TECTONOTHERMAL EVOLUTION OF CHHOTANAGPUR GRANITE GNEISS COMPLEX FROM NORTHEASTERN PART OF PURULIYA DISTRICT, WEST BENGAL, EASTERN INDIA

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1 Indian Journal of Geology Vol.80, Nos. 1-4, (2008) p (Published January 2010) TECTONOTHERMAL EVOLUTION OF CHHOTANAGPUR GRANITE GNEISS COMPLEX FROM NORTHEASTERN PART OF PURULIYA DISTRICT, WEST BENGAL, EASTERN INDIA B. GOSWAMI 1 AND C. BHATTACHARYYA 2 2 Department of geology, University of Calcutta 35, Ballygunge Circular Road, Kolkata bapigoswami69@yahoo.co.in; 2 chittaranjangeology@gmail.com Abstract The principal rocks of northwestern part of the present area are non-porphyritic granitoid gneisses having enclaves of metamorphites belonging to amphibolite facies while the southeastern part is composed predominantly of leptynitic granitoid gneisses and porphyritic granitoid gneisses with enclaves of metamorphites belonging to granulite facies. The amphibolite - granulite facies boundary represents a structural discontinuity and a shear zone. The area suffered at least three phases of deformations. Rarely preserved, D 1 minor folds are produced on bedding plane/lithocontact. The regional foliation (S 1 ) is axial planar to the D 1 folds, and was initially subhorizontal. The regional metamorphism (M 1 ) and D 1 are coeval. After the peak metamorphic condition during M 1 was attained, the anatectic granitoid melt was profusely injected along the S 1 of older metamorphites giving rise to non-porphyritic/leptynitic granitoid gneisses. S 1 -foliation was folded by D 2 -deformation into ENE- WSW trending overturned, nearly non-plunging, tight folds. Nepheline syenites were emplaced in the granulite facies terrain, parallel to the axial plane of D 2 -folds before the end of M 1 metamorphism ( Ma).The D 3 -deformation produced two steep, northerly-inclined ENE - WSW regional shear zones. The granulite facies terrain has been uplifted from deeper level along the shear zones and juxtaposed with amphibolite facies terrain during D 3 -phase. Porphyritic granitoid batholith emplaced in these shear zones caused thermal metamorphism (M 2 ) of regionally metamorphosed rocks at hornblende hornfels facies ( Ma).The peak temperatures of regional metamorphism (M 1 ) attained during amphibolite and granulite facies are estimated to be around 700 o C and 820 o C respectively while the peak thermal metamorphic (M 2 ) temperature attained during later emplacement of porphyritic granitoid magma is around C. The peak pressures of amphibolite and granulite facies 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 of Puruliya district is composed mainly of granitoid gneisses (Sen, 1956, 1959; GSI, 1977) which contain enclaves of pelitic rocks such as mica schists and khondalite, with varying proportions of psammitic and calcareous intercalations, and metabasic rocks (amphibolites and basic granulite). The metabasic rock bodies are generally aligned parallel to regional foliation and have initially

2 42 Tectonothermal Evolution of Chotonagpur Granite Gneiss Complex intruded into the above metasediments (Fig. 1). The intrusive units within the granitoid gneisses include nepheline syenite gneisses (Bhaumik et al., 1990; Chattopadhyay and Bhattacharyya, 1990; Ray and Mondal, 2002), porphyritic granitoid gneisses of batholithic dimension, pegmatite and quartz veins, ultramafics and dolerites. The structural and metamorphic aspects of the southern and eastern parts of the present area (Fig. 1) were dealt with about 50 years back by Sen (1956, 1959). Mahadevan (1992) studied tectonostratigraphic evolution of the adjoining southern part of the area. The purpose of this paper is to report the results of detailed studies on structures, petrology and metamorphism along with thermobarometric data which throw new light on the tectonothermal evolution of the northeastern Puruliya as a whole. 2. Geological Setting The area belongs to the eastern extension of Proterozoic Chhotanagpur granite gneiss complex (CGGC). This gneissic complex is of composite character consisting mainly of granitoid gneisses, migmatites, and massive granites with enclaves of metasedimentary and meta-igneous rocks, and intrusive basic and intermediate rock types (Ghose, 1983, 1992). Tentative general stratigraphic succession for the Puruliya district is given in Table 1. In the present area (Fig. 1) nonporphyritic granitoid gneisses (NPGG) occupies the major part of the northern half and contains enclaves of mica schists and khondalites, marbles, calc-silicate schists/ gneisses, para- and ortho-amphibolites belonging to amphibolite facies. The NPGG is followed to the south by an E - W trending porphyritic granitoid gneiss batholith (PGG). Leptynitic granitoid gneisses (LGG) occur to the south and east of PGG. The leptynitic granitoid gneiss country contains minor patches of migmatites, basic granulites, charnockitic rocks, calc-silicate rocks and anorthosites representing granulite facies. Both PGG and LGG contain small patches of basic granulites and charnockitic rocks. Hence the central part of the area is a granulite facies terrain. Again in the southern-most part of the area LGG contain enclaves of amphibolites and calc-silicate rocks belonging to amphibolite facies. A general E-W strike with northerly dip (less commonly southerly) of the dominant foliation in the present area (Sen, 1956) is comparable to the orientation of regional foliation in the Chhotanagpur granite gneiss belt (Sengupta and Sarkar, 1964, 1968; Ghose, 1983; Mazumder, 1988; Sarkar, 1988; Baidya et al., 1989). The dominant northerly dip of foliation in the present area has been earlier ascribed to regional antiformal folding with overturned southern limb (Sen, 1956). 3. Structural Geology The present area of study has suffered two phases of folding deformation and two phase of shear related deformation. First generation (D 1 ) folds on bedding in metasedimentary rocks show moderate northerly dip of axial plane, which is parallel to regional foliation. D 1 -folds are represented rarely as tight to isoclinal, intrafolial, and rootless, minor folds within non-porphyritic/ composite/migmatitic granitoid gneisses (Fig. 2). Non-porphyritic granitoid magma were injected parallel to the axial plane of the D 1 -

3 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 gneissic banding before the end of D 1 -phase. The dominant/regional foliation (S 1 ) has been folded into overturned (the southern limb being overturned), nearly non-plunging, tight folds (D 2 ) with northerly dipping axial plane (S 2 ; Fig. 3a, 3b). Superposition of D 2 on D 1 folds has led to the formation of arrow head/ mushroom shaped outcrop patterns affecting the gneissic banding in non-porphyritic granitoid gneisses (Fig. 4). However, occasional formation of domes and basins and hook-shaped patterns due to the change of orientation of S 0 surfaces during the

4 44 Tectonothermal Evolution of Chotonagpur Granite Gneiss Complex 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 Formation Panchet 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 quartzmagnetite veins Precambrian Granitic rocks Metabasic rocks Metasedimentary rocks Porphyroblastic granite gneiss, biotite granite, composite gneiss, migmatites, garnetiferous granite gneiss Amphibolite, metanorite, hornblende schist Calc-granulite, crystalline limestone, garnetiferous sillimanite schist (graphite and kyanite-bearing at places) emplacement of non-porphyritic granitoid magma along the S 1 axial plane of D 1 folds (described later). D 2 folds are common in calcsilicate rocks, amphibolites and migmatites in the northwestern part of the area (Fig. 3). D 2 folds are uncommon in the granulite facies terrain lying in the southern and eastern part where leptynitic foliation and quartz-ribbon and rods are common features. The third phase deformation produced two parallel ENE-WSW trending regional shear zones, which are steeply dipping to the N, with steeply plunging mineral lineation. Both of these shear zones are branches of the North Puruliya Shear Zone (NPSZ). The northern shear zone (NSZ, Fig. 1), close to the northern boundary of the porphyritic granitoid gneiss batholith demarcates roughly the amphibolite facies of the northwestern part from the granulite facies of the southeastern part of the area. The boundary between the amphibolite facies terrain (WNW) and the granulite facies terrain (ESE) is sharp, and there is no gradational change in mineral composition of amphibolites from amphibolite facies terrain to basic granulites of the southeastern part. There is also significant difference in trace element compositions of basic granulites and of the amphibolites. Shear zone thickness ranges from few centimetres to more than 100 metres. The gradual change from host granitoid rocks to intensely foliated rocks and intense grain size reduction, S-C band, pinch-and-swell structure, rotations of porphyroclast mark the shear zone (Fig. 5a). The shear-sense indicators within the mylonitic rocks indicate top-to-se sense of shear (Fig. 5b). The southern shear zone (SSZ),

5 B. Goswami and C. Bhattacharyya 45 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 PGG batholith, is characterized by intrusion of profuse pegmatite bodies and development of locally close spaced shear folds (subparallel axial planes). Epidotization of granite gneiss is a common feature of SSZ. This shear zone is nearly vertical or steep northerly/southerly dipping and shows upliftment of northern side. In the present study area SSZ passes in the LGG country, but enclaves of basic granulites and charnockitic rocks are limited only to the northern sides of SSZ (Fig. 1). 4. Emplacement of Magmas in Relation to Deformation 4.1. Non-Porphyritic Granitoid Gneisses (NPGG) Graphic intergrowth of quartz and K- feldspar, and weak zoning in plagioclase with Albite rim suggest that NPGG of largely syeno-granite composition have crystallized from a melt. However, folds and boudinage on leucosomal bands of NPGG together with partial recrystallization and undulose extinction in quartz and feldsapr suggest subsolidus plastic strain and recrystallization. Trails of biotite are often seen to be warping around porphyroclasts of feldspar in shear zones. S 1 in the amphibolites/ mica schists enclaves (cf. paleosome) within the migmatitic gneisses is parallel to the gneissosity in the non-porphyritic granitoid gneisses (cf.

6 46 Tectonothermal Evolution of Chotonagpur Granite Gneiss Complex Fig 3. a) Overturned D2 folds in migmatitic granitoid gneiss (non-porphyritic). Boxed area is shown separately by an enlarged photograph in Fig. 3b. About 1 km NNE of Cheliyama. Length of the axe = 1.2m Fig 4. Arrowhead pattern in migmatitic non-porphyritic granitoid gneiss. About 600 m SSE of Cheliyama. Ruler = 14 cm. leucosomes). This common parallelism of foliation and centimetr to metr-scale granitic layers in stromatic migmatites suggests "percolative flow of (granitic) melt along the flattening fabric using the dynamic permeability produced during active deformation" (Brown and Solar, 1998). The above suggests emplacement of granitoid magma during development of S 1 fabric (D 1 ). Later D 2 -folding deformed both S 1 -foliation and sheets of non-porphyritic granitoid gneisses (Fig. 3) Nepheline syenite gneisses The nepheline syenite (monzosyenite) gneisses form two E-W elongate lensoid bodies which intruded concordantly into the khondalitic and leptynitic granitoid gneiss close to ENE trending northern shear zone (Fig. 1). Preferred alignment of long axes of euhedral K-feldspar megacrysts defines a magmatic foliation in the nepheline syenite gneisses. However, porphyroclasts of K- feldspar and nepheline along with biotite and amphibole define a gneissosity in deformed samples. Anhedral to subhedral megacrysts of deformed microperthite and nepheline are enclosed in aggregates of fine to medium grained felsic minerals. Undulose extinction, polygonization and triple-point junction in the felsic groundmass indicate recrystallization. Bending of twin lamellae and marginal granulation of microcline megacrysts and kinks in biotite are frequently noticed. Amphibole generally occurs as tabular megacrysts and contains inclusions of albite. It occurs also as inclusion within nepheline and alkali feldspar. Sphene occurs commonly as thin rim around ilmenite. These nepheline syenite bodies with common recrystallization

7 B. Goswami and C. Bhattacharyya 47 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 northern fringe of porphyritic granitoid gneiss batholith, near Isradanga village, about 5 Km NE of Raghunathpur. features in constituent minerals indicate their emplacement prior to the waning phase of regional metamorphism (M 1 ) Porphyritic Granitoid Gneiss Batholith The porphyritic granitoid gneisses (PGG) of largely monzogranitic to syenogranitic composition intruded the country of non-porphyritic and leptynitic granitoid gneisses (Sen, 1956; Goswami, 2007). Mica schists, khondalites, quartzites, calc-silicate rocks, amphibolites and basic granulites occur as enclaves within the PGG. The planar fabric in the enclaves was formed during the first deformation, D 1. Hence the local discordance between the planar fabric in the host PGG and that in the enclaves of the metamorphites and the NPGG with D 2 folds suggest that the emplacement of the PGG batholith took place at least after D 2 deformation. The magmatic flow planes in PGG batholith have ENE-WSW parallel to the overall elongation of the batholith. Magmatic fabric is defined by preferred orientation of euhedral to subhedral feldspar (mainly microcline) +biotite +hornblende (Fig. 6). Deflections of foliation of porphyritic granitoids around the xenoliths of basic granulite are also consistent with flow of magma around solid objects. In the central part of the batholith orientations of feldspar megacrysts are haphazard. C/S structures are also seen at places (Fig. 7; Berthe et al., 1979). Presence of both magmatic and solidstate deformation features in PGG batholith demonstrates that the shearing has been active during and after complete crystallization of the magma. The control of shear zone/fault zone/ thrust zone for emplacement of granitoid magma, giving rise to porphyritic granitoid rocks has been suggested in published literature (e.g., D'Lemos et al., 1992; Rajesh, 2000). 5. Episodes of Metamorphism The dominant schistosity (S 1 ) in the enclaves is defined by the preferred arrangement of metamorphic minerals (e.g., sillimanite in khondalite, pyroxene and hornblende in basic granulite, and hornblende in amphibolite). These suggest that the M 1

8 48 Tectonothermal Evolution of Chotonagpur Granite Gneiss Complex 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

9 B. Goswami and C. Bhattacharyya 49 Table 2. Thermobarometric estimates for the near-peak metamorphic stage of regional metamorphism (M 1 ) and thermal metamorphism (M 2 ) M 1 M 2 Amphibolite facies domain Granulite facies domain Porphyritic granitoid domain Temperature ± 50 ( 0 C) Rock Type (No. of Samples) Amphibolite (3) Calc-silicate rocks (5) Mica schist (5) Calc-silicate gneiss** Method BH W P.G. 600 a 600 b 820 c FS 669 B, G 742* 725* GP 610* SB 615** 653 H 680** 640* 600 LG 730** 729 B, L 677 FK 705 S WS 776 Peak temp. (ºC)~700 ~820 ~800 BH MP NP 6.6* B, L 6.4 P.G PC 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 and Patino 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 Grid a Petrogenetic Grid of calc-silicate rocks: Winkler (1988, Fig. 15.3, p.247); b Petrogenetic Grid of metapelitic rocks: Carmichael (1978); c Petrogenetic Grid of nepheline syenite gneisses: Goswami and Bhattacharyya (2008) Pressure ± 0.5 (kbar) Khondalite Basic granulite (3) Enderbite (1) Nepheline syenite gneiss (12) Basic granulite(hbl hornfels) (3) Enderbite (1) Porphyritic granitoid (Hbl-Crystallization) (2)

10 50 Tectonothermal Evolution of Chotonagpur Granite Gneiss Complex metamorphism is coeval with the first phase of deformation (D 1 ). M 1 continued upto D 2 deformation event. During the emplacement of porphyritic granitoid batholith in the regionally metamorphosed country rocks, the latter were subjected to contact metamorphism (M 2 ) in hornblende-hornfels facies leading to migmatites in some places. Widespread alteration of pyroxene to hornblende and of both pyroxene and hornblende to biotite in basic granulites occurring as enclaves in porphyritic granitoid gneiss batholith resulted from M 2 (Goswami, 2007). Thermobarometric estimates for the near-peak metamorphic stage of regional metamorphism (M 1 ) and contact metamorphism (M 2 ) are given in Table 2 (Goswami, 2007). The preferred estimate of M 1 metamorphic conditions in granulites is suggested to be ~800 0 C and kbar. The absence of garnet in basic granulites of present study indicates that the peak-m 1 pressure was unlikely to be high. Enclaves of enderbites and basic granulites within porphyritic granitoids are metamorphosed by M 2 event. They give variable estimates of pressure which is due to lack of complete resetting of M 1 textures (especially for garnet bearing assemblage of enderbites) during M 2 event. 6. Summary of Tectonothermal History For a long time the Chhotonagpur Grnaite Gneissic Complex (CGGC), which is tectonically attached with Singhbhum craton towards south, has been thought of as a product of late Archaean to Paleoproterozoic crustal consolidation (Sarkar, 1988). Available age data have indicated, however, a dominant 'Late Grenvillean' tectonothermal event at Ma in CGGC (Ghose, 1992 and references therein). An earlier Mesoproterozoic metamorphism ( Ga; Ray Barman et al., 1994) was registered only at a few places of the CGGC. Recently Chatterjee et al. (2008) have shown that emplacement of gabbroic anorthosite rocks of the Saltora area (lying a few km east of the study area) took place at 1550±12 Ma subsequent to D 1 but before D 2 deformation event. Subsequent metamorphism in the CGCC is recorded by the 947±27 Ma zircon growth in the anorthosite, and 950±20 Ma and 995±24 Ma monazite growths in the matrix and within garnet of metapelite granulite located north of the anorthosite (Chatterjee et al., 2008). The prograde pre/syn- D 1 granulite facies metamorphism (M 1 ) in the enclave suite ( C and 4-6 kbar) is suggested to be >1.4 Ga (EPMA monazite age) (Maji et al., 2008). EPMA dating of monazites in sheared granites yielded ~ Ga (Maji et al., op. cit). However, for the migmatitic gneisses of adjoining Muruguma area it has been suggested that regional metamorphism (M 1 ) and formation of associated non-porphyritic granitoids took place about 1178±61 Ma ago (Rb-Sr whole rock isochron, Ray Barman et al., 1994). Radiometric age data of samples from Bero suggest that the age of emplacement of porphyritic granitoid magma was 1071 ± 64 Ma (Rb-Sr whole rock isochron, Ray Barman et al., 1994). The K-Ar ages of biotite from porphyritic granitoid gneisses (870 ± 40 Ma) and that of muscovite from leucogranite (810 ± 40 Ma) of the Jaipur area, about 43 km SW from the western boundary of the present area, gives somewhat younger ages due to lower blocking temperatures of K-Ar (Baidya et al., 1987). Based upon the above meagre age

11 B. Goswami and C. Bhattacharyya 51 information of rocks/minerals of Puruliya district and CGGC as a whole, the summary of metamorphism, deformation and magmatism particularly with reference to the present area may now be drawn as follows. Three major phases of ductile deformation have been recognized in the region. The earliest phase (D 1 ) is responsible for the dominant metamorphic penetrative foliation (S 1 ) in amphibolite and granulite facies rocks occurring mainly as enclaves (about 1400 Ma ago?). After attaining the peak metamorphic condition the anatectic granitoid melt was profusely injected along the S 1 foliation plane of some metasediments and amphibolites giving rise to non-porphyritic granitoid gneisses/composite gneisses. The country rocks in which the granitoid magma was emplaced were already metamorphosed under amphibolite facies. NPGG and LGG magmas generated at C and C respectively (zircon saturation temperatures) at 100 to167 Mpa P H2O (Goswami, 2007). Deformation (and metamorphism) continued, so that the S 1 schistosity along with injected granitoid bands and veins were folded during D 2, possibly before the end of M 1. The D 3 phase generated the regional NPSZ having two branches NSZ and SSZ passing northern and southern peripheries of PGG body and showing excellent development of mylonitic texture. The granulite facies terrain has been uplifted from deeper level along the shear zones and juxtaposed with amphibolite facies terrains (Fig. 1, see cross-section) during D 3 phase. The porphyritic granitoid melt (900 to 1000Ma) was responsible for the thermal metamorphism/ metasomatism of the country rock under hornblende-hornfels facies (M 2 ). Emplacement of porphyritic granitoid magma took place along the shear zones. Available geochronological data suggest that M 2 episode continued for about 1000 Ma to 900 Ma (Chatterjee et al., 2008). The CGGC in general, and the present area in particular, seems to have experienced at least two major tectonothermal episodes during the Proterozoic (Chatterjee et al., 2008), having a large time-gap between the two episodes. The regionally metamorphosed granulites (M 1 = >1400 Ma) have shown an ITD path (Ray Barman et al., 1994) followed by an emplacement of PGG and contact/ thermal metamorphism (M 2 at Ma). It is interesting to note that there is a large time gap between regional metamorphism granulite ( Ma) and thermal metamorphism due to emplacement of the porphyritic granitoids ( Ma). The D 1 and D 2 folds have been interpreted as coeval with the M 1 metamorphism. Usual interpretation of large overturned folds and pervasive schistosity (in the metasediemntary enclaves) involve crustal shortening. In the absence of more reliable precise dates for the different components of the study area/region, it would be impossible to exclude the possibility of crustal convergence before the onset of regional metamorphism. However, high T (~800 o C) but relatively low P (~7 kbar, present study) estimates of granulites (Maji et al., 2008 estimated o C and 4-6 kbar from a larger data base) and amphibolites (700 o C at ~4.5 kbar) do not suggest crustal overthickening. High thermal gradient, abundant mafic/ultramafic intrusions (Mandal et al., 2007), and geophysical evidence for the presence of sill-like mafic body at about 5 km depth (Verma et al., 1988) on the other hand, can suggest magma under-plating. Except

12 52 Tectonothermal Evolution of Chotonagpur Granite Gneiss Complex biotitization and hornblendization related to the emplacement of PGG (M 2 ) the granulite facies rocks of present study do not show much sign of retrogression, which, in turn, supports rapid exhumation immediately after M 1 episode. Further, granulite facies rocks are separated from adjacent amphibolite facies rocks by two shear zones (Fig. 1). The granulite facies rocks during exhumation formed a horst like structure within the amphibolite facies rocks (Fig. 1, see cross-section). Acknowledgements We thank the authorities of Calcutta University and the Principal, J.K. College, Puruliya for providing research facilities. We are also very much thankful to Prof. D. Mukhopadhyay, and Prof. A.R. Basu for reviewing the manuscript critically and suggesting improvements. Editorial review by Prof. D. Saha has upgraded the ms. to a great extent. Help rendered by Smt. Chandreyee Goswami is also gratefully acknowledged. This study was funded by UGC through a Minor Research Project to BG. References Baidya, T. K., Maity, N. and Biswas, P Tectonic phases and crustal evolution in a part of the Eastern Chhotanagpur Gneissic Complex. Journal of the Geological Society of India 34, Baidya, T.K., Chakraborty, S., Drubetskoy, E. and Khilatova, I New geochronologic data on some granitic phases of the Chhotanagpur granite gneiss complex in the northwestern Purulia district. West Bengal. Indian Journal of Earth Sciences 14, Bhattacharya, A., Krishnakumar, K.R., Raith, M., and Sen, S.K An improved set of a - X parameters for Fe-Mg-Ca garnets and refinements of the orthopyroxene - garnet - plagioclase - quartz barometer. Journal of Petrology 32, Bhattacharya, A., Mohanty, L., Maji, A., Sen, S.K., and Raith, M Non-ideal mixing in the phlogopiteannite binary; constrainta from experimental data on Mg-Fe partitioning and a reformulation of the biotitegarnet geothermometer. Contributions to Mineralogy and Petrology 111, Bhattacharyya, P.K. and Mukherjee, S Granulites in and around Bengal anorthosite, Eastern India: genesis of coronal garnet and evolution of the granulite - anorthosite complex. Geological Magazine 124, Bhaumik, T., Mukherjee, S. and Bose, A Petrology of Nepheline Syenites from Santuri Puruliya District, West Bengal. Journal Geological Society of India 36, Blundy, J.D. and Holland, T.J.B Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer. Contributions to Mineralogy and Petrology 104, Brown, M. and Solar, G.S Granite ascent and emplacement during contractional deformation in convergent orogens. Journal of Structural Geology 20, Carmichael, D.M Metamorphic bathozones and bathograds: a measure of the depth of postmetamorphic uplift and erosion on a regional scale. American Journal of Science 278, Chatterjee, N., Crowley, J.L., and Ghose, N.C Geochronology of the 1.55 Ga Bengal anorthosite and Grenvillian metamorphism in the Chotanagpur gneissic complex, eastern India. Precambrian Research 161, Chattopadhyay, R. and Bhattacharyya, C Petrology of 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 Granite magma generation, ascent and emplacement within a transpressional orogen. Journal of Geological Society of London 149, Dunn, J.A Geology of North Singhbhum including parts of Ranchi and Manbhum districts. Memoir of Geological Survey of India 54(1). Ferry, J.M. and Spear, F.S Experimental calibration of portioning of Fe and Mg between garnet and biotite. Contributions to Mineralogy and Petrology 66,

13 B. Goswami and C. Bhattacharyya 53 Fonarev, V.I. and Konialov, A.N Experimental study of Fe-Mg distribution between biotite and orthopyroxene at P=490Mpa. Contributions to Mineralogy and Petrology 93, Ganguly, J Garnet-clinopyroxene solid solutions and geothermometry based on Fe-Mg distribution coefficient. Geochimia et Cosmochimia Acta 43, Ghose N.C., Shmaknin, B.M. and Smirnov, V.N Some geochronological observations on the Precambrians of Chhotanagpur, Bihar, India. Geological Magazine 110, Ghose, N.C Geology, tectonics and evolution of the Chhotanagpur granite gneiss complex, Eastern India. In: Recent Researches in Geology, S. Sinha Roy (ed.) 10, Ghose, N.C Chhotanagpur gneiss-granulite complex, Eastern India: Present status and future prospect. Indian Journal of Geology 64, Goswami, B Petrology and Geochemistry of the Precambrian Rocks of Cheliyama - Raghunathpur - Santuri area, Puruliya district, West Bengal. Unpublished Ph.D. Thesis. University of Calcutta. Goswami, B. and Bhattacharyya, C Metamorphism of Nepheline Syenite Gneisses from Chhotanagpur Granite Gneiss Complex, Northeastern Puruliya district, Eastern India, Journal of the Geological Society of India 71, Goswami, B. and Bhattacharyya, C Petrology and Geochemistry of Late-Tectonic Raghunathpur porphyritic granitoid batholith of Chhotanagpur Granite Gneiss Complex from the northeastern part of Puruliya district, West Bengal: Evidences of Mantle Input. Abst. In the Seminar on "Plume Signature in Geological History" held on 8th Jan., 2008 at the Dept of Geol, University of Calcutta, p.4. Graham, C.M. and Powell, R A garnet-hornblende geothermometer: calibration, testing, and application to the Pelona schist, Southern California. Journal of Metamorphic Geology 2, GSI Role of minerals in the development of Puruliya district, West Bengal. Journal of Mines, Metals and Fuels, Special Publication, Puruliya seminar Harley, S.L An experimental study of the partitioning of Fe and Mg between garnet and orthopyroxene. Contributions to Mineralogy and Petrology 86, Lal, R.K Internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnet - orthopyroxene - plagioclase - quartz assemblages and their application to the South Indian granulites. Journal of Metamorphic Geology 11, Lee, H.Y. and Ganguly, J Equilibrium composition of coexisting garnet and orthopyroxene: Experimental determinations in the system FeO- MgO-Al2O3-SiO2 and applications. Journal of Petrology 29, Mahadevan, T.M Geological evolution of the Chhotanagpur Gneissic complex in a part of purulia district, West Bengal. Indian Journal of Geology 64, Maji, A.K., Goon, S., Bhattacharya, A., Mishra, B., Mahato, S. and Bernhardt, H Proterozoic polyphase metamorphism in the Chhotanagpur Gneissic Complex (India), and implication for transcontinental Gondwanaland correlation. Precambrian Research 162, Mandal, A., Goswami, B., Mukherjee, S. Das, S. Bhattacharyya, I. and Bhattacharyya, C Mantle metasomatism of Ultramafic intrusives in Chhotanagpur Granite Gneiss Complex, Puruliya district, West Bengal, Eastern India: Evidence from trace element and REE geochemistry. In: Igneous Petrology: 21st Century Perspective, J.S. Ray and C. Bhattacharyya (eds.), Allied Publishers Pvt. Ltd Mazumder, S.K Crustal evolution of Chhotanagpur gneissic complex and the mica belt of Bihar. In: Precambrian of the Eastern Indian Shield, D. Mukhopadhyay (ed.), Memoir, Geological Society of India 8, McCarthy, T.C. and Patino Douce, A.E Empirical calibration of the silica-ca-tschermak-anorthite (SCAn) geobarometer. Journal of Metamorphic Geology 16, McMurry, J Crystal accumulation and shearing in a megacrystic quartz monzonite: Bodocó Pluton, Northeastern Brazil. Journal of Petrology 42,

14 54 Tectonothermal Evolution of Chotonagpur Granite Gneiss Complex Moecher, D.P., Essene, E.J., and Anovitz, L.M Calculation and application of clinopyroxene - garnet - plagioclase - quartz geobarometers. Contributions to Mineralogy and Petrology 100, Newton, R.C. and Hastleton, H.T Thermodynamics of the garnet-plagioclase-al 2 SiO 5 - quartz geobarometer. In: Thermodynamics of Minerals and Melts, R.C. Newton, A. Navrotsky and B.J. Wood (eds.) Newton, R.C. and Perkins, D.I Thermodynamic calibration of geobarometers based on the assemblages garnet-plagioclase-orthopyroxene (clinopyroxene)-quartz. American Minerals 67, Paterson, S.R., Vernon, R.H. and Tobisch, O.T A review of criteria for the identification of magmatic and tectonic foliations in granitoids. Journal of Structural Geology 11, Perkins, D.I. and Chipera, S Garnetorthopyroxene-plagioclase-quartz barometry: Refinement and application to the English River subprovince of the Minnesota River Valley. Contributions to Mineralogy and Petrology 89, Rajesh, H.M Characterization and origin of a compositionally zoned aluminous A-type granite from South India. Geological Magazine 137, Ray Barman, T., Bishui, P.K., Mukhopadhyay, K. and Ray, J.N Rb-Sr geochronology of the high grade rocks from Purulia, West Bengal and Jamua- Dumka sector, Bihar. Indian Minerals 48, Ray, A. and Mondal, R.P Petrological account of nepheline syenites and associated rocks around Kankarkiari, Purulia district, West Bengal. Indian Journal of Earth Sciences 29, Sarkar, A.N Tectonic evolution of the Chhotanagpur plateau and Gondwana basins in Eastern India: An interpretation based on suprasubduction geological processes. In: Precambrian of the Eastern Indian Shield, D. Mukhopadhyay (ed.), Memoir, Geological Society of India 8, Sarkar, S.N Precambrian stratigraphy and geochronology of peninsular India. Dhanbad Publishers, Dhanbad 33p. Sarkar, S.N., Polkanov, A.A., Gerling, E.K. and Chukrov, F Precambrian geochronology of Peninsular India. A synopsis. Science and Culture 30, Sen, S Structures of porphyritic granite and associated rocks in east Manbhum, Bihar, India. Bulletin of the Geological Society of America 67, Sen, S Mineralogic trends in the evolution of metamorphic rocks and origin of granites of East Manbhum, India. Proceedings: National Institute of Science of India 25A, No.2. Sen, S.K. and Bhattacharya, A An orthopyroxenegarnet thermometer and its application to the Madras charnockites. Contributions to Mineralogy and Petrology 88, Sengupta, D.K. and Sarkar, S.N Structure of the granitic rock and associated metamorphites of the area around Muri-Silli-Jhalida, Ranchi and Purulia Districts, India. Proceedings, 22nd International Geological Congress 4, Sengupta, D.K. and Sarkar, S.N Structure of granitic rocks and associated metamorphites of the area around Muri-Silli-Jhalida, Ranchi and Purulia Districts, India. Transactions Geological Metallurgical Institute of India 65, Sengupta, P., Dasgupta, S., Bhattacharyya, P.K., and Mukherjee, M An orthopyroxene-biotite geothermometer and its application in crustal granulites and mantle derived rocks. Journal of Metamorphic Geology 8, Verma, R.K., Mukhopadhyay, A., Ashraf, M.H., Nag, A.K. and Satyanarayana, Y Analysis of gravity field over Chhotanagpur plateau and Bihar Mica Belt. Memoir Geological Society of India 8, Wells, P.R.A Pyroxene thermometry in simple and complex systems. Contributions to Mineralogy and Petrology 62, Whitney, J.A. and Stormer, J.C The distribution of NaAlSi 3 O 8 between coexisting microcline and plagioclase and its effect on geothermometric calculations. American Minerals 62, Winkler, H.G.F Petrogenesis of metamorphic rocks. Narosa Publishing House 344p.