Chapter 20 Provenance, timing of sedimentation and … · Chapter 20 Provenance, timing of sedimentation and metamorphism of metasedimentary rock suites from the Southern Granulite

Chapter 20

Provenance, timing of sedimentation and metamorphism of metasedimentaryrock suites from the Southern Granulite Terrane, India

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, India2Steinmann Institut, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany

3Institut für Mineralogie, Universität Münster, Corrensstrasse 24, 48145 Münster, Germany4Department of Geosciences, Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden

5Institut für Geologie, Universität Bern, Baltzerstrasse 1þ 3, 3012 Bern, Switzerland6Raman 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 Madagascar. 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 Craton and theadjoining parts of the Indian shield. The similarity of the provenance 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 likelybasin formation age of the metasedimentary rocks of the Southern Granulite Terrane show remarkable similarity to the rock assemblageand timing of sedimentation of the Palaeoproterozoic to Neoproterozoic shallow-marine deposits of the Purana basins lying severalhundred 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 sandstone,shale and limestone that cover a vast expanse of the crystallinebasement of peninsular India are known as the Purana sedimentaryrocks (Fig. 20.1, reviewed in Malone et al. 2008). Detailed faciesanalyses suggest that the Purana sediments were deposited in anintracontinental shallow-marine environment (Basu et al. 2008;Malone et al. 2008; Bickford et al. 2011a, b). U–Pb isotope analy-sis of detrital zircon grains in the clastic sediments suggests that thesediments of the Purana basins were sourced from the exhumedmetamorphic rocks of the Indian shield (reviewed in Basu et al.2008; Malone et al. 2008). The Southern Granulite Terrane (SGT)that lies several hundred kilometres south of the present-dayexposure limit of the Purana rocks shows detached outcrops ofinterbanded quartzite, metapelite (sensu stricto) and marble þcalc-silicate rocks (Fig. 20.2). The temporal and genetic relationsof this metasedimentary rock ensemble, which is generally con-sidered to be the metamorphosed equivalent of the Purana sand-stone–shale–limestone sequence (reviewed in Sengupta et al.2009), are not understood. As metamorphic overprints obliteratedmost of the sedimentary features, U–Pb isotope analyses of detritalzircon grains and their in situ overgrowths can provide valuableinformation on the provenance, timing of sedimentation andmetamorphism of the sedimentary sequences of the SGT. Thisinformation may also help to establish whether these metasedi-mentary rocks are temporally and genetically related to thePurana sediments exposed further north. U–Pb isotope data fromdetrital zircon grains also help in tracing the loci of trans-continental orogenic belts and sutures that stitched together dispa-rate continental fragments to form supercontinents (e.g. Collinset al. 2003; de Waele et al. 2011). The Mozambique suture,along which the supercontinent Gondwanaland may have been

amalgamated, is a case in point. Based on contrasting sedimentarysources of detrital zircon populations in a suite of high-grade meta-pelitic rocks, Collins et al. (2003) argued that the NeoarchaeanAntananarivo block and the Meso- to Neoarchaean Antongilblock of Madagascar represent opposite sides of the MozambiqueOcean that closed during the Pan-African orogeny. The junctionbetween the two blocks, the Betsimisaraka Suture Zone (BTSZ),has been extended to merge with the Palghat Cauvery ShearZone (PCSZ) of the SGT (Collins et al. 2003, 2007a, b; seeFig. 20.1). One implicit assumption in this correlation is that thecrustal blocks that lie to the north (the Salem Block, Clark et al.2009a) and to the south (Madurai Block, Ghosh et al. 2004) ofthe PCSZ represent disparate crustal terranes that were juxtaposedonly during the Late Neoproterozoic orogeny. Extant geologicaland geochronological studies present conflicting views on thestatus of the PCSZ. Vestiges of mafic–ultramafic rock associationswere interpreted to represent slices of Neoproterozoic ocean crust(Sajeev et al. 2009; Yellappa et al. 2012). Inferred high-pressureto ‘eclogite’ facies metamorphism of late Neoproterozoic age(Sajeev et al. 2009) and crustal-scale shear deformation alongthe PCSZ (Chetty 1996) have been cited as supportive evidenceof the geodynamic model proposed by Collins et al. (2003). Onthe other hand, continuation of 2.5 Ga old felsic crust across theso-called PCSZ (Braun & Kriegsman 2003; Ghosh et al. 2004;for review of references up to 2003; Bhaskar Rao et al. 2003;Brandt et al. 2014), lack of evidence in favour of crustal-scaleshear deformation in many parts of the PCSZ (Mukhopadhyayet al. 2003) and the occurrence of middle Neoproterozoic pluton-ism and metamorphism on both sides of the PCSZ are in grossdisagreement with proposed continuation of the BTSZ ofMadagascar 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

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laser ablation-inductively coupled plasma mass spectrometry(LA-ICPMS) U–Pb isotope dates of zircon populations fromthree key rock samples from the SGT. Two of them are quart-zose–metasedimentary rocks collected from the northern part ofthe PCSZ (Fig. 20.2). The third sample is a porphyritic granitethat intruded and was subsequently deformed and metamorphosedtogether with an intercalated sequence of orthoquartzite–metape-lite–metacarbonate lying to the south of the PCSZ (i.e. on theMadurai Block, Fig. 20.2). The geochronological data of thezircon populations from the three key rock samples are comparedwith zircon U–Pb ages reported from south of the PCSZ, theadjoining parts of the Indian shield and northern Madagascar.The motivation for this is (a) to test if the metasedimentaryrocks within the SGT are temporally and genetically related tothe Purana sediments lying further north, (b) to put constraintson the provenance of the sedimentary protoliths, the timing of for-mation and inversion of the sedimentary basins that developedacross the PCSZ, (c) to test the hypothesis that the PCSZ is a Neo-proterozoic terrane boundary and represents the eastward con-tinuation of the BTSZ and (d) to compare the crustal architectureand age provinces of northern Madagascar with those within theSGT with the goal of understanding the disposition of differentcrustal domains in the Indo-Madagascar landmass prior to thePan-African orogeny.

Geological background

The Precambrian shield of peninsular India consists of Palaeo- toNeoarchaean nuclei that are girdled by Proterozoic fold belts/granulite terranes (Fig. 20.1; reviewed in Naqvi & Rogers 1987;Ramakrishnan & Vaidyanathan 2008). A large part of the meta-morphosed basements is covered by Meso- to Neoproterozoicunmetamorphosed sedimentary sequences consisting of sand-stone–shale–carbonate associations which, in the Indian litera-ture, are known as the rocks of Purana basins (Holland 1909).The Vindhyan, Chhattisgarh and Cuddapah basins are examplesof the most extensive Purana basins (Fig. 20.1). Recent studieson several Purana basins have revealed that, barring a few partswhere sedimentation continued until late Neoproterozoic time,sedimentation in these basins ended before 900 Ma (reviewedin Basu et al. 2008; Malone et al. 2008; Bickford et al. 2011a;Turner et al. 2014). U–Pb ages of detrital zircon grains in theclastic sediments of the Vindhyan and Chhattisgarh basins suggestthat sediments were sourced from the adjoining metamorphic ter-ranes of the Indian shield (Patranabis-Deb et al. 2007; Maloneet al. 2008; Bickford et al. 2011a; Turner et al. 2014). Sedimento-logical attributes of the Purana sediments suggest their depositionin a shallow-marine environment (reviewed in Patranabis-Debet al. 2007; Malone et al. 2008). The southern margin of the

Fig. 20.1. Reconstructed east Gondwanaland

showing the positions and broad tectonic architecture

of India, 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,

Bundelkhand Craton; ChB, Chhattisgarh Basin; SC,

Singhbhum Craton; BaC, Bastar Craton; EDC,

Eastern Dharwar Craton; WDC, Western Dharwar

Craton; CG, Closepet Granite; 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, Lützow–Holm-Bay Complex. (1)

Moyar–Attur 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).

P. SENGUPTA ET AL.298



Cuddapah Basin defines the southernmost limit of the preservedPurana basins (Fig. 20.1). The SGT girdles the southern edge ofthe Palaeo- to the Mesoarchaean Western and NeoarchaeanEastern Dharwar Craton (Fig. 20.1). The geological history ofthe SGT is complex and is succinctly reviewed in a number ofpublications (Braun & Kriegsman 2003; Ghosh et al. 2004;Plavsa et al. 2012; Brandt et al. 2014). The major magmatic andmetamorphic episodes in the SGT and the adjoining crustaldomains of the Indian shield are graphically presented inFigure 20.3. On the basis of the extant geological information,the SGT can be divided into three roughly east–west elongatedblocks (Fig. 20.2). In this communication the term ‘block’ isused in a loose sense to imply a segment of continental crusthaving characteristic geological features. These ‘blocks’ mayor may not be bound by terrane boundaries or shear zones. Thenorthernmost part, the Salem Block, is composed dominantly ofNeoarchaean felsic orthogneisses (charnockite/enderbite/horn-blende–biotite gneiss) and contains enclaves of mafic–ultramaficrocks and minor banded iron formation. The rocks of the SalemBlock underwent high-grade metamorphism during the earliestPalaeoproterozoic time (Clark et al. 2009a; Peucat et al. 2013).The southernmost part is composite in nature and can be subdi-vided into the northern Madurai Block and the southern Trivan-drum Block that are presumed to be separated by the Achankovil

Shear Zone (Fig. 20.2; reviewed in Braun & Kriegsman 2003).In contrast to the Salem Block, the Madurai Block and the Trivan-drum Block show abundant metasedimentary rocks that wereaffected by high-grade metamorphism during Late Neoproterozoictime (Ediacaran–Cambrian, reviewed in Braun & Kriegsman2003; Ghosh et al. 2004; Collins et al. 2007b among others). Ac. 300 km-long and 70 km-wide zone that separates the SalemBlock from the Madurai Block is composed of a plethora ofrocks that are akin to both the Salem Block and the MaduraiBlock and have a protracted magmatic, deformation and meta-morphic history ranging from c. 2900 to c. 500 Ma (reviewed inChetty 1996; Mukhopadhyay et al. 2003; Ghosh et al. 2004;Dutta et al. 2011; Plavsa et al. 2012; Brandt et al. 2014). It hasbeen suggested in many studies that a number of crustal-scaleshear zones (collectively termed the Cauvery Shear System,CSS) have dissected the rocks of this narrow belt (Fig. 20.2;reviewed in Chetty 1996; Dutta et al. 2011; Plavsa et al. 2012;Brandt et al. 2014). The PCSZ, which marks the southern boundaryof the CSS, is perceived to be a terrane boundary along whichthe Madurai and Trivandrum blocks were amalgamated withthe Salem Block, and remaining part of the CSS during thePan-African orogeny (reviewed in Collins et al. 2007a, b; Yellappaet al. 2012). An interlayered suite of metasedimentary rocks con-sisting of orthoquartzite–metapelite–marble–calc-silicate rocks

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) Cardamom Hills; (3) Biligirirangan

Hills; (4) Nilgiri Hills; (5) Shevaroy Hills; (6) Kollimalai Hills; (7) Palni Hills; (8) Coorg Hills. All other acronyms are as in Figure 20.1.

SOURCE AND TIME OF SEDIMENTATION 299



occurs as detached outcrops in a country of Neoarchaean to Palaeo-proterozoic felsic orthogneisses (reviewed in Meissner et al. 2002;Ghosh et al. 2004; Collins et al. 2007a, b; Sengupta et al. 2009;Kooijman et al. 2011; Plavsa et al. 2012; Brandt et al. 2014).The metasedimentary rocks occur on both sides of the PCSZ(Fig. 20.2). U–Pb ages of zircon in a few metasedimentaryrocks suggest that the latest deformation and metamorphismof the metasedimentary rocks occurred during Pan-African time(c. 650–520 Ma, Raith et al. 2010; Kooijman et al. 2011). Petrolo-gical studies on the metasedimentary rocks, albeit from only twolocalities near the PCSZ, demonstrate that the metasedimentarypackets were buried to great depth (corresponding to .9 kbar)and were metamorphosed between 700 and 800 8C (Senguptaet al. 2009; Raith et al. 2010). The northernmost outcrop of themetasedimentary rocks of the SGT lies several hundred kilometressouth of the unmetamorphosed sedimentary sequence of theCuddapah Basin (Figs 20.1 & 20.2). In a reconstruction of Gond-wanaland (Fig. 20.1), the Western Dharwar Craton and the SGTare juxtaposed against the Mesoarchaean Antongil–Masora andthe Neoarchaean Antananarivo blocks of Madagascar, respect-ively. On the basis of contrasting sources of protoliths of the meta-sedimentary rocks overlying the Meso- and Neoarchaean blocksof Madagascar, it is assumed that a suture zone (BTSZ) existsbetween these two Archaean blocks (reviewed in Collins et al.2003). The BTSZ is considered as a late Neoproterozoic suturezone along which the Mozambique Ocean might have been con-sumed (Collins et al. 2003, 2007a). Based on a few U–Pb ages ofdetrital zircon grains in metasedimentary rocks of the SGT,

Collins et al. (2007b) postulated that the PCSZ represents the east-ward continuation of the BTSZ (Fig. 20.1; reviewed in Collins et al.2003). The idea proposed by Collins et al. (2007a) has beensupported (Chetty 1996; Sajeev et al. 2009; Yellappa et al. 2012)and contradicted (Ghosh et al. 2004; Plavsa et al. 2012; Brandtet al. 2014) by subsequent studies. Lack of convincing field evi-dence in favour of crustal-scale shear zones in the eastern part ofthe PCSZ (Mukhopadhyay et al. 2003) also contradicts the modelof Collins et al. (2007a).

Analytical methods, description of samples

Zircon grains were separated from the studied rock samplesemploying standard procedures (crushing, sieving, magnetic andheavy liquid separation). Representative zircon grains were thenhand-picked using a binocular microscope. The separated zirconfractions included all sizes and morphologies and for eachsample over 100 grains were selected and mounted on one-inchepoxy discs. The internal structures of the grains were documentedby cathodoluminescence and back-scattered electron imagingusing a Jeol 8900 JXA Superprobe. The U–Pb isotope ratios ofthe grains were measured using a Thermo-Fisher Element 2sector field ICP-MS coupled to a New Wave UP193HE ArFexcimer laser system at the Institut für Mineralogie, WestfälischeWilhelms-Universität, Münster, Germany. Details of the analyticaltechniques are given in Kooijman et al. (2012). Repeat in-runmeasurements of the 91500 standard zircon (Wiedenbeck 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).




1995) yielded an external reproducibility (2s, n ¼ 20) of 1.5% for206Pb/238U and 2.8% for 207Pb/206Pb. All uncertainties arereported at the 2s level.

Three rocks, two metapelites and one porphyritic granite, werechosen for this study. The metapelite samples were collectedclose to the northern margin of the PCSZ (Fig. 20.2), whereasthe porphyritic granite that intruded and co-folded with the ortho-quartzite–pelite–carbonate unit of Alambadi was collected southof the PCSZ (Fig. 20.2). In the following section we present a briefdescription of the field features and highlight the morphology andzoning patterns of the zircon grains recovered from the three rocks.

Metapelite UNG64 (N11827.9600E77827.9600)

The sample was collected from a metasedimentary enclave withinthe virtually undeformed Pan-African (c. 560 Ma, U–Pb zircondate, Brandt et al. 2010) alkaline granite complex of Sankagiri(Fig. 20.4a). This is a garnet–biotite–kyanite schist that is profu-sely veined by quartz (Fig. 20.4b). Intercalated marble and calc-silicate rocks were deformed and metamorphosed together withthe metapelite. Petrological study demonstrated that the metasedi-mentary sequence was buried to a depth corresponding to 9 kbar(700–750 8C), followed by uplift along a steeply decompres-sive pressure–temperature path (Sengupta et al. 2009). Contactmetamorphism by the pegmatitic granite caused replacement ofbladed kyanite by fibrous sillimanite (Sengupta et al. 2009).

Morphology and zoning patterns of zircon. The zircon grains haverounded to irregular outlines but a few elongated grains (width–length ratio c. 1:2) show a subhedral to euhedral shape. The oscil-latory zoning in most grains shows planar truncations, suggestingthat they are detrital grains (Fig. 20.5a). Overgrowths on oscil-latory zoned detrital grains show higher luminescence and a‘patchy’ internal structure (Fig. 20.5a). They are very thin alongthe prisms but rather broad at the crystal ends. At the immediateinterface with the detrital grains, very thin high and low lumines-cence zones are developed. Ragged interfaces indicate minorresorption of the detrital grains through dissolution by porefluids. The patchy zoning of the rims argues against their crystal-lization from a melt for which the rock lacks evidence.

Psammitic paragneiss N58 (N1185.9490E7888.9640)

This metasedimentary rock sample was collected from a roadsideexposure near the northern bank of the Cauvery River (Fig. 20.2).A compositional banding is manifested by centimetre- to deci-metre-thick quartz-rich (psammitic) bands that are intercalatedwith quartz-poor pelitic schist (Fig. 20.4c, d). In the psammiticlayers, a prominent schistosity subparallel with the composi-tional banding is defined by preferred orientation of biotite. Theschistosity and the compositional banding show outcrop-scalefolds (Fig. 20.4c). Porphyroblasts of kyanite and garnet are devel-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.




thin (,1 cm-thick) leucosomes that never exceed more than15 vol% of the rock.

Morphology and zoning patterns of zircon. Zircon grains showdiverse morphologies and internal structures (Fig. 20.5b). Mostof the grains are rounded to anhedral with a few grains showingeuhedral prismatic shape. Core regions of both euhedral and anhe-dral grains show fine oscillatory zoning that is truncated by grainoutline as well as highly luminescent, structureless, discontinuousrims (Fig. 20.5b). In a few euhedral grains, the oscillatory zonedcores are truncated and overgrown by thin low-luminescencemantles and the latter are truncated by high-luminescence rims(Fig. 20.5b). Featureless low-luminescence domains on high-luminescence cores (with or without oscillatory zoning) arepresent in a few grains. The morphology of the oscillatory andhigh-luminescence cores and their truncation by high-lumines-cence rims suggests that the cores are detrital grains on whichthe luminescent rims grew in situ during metamorphism.

Porphyritic granite A72 (N11845.2020E7883.4280)

The sample was collected from a porphyritic granite whichintruded the folded metasedimentary (orthoquartzite–shale–carbonate) unit near Alambadi, south of the supposed PCSZ(Fig. 20.2). Rafts of calc-silicate rocks and quartzite in thegranite further corroborate the intrusive relation with the metasedi-mentary unit (Fig. 20.4a). The granite and the metasedimentary

country rocks were subsequently deformed and metamorphosed.A distinct L-S fabric and augen-structure was developed in thegranite (Fig. 20.4e, f ), although in low-strain domains a magmaticflow structure defined by euhedral K-feldspar phenocrysts in afiner-grained matrix of plagioclase, quartz and biotite may stillbe preserved. The L-S fabric is folded and the geometry of thefolds is similar to the regional F2 folds defined by the adjoiningmetasedimentary package (Mukhopadhyay et al. 2003; our data).

Morphology and zoning patterns of zircon. Zircon grains exhibit avariety of morphology with most grains showing euhedral pris-matic shapes (Fig. 20.5c). Aspect ratios of these grains are c. 3:1.The morphology of euhedral grains is consistent with their crystal-lization from a melt phase, presumably during magmatic crystalli-zation of the protolith of the rock. The euhedral shape is lessdistinct and the aspect ratio varies considerably in other grains(Fig. 20.5c). Distinct oscillatory zoning patterns largely preservethe magmatic crystal shape and growth structures. A few grainsshow featureless low-luminescence cores. Highly luminescentand featureless overgrowths are restricted to the ends of the crys-tals and rarely truncate the oscillatory zoning (Fig. 20.5c).

Results

In this section 206Pb/238U and 207Pb/206Pb dates of the differenttextural domains of the representative zircon grains from eachsample are presented. Details of the analyses are represented

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 as206Pb/238U ages, whereas for older zircondomains, the 207Pb/206Pb ages are given.




in Figure 20.6. The U–Pb data of zircon grains in each sample areplotted in concordia diagrams. Only concordant to near-concor-dant ages (,10% discordance) were considered for provenanceanalysis and bracketing the age of sedimentation. Furthermore,207Pb/206Pb ages are used for spots that yielded ages older than1000 Ma whereas 206Pb/238U ages are used for spots that yieldedages younger than 1000 Ma. The program Isoplot-3.00 (Ludwig2003) was used for statistical analysis of the age data and con-struction of Concordia plots.

Metapelite (sample UNG 64)

A total of 61 spots were analysed on 23 zircon grains; 40 out of 61analyses show ,10% discordance. The truncated oscillatory detri-tal grains show an age spectrum ranging from 2509 to 836 Ma(Fig. 20.5a). Near-concordant ages of detrital zircons cluster intwo distinct populations: 2448+7 and 897 + 20 Ma (Fig. 20.6a).The ages of thin overgrowths that presumably date the in situgrowth of metamorphic zircon fall within 633–487 Ma.

Psammitic paragneiss (sample N58)

In sample N58 69 spots were analysed on 34 grains. Out of 69analyses, 46 dates show ,10% discordance while the remain-ing analyses show 30–89% discordance. The detrital coresyield a spectrum of ages (3030–2230 Ma for ,10% discordantgrains; Figs 20.6 & 20.5b). Compared with the metapelite UNG64, the detrital zircon population of this sample does not containNeoproterozoic components and shows a tight cluster around2395 + 6 Ma (81% of the analyses, Fig. 20.6b). Three-spot ana-lyses of the highly luminescent rims yielded 206Pb/238U ages inthe range of 567–525 Ma (two reversely concordant and one,10% discordant).

Porphyritic granite (sample A72)

A total of 81 spots were analysed on 27 grains out of which onlyeight analyses show .10% discordance and are not considered

for discussion. The oscillatory zoned domains give ages tightlyclustering between 790 and 760 Ma with an average of768 + 3 Ma. The rare dark luminescent cores give slightly olderdates (803–815 Ma). The three overgrowths on the oscillatoryzoned domains give an average age of 570 + 19 Ma.

Discussion

Timing of sedimentary basin formation on the SGT and the

temporal relation with the Purana basins of the Indian shield

Constraining the time of sedimentation of the metamorphosedorthoquartzite–shale–carbonate sequence is not straightforward.Nevertheless, the age of the oldest metamorphic event that affectedthe sediments and the youngest age of detrital zircon grainsprovide the temporal window within which the sedimentation andhence the basin formation took place (Collins et al. 2003, 2012;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-morphic zircons from two metasedimentary samples (UNG64 andN58) from the CSS. Also included in Figures 20.7 and 20.8 arepublished ages of metamorphic and detrital zircon grains frommetasedimentary sequences of northern Madagascar, the CSS,the Madurai Block, the Trivandrum Block and the Vindhyanbasin – the largest Purana basins in the Indian shield. The detritaland metamorphic zircon populations in sample UNG 64 indicatedeposition of the sedimentary suite in the narrow time spanbetween 836 and 633 Ma (Fig. 20.6a). This age bracket of sedi-mentary basin formation is in good agreement with the agebracket of sedimentation (689–513 Ma) of the protoliths ofsome metapelites from the Madurai and Trivandrum blocks(Fig. 20.7; Collins et al. 2007b). Owing to the paucity ofyounger detrital zircon grains, a wide time bracket for sedimentarybasin formation is obtained from the zircon data of two samplesthat lie close to the PCSZ (N58: 2280–567 Ma; Ayy-1, Raithet al. 2010: 1992–615 Ma). The four metasedimentary samplesfrom the Madurai Block analysed by Kooijman et al. (2011)suggest that sedimentation in these areas occurred within a broad

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 and639 + 4 Ma are evident.




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).




time span of c. 1750–990 Ma (Fig. 20.7). A similar wide timebracket for sedimentary basin formation (1900–515 Ma) is alsoreported by Collins et al. (2007b) from the Trivandrum Block(Fig. 20.7). All of these observations support the view that sedi-mentation of the protoliths of the metasedimentary rocks thatdeveloped on the Archaean/Palaeoproterozoic gneissic basementof the CSS and Madurai Block occurred over a protracted periodof time. The intercalated sequence of quartzite–metapelite–meta-carbonate rocks that are exposed in the CSS and Madurai Block islikely to represent the metamorphosed equivalents of platformalshallow-marine sandstone–shale–carbonate suites. The precursorsediments of these sequences, therefore, bear a resemblance to thePurana basin fills in terms of broad depositional environment, theage populations of detrital zircons and the timing of sedimentarybasin formation (see Figs 20.6 & 20.7). The southernmostexposure limit of the Purana basins, the Cuddapah Basin and itsequivalents occurs several hundred kilometres north of the north-ernmost exposure limits of the Proterozoic metasedimentaryrocks of the SGT (Fig. 20.1). It seems, therefore, likely that thePurana basins extended further south, at least up to Tirunelveli,where exposures of interbanded quartzite, metapelite and marbleare widespread (Fig. 20.7). In this scenario, the southern part ofthe vast shallow-marine intracontinental deposits of the extendedPurana basins were deformed and metamorphosed during thePan-African orogenesis (cf. Sengupta et al. 2009; Raith et al.2010; Kooijman et al. 2011; Sato et al. 2011a; Brandt et al. 2014).

Provenance of precursor sediments of the studied

metasedimentary rocks

Although the inferred precursors of the metasedimentary units thatoccur across the PCSZ resemble the stable shelf deposits of thePurana basin fills, superimposed deformation and metamorphismhave obliterated most sedimentary structures (e.g. palaeoslopemarkers) of the former rocks. Nevertheless an attempt has beenmade to identify the likely hinterlands of the metasedimentaryassemblage integrating the U–Pb/Pb–Pb ages of the detritalzircon grains and geochronological information of the crystallinebasement rocks from the adjoining parts of the Indian shield andfrom the continents that were once contiguous within the recon-structed Gondwanaland (Fig. 20.3). Concordant and nearlyconcordant 206Pb/238U and 207Pb/206Pb ages of detrital zirconpopulations in the spatially adjoining samples UNG64 and N58can be divided into four age clusters: 3030–2600, 2550–2450,2400–2200 and 1092–836 Ma. Zircon grains of the oldest popu-lation, although few in number and restricted to sample N58, arelikely to have their source in the Western Dharwar Craton andthe CSS where rocks of this age are reported (Fig. 20.3). TheVindhyan sedimentary sequence is considered to be representativeof the Purana basins and it contains detrital zircons that exhibitages ranging from c. 3.3 to 1.0 Ga. Therefore, the possibilitycannot be completely ruled out that these oldest zircon popu-lations, at least part of them, could also have been derived fromrecycling of the Purana sedimentary sequences. It is intriguingthat sample UNG64, which marks the northernmost outcrop ofthe quartzite–metapelite–marble ensemble in the SGT, does notcontain any contribution from the Western Dharwar Craton (orfrom Purana sedimentary rocks), whereas the quartzite sampleMR-48 of Kooijman et al. (2011) that comes from an area southof the PCSZ has this older component. Sampling bias and varieddrainage patterns may explain this diversity of detrital zirconpopulation. Detrital zircons of the 2550–2450 Ma age cluster aremost likely derived from the Eastern Dharwar Craton and theCSS where rocks of this age range are reported (Fig. 20.3). Identi-fication of the source of zircon grains of the 2400–2200 Ma agecluster is not straightforward. Extant geochronological infor-mation, although sparse, suggests that, barring one locality fromRajasthan, where 2240 Ma old felsic rocks are found (Khairmalia

felsites; Malone et al. 2008), evidence for 2400–2200 Ma oldigneous or partial melting events that led to zircon growthare hitherto not reported from the Indian shield. Intriguingly,2400–2200 Ma-old detrital zircons are abundant in the Vindhyansedimentary sequence that covers the northern Indian shield(Turner et al. 2014; Figs 20.1 & 20.6). This observation raisesthree possibilities: (1) the zircon grains in the Vindhyan sequenceare derived from the Aravalli Craton; (2) rocks of this age rangehave not yet been dated from the Indian shield; or (3) rocks ofthis age range are lying beneath the Phanerozoic volcanic and sedi-mentary rock cover (see Fig. 20.2). In recent communications, deWaele et al. (2011) and Tucker et al. (2011) argued that c. 2300–2200 Ma-old detrital zircon grains that are present in the meta-sedimentary rocks of northern Madagascar were sourced fromthe Aravalli Craton. It is, therefore, possible that the detritalzircon grains from the third cluster could have been sourcedfrom the Aravalli Craton, although recycling of Purana sedimen-tary components remains an alternative possibility. Evidently, amore detailed study is warranted to support or reject the othertwo possibilities. The Neoproterozoic detrital zircon populationdefining the fourth age cluster (1092–836 Ma) could have beenderived from several areas within India and adjoining Sri Lanka,namely the Madurai Block, the Wanni and Vijayan Complexesof Sri Lanka, and the Eastern Ghats Belt, where crustal growthduring this period has been documented (Fig. 20.3 and the refer-ences cited therein). It is, however, not known if all of these ter-ranes were exhumed at this time to provide detrital componentsof the sedimentary precursor of sample UNG64.

Timing of metamorphism

Concordant to near-concordant (,10% discordance) ages ofzircon rims that developed on detrital (in samples UNG64 andN58) or magmatic zircon grains (sample A72) are likely to datethe metamorphism that affected these rocks. In view of overlap-ping 206Pb/238U ratios in zircon rims in all the three samples, thedata are combined for statistical analysis. The data define two dis-tinct age populations that fall within tight clusters at 639 + 4 and527 + 2 Ma (Fig. 20.6d), supporting the view that the metasedi-mentary rocks and the porphyritic granitoids underwent two dis-tinct metamorphic episodes. Two phases of metamorphism in theSGT across the Ediacaran–Cambrian boundary are also reportedby Raith et al. (2010; 615 + 11 and 529 + 9 Ma) and Kooijmanet al. (2011; clustering at 590 and 520 Ma). The older phase ofmetamorphism is also supported by the occurrence of the metape-lite (sample UNG64) as enclave within c. 560 Ma-old alkalinegranite in the Sankagiri area, and the thermal effects caused bythe granite host (Brandt et al. 2010). In view of this observation,the 639 + 4 Ma age is interpreted to date the metamorphism andcoeval deformation of the garnetþ kyanite-bearing metapeliteenclave (sample UNG64, Sengupta et al. 2009). Effects of Edia-caran or Cambrian metamorphic events are also widespreadthroughout the SGT (reviewed in Braun & Kriegsman 2003;Ghosh et al. 2004; Santosh et al. 2006; Collins et al. 2007a;Clark et al. 2009b; Kooijman et al. 2011; Brandt et al. 2014).

Can the PCSZ be a Neoproterozoic terrane boundary?

Several recent studies have argued that the PCSZ represents a Neo-proterozoic terrane boundary along which the southern MaduraiBlock was welded to the northern Salem Block during thePan-African orogenesis (e.g. Collins et al. 2003; Sajeev et al.2009; Yellappa et al. 2012). In the context of this suggestion thefollowing observations are relevant:

1. Orthoquartzite–shale–carbonate sequences of shallow-marine origin occur as deformed and metamorphosed keelsin the orthogneiss basement on both sides of the PCSZ.




2. Age populations of detrital zircons in quartzites and metape-lites indicate a similar provenance of the clastic material,with a substantial contribution from the Western DharwarCraton (and/or Purana sedimentary sequence).

3. The c. 955 Ma metamorphism of the metasedimentarypackages exposed south of the supposed PCSZ suggests thatsedimentation was completed before this time. This then sup-ports the interpretation that the Madurai Block, which hoststhese metasedimentary rocks, was connected to the DharwarCraton (the source of .3.3 Ga old detrital zircon grains)before c. 955 Ma (Fig. 20.8).

4. The 800–700 Ma old magmatic rocks (A-type porphyriticgranite, massif-type anorthosite, carbonatite–alkaline rocks,reviewed in Braun & Kriegsman 2003; Ghosh et al. 2004;Brandt et al. 2014) that represent magmatism in an exten-sional setting are present on both sides of the PCSZ(Fig. 20.8). Taken together these observations contradict theinterpretation that the PCSZ is a Neoproterozoic terraneboundary. Our study, therefore, supports the view of Mukho-padhyay et al. (2003), Ghosh et al. (2004) and Plavsa et al.(2012) that no significant break in terms of geological andgeochronological characters exists across the PCSZ.

Implications of this study in the context of the

Indo-Madagascar connection

In the reconstructed Gondwanaland the Indian shield is juxtaposedagainst Madagascar (Fig. 20.8). In this reconstruction the Palaeo-to Mesoarchaean Western Dharwar Craton faces the Antongil–Masora Block of the same age, whereas the Salem Block, theCSS and the Madurai Block face the Neoarchaean AntananarivoBlock of east Madagascar (Fig. 20.8, reviewed in Rekhaet al. 2013). Highlighting contrasted age populations of detritalzircons in metasedimentary units lying across the BTSZ(.3300 Ma ages in the eastern part and 2300–1800 Ma ages inthe western part of the BSTZ), Collins et al. (2003) consideredthe BTSZ to represent a fossil ‘suture zone’ along which theMozambique Ocean closed during the Pan-African orogeny. Theimplication of this model is that the western Antananarivo Blockand eastern Mesoarchaean Antongil–Masora Block representtwo sides of the Mozambique Ocean and the two blocks were jux-taposed only during the late Neoproterozoic time. The model ofCollins et al. (2003), however, has been recently challenged byTucker et al. (2011) and de Waele et al. (2011) on the groundsthat Mesoarchaean detrital zircon populations are also present inthe metasediments that developed on the western AntananarivoBlock (the Itremo unit). According to these authors a coherentlandmass, including the Antongil–Masora–Antananarivo blocks,existed well before Neoproterozoic time. This observation thuscontradicts the interpretation that the BTSZ represents a Neopro-terozoic terrane boundary. The Antananarivo Block of Madagascarshares many geological features with the crust exposed in theSalem Block, the CSS and the Madurai Block (Fig. 20.8). Theseinclude:

1. A c. 2.5–2.6 Ga orthogneiss basement is common to all ofthese terranes (Ghosh et al. 2004; de Waele et al. 2011;Plavsa et al. 2012; Brandt et al. 2014).

2. The Neoarchaean crust of the Antananarivo Block, the CSSand the Madurai Block was covered by shallow-marine sand-stone–shale–carbonate sequences that were deposited poss-ibly in two cycles, between 1800–800 and 800–620 Ma(Figs 20.7 & 20.8). Part of the detrital components in thesesediments (now deformed and metamorphosed) was sourcedfrom a unified Mesoarchaean landmass (Western DharwarCraton–Antongil–Masora) lying to the north (Fig. 20.8).

3. c. 2300–1800 Ma detrital zircon populations are common inthe sedimentary sequences that developed on the gneissic

basement of the Antananarivo Block, the CSS (samplesUNG64 and N58) and the Madurai Block (Fig. 20.7).

4. c. 800–700 Ma rift-related magmatism (A-type granitoids,alkaline rocks and massif-type anorthosite) and associatedthermal metamorphism are common to all of the blocks(Collins et al. 2003; Ghosh et al. 2004; de Waele et al.2011; Kooijman et al. 2011; Brandt et al. 2014; and thisstudy).

5. The Proterozoic basement, the sedimentary cover rocks andthe c. 700–800 Ma magmatic rocks of the AntananarivoBlock, the CSS and the Madurai Block became intenselydeformed and metamorphosed during the c. 600–500 MaEdiacaran/Cambrian tectonothermal events (Figs 20.7 &20.8).

In view of the marked similarity in geological histories of northernMadagascar and the SGT, it seems likely that the Precambrianrocks of the Antananarivo Block, the Salem Block, the CSS andMadurai Block formed a unified landmass (Greater DharwarCraton) at least before 800 Ma. If the c. 955 Ma metamorphicimpressions on some metasedimentary units of the MaduraiBlock are considered, this unified Indo-Madagascar landmassseems 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 asnorthern Madagascar supports the proposition of Tucker et al.(2011, 2014) and de Waele et al. (2011) that the WesternDharwar Craton and adjoining parts of the Indian shield provideddetritus for the Proterozoic sedimentary basins developed on theunified Indo-Madagascar landmass. If this model remains valid,a large part of the Neo- and Meso-Archaean crusts of central andnorthern Madagascar (at least up to the Ranotsara–BongolavaShear Zone, Fig. 20.8) and the SGT of India would represent onecontinuous strand line of the perceived Mozambique Ocean. Thesouthern limit of this shallow-marine sedimentary sequenceshould be marked by metasedimentary rocks where the ‘detritalfootprint’ of Mesoarchaean rocks would be recognized(Fig. 20.8). At present this signature is traceable up to the Siruma-lai hills in the Madurai Block (Fig. 20.8). Geological evidence forthe closure of this ocean and, hence, the location of the late Neo-proterozoic suture should be searched in areas lying furthersouth of this unified crustal block. Detailed geological and geo-chronological studies on rocks from the southern part of theMadurai Block and the Trivandrum Block are required to ascertainif the Mozambique Ocean suture passed through the SGT.

P. S., M. T., P. C. and S. S. acknowledge the financial support from the Council of

Scientific and Industrial Research New Delhi. S.S. and P.S. also acknowledge the

financial assistance from the Center of Advance Studies, Department of Geologi-

cal Sciences, Jadavpur University and University Potential for Excellence (Phase

II), Jadavpur University. D. M. acknowledges support through the Honorary

Scientist Project of the Indian National Science Academy. The fieldwork of M.

R. was financially supported through RWE-grant PN 71734. E. K. and the analyti-

cal work were supported by a Leibniz Award (DFG, Germany) to K. M. We thank

Dr U. K. Bhui (Pandit Deendayal Petroleum University, India) and Dr U. Dutta

(Indian School of Mines, India) for their help in the field and many stimulating

discussions. We sincerely thank Professor A. Hofmann and an anonymous

reviewer for their critical but very constructive comments, which have improved

the clarity of the manuscript. We also thank Professor R. Mazumder for inviting

us to contribute to this Geological Society of London Memoir and for his

editorial remarks.

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Chapter 20 Provenance, timing of sedimentation and … · Chapter 20 Provenance, timing of sedimentation and metamorphism of metasedimentary rock suites from the Southern Granulite