Goswami Bhattacharyya Chapter 3

VOL. 80, Nos.1-4, 2008 (Published January 2010) ISSN 0970-1354

KOLKATA

INDIAN JOURNAL OF GEOLOGY

Special Issue

Geodynamic Regimes, Global Tectonics and Evolution of Precambrian Cratonic Basins in India

Indian Journal of GeologyVol.80, Nos. 1-4, (2008) p.41-54 (Published January 2010)

TECTONOTHERMAL EVOLUTION OF CHHOTANAGPUR GRANITEGNEISS COMPLEX FROM NORTHEASTERN PART OF PURULIYA

DISTRICT, WEST BENGAL, EASTERN INDIA

B. GOSWAMI1 AND C. BHATTACHARYYA2

2Department of geology, University of Calcutta 35, Ballygunge Circular Road, Kolkata -700019

e-mail: 1 [email protected];2 [email protected]

Abstract

The principal rocks of northwestern part of the present area are non-porphyritic granitoidgneisses having enclaves of metamorphites belonging to amphibolite facies while the southeasternpart is composed predominantly of leptynitic granitoid gneisses and porphyritic granitoid gneisseswith enclaves of metamorphites belonging to granulite facies. The amphibolite - granulite faciesboundary represents a structural discontinuity and a shear zone. The area suffered at least threephases of deformations. Rarely preserved, D1 minor folds are produced on bedding plane/lithocontact.The regional foliation (S1) is axial planar to the D1 folds, and was initially subhorizontal. The regionalmetamorphism (M1) and D1 are coeval. After the peak metamorphic condition during M1 was attained,the anatectic granitoid melt was profusely injected along the S1 of older metamorphites giving rise tonon-porphyritic/leptynitic granitoid gneisses. S1-foliation was folded by D2-deformation into ENE-WSW trending overturned, nearly non-plunging, tight folds. Nepheline syenites were emplaced inthe granulite facies terrain, parallel to the axial plane of D2-folds before the end of M1 metamorphism(1400-1500 Ma).The D3-deformation produced two steep, northerly-inclined ENE - WSW regionalshear zones. The granulite facies terrain has been uplifted from deeper level along the shear zonesand juxtaposed with amphibolite facies terrain during D3-phase. Porphyritic granitoid batholithemplaced in these shear zones caused thermal metamorphism (M2) of regionally metamorphosedrocks at hornblende hornfels facies (900 - 1000 Ma).The peak temperatures of regional metamorphism(M1) attained during amphibolite and granulite facies are estimated to be around 700oC and 820oCrespectively while the peak thermal metamorphic (M2) temperature attained during later emplacementof porphyritic granitoid magma is around 8000C. The peak pressures of amphibolite and granulitefacies metamorphism are 5 ± 0.5 and 7 ± 0.5 kb and during thermal metamorphism, 7 ± 0.5 kb.

Keywords: Chotonagpur granite gneiss complex, regional metamorphism, shear zones

1. Introduction

The terrain in the northeastern part ofPuruliya district is composed mainly ofgranitoid gneisses (Sen, 1956, 1959; GSI,1977) which contain enclaves of pelitic rocks

such as mica schists and khondalite, withvarying proportions of psammitic andcalcareous intercalations, and metabasic rocks(amphibolites and basic granulite). Themetabasic rock bodies are generally alignedparallel to regional foliation and have initially

Tectonothermal Evolution of Chotonagpur Granite Gneiss Complex42

intruded into the above metasediments (Fig.1). The intrusive units within the granitoidgneisses include nepheline syenite gneisses(Bhaumik et al., 1990; Chattopadhyay andBhattacharyya, 1990; Ray and Mondal, 2002),porphyritic granitoid gneisses of batholithicdimension, pegmatite and quartz veins,ultramafics and dolerites.

The structural and metamorphic aspectsof the southern and eastern parts of the presentarea (Fig. 1) were dealt with about 50 yearsback by Sen (1956, 1959). Mahadevan (1992)studied tectonostratigraphic evolution of theadjoining southern part of the area. Thepurpose of this paper is to report the results ofdetailed studies on structures, petrology andmetamorphism along with thermobarometricdata which throw new light on thetectonothermal evolution of the northeasternPuruliya as a whole.

2. Geological Setting

The area belongs to the easternextension of Proterozoic Chhotanagpur granitegneiss complex (CGGC). This gneissiccomplex is of composite character consistingmainly of granitoid gneisses, migmatites, andmassive granites with enclaves ofmetasedimentary and meta-igneous rocks, andintrusive basic and intermediate rock types(Ghose, 1983, 1992). Tentative generalstratigraphic succession for the Puruliyadistrict is given in Table 1.

In the present area (Fig. 1) non-porphyritic granitoid gneisses (NPGG)occupies the major part of the northern halfand contains enclaves of mica schists andkhondalites, marbles, calc-silicate schists/gneisses, para- and ortho-amphibolitesbelonging to amphibolite facies. The NPGG

is followed to the south by an E - W trendingporphyritic granitoid gneiss batholith (PGG).Leptynitic granitoid gneisses (LGG) occur tothe south and east of PGG. The leptyniticgranitoid gneiss country contains minorpatches of migmatites, basic granulites,charnockitic rocks, calc-silicate rocks andanorthosites representing granulite facies.Both PGG and LGG contain small patches ofbasic granulites and charnockitic rocks. Hencethe central part of the area is a granulite faciesterrain. Again in the southern-most part of thearea LGG contain enclaves of amphibolitesand calc-silicate rocks belonging toamphibolite facies.

A general E-W strike with northerly dip(less commonly southerly) of the dominantfoliation in the present area (Sen, 1956) iscomparable to the orientation of regionalfoliation in the Chhotanagpur granite gneissbelt (Sengupta and Sarkar, 1964, 1968; Ghose,1983; Mazumder, 1988; Sarkar, 1988; Baidyaet al., 1989). The dominant northerly dip offoliation in the present area has been earlierascribed to regional antiformal folding withoverturned southern limb (Sen, 1956).

3. Structural Geology

The present area of study has sufferedtwo phases of folding deformation and twophase of shear related deformation. Firstgeneration (D1) folds on bedding inmetasedimentary rocks show moderatenortherly dip of axial plane, which is parallelto regional foliation. D1-folds are representedrarely as tight to isoclinal, intrafolial, androotless, minor folds within non-porphyritic/composite/migmatitic granitoid gneisses (Fig.2). Non-porphyritic granitoid magma wereinjected parallel to the axial plane of the D1-

B. Goswami and C. Bhattacharyya 43

Fig. 1. Geological map of the area showing planar structure and shear zones.

folds and metamorphosed to form the gneissicbanding before the end of D1-phase.

The dominant/regional foliation (S1) hasbeen folded into overturned (the southern limbbeing overturned), nearly non-plunging, tightfolds (D2) with northerly dipping axial plane(S2; Fig. 3a, 3b). Superposition of D2 on D1

folds has led to the formation of arrow head/mushroom shaped outcrop patterns affectingthe gneissic banding in non-porphyriticgranitoid gneisses (Fig. 4). However,occasional formation of domes and basins andhook-shaped patterns due to the change oforientation of S0 surfaces during the


Table 1. Tentative geological succession of Puruliya district (after GSI, 1977)

Quaternary Coarse to fine sand, silt and clay, lithomargic clay, yellow clay,calcareous nodules and laterite

---------------------------------------Unconformity-----------------------------------------Upper Gondwana Supra-Panchet (Mahadeva) Sandstone

FormationPanchet Formation Sandstone and shale

----------------------------------------Unconformity----------------------------------------Lower Gondwana Raniganj Formation Sand, shale and coal seam(Damuda Group) Barren Measures Formation Ironstone shale and ferruginous sandstone

------------------------------------Unconformity--------------------------------------------Pegmatite, quartz and chert veins, epidote-

Pegmatite and allied rocks feldspar-quartz veins, apatite and quartz- magnetite veins

Granitic rocks Porphyroblastic granite gneiss, biotitePrecambrian granite, composite gneiss, migmatites,

garnetiferous granite gneiss

Metabasic rocks Amphibolite, metanorite, hornblende schist

Metasedimentary rocks Calc-granulite, crystalline limestone,garnetiferous sillimanite schist

(graphite and kyanite-bearing at places)

emplacement of non-porphyritic granitoidmagma along the S1 axial plane of D1 folds(described later). D2 folds are common in calc-silicate rocks, amphibolites and migmatites inthe northwestern part of the area (Fig. 3). D2folds are uncommon in the granulite faciesterrain lying in the southern and eastern partwhere leptynitic foliation and quartz-ribbonand rods are common features.

The third phase deformation producedtwo parallel ENE-WSW trending regionalshear zones, which are steeply dipping to theN, with steeply plunging mineral lineation.Both of these shear zones are branches of theNorth Puruliya Shear Zone (NPSZ). Thenorthern shear zone (NSZ, Fig. 1), close to thenorthern boundary of the porphyritic granitoidgneiss batholith demarcates roughly theamphibolite facies of the northwestern part

from the granulite facies of the southeasternpart of the area. The boundary between theamphibolite facies terrain (WNW) and thegranulite facies terrain (ESE) is sharp, andthere is no gradational change in mineralcomposition of amphibolites from amphibolitefacies terrain to basic granulites of thesoutheastern part. There is also significantdifference in trace element compositions ofbasic granulites and of the amphibolites. Shearzone thickness ranges from few centimetresto more than 100 metres. The gradual changefrom host granitoid rocks to intensely foliatedrocks and intense grain size reduction, S-Cband, pinch-and-swell structure, rotations ofporphyroclast mark the shear zone (Fig. 5a).

The shear-sense indicators within themylonitic rocks indicate top-to-SE sense ofshear (Fig. 5b). The southern shear zone (SSZ),


Fig 2. Tight to isoclinal folds in quartzite. Note that the granitic material intruded parallel to the axial plane of the minor folds.About 1 km S of Cheliyama. Pen length = 14.5cm.

passing along the southern boundary of PGGbatholith, is characterized by intrusion ofprofuse pegmatite bodies and development oflocally close spaced shear folds (subparallelaxial planes). Epidotization of granite gneissis a common feature of SSZ. This shear zoneis nearly vertical or steep northerly/southerlydipping and shows upliftment of northern side.In the present study area SSZ passes in theLGG country, but enclaves of basic granulitesand charnockitic rocks are limited only to thenorthern sides of SSZ (Fig. 1).

4. Emplacement of Magmas in Relation toDeformation

4.1. Non-Porphyritic Granitoid Gneisses

(NPGG)Graphic intergrowth of quartz and K-

feldspar, and weak zoning in plagioclase withAlbite rim suggest that NPGG of largelysyeno-granite composition have crystallizedfrom a melt. However, folds and boudinageon leucosomal bands of NPGG together withpartial recrystallization and unduloseextinction in quartz and feldsapr suggest sub-solidus plastic strain and recrystallization.Trails of biotite are often seen to be warpingaround porphyroclasts of feldspar in shearzones.

S1 in the amphibolites/ mica schistsenclaves (cf. paleosome) within the migmatiticgneisses is parallel to the gneissosity in thenon-porphyritic granitoid gneisses (cf.


Fig 4. Arrowhead pattern in migmatitic non-porphyriticgranitoid gneiss. About 600 m SSE of Cheliyama.Ruler = 14 cm.

Fig 3. a) Overturned D2 folds in migmatitic granitoid gneiss (non-porphyritic). Boxed area is shown separately by an enlargedphotograph in Fig. 3b. About 1 km NNE of Cheliyama. Length of the axe = 1.2m

leucosomes). This common parallelism offoliation and centimetr to metr-scale graniticlayers in stromatic migmatites suggests"percolative flow of (granitic) melt along theflattening fabric using the dynamicpermeability produced during activedeformation" (Brown and Solar, 1998). Theabove suggests emplacement of granitoidmagma during development of S1 fabric (D1).Later D2-folding deformed both S1-foliationand sheets of non-porphyritic granitoidgneisses (Fig. 3).

4.2. Nepheline syenite gneissesThe nepheline syenite (monzosyenite)

gneisses form two E-W elongate lensoidbodies which intruded concordantly into thekhondalitic and leptynitic granitoid gneissclose to ENE trending northern shear zone(Fig. 1). Preferred alignment of long axes ofeuhedral K-feldspar megacrysts defines amagmatic foliation in the nepheline syenitegneisses. However, porphyroclasts of K-feldspar and nepheline along with biotite andamphibole define a gneissosity in deformedsamples. Anhedral to subhedral megacrysts ofdeformed microperthite and nepheline areenclosed in aggregates of fine to mediumgrained felsic minerals. Undulose extinction,polygonization and triple-point junction in thefelsic groundmass indicate recrystallization.Bending of twin lamellae and marginalgranulation of microcline megacrysts andkinks in biotite are frequently noticed.Amphibole generally occurs as tabularmegacrysts and contains inclusions of albite.It occurs also as inclusion within nephelineand alkali feldspar. Sphene occurs commonlyas thin rim around ilmenite. These nephelinesyenite bodies with common recrystallization


features in constituent minerals indicate theiremplacement prior to the waning phase ofregional metamorphism (M1).

4.3. Porphyritic Granitoid Gneiss BatholithThe porphyritic granitoid gneisses

(PGG) of largely monzogranitic tosyenogranitic composition intruded thecountry of non-porphyritic and leptyniticgranitoid gneisses (Sen, 1956; Goswami,2007).

Mica schists, khondalites, quartzites,calc-silicate rocks, amphibolites and basicgranulites occur as enclaves within the PGG.The planar fabric in the enclaves was formedduring the first deformation, D1. Hence thelocal discordance between the planar fabricin the host PGG and that in the enclaves of themetamorphites and the NPGG with D2 foldssuggest that the emplacement of the PGGbatholith took place at least after D2deformation.

The magmatic flow planes in PGGbatholith have ENE-WSW parallel to theoverall elongation of the batholith. Magmaticfabric is defined by preferred orientation ofeuhedral to subhedral feldspar (mainly

microcline) +biotite +hornblende (Fig. 6).Deflections of foliation of porphyriticgranitoids around the xenoliths of basicgranulite are also consistent with flow ofmagma around solid objects. In the central partof the batholith orientations of feldsparmegacrysts are haphazard. C/S structures arealso seen at places (Fig. 7; Berthe´ et al., 1979).

Presence of both magmatic and solid-state deformation features in PGG batholithdemonstrates that the shearing has been activeduring and after complete crystallization of themagma. The control of shear zone/fault zone/thrust zone for emplacement of granitoidmagma, giving rise to porphyritic granitoidrocks has been suggested in publishedliterature (e.g., D'Lemos et al., 1992; Rajesh,2000).

5. Episodes of Metamorphism

The dominant schistosity (S1) in theenclaves is defined by the preferredarrangement of metamorphic minerals (e.g.,sillimanite in khondalite, pyroxene andhornblende in basic granulite, and hornblendein amphibolite). These suggest that the M1

Fig 5. a) Mylonitic augen showing porphyroclasts of K-feldspars, northern boundary of porphyritic granitoid gneiss batholith,Khajura stone quarry. Bar length = 12 cm. b) Schematic diagram of S-C mylonite with vertical C-surface in the northernfringe of porphyritic granitoid gneiss batholith, near Isradanga village, about 5 Km NE of Raghunathpur.


Fig 6. Imbrication ("tilting") of some elongate euhedral megacryst of K-feldspar, Raghunathpur College. Ruler = 14 cm.

Fig 7. Centimetre-thick shear zone in porphyritic granitoid, Raghunathpur college campus. Pen = 14.5


Table 2. Thermobarometric estimates for the near-peak metamorphic stage of regional metamorphism (M1) andthermal metamorphism (M2)

M1 M2

Amphibolite facies domain Granulite facies domain Porphyritic granitoid domain

MethodBH 725 820 860 720 800W 784 800

P.G. 600a 600b 820c

FS 669B, 92 641

G 742* 725*GP 610*SB 615** 653H 680** 640* 600

LG 730** 729B,91 653

L 677FK 705S 722 794

WS 776Peak temp. (ºC)~700 ~820 ~800

BH 4.6 7.2 9.9 5.8 6.9MP 7.1 7.1NP 6.6*

B, 91 6.8L 6.4

P.G. 4.5 4.5 5 -7PC 7.4**M 7.5**M 7.2**

NH 6.5*Peak Pressure (kbar) ~ 4.5 ~ 7.0 ~ 5.5

Data from Bhattacharyya and Mukherjee (1987); ** Data from Sen and Bhattacharya (1993); rest of the data from Goswami (2007). Abbreviations:B, 91: Bhattacharya, A. et al.(1991); B,92: Bhattacharya, A. et al. (1992); BH: Blundy, J.D. and Holland, T.J.B. (1990); C: Carmichael, D.M. (1970);FK: Fonarev, V.I. and Konialov, A.N. (1986); FS: Ferry, J.M. and Spear, F.S. (1978); G: Ganguly, J. (1979); GP: Graham, C.M. and Powell, R.(1984); H: Harley, S.L. (1984); L: Lal, R.K. (1993); LG: Lee, H.Y. and Ganguly, J. (1988); M:Moecher, D.P. et al. (1988); MP: Mc Carthy andPatino Douce(1998); NH: Newton, R.C. and Hastleton, H.T. (1981); NP: Newton and Perkins (1982); PC: Perkins, D.I. and Chipera, S. (1985); S:Sengupta, P.K. et al. (1990); SB: Sen, S.K. and Bhattacharya, A. (1984); W: Wells (1977); WS: Whitney, J.A. and Stormer, J.C. (1977). P.G.:Petrogenetic GridaPetrogenetic Grid of calc-silicate rocks: Winkler (1988, Fig. 15.3, p.247); bPetrogenetic Grid of metapelitic rocks: Carmichael (1978); cPetrogeneticGrid of nepheline syenite gneisses: Goswami and Bhattacharyya (2008)

Rock

Typ

e(N

o. o

f Sam

ples

)

Amph

ibol

ite (3

)

Calc-

silica

te ro

cks (

5)

Mica

schi

st (5

)

Calc-

silica

te gn

eiss*

*

Khon

dalit

e

Basic

gra

nulit

e (3)

Ende

rbite

(1)

Neph

eline

syen

itegn

eiss (

12)

Basic

gra

nulit

e(Hb

lho

rnfe

ls) (3

)

Ende

rbite

(1)

Porp

hyrit

ic gr

anito

id(H

bl-C

rysta

lliza

tion)

(2)

Tem

pera

ture

± 5

0 (0 C

)Pr

essu

re ±

0.5

(kba

r)


metamorphism is coeval with the first phaseof deformation (D1). M1 continued upto D2deformation event. During the emplacementof porphyritic granitoid batholith in theregionally metamorphosed country rocks, thelatter were subjected to contact metamorphism(M2) in hornblende-hornfels facies leading tomigmatites in some places. Widespreadalteration of pyroxene to hornblende and ofboth pyroxene and hornblende to biotite inbasic granulites occurring as enclaves inporphyritic granitoid gneiss batholith resultedfrom M2 (Goswami, 2007).

Thermobarometric estimates for thenear-peak metamorphic stage of regionalmetamorphism (M1) and contactmetamorphism (M2) are given in Table 2(Goswami, 2007). The preferred estimate ofM1 metamorphic conditions in granulites issuggested to be ~8000C and 6.5-7.5 kbar. Theabsence of garnet in basic granulites of presentstudy indicates that the peak-M1 pressure wasunlikely to be high. Enclaves of enderbites andbasic granulites within porphyritic granitoidsare metamorphosed by M2 event. They givevariable estimates of pressure which is due tolack of complete resetting of M1 textures(especially for garnet bearing assemblage ofenderbites) during M2 event.

6. Summary of Tectonothermal History

For a long time the ChhotonagpurGrnaite Gneissic Complex (CGGC), which istectonically attached with Singhbhum cratontowards south, has been thought of as a productof late Archaean to Paleoproterozoic crustalconsolidation (Sarkar, 1988). Available agedata have indicated, however, a dominant 'LateGrenvillean' tectonothermal event at 900 -1000 Ma in CGGC (Ghose, 1992 and

references therein). An earlierMesoproterozoic metamorphism (1.6-1.4 Ga;Ray Barman et al., 1994) was registered onlyat a few places of the CGGC.

Recently Chatterjee et al. (2008) haveshown that emplacement of gabbroicanorthosite rocks of the Saltora area (lying afew km east of the study area) took place at1550±12 Ma subsequent to D1 but before D2deformation event. Subsequent metamorphismin the CGCC is recorded by the 947±27 Mazircon growth in the anorthosite, and 950±20Ma and 995±24 Ma monazite growths in thematrix and within garnet of metapelitegranulite located north of the anorthosite(Chatterjee et al., 2008). The prograde pre/syn-D1 granulite facies metamorphism (M1) in theenclave suite (750-8500C and 4-6 kbar) issuggested to be >1.4 Ga (EPMA monazite age)(Maji et al., 2008). EPMA dating of monazitesin sheared granites yielded ~0.9-1.0 Ga (Majiet al., op. cit).

However, for the migmatitic gneisses ofadjoining Muruguma area it has beensuggested that regional metamorphism (M1)and formation of associated non-porphyriticgranitoids took place about 1178±61 Ma ago(Rb-Sr whole rock isochron, Ray Barman etal., 1994). Radiometric age data of samplesfrom Bero suggest that the age of emplacementof porphyritic granitoid magma was 1071 ±64 Ma (Rb-Sr whole rock isochron, RayBarman et al., 1994). The K-Ar ages of biotitefrom porphyritic granitoid gneisses (870 ± 40Ma) and that of muscovite from leucogranite(810 ± 40 Ma) of the Jaipur area, about 43 kmSW from the western boundary of the presentarea, gives somewhat younger ages due tolower blocking temperatures of K-Ar (Baidyaet al., 1987).

Based upon the above meagre age


Emplacement of porphyritic granitoid magmatook place along the shear zones. Availablegeochronological data suggest that M2 episodecontinued for about 1000 Ma to 900 Ma(Chatterjee et al., 2008).

The CGGC in general, and the presentarea in particular, seems to have experiencedat least two major tectonothermal episodesduring the Proterozoic (Chatterjee et al., 2008),having a large time-gap between the twoepisodes. The regionally metamorphosedgranulites (M1 = >1400 Ma) have shown anITD path (Ray Barman et al., 1994) followedby an emplacement of PGG and contact/thermal metamorphism (M2 at 900-1000 Ma).It is interesting to note that there is a large timegap between regional metamorphism granulite(1400 - 1500 Ma) and thermal metamorphismdue to emplacement of the porphyriticgranitoids (900 - 1000 Ma). The D1 and D2folds have been interpreted as coeval with theM1 metamorphism. Usual interpretation oflarge overturned folds and pervasiveschistosity (in the metasediemntary enclaves)involve crustal shortening. In the absence ofmore reliable precise dates for the differentcomponents of the study area/region, it wouldbe impossible to exclude the possibility ofcrustal convergence before the onset ofregional metamorphism. However, high T(~800oC) but relatively low P (~7 kbar, presentstudy) estimates of granulites (Maji et al., 2008estimated 750-850oC and 4-6 kbar from alarger data base) and amphibolites (700oC at~4.5 kbar) do not suggest crustal over-thickening. High thermal gradient, abundantmafic/ultramafic intrusions (Mandal et al.,2007), and geophysical evidence for thepresence of sill-like mafic body at about 5 kmdepth (Verma et al., 1988) on the other hand,can suggest magma under-plating. Except

information of rocks/minerals of Puruliyadistrict and CGGC as a whole, the summaryof metamorphism, deformation andmagmatism particularly with reference to thepresent area may now be drawn asfollows.

Three major phases of ductiledeformation have been recognized in theregion. The earliest phase (D1) is responsiblefor the dominant metamorphic penetrativefoliation (S1) in amphibolite and granulitefacies rocks occurring mainly as enclaves(about 1400 Ma ago?). After attaining the peakmetamorphic condition the anatectic granitoidmelt was profusely injected along the S1foliation plane of some metasediments andamphibolites giving rise to non-porphyriticgranitoid gneisses/composite gneisses. Thecountry rocks in which the granitoid magmawas emplaced were already metamorphosedunder amphibolite facies. NPGG and LGGmagmas generated at 7000C and 8000Crespectively (zircon saturation temperatures)at 100 to167 Mpa PH2O (Goswami, 2007).Deformation (and metamorphism) continued,so that the S1 schistosity along with injectedgranitoid bands and veins were folded duringD2, possibly before the end of M1. The D3phase generated the regional NPSZ having twobranches NSZ and SSZ passing northern andsouthern peripheries of PGG body andshowing excellent development of mylonitictexture. The granulite facies terrain has beenuplifted from deeper level along the shearzones and juxtaposed with amphibolite faciesterrains (Fig. 1, see cross-section) during D3phase.

The porphyritic granitoid melt (900 to1000Ma) was responsible for the thermalmetamorphism/ metasomatism of the countryrock under hornblende-hornfels facies (M2).


biotitization and hornblendization related tothe emplacement of PGG (M2) the granulitefacies rocks of present study do not show muchsign of retrogression, which, in turn, supportsrapid exhumation immediately after M1episode. Further, granulite facies rocks areseparated from adjacent amphibolite faciesrocks by two shear zones (Fig. 1). The granulitefacies rocks during exhumation formed a horstlike structure within the amphibolite faciesrocks (Fig. 1, see cross-section).

Blundy, J.D. and Holland, T.J.B. 1990. Calcic amphiboleequilibria and a new amphibole-plagioclasegeothermometer. Contributions to Mineralogy andPetrology 104, 208-224.

Brown, M. and Solar, G.S. 1998. Granite ascent andemplacement during contractional deformation inconvergent orogens. Journal of Structural Geology20, 1365-1393.

Carmichael, D.M. 1978. Metamorphic bathozones andbathograds: a measure of the depth of post-metamorphic uplift and erosion on a regional scale.American Journal of Science 278, 769-797.

Chatterjee, N., Crowley, J.L., and Ghose, N.C. 2008Geochronology of the 1.55 Ga Bengal anorthositeand Grenvillian metamorphism in the Chotanagpurgneissic complex, eastern India. PrecambrianResearch 161, 303-316.

Chattopadhyay, R. and Bhattacharyya, C. 1990 Petrologyof foid syenites from Kankarkiari, Puruliya District,West Bengal. Proc. 77th Ind. Sci. Cong. pt. II(abstract), p. 12.

D'Lemos, R.S., Brown, M. and Strachan, R.A. 1992.Granite magma generation, ascent and emplacementwithin a transpressional orogen. Journal ofGeological Society of London 149, 487-490.

Dunn, J.A. 1929. Geology of North Singhbhum includingparts of Ranchi and Manbhum districts. Memoir ofGeological Survey of India 54(1).

Ferry, J.M. and Spear, F.S. 1978. Experimentalcalibration of portioning of Fe and Mg betweengarnet and biotite. Contributions to Mineralogy andPetrology 66, 113-117

References

Baidya, T. K., Maity, N. and Biswas, P. 1989. Tectonicphases and crustal evolution in a part of the EasternChhotanagpur Gneissic Complex. Journal of theGeological Society of India 34, 318-324.

Baidya, T.K., Chakraborty, S., Drubetskoy, E. andKhilatova, I. 1987. New geochronologic data onsome granitic phases of the Chhotanagpur granitegneiss complex in the northwestern Purulia district.West Bengal. Indian Journal of Earth Sciences 14,136-141.

Bhattacharya, A., Krishnakumar, K.R., Raith, M., andSen, S.K. 1991. An improved set of a - X parametersfor Fe-Mg-Ca garnets and refinements of theorthopyroxene - garnet - plagioclase - quartzbarometer. Journal of Petrology 32, 629-656.

Bhattacharya, A., Mohanty, L., Maji, A., Sen, S.K., andRaith, M. 1992. Non-ideal mixing in the phlogopite-annite binary; constrainta from experimental data onMg-Fe partitioning and a reformulation of the biotite-garnet geothermometer. Contributions to Mineralogyand Petrology 111, 87-93.

Bhattacharyya, P.K. and Mukherjee, S. 1987. Granulitesin and around Bengal anorthosite, Eastern India:genesis of coronal garnet and evolution of thegranulite - anorthosite complex. GeologicalMagazine 124, 21-32.

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AcknowledgementsWe thank the authorities of Calcutta

University and the Principal, J.K. College, Puruliyafor providing research facilities. We are also verymuch thankful to Prof. D. Mukhopadhyay, and Prof.A.R. Basu for reviewing the manuscript criticallyand suggesting improvements. Editorial review byProf. D. Saha has upgraded the ms. to a great extent.Help rendered by Smt. Chandreyee Goswami is alsogratefully acknowledged. This study was fundedby UGC through a Minor Research Project to BG.


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