the SGT. Tucker et al. (2011) and de Waele et al. (2011) on the grounds that Mesoarchaean detrital zircon populations are also present in P. S., M. T., P. C. and S. S. acknowledge the financial support from the Council of the metasediments that developed on the western Antananarivo Scientific and Industrial Research New Delhi. S.S. and P.S. also acknowledge the Block (the Itremo unit). According to these authors a coherent financial assistance from the Center of Advance Studies, Department of Geologi- landmass, including the Antongil–Masora–Antananarivo blocks, cal Sciences, Jadavpur University and University Potential for Excellence (Phase existed well before Neoproterozoic time. This observation thus II), Jadavpur University. D. M. acknowledges support through the Honorary contradicts the interpretation that the BTSZ represents a Neopro- Scientist Project of the Indian National Science Academy. The fieldwork of M. terozoic terrane boundary. The Antananarivo Block of Madagascar R. was financially supported through RWE-grant PN 71734. E. K. and the analyti- shares many geological features with the crust exposed in the cal work were supported by a Leibniz Award (DFG, Germany) to K. M. We thank Salem Block, the CSS and the Madurai Block (Fig. 20.8). These Dr U. K. Bhui (Pandit Deendayal Petroleum University, India) and Dr U. Dutta include: (Indian School of Mines, India) for their help in the field and many stimulating discussions. We sincerely thank Professor A. Hofmann and an anonymous 1. A c. 2.5–2.6 Ga orthogneiss basement is common to all of reviewer for their critical but very constructive comments, which have improved these terranes (Ghosh et al. 2004; de Waele et al. 2011; the clarity of the manuscript. We also thank Professor R. Mazumder for inviting Plavsa et al. 2012; Brandt et al. 2014). us to contribute to this Geological Society of London Memoir and for his 2. The Neoarchaean crust of the Antananarivo Block, the CSS editorial remarks. and the Madurai Block was covered by shallow-marine sand- stone–shale–carbonate sequences that were deposited poss- References ibly in two cycles, between 1800–800 and 800–620 Ma (Figs 20.7 & 20.8). Part of the detrital components in these Basu,A.,Patranabis-Deb, S., Schieber,J.&Dhang, P. C. 2008. Stra- sediments (now deformed and metamorphosed) was sourced tigraphic position of the  1000 Ma Sukhda Tuff (Chhattisgarh from a unified Mesoarchaean landmass (Western Dharwar Supergroup, India) and the 500 Ma question. Precambrian Research, Craton–Antongil–Masora) lying to the north (Fig. 20.8). 167, 383–388. 3. c. 2300–1800 Ma detrital zircon populations are common in Bhaskar Rao, Y. J., Chetty, T. R. K., Janardhan,A.S.&Gopalan,K. the sedimentary sequences that developed on the gneissic 1996. Sm–Nd and Rb–Sr ages and P-T history of the Archaean Downloaded from http://mem.lyellcollection.org/ by guest on September 18, 2018

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