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Petrotectonic framework of granulites from northernpart of Chilka Lake area, Eastern Ghats Belt, India:

Compressional vis-a-vis transpressional tectonics

Kaushik Das1,∗, Sankar Bose

2,∗, Subrata Karmakar3 and Supriya Chakraborty

2

1Department of Earth Sciences, Bengal Engineering and Science University, Shibpur, Howrah, India.2Department of Geology, Presidency University, Kolkata, India.

3Department of Geological Sciences, Jadavpur University, Kolkata, India.∗Corresponding author. e-mail: [email protected]

Granulite-facies rocks occurring north-east of the Chilka Lake anothosite (Balugan Massif) show a com-plex metamorphic and deformation history. The M1–D1 stage is identified only through microscopicstudy by the presence of S1 internal foliation shown by the M1 assemblage sillimanite–quartz–plagioclase–biotite within garnet porphyroblasts of the aluminous granulites and this fabric is obliterated in outcropto map-scale by subsequent deformations. S2 fabric was developed at peak metamorphic condition (M2–D2) and is shown by gneissic banding present in all lithological units. S3 fabric was developed due to D3

deformation and it is tectonically transposed parallel to S2 regionally except at the hinge zone of the F3

folds. The transposed S2/S3 fabric is the regional characteristic structure of the area. The D4 event pro-duced open upright F4 folds, but was weak enough to develop any penetrative foliation in the rocks exceptfew spaced cleavages that developed in the quartzite/garnet–sillimanite gneiss. Petrological data suggestthat the M4–D4 stage actually witnessed reactivation of the lower crust by late distinct tectonothermalevent. Presence of transposed S2/S3 fabric within the anorthosite arguably suggests that the pluton wasemplaced before or during the M3–D3 event. Field-based large-scale structural analyses and microfabricanalyses of the granulites reveal that this terrain has been evolved through superposed folding eventswith two broadly perpendicular compression directions without any conclusive evidence for transpres-sional tectonics as argued by earlier workers. Tectonothermal history of these granulites spanning inNeoproterozoic time period is dominated by compressional tectonics with associated metamorphism atdeep crust.

1. Introduction

Reconstructing structural history of a poly-meta-morphosed and poly-deformed high-grade terrainis often a difficult task as imprints of earlierevents are seldom preserved during subsequenthigh-temperature metamorphism. Mineralogicalchange, partial melting, intensive deformation andrecrystallization of the earlier foliation-defining

assemblages are collectively responsible for oblit-erating earlier structures in granulite-grade rocks.This is particularly a problem for high-temperatureand ultra-high temperature granulites. Deforma-tion and kinematic analysis of lower crustal rocksis often fraught with uncertainty but success-fully adopted in many regional granulite terranes(Goscombe 1992; Kelly et al 2000; Forbes et al2007; Sarkar et al 2007 among others). Integrated

Keywords. Poly-phase metamorphism; superposed folding; Chilka Lake granulites; Eastern Ghats Belt; oblique collision.

J. Earth Syst. Sci. 121, No. 1, February 2012, pp. 1–17c© Indian Academy of Sciences 1

2 Kaushik Das et al

metamorphic and deformation history with a rea-sonable support from geochronological data, canbe used as a significant tool to understand evolu-tionary history of orogenic belts (cf. Moller et al2007). Such orogenic belts of Precambrian eraare presently manifested as eroded mountain beltsbordering cratons and are key witnesses to ever-changing lithospheric processes (Saha et al 2008;Karmakar et al 2009). The history of continen-tal growth and eventual juxtaposition of ancientcontinents during supercontinent assembly can betraced back from such mountain belts exposed indifferent continents (Zhao et al 2004; Rogers andSantosh 2002, 2009). Continental growth, either bycollisional or by accretional orogeny depends onrelative positions of the continental and/or oceanicplates, having distinct magmatic, metamor-phic and deformation styles. Understanding thecomprehensive history of deformation and meta-morphism during orogenesis in Precambrian timeis a major challenge.

Eastern Ghats Belt (EGB) of India witnessedmultiple cycles of orogeny throughout the Protero-zoic eon (Dasgupta and Sengupta 2003; Dobmeierand Raith 2003; Bhadra et al 2004; Sarkar et al2007). It is now understood that each of theseorogenies was connected to global-scale supercon-tinent cycle. As a consequence, the EGB standsalone as a witness to the formations of Columbia,Rodinia and Gondwana (cf. Karmakar et al 2009;Bose et al 2011a). Chilka Lake area is situatedat the northern fringe of EGB close to the Singh-bhum Craton. This area exposes rocks of petrolog-ical, structural and isotopic characteristics whichare quite distinctive compared to the rest of EGB.This distinction led earlier workers to conceiveChilka Lake area as a separate block whose ances-try is poorly known. The deformation history isarguably dominated by transpressional tectonicsunlike other parts of EGB (Dobmeier and Simmat2002). This particular point needs to be reap-praised because an entire geodynamic evolution

Figure 1. General lithological map showing the Eastern Ghats granulite belt and the present study area. The subdivisionsare after Ramakrishnan et al (1998). Isotopic domains (shown as roman numerals within circles) are after Rickers et al(2001). Abbreviations used are MR: Mahanadi Rift, GR: Godavari Rift, SSZ: Sileru Shear Zone, KSZ: Koraput–SonepurShear Zone, EGBSZ: Eastern Ghats Boundary Shear Zone, MSZ: Mahanadi Shear Zone, NSZ: Nagavalli Shear Zone, VSZ:Vamasdhara Shear Zone.

Petrotectonic framework of granulites from Chilka Lake area 3

was based on it. A new set of independentlyacquired field data and its evaluation would bethe foremost and simplest proposition to startwith.

The present study area is located at the north-eastern part of Chilka Lake, EGB, and extendsalong a linear belt from Khurda to Sunakhalacovering approximately 700 km2 area (figure 1).For convenience in presenting structural data anddetailed structural analyses, the present study areais divided into two sectors. Sector-1 constitutes thenorth-eastern part of the study area within the lati-tude 20◦00′–20◦15′N and longitude 85◦30′–85◦45′E.Sector-2 constitutes the south western part ofstudy area within the latitude 19◦45′–20◦00′N andlongitude 85◦30′–85◦15′E.

2. Geological background

The EGB occurs along the eastern coast of India(figure 1) and exposes diverse rock types that expe-rienced granulite-grade metamorphism. In an ear-lier study, this terrain was longitudinally dividedinto four zones based on predominant lithologi-cal association, viz., Western Charnockite Zone(WCZ), Western Khondalite Zone (WKZ), CentralMigmatite Zone (CMZ) and Eastern KhondaliteZone (EKZ) (Ramakrishnan et al 1998). On theother hand, Nd-isotopic mapping over the entireEGB and based on Sm–Nd, Rb–Sr and Pb207–Pb206 isotopic data; Rickers et al (2001) identi-fied four distinct crustal domains whose geograph-ical boundaries grossly contradict the longitudinalsubdivision. Domain 1 is located at the northernand southern parts of the Godavari Rift. However,there are notable differences in isotopic signaturesbetween the northern (Domain 1B) and the south-ern domains (Domain 1A). Domain 1B is char-acterized by strong Archaean protolith signatures(3.9–3.2 Ga for orthogneisses) while the Domain1A preserves prominently Proterozoic protolith sig-natures and is characterized by homogeneous Nd-model ages for orthogneisses (2.5–2.3 Ga) andparagneisses (2.8–2.6 Ga). The other three domainsoccur on the north of the Godavari Rift. Domain3 occurs surrounding the Chilka Lake and showshomogeneous isotopic signatures of Proterozoic age(2.2–1.8 Ga for both orthogneisses and metasedi-ments). Domain 2 is sandwiched between Domain1 and Domain 3 and is isotopically the most het-erogeneous one (2.5–2.1 Ga for metasediments and3.2–1.8 Ga for orthogneisses). Domain 4 occursat the northern extremity of the EGB close tothe Mahanadi Rift with Nd-model ages 3.2 Gafor orthogneisses and 2.8–2.2 Ga for metasedi-ments. In a more recent classification, the entireEGB was subdivided into several crustal provinces

each containing several domains (Dobmeier andRaith 2003). It is notable that the crustal domainsdescribed by Rickers et al (2001) are different fromthose of Dobmeier and Raith (2003). Apart fromcontrasts in protolith histories, these domains andprovinces are characterized by contrasting styleand timing of deformation and petrological pro-cesses. It is now realized that evolution of EGBis to be reconciled in a time- and space-specificmanner.

The Chilka Lake Domain of Dobmeier and Raith(2003) (part of the Domain 3 of Rickers et al2001) occurs at the north-eastern part of the EGBwhere the present study is focused. This domainis characterized by the presence of a large body ofanorthosite massif in the central region. The rocksexposed north-east of this anorthosite massif com-prise high-grade granulites that witnessed multiplestages of deformation. Metapelitic granulites andcharnockites of this area were previously studied bySen et al (1995) who documented several phases ofdecompression and cooling-dominated P−T pathensued from a peak condition of ∼1100◦C and>10 Kbar. The geodynamic significance of sucha metamorphic history is neither supported fromthat particular study, nor was substantiated bylater work (as reviewed in Dasgupta and Sen-gupta 2003). Recent discovery of ultrahigh temper-ature (UHT) (>1000◦C) contact metamorphismnear the anorthosite massif (Raith et al 2007; Sen-gupta et al 2008) provided additional petrolog-ical data, but could not resolve the uncertaintyinvolving regional metamorphism and deforma-tion pattern shown by all the rocks. A furtherbone of contention is the tectonic setting vis-a-vis intrusion age of the anorthosite magma. Thelatter was initially fixed at ca. 792 Ma (Krauseet al 2001), but recent data of ca. 983 Ma(Chatterjee et al 2008) suggest an older emplace-ment age. The tectonic setting of the anorthositemagmatism is a contentious issue as such magmanormally is emplaced at extensional setting unlikethe proposed scenario in the Chilka Lake Domain.However, the adjacent Bolangir anorthosite mag-matism at ca. 933 Ma is interpreted to occur atcompressional setting (Dobmeier 2006).

Structural analysis around Chilka Lakeanorthosite by previous researchers identifiedevidence of multiple deformational episodes. Bhat-tacharya et al (1993, 1994) identified imprints ofthree deformational episodes based on structuralmapping and limited remote sensing data. Theyargued that the dominant NE–SW trending folia-tion (S1) of the Chilka Lake area can be correlatedwith regional trend of the EGB that was formedduring D1 deformational event. On the other hand,a four-fold deformational style was proposed byothers mostly based on satellite image analyses

4 Kaushik Das et al

with limited ground checks (Dobmeier and Raith2000; Dobmeier and Simmat 2002). Bhattacharyaet al (1994) identified early compositional lay-ering as S0 within khondalite, which eventuallywas isoclinally folded during D1 deformation. Onthe other hand, Dobmeier and Raith (2000) andDobmeier and Simmat (2002) designated the rocksuite of charnockite, enderbite and khondalite as‘migmatite complex’. According to these workers,the composite layering in this rock suite resultedfrom foliation-parallel injection of anatectic meltthat was produced during partial melting of thelower crust. It is not clear how such anatectic meltcould maintain uniform chemistry when producedfrom such a heterogeneous protolith. This compo-sitional layering (S0) was tightly folded during D1

deformation producing layer-parallel axial planarS1 fabric. Although both these working groupsinvoked a primary compositional plane (S0), it isdifficult to understand how such a primary struc-ture could be preserved after high-temperaturemetamorphism that all the rocks underwent subse-quently. F2 folding is isoclinal where S1 is partiallytransposed by S2 (Bhattacharya et al 1994). F3

folding is open and upright and the S3 plane isexposed as the fracture cleavage developed inboth metapelites and the ‘migmatite complex’along the axial plane of F3 folds (Bhattacharyaet al 1994). Dobmeier and Raith (2000) opinedthat S2 and S3 are transposed in nature and aremarked by fine-grained leucosome layer in the‘migmatite complex’. Dobmeier and Raith (2000)described a steeply dipping NNE–SSW trendingS4 fabric which is formed during F4 folding of D4

deformational event in local-scale. Pegmatites arealso emplaced along this S4 foliation. Geometry ofthis F4 folding is different from the last-formed F3

folding described by Bhattacharya et al (1994).There was also a controversy regarding the sta-tus of the patchy charnockites of Chilka Domain.The relict and vestigial ancestry of this rock(Bhattacharya et al 1994) was later refuted byDobmeier and Raith (2000) who argued thatcharnockites are formed in situ in leptynite dueto the channelization of synkinematic fluid dur-ing D3 deformational episode. A further problemstems from the status of the anorthosite pluton-ism as some argue its emplacement is post-D1

(Bhattacharya et al 1994), whereas others arguefor a post-D2 emplacement (Dobmeier and Simmat2002). The diverse opinions amongst the earlierresearchers on these petrological and structuralissues call for a re-evaluation and the present studyaims at unravelling the structural evolution of theChilka Lake granulites with new data. We wouldalso strive to compare this structural interpreta-tion with petrological and geochronological dataavailable from this domain.

3. Salient petrological featuresof the studied granulites

Garnet–sillimanite gneiss or khondalite is the mostcommon rock type of the studied area hostinga number of other rock types. Mineralogically,it contains quartz, alkali feldspar, garnet, silli-manite and opaque. The rock is strongly foliatedwith characteristic gneissic banding marked byalternate garnet–sillimanite-rich layer with quart-zofeldspathic layers.

Garnetiferous quartzofeldspathic gneiss or lep-tynite is leucocratic rock composed of quartz, alkalifeldspar, plagioclase, garnet and ilmenite with orwithout biotite. The rock is well-foliated with typi-cal gneissic fabric. The gneissic fabric is defined bythe concentration of mafic minerals such as garnet,biotite and ilmenite alternating with quartzofelds-pathic bands.

Aluminous granulite occurs in two contrastingmodes. One of its varieties is massive in appearanceand podiform in occurrence. The massive varietyof aluminous granulite occurs as centimetre-thicklenses, pinched and swollen layers and torn boudinswithin garnetiferous quartzofeldspathic gneisses.The immediate vicinity of the former is occupied bygarnet-free quartzofeldspathic aggregates identi-fied as leucosome fraction left out from the melano-cratic layers/lenses of the aluminous granuliteduring partial melting. The aluminous granuliteis characterized by coarse-grained garnet, orthopy-roxene, plagioclase and cordierite grains with occa-sional presence of spinel and sapphirine. Localabundance of quartz and alkali-feldspar is dis-cernable. Presence of biotite in this rock is con-spicuous. The other variety of the aluminousgranulite is distinctly migmatitic in nature.In the migmatitic variety, centimetre–millimetre-thick garnet–sillimanite–ilmenite layer alternateswith quartzofeldspathic layers.

Orthopyroxene-bearing quartzofeldspathic gneissoccurs as foliation-parallel layers and lenses withinthe garnetiferous quartzofeldspathic gneiss andtheir mode of occurrence is similar to those ofmassive variety of aluminous granulite. The rock ismedium- to fine-grained containing orthopyoxene,plagioclase, alkali-feldspar, quartz and biotite. Itshows strong gneissic fabric by alternate pyroxene-rich and quartz-feldspar-rich millimetre-thicklayering. The contact of this rock with thehost garnetiferous quartzofeldspathic gneiss ismostly sharp without any presence of chilled-margin. Mineralogically, this rock is akin to thecharnockite-enderbite group of rocks, but theirigneous ancestry is not proved. In addition tothis, patchy charnockites with broadly similarmineralogy are found within the garnetiferousquartzofeldspathic gneiss host. These charnockite


patches locally pseudomorph the garnetiferousquartzofeldspathic gneiss, but the foliation in thelatter could be traced in the former as continu-ous structure. These field features thus supportthe contention that the patchy charnockiteswere formed by in situ fluid-assisted metamor-phic process as invoked by Dobmeier and Raith(2000).

Two-pyroxene granulite is another memberof rock suite of garnetiferous quartzofeldspathicgneiss–aluminous granulite–orthopyroxene-bearingquartzofeldspathic rock ensemble. This rock con-tains both orthopyroxene and clinopyroxene alongwith plagioclase and ilmenite. Presence of quartzis nominal, but its abundance increases towardsits contact with the host leptynite and migmatitic

aluminous granulite. Presence of sulphide veinscontaining chalcopyrite and pyrrhotite is observedunder microscopic study. The rock is massive,fine-grained and recrystallized in nature. Pres-ence of possible chilled margin (now character-ized by profuse concentration of garnet) suggestsintrusive relation with the migmatitic aluminousgranulite. Petrological and geochemical character-istics of this rock are discussed in detail by Boseet al (2011b).

4. Regional-scale structures

In the study area (north–northeast of BalugaonMassif anorthosites), the major mapable lithounits are

Figure 2. Lithological maps of the study area. (a) North-eastern sector (sector 1) and (b) south-western sector (sector 2).

6 Kaushik Das et al

garnet–sillimanite gneiss (khondalite), garnetiferousquartzofeldspathic gneiss (leptynite) and a ‘mixedrock suite’ (composed of variable proportion ofaluminous granulites, two-pyroxene granulites,garnetiferous quartzofeldspathic gneisses andcharnockite–enderbite rocks) (figure 2a, b). Anarea covering ∼700 km2 has been structurallymapped in the scale of 1: 50,000 (figure 3a, b). The‘mixed rock suite’ occurs as an enclave or a pocketwithin garnet–sillimanite gneiss and garnetifer-ous quartzofeldspathic gneiss in regional scale.Structurally, four phases of penetrative defor-mation are identified. The primary depositionalsurface (S0) is not preserved either macroscopi-cally or microscopically probably due to intensivemultiphase deformation and transposition of S0

with successive structural fabrics in high-grademetamorphism. As a result, the transformationmechanism from S0 to S1 during D1 is completelylost. The S1 fabric, formed because of D1 deforma-tional event is rarely preserved in microscopic scalein aluminous granulites of ‘mixed rock suite’. So,signatures of D1 deformation could only be stud-ied from microstructures. S1 acts as a form-surfacewhich was affected by later deformational events(D2–D4). However, absence of S1 in outcrop- toregional-scale precludes identification of the secondfolding (F2) during structural mapping. On thecontrary, the pervasive penetrative S2 gneissicfoliation, developed during D2 deformation ispresent in all the rock units. The gneissic foliation

S2 in khondalite, leptynite and ‘mixed rock suite’are regionally folded during D3 deformationalevent. The overall disposition of the strike of S2

gneissic foliation systematically varies from E–Wto NE–SW and finally to N–S orientation fromthe northeastern part to the southwestern partof the study area (figure 3a, b). Regional-scaleclose to tight fold closures (F3) are envisagedfrom systematic variation of the S2 foliation.The F3 folds are gently-plunging inclined in nature.F3 fold axis regionally trends along NE–SW withplunge reversal with variable amounts of plunge(from 2◦ up to 40◦). The inter-limb angles varyregionally and the F3 folds are open at southernpart of the area. The newly-developed S3 foliationformed during D3 is almost parallel to S2 gneissicfoliation at limbs of tight to isoclinal F3 folds onoutcrop-scale. Therefore, the regional penetrativefoliation can be designated as ‘transposed S2/S3

fabric’ except at the regional F3 fold closures (suchas in Basanta Mundia). The absence of F2 foldingin outcrop-scale is conspicuous and only observedunder microscopic scale. The general NE–SWtrend of the S2 foliation is deflected near Kuhurigiving a southern fold closure adjacent to Balu-gaon Massif anorthosite complex (figure 3b). Thelast D4 deformational event is evident from thedevelopment of F4 folds on either S2 or transposedS2/S3 foliation that interfere with F3 throughoutthe area in outcrop- to regional-scale. The variableorientations of the F3 fold axes and axial traces on

Figure 3. Structural maps of the study area. (a) North-eastern sector (sector 1) and (b) south-western sector(sector 2).


regional scale (figure 3a, b) are probably due tomodifications during later deformational event(D4). However, the general trend of F4 axes isbroadly in N–S to NW–SE direction on regional-scale, whereas the F4 axial traces in someoutcrops are parallel to N–S. S4 was developedduring D4 and appears as spaced cleavage inoutcrop-scale. Detailed structural studies withmapping of the area in regional scale (1:50,000),outcrop-scale studies with plane paper mappingin amplified scale (1:100), vertical section drawingand sector-wise stereographic plotting have beencarried out to decipher the interference pattern ofF3 and F4 folds. The interference pattern of F3

and F4 on S2 foliation as well as transposed S2/S3

shows the non-plane non-cylindrical to plane non-cylindrical patterns in different sectors dependingon the initial geometry of F3 folding and local-scale strain variation. Evidence of superimposeddeformation between F3 and F4 are preserved inoutcrop-scale in aluminous granulites of ‘mixedrock suite’, khondalite and leptynite. Only aweak gneissic foliation, visible in anorthosite, atits contact of the above-mentioned granulites insouthwestern part of the study area is orientedbroadly either in E–W or NW–SE direction. Thefoliation in anorthosites was presumably devel-oped during the last intense deformation D3 thatgave rise to a strong transposed S2/S3 gneissicfoliation in the granulites. Weak variation in foli-ation traces in anorthosites is probably the resultof the D4 deformation.

5. Mesoscopic and microscopicstructures

Imprints of four penetrative deformations areobserved and have been designated as D1, D2,D3 and D4. Development of S1 foliation duringD1 event and development of F2 folds on S1

foliation during D2 event are documented onlyfrom microstructures. Development of S2 gneissicfoliation and successive structural signatures cor-responding to D2–D4 are evident in mesoscopic-as well as megascopic-scale. Later folds aredesignated as F3 and F4 corresponding to D3 andD4 deformational events respectively, whereas theaxial planar foliations of corresponding folds aretermed as S3 and S4, respectively. The character-istic metamorphic assemblages developed duringdifferent stages of deformational history are alsodescribed here in detail to correlate these two pro-cesses. From the microstructural study, it is evi-dent that M2–D2 assemblage was developed duringthe peak metamorphic stage, whereas M3–D3

and M4–D4 assemblages were developed duringretrograde stage.

5.1 D1 deformation and pre-peak M1–D1

assemblage

The signature of S0, the primary compositionalbanding is completely lost and earliest planar fab-ric, identifiable only under microscope, is a schis-tosity, which is now designated as S1. This earliestplanar fabric S1 is absent in the macroscopic- tomegascopic-scale in all rock units due to overprint-ing of subsequent deformation under high-grademetamorphic conditions.

Microscopically, traces of S1 fabric are preservedin the aluminous granulites. Inclusion trails ofsillimanite are present within garnet porphyro-blasts of both the types of aluminous granulites.In the massive variety, such garnet porphyroblastslocally also contain rounded inclusion of quartz,plagioclase and biotite. The included sillimanite–biotite–ilmenite assemblage within garnet porphy-roblasts in the migmatitic variety is often folded(figure 4a). This internal foliation composed ofbiotite, sillimanite, ilmenite and quartz, repre-sents the traces of an earlier planar fabric (S1).Thus, the early M1–D1 assemblage is broadlyrepresented by biotite–sillimanite–quartz–plagio-clase developed during pre-peak metamorphicconditions.

5.2 D2 deformation and peak M2–D2

assemblage

Mesoscopically, a penetrative foliation (S2), definedby gneissic fabric has been developed during D2

deformational event. Folded internal foliationshave been documented only under microscope(figure 4a). Gneissic foliation, S2 is developednearly in all the rock units present in the studyarea. In the aluminous granulites, S2 fabric ischaracterized by alternate banding between por-phyroblastic garnet–sillimanite–cordierite andquartzofeldspathic layers. In khondalite, thisfoliation is defined by alternate garnet–sillimanite±biotite bearing mesocratic and quart-zofeldspathic leucocratic layers. On the otherhand, in leptynite, S2 fabric is characterized bygarnet-rich mesocratic layer and quartzofelds-pathic leucosomal layers. In regional-scale, theorientation of S2 gneissic foliation systematicallyvaries from northeast part of the study area to thesouthwestern part, which resulted from superim-position of later deformational events, discussedlater.

Microscopically, the peak assemblage (M2) isrepresented by porphyroblastic orthopyroxene–garnet-cordierite–plagioclase in massive aluminousgranulites which is surrounded by quartzofelds-pathic leucosome. In the migmatitic aluminousgranulite, peak assemblage is represented by

8 Kaushik Das et al

Figure 4. Photomicrographs of studied granulites. (a) Sillimanite–biotite–ilmenite (Sil+Bt+Ilm) S1-assemblage within gar-net (Grt) porphyroblast in the migmatitic variety of aluminous granulite. Note the folded nature of S1 and its relationshipwith S2 in the studied section. (b) Garnets porphyroblast are flattened along the foliation in leptynite.

garnet–sillimanite–ilmenite-bearing dark layersalternating with quartzofeldspathic leucosomelayers. The leucocratic layers in both the varietiesof aluminous granulites are composed of quartz–perthite–plagioclase. Their relative proportionvaries considerably in different samples.

In the charnockite/enderbite gneiss, a weak foli-ation is defined by the flattened grains of quartz,orthopyroxene and feldspar grains whereas in theleptynite, this foliation is characterized by flat-tened quartz and feldspar grains. Garnet por-phyroblasts occasionally show flattening along thefoliation in this rock (figure 4b).

Two-pyroxene granulites are characterized bya weak foliation defined by the assemblageorthopyroxene–clinopyroxene–plagioclase–ilmenite,which were stabilized at peak metamorphiccondition. Minor amount of quartz is also present.

5.3 D3 deformation and post-peakM3–D3 assemblage

In the outcrop-scale, all the lithounits systemati-cally show folded S2 assemblage in the same styleas the regional folding. The F3 folds are tightto isoclinal, gently inclined and gently plunging(fold axis plunges 18◦ towards 260◦) on outcrop-scale in khondalite near Barunai (figure 5a). Atti-tude of fold axes also indicates plunge reversal(15◦ towards 85◦). Newly developed S3 planarfabrics due to this deformational event are dif-ferent for different lithounits. In khondalite, S3

fabric is defined by flattening of preexisting quartzand feldspar grains axial planar to the F3 folds(figure 5b). At the hinge zones of the F3, S2 andS3 are at high angles to each other. On the other

hand, S2 and S3 are lying parallel to each otherat the limb zones of the F3 isoclinal folds. Inoutcrop-scale, the form surfaces (S2 gneissic foli-ation) of F3 folds were tectonically transposedwith the newly developed axial planar foliation, S3.This tectonic transposition of S2/S3 foliations overlarge area is the general character. The segregatedgranitic bands of gneissic compositional bandingsuffered extensional fracturing at high angles to thetransposed S2/S3 foliation. At Basanta Mundia,the S2 gneissic foliation in aluminous granulitesis folded to develop F3 small folds where garnet–cordierite-bearing leucosome bands are segregatedaxial planar to these folds (figure 5c). Similar isocli-nal recumbent to gently inclined subhorizontal togently plunging F3 folds are also observed in lep-tynite near Kuhuri. Coarse-grained granitic mate-rials were segregated or emplaced parallel to thistransposed S2/S3 foliation and occasionally inter-sect the F3 fold hinges (figure 5d). Near Jatni,S2 gneissic foliation in khondalite/quartzite showssmall isoclinal, steeply inclined (65◦), moderatelyplunging (48◦) folds. Axial planar fabric (S3) isdeveloped which becomes parallel to S2 due totectonic transposition (figure 5e). The quartz-richband showed pinch-and-swell structures during thisdeformational event. The near-perpendicular rela-tionship between fold and boudin axes (deducedfrom bending fold orientations) suggests that theboudinage occurs due to the extension parallel tofold axis. At Kuhuri, the migmatitic rock ensembleconsisting of leptynite, charnockite/enderbite andaluminous granulites (mapped as mixed unit) arestrongly folded to exhibit transposed S2/S3 gneis-sic foliation where the competent restitic alumi-nous granulite bands are boudinaged (figure 5f).The individual boudins as well as boudinaged


Figure 5. Field photographs of the studied granulites. (a) Tight to isoclinal F3 fold in khondalite near Barunai, (b) S3fabric defined by flattened quartz and feldspar grain axial planar to isoclinal F3 fold in khondalite near Barunai, (c) garnet–cordierite-bearing leucosomal part axial planar to F3 small folds in aluminous granulites, (d) isoclinal recumbent gentlyplunging F3 folds in leptynite, (e) small isoclinal, steeply inclined, moderately plunging folds within khondalite/quartziteband showing transposed S2/S3 fabric and S4 as spaced cleavage (inset showing the pole plots of fabric as well as axialorientations) and (f) folded boudins of aluminous granulite encased in leptynite.

layers are folded during subsequent deformation(D4). The cordierite-bearing leucosome layers areconcentrated at the neck zone of the foldedboudins. Occasionally, cordierite-bearing quart-zofeldspathic layers are found to be segregatedwithin boudins as well as its surroundings as net-work veins. The boudinage process appears topost-date the segregation.

Minerals stabilized during the peak metamor-phic stage show partial to complete decomposition

to intergrowths surrounding the M2 phases. Inthe massive-type aluminous granulite, porphyrob-lastic garnet commonly exhibits partial to com-plete decomposition texture surrounding it indifferent samples. Delicate symplectites of orthopy-roxene and cordierite often corrodes and embaysgarnet grains. In other samples, coarse garnetshows breakdown to coarse intergrowth of sap-phirine, spinel, cordierite and plagioclase (figure6a). Garnet also shows breakdown to sapphirine–

10 Kaushik Das et al

cordierite. In few samples, garnet is surrounded bya double-layer corona containing spinel–cordieriteand sapphirine–cordierite symplectite, respectively(figure 6b). Garnet also shows partial breakdownto skeletal to symplectic intergrowth of spinel–orthopyroxene–cordierite in other samples. Suchcomplex intergrowth textures are considered to bea product of M3 event.

All the porphyroblastic phases in both the vari-eties of aluminous granulites show evidence of

grain-scale deformation, viz., partial grain refine-ment, undulosity, bending of cleavage traces aswell as fracturing. In leucosome layers, quartzgrains show lobate and cuspate mutual contacts(figure 6c). This kind of mutual contact betweengrains are characteristic of high-temperature meta-morphic condition where the grain boundariesbecome highly mobile and may sweep the mate-rial in any direction to remove dislocations andsubgrain boundaries (Gower and Simpson 1992;

Figure 6. Photomicrographs showing textures of the studied granulites. (a) Coarse garnet (Grt) shows breakdown to coarseintergrowth of sapphirine (Spr), spinel (Spl), cordierite (Crd) in aluminous granulite, (b) garnet (Grt) is surrounded by adouble layer corona containing spinel+cordierite (Spl+Crd) and sapphirine+cordierite (Spr+Crd) symplectite, respectively,(c) Individual quartz grain shows lobate contact to each other in leptynite, (d) quartz grain underwent recrystallization asevident from polygonal grain boundary, (e) ribbon-shaped quartz grain in charnockitic gneiss.


Passchier and Trouw 2005). Such lobate shapefurther suggests that recrystallization of quartzgrains is mainly guided by high-temperatureGrain Boundary Migration (GBM). Quartz andfeldspar underwent static recrystallization as evi-dent from polygonal grain boundaries (figure 6d).The process responsible could be Grain BoundaryArea Reduction (GBAR). Quartz grains in bothcharnockite/enderbite and leptynite are occasion-ally ribbon-shaped (figure 6e). Undulose extinctionand cuspate grain boundaries are also common inquartz and feldspar grains.

All these characteristic features indicate thatthe peak assemblage was subjected to crystal–plastic deformation during D3 deformationalevent.

Figure 7. Schematic diagram showing successive develop-ment of different stages of deformation and associated fab-ric. (a) S2 fabric is a penetrative gneissic foliation defined byporphyroblastic phases. F2 fold is only identified in micro-scopic scale within porphyroblastic phases. (b) Tight to iso-clinal F3 fold on axial-plane perpendicular profile section. S3fabric is axial planar to F3 fold in outcrop scale. Note thatS2 and S3 are parallel to each other except in hinge zoneof F3 fold. This is designated as transposed S2/S3 fabric.(c) Horizontal section showing D4 deformation event thatregionally folded the transposed S2/S3 fabric. S4 is definedby a spaced cleavage on outcrop scale.

5.4 D4 deformation and M4–D4 assemblage

The S2 gneissic foliation along with the trans-posed S2/S3 fabric was affected by the D4 deforma-tion. Overall orientation of F4 axial plane is alongNW–SE direction though it is slightly variable inregional-scale. The F4 folds are characteristicallyopen and upright to steeply inclined, with variableamount of plunge (12◦–53◦) towards NW direction(figure 5e, inset). The deformation is too weak todevelop any penetrative fabric in the rocks exceptoccurrences of spaced cleavage along axial planes ofF4 in some instances. This last deformational eventmodified the F3 folds depending on variable ori-entation of folded S2 and S2/S3 transposed fabric,inter-limb angle of F3 folding, orientation of localstrain ellipsoid and its relation with variable F3

fold axes. Attitude of F4 fold axes are also variablein large-scale. Angular relationship exists betweenaxial planes of F3 and F4 folds. The regional dis-position of F3 and F4 folds clearly documents over-all non-plane non-cylindrical interference pattern

Figure 8. Non-plane non-cylindrical fold interference pat-tern of F3 and F4 near Basanta Mundia. Upper figure repre-sents the field photograph while the lower figure representsthe hand sketch of same outcrop showing disposition of S2foliation, F3 fold, S3 foliation and S4 foliation.


(figure 3a, b). Schematic development of succes-sive stages of deformation is illustrated in figure 7.D4 deformation causes rotation of the early megas-copic F3 fold axes as evident from plunge reversalof F3 in outcrop and also from the regional mappattern. Such rotation of the F3 fold axis is notuniform in nature across the whole area.

Near Jatni, open, steeply inclined plunging F4

folds are shown by the transposed S2/S3 gneissicfoliation in khondalite. Spaced cleavage developsalong axial plane to F4 fold with N–S trend hav-ing moderate to steep dip (varies from 65◦–70◦)inclined towards west (figure 5e). The fold interfer-ence pattern of F3 and F4 also systematically variesin outcrop-scale from northeast to southwest of thearea depending upon the initial fold geometry andlocal disposition with respect to F4. Near BasantaMundia, the interference pattern is shown by thefolding of the aluminous granulites exposed in twoorthogonal sections; one broadly perpendicular toinitial F3 fold axis and other one broadly parallelto it. Axial trace of F3 fold shows curvature on foldaxis, perpendicular section indicates its non-planenature and its non-cylindrical character is evidenton the section that is at low angle to the fold axis(figure 8). F3 plunges 12◦ towards 260◦, whereas F4

axial plane strikes along 105◦ and dips 70◦ towardsnorth. The initial gently plunging, tight to isocli-nal inclined F3 folds were modified during inter-ference of an open upright F4 folds at an angle to

it. At Kuhuri, non-plane non-cylindrical interfer-ence pattern of F3 and F4 are also observed on twoorthogonal sections in leptynite. On vertical sec-tion, occurring at high angle to F3 fold axis, theF3 folds are tight to isoclinal, gently plunging (10◦

towards 90◦), recumbent to gently inclined. In nearhorizontal plane, the transposed S2/S3 gneissosityare folded by subsequent D4 deformation forming anon-plane non-cylindrical interference pattern (fig-ure 9). The nature of F4 is open upright gentlyplunging (15◦ towards 30◦). However, type-2 inter-ference pattern is a common feature of this areaand is evident in regional-scale and in most of theoutcrop sections. The F3/F4 interference patternnear Sunakhala exhibits plane non-cylindrical, i.e.,type-1 in nature. The interference pattern is shownin a plane paper map of the outcrop with a scaleof 1:100 (figure 10). F4 folds were developed on S2

at an angle with a set of open F3 folds formingthis kind of interference. It is evident that styleof interference pattern is variable in outcrop-scaledepending upon the initial fold geometry and prob-ably local strain ellipse. Moreover, the individualboudins as well as boudinaged layers of aluminousgranulites hosted within leptynite, developed dur-ing D3 event, have been subsequently folded duringF4 event on Kuhuri quarry face (figure 5f).

The M2 and M3 assemblages are overprintedby biotite-bearing assemblages at post-peak meta-morphic condition. Most commonly, garnet and

Figure 9. Schematic block diagram of the fold interference patterns in Kuhuri area. The field photographs of the horizontalsurface and vertical section are shown in (a) and (b), respectively.


Figure 10. Plane table map near Sunakhala area in scale of 1:100. The mapped rock is khondalite showing the planenon-cylindrical interference patterns of F3 and F4.

orthopyroxene porphyroblasts are replaced bycoarse- to medium-grained biotite flakes. Biotitegrowth is post-tectonic with respect to D2 garnetporphyroblast. In pyroxene granulites, amphibolegrowth is identified as D4 assemblages.

6. Discussion

Bhattacharya et al (1994) argued that the primarycompositional layering (S0) is preserved in khon-dalite. On the other hand, Dobmeier and Raith(2000) and Dobmeier and Simmat (2002) proposedthat the compositional layering (SSL) actuallyrepresents a package of enderbite, khondalite andcomposite layers of leucosome and restite in themigmatitic aluminous granulites, all of which werederived through partial melting of metasediments.Bhattacharya et al (1994) opined that the S1 fab-ric developed in khondalites and charnockites dueto F1 folding represents the major tectonic trendwhich is broadly parallel to the dominant trend

of Eastern Ghats granulites belt. They claimedthat the study area evolved through multiplephases of deformation (Bhattacharya et al 1994)and metamorphism (Sen et al 1995), where peakmetamorphic condition was attained as 1100◦Cand >10 Kbar. Survival of the primary sedimen-tary banding is difficult in this kind of high-grademetamorphic terrain with multiple phases ofdeformation accompanied by partial melting anddevelopment of metamorphic segregation. Unlessproved using any field or laboratory techniques(such as petrography), the preserved compositionalbanding in high-grade metamorphic rocks may beconsidered as metamorphic compositional band-ing. On the contrary, our field observation as wellas petrographic studies clearly indicate that thecompositional banding, defined by peak porphy-roblastic phases was developed by segregation ofpartial melt (the leucosome layers) during M2–D2

event, and therefore has been designated as S2.Our structural study suggests that S1 foliation,defined by biotite–sillimanite–ilmenite–quartz,


preserved in aluminous granulites, was developedduring pre-peak M1–D1 event. The S1 foliationis thus only preserved in microscopic-scale and iscompletely absent in outcrop- and regional-scale.The subsequent high-temperature M2–D2 eventcompletely obliterated the S1 foliation, except fromfew microdomains. Lack of S1 fabric in mesoscopic-scale does not support the contention ofBhattacharya et al (1994) that the major NE–SWtectonic trends of the Eastern Ghats was producedduring the first episode of folding. We have collatedall the structural data of the present study in two 3-D block diagrams representing the fold interferencepatterns and showing the relationship between dif-ferent structural fabrics. These two interpretativeblock diagrams represent the mapped north-eastern and south-western parts of the studiedsector (figure 11a, b). Mesoscopic gneissic fab-ric, S2 is dominant in all lithological units andregionally parallel to S3 fabric (excepting the foldhinges). The transposed S2/S3 fabric can be cor-related with the major tectonic trend of the EGB.Syn-M2 event would cause grain-scale deformationin porphyroblastic phases, but those signaturesmight be erased by static grain growth at hightemperature near-peak metamorphic conditionsthat also accompanied melt/fluid. Probably mostof the grain-scale signatures of deformation pre-served in the rocks results from subsequent M3–D3

event.Broadly E–W to WNW–ESE shortening is evi-

dent from F3 fold patterns during M3–D3 event.Garnetiferous cordierite-bearing leucosomes occur-ring axial planar to F3 folds in aluminous granulitesand along D3 boudinaged layers, developed dur-ing this event. Coarse-grained granitic materialsare segregated/emplaced along transposed S2/S3

foliation in leptynite. Incipient charnockitizationprobably occurred during or after M3–D3 event inthis rock as the charnockitized patches are super-posed on the existing foliation of transposed S2/S3.Moreover, the charnockitized patches as well asnon-charnokitic greasy metasomatic patches in theleptynitic (with/without garnet) host rock do notshow any evidence of further deformation, at leastin the outcrop-scale. Small-scale shear bands weredeveloped parallel to S3 foliation planes.

Anorthosite bodies occurring at the contactsof granulites record only one generation of weakfoliation that shows warping. This foliation wasprobably developed during M3–D3 event when S2

gneissosity was transposed with S3 foliation inassociated granulites. The last D4 event was weakenough to develop any penetrative foliation inthe granulites except weak spaced-cleavages thatdeveloped in khondalite. The weak warping of theS3 foliation in anorthosite implies superposition ofD4 event. Therefore, anorthosite plutonism seems

Figure 11. Schematic block diagrams of (a) north-easternblock and (b) south-western block of the studied areashowing 3-D orientation of superposed planar geometry.The horizontal planes represent the mapped patterns ofthe respective blocks as in figure 3(a, b).

to post-date M2–D2 stage and probably pre- tosynchronous with M3–D3 event.

Our structural study points towards the pres-ence of non-plane nature of folding developed dueto interference of F3 and F4 during subsequent andlast phase of D4 deformation. It is to be notedthat Bhattacharya et al (1994) reported hook-shaped non-plane interference pattern in khon-dalite of their F1 and F2 folds. However, carefulobservation from outcrop structures as well as fromregional structural map suggests the interference ofF3 and F4 are of non-cylindrical in addition to that.


Thus in general non-plane non-cylindrical natureof interference pattern of F3 and F4 is suggestedhere, though it varies to plane non-cylindrical fold-ing in few places. Broadly, N–S to NNE–SSWcompression is evident during the last phase offolding (F4) that produces a series of open uprightsuperimposed folds on F3 gently plunging, recum-bent to gently inclined and tight to isoclinals folds.Microscopically, brittle behaviour of quartz andfeldspar grains suggests low-temperature deforma-tional episode probably during D4 event.

Dobmeier and Simmat (2002) interpreted thestructural evolution of the Chilka Lake areain terms of transpressional tectonics presumablyresulting from oblique collision of Indian and eastAntarctic Craton at ca. 690–660 Ma. The argu-ments put forward by these workers are:

• presence of sub-horizontal stretching lineation,• a prominent lineament pattern observed from

satellite imagery data,• distribution of Mesozoic faults in the Mahanadi

Offshore basin, and• identification of two opposite sets of shear fab-

rics that implied reactivation of the erstwhiletranspressional zone.

Transpression is a topic of intense researchinterest because of its large-scale implicationin crustal dynamics (Sanderson and Marchini1984; Robin and Cruden 1994; Jones et al1997; Dewey et al 1998; Lardeaux et al 2001;Goscombe et al 2003, 2005). The fabric orien-tation, which is a key element of transpression,depends on factors such as angle of obliquity(α), intensity of finite strain (coaxial and/or non-coaxial) and kinematic partitioning. The angleof obliquity further controls the thermal regimeand erosion rate of a transpressional system(Thompson et al 1997; Batt and Braun 1997).Moreover, the principal shortening surface (i.e.,foliation) might show strike variation with the non-coaxial component of the strain, but dip remainsvertical (cf. Druguet and Hutton 1998). Associatedstretching lineation might switch from horizontalto vertical when α<20◦ in case of monoclinic trans-pression (Sanderson and Marchini 1984). However,if there is a component of lateral (strike-parallel)stretch in the deformation zone, such lineationswitch does not occur at all (Chattopadhyay andKhasdeo 2011). Thus, the relationship betweenstretching lineation and tectonic transport aloneis not conclusive to distinguish imprints of trans-pression from that of compressional deformation.This implies that before relating regional lineamentpattern to plate motion, a thorough appraisal ofoutcrop-based structural data on a regional-scaleis needed to critically evaluate the proper tectonic

build-up of crustal boundary zones. The idea oftranspressional tectonics in Chilka Domain possi-bly emerged from the proposed collision of Indianand east Antarctic blocks. The interpretation ofDobmeier and Simmat (2002) regarding ChilkaLake granulites is significant. However, during thepresent study, we observed stretching lineation inlocalized shear zones and this is not a regional-scale fabric in the granulites. Structural featuressuch as tight isoclinal folds, nappes, overfolds andthrusts that are common in transpression system(Tavarnelli et al 2004; Goscombe et al 2005) arenot observed in Chilka Lake area. The regional lin-eament pattern in Dobmeier and Simmat’s studymatches with the regional foliation pattern ofour study. The correlation of Mesozoic fault sys-tems as reactivated transpressional boundariesappears to be extrapolation. As an alternative,the style of structural development in the ChilkaLake granulites can be explained from superposedfold geometries in response to compressional tec-tonics. It should be noted that the adjacent AngulDomain witnessed deformation pattern similar toChilka Lake Domain, but no imprints of trans-pression is evoked from there (Sarkar et al 2007).Despite all these factors, it is difficult to discountregional transpression on the basis of absence ofstretching lineation, etc., as the signatures of trans-pression are varied and the implied scale of trans-pressional tectonics (plate boundary scale) is largerthan the regional- to mesoscopic/microscopic-scalestructural analysis presented in this study.

Recently, Chatterjee et al (2010) identified N–Strending Eastern Indian Tectonic Zone (EITZ)encompassing highly deformed locales of Singh-bhum and Chhotanagpur Cratons, north of EGB.Based on monazite ages, these authors put anage bracket of ca. 870–780 Ma for this event.Although geographically far apart, sheer coinci-dence of monazite ages from Chilka Lake area com-pels the authors to include the northern fringe ofEGB within the EITZ. According to Chatterjeeet al (2010), the ca. 870–780 Ma deformation andanatexis in EITZ was related to a distinct periodof southward movement of India after its collisionwith east Antarctica during the Rodinia assembly(by ca. 900 Ma). This further implies that rift-ing related to Rodinia did not initiate before ca.780 Ma. On the other hand, the ca. 715–655 Mamonazite age from the Chilka Lake anorthosite isinterpreted to be related to the convergence andpossible collision between the EGB–Rayner Blockand Western Australian Craton (Chatterjee et al2008).

Following all these interpretations, it appearsthat the Chilka Block, originally attached toEGB–Rayner Block during Rodinia assembly, wit-nessed compressional regime during its southward


movement until its eventual collision with WesternAustralia. Available geochronological data indi-cate a distinct Neoproterozoic tectonothermal his-tory for granulites of the Chilka Domain. In thepresent study, we identified four distinct meta-morphic and deformation events (D1–D4 events).Megascopic and microscopic data reveal that thepetrological and structural imprints of these gran-ulites are a product of these superposed events(two directions of compression being broadlyE–W for the earlier event and broadly N–S for laterevent). As far as the structural geometry and fabricdevelopment patterns are concerned, we proposethat Chilka Domain evolved through superpositionof folds in overall compressional regimes at highangle, without significant transpression.

Acknowledgements

The authors acknowledge the financial grant fromthe Department of Science and Technology, Govt.of India through a research project (Grant no.ESS/16/251/2005). SK acknowledges the financialassistance to CAS, Department of Geological Sci-ences, Jadavpur University. Fieldwork assistanceof Sayani Sarkar, Syamantak Chatterjee, BiplabMahato, Souvik Sengupta and Partha Sarathi Dasproved valuable in this work. Constructive andinformative reviews of Dr S K Bhowmick and ananonymous reviewer helped to improve the qualityof the paper.

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MS received 26 April 2011; revised 10 September 2011; accepted 16 September 2011

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