GR Gondwana Research

Evidence of Superposed Metamorphism from the Gokavaram Area, Eastern Ghats Belt, and its Relation

with the Kemp Land Coast, East Antarctica

Somnath Dasgupta', Pulak Sengupta', Supratim Pal' and Masato Fukuoka2

'Depcirtnzent of Geological Sciences, Jadavpur University, Calcutta-700 032, India

Department of Earth and Planetary Sciences, Hiroshima University, Hiroshima, Japan

(Marzuscript received AuguJt 24,1998;accepted September 25, 1998)

Abstract In this paper, we compare tlie petrological histories of the Kemp Land Coast (east Antarctica) and Gokavaram area

(Eastern Ghats), that were supposed to have been juxtaposed. The area around Gokavaram is dominated by different varieties of paragneisses (pelitic, quartzofeldspathic and calcareous composition) with relatively minor amounts of orthogneisses (mafic, enderbitic and granitic composition). The rocks were involved in three major phases of deformation and were finally affected by localised shear movement. On the basis of reaction textures, well preserved in high Mg-A1 granulites and calc-silicate granulites, and geothermobarometric data we deduce a polymetamorphic evolution of tlie rocks. Following an early M, metamorphism culminating at 9.2-9.4 kbar, > 950"C, the rocks cooled nearly isobarically down to 850°C. During a subsequent M, metamorphism, near isothermal decompression to 5-6 kbar occurred. This was followed by near isobaric cooling down to 600-650°C. M, is a weak ampliibolite facies overprint, largely restricted to late shears, which involved hydration as well. Available radiometric data from this area can be interpreted in terms of partial resetting of U-Pb systematics in older sphenes due to M, metamorphism at ca. 550 Ma. Despite tlie absence of sufficient isotopic data on tlie Eastern Ghats granulites, we document a remarkable similarity in the petrological history of the two supposedly erstwhile neighbours.

Key Words: Eastern Ghats, Indo-Antarctic correlation, Pan-African metamorphism.

Introduction

In most palaeogeographic reconstructions from the Mesoproterozoic to Mesozoic, the Eastern Ghats Belt along the east coast of India (Fig. 1) is juxtaposed against portions of east Antarctica and forms a part of the 950-1000 Ma SWEAT orogen (reviewed in Yoshida, 1995; Unrug, 1996). Detail structural, petrological and geochronological work on the east Antarctic granulites along Mac Robertson Land and Kemp Land coasts have identified two tectonometamorphic provinces, the Archaean Napier Complex and the Proterozoic Rayner Complex (Harley and Hensen, 1990). It is now becoming clear that a large tract of east Antarctica was affected by ca. 550 Ma Pan-African orogeny and metamorphism (Shiraishi et al., 1994). In contrast to the detail geological investigations in east Antarctica, the Eastern Ghats Belt has received geological attention only recently. Although petrological studies on the Eastern Ghats Belt have been carried out in considerable

details in the last decade (reviewed in Dasgupta, 1995; Dasgupta and Sengupta, 1998), geochronological data correlatable with specific geologic events have started to flow in very recently Since the early work of Grew and Manton (1986), it was established that the northern portion of the Eastern Ghats Belt (in the area near Visakhapatnam and further north, Fig. 1) were affected by cu. 1000 Ma orogeny and granulite facies metamorphism (Aftalion et al., 1988; Paul et al., 1990). Petrologic and geochronologic data of Grew and Manton (1986) have shown that the ca. 1000 Ma granulite metamorphism was preceded by another high temperature event, whose age remains indeterminate. Recently, Mezger et al. (1996) have shown that the area north of the Godavari Rift in the Eastern Ghats Belt (NEGB, Fig. 1) had a different thermal history with respect to the southern portion (SEGB). In the NEGB, a major granulite facies metamorphism occurred at ca. 960 Ma and a weaker amphibolite facies overprint at cn.550 Ma was detected. To the contrary, the SEGB is characterised by the Middle

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228 S. DASGUPTA ET AL.

MR

Fig. 1. Disposition of the Eastern Ghats Belt showing tlie location of areas mentioned in text and Fig. 5 . NEGB : Northern Eastern Ghats Belt. SEGB : Southern Eastern Ghats Belt. MR : Mahanadi Rift. GR : Godavari Rift. Locations : 1 : Chilka, 2 : Rayagada, 3 : Salur, 4 : Garbham, 5 : Vizianagaram, 6 : Anantagiri-Araku, 7 : Sunkarametta, 8 : Paderu, 9 : G. Madugula, 10 : Anakapalle, 11 : Gokavaram (study area), 12 : Kondapalle, 13 : Chimakurthy. Inset showing location of the Eastern Ghats Belt in India

Proterozoic granulite facies metamorphism (>1600 Ma) and no imprint of the ca..1000 Ma Grenvillian (=Rayner) and ca. 550 Ma Pan-African orogeny has so far been detected. This raises the possibility that the Napier-Rayner boundary in East Antarctica should pass through an area close to the Godavari Rift (discussed by Sengupta et al., 1998). The contrasting tectonothermal histories of the NEGB and SEGB separated by the Godavari Rift necessitate detail structural, petrological and geochronological work on the areas lying close to the Rift. We have, therefore, chosen an area near Gokavaram (Lat. 17'15'30" N, Long. 81'51' E) (Fig. l), which is located at the southern extreme of NEGB for investigation. If in reality the Godavari Rift is an extension of the Napier- Rayner Boundary, this area would have been juxtaposed against the Kemp Land coast of east Antarctica (Fig.2). In

this communication, we will compare the petrological histories of the area near Gokavaram and that along the Kemp Land coast to test this hypothesis. From the Gokavaram area, Mezger and Cosca (1998) obtained U-Pb cooling age of 863 +_ 8 Ma from sphene occurring in calc- silicate granulites.

Geological History of the Kemp Land Coast, East Antarctica

The Napier-Rayner boundary in Kemp Land is gradational (Harley and Hensen, 1990) and consists of Archaean (Napier) relicts and their reworked equivalents, particularly in the Edward VIII Gulf area and Stillwell Hills (Sheraton and Black, 1983; Sheraton et a1.,1987). Although

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SUPERPOSED METAMORPHISM, EASTERN GHATS 229

Fig. 2. A possible correlation of Eastern Ghats Belt with east Antarctica. Abbreviations are : RC : Rayner Complex, NC : Napier Complex, EER : East Enderby Rift, S : Stillwell Hills, PB : Prydz Bay, LR : Lambert Rift, RG : Rauer Group, VHC : Vestfold Hills Complex, NEGB : Northern Eastern Ghats Belt, SEGB : Southern Eastern Ghats Belt, G : Gokavaram, V : Visakhapatnam, SC : Singhbhum Craton, BC : Bastar Craton, DC : Dharwar Craton, CB : Cuddapali Basin, MG : Madras Granulite, MR : Mahanadi Rift, GR : Godavari Rift, PCSZ : Palgliat Cauvery Shear Zone

the proposition that whole of the Rayner Complex represents reworked Archaean material has been ruled out, the boundary between the two complexes often contain relict Archaean material (Black et a1.,1987). Clarke (1987,1988) outlined the geological evolution of these Archaean-Proterozoic complexes in Kemp Land and adjoining MacRobertson Land and reports the occurrence of a cn. 2700 Ma (Rb-Sr whole rock) felsic orthogneiss within an area dominated by paragneisses. The proportion of reworked Archaean material compared with metamorphosed equivalent of younger intrusives and sedimentary cover sequence is not yet established (Harley

and Hensen,l990). The P-T trajectory of evolution of the granulites from Stillwell Hills and adjoining parts of Kemp Land showed polymetamorphic evolution of the rocks (Harley and HensenJ990). Subsequent to an early granulite facies metamorphism (Archaean?) the rocks cooled nearly isobarically (Clarke, 1987). The Rayner orogeny (ca. 1000 Ma) is reflected in three generations of deformation, second granulite facies metamorphism and post-peak near isothermal decompression down to 4-5 kbar, 700-750°C (Ellis, 1983; Clarke, 1988). Subsequently, retrograde shear zones and mylonite-pseudotachylite zones developed during the Pan-African orogeny (Clarke,1988).

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230 S. DASGUPTA ET AL

Geologic Setting of the Gokavaram Area

Lithology and structure

A sketch geological map of the study area is shown in Fig. 3. The study area is dominated by several types of paragneisses. These include khondalite (garnet + sillimanite + perthite + quartz + plagioclase) and leptynite (garnet + quartz + plagioclase + K-feldspar), the latter containing isolated outcrops of spinel-sapphirine bearing Mg-A1 granulite and scapolite-wollastonite bearing calc-silicate granulite. Magmatic rocks in the study area are represented by mafic granulite (orthopyroxene + clinopyroxene + plagioclase + ilmenite), metanorite (orthopyroxene + plagioclase + Fe-Ti oxides + garnet), enderbitic orthopyroxene granulite (orthopyroxene + plagioclase + quartz + K-feldspar +- garnet) and granitic rocks (quartz + K-feldspar + plagioclase +biotite + garnet). These magmatic rocks occur as detached bands and lenses in leptynite (Fig. 3). South of B ugubanda (Fig.3), a pegmatitic rock (quartz

INDEX 8 1 '447'

Leptynite

HKhondalite

Enderbite

Granite

talc granulite

mMafic granulite

@J Spinel granulite

Pegmatite

["Soil cover 4 Strike of foliation /p Strike of shear foliation A& Shear zone

+ K-feldspar + plagioclase) occurs within an E-W trending shear zone cutting across enderbite. This pegmatite contains wolframite + graphite mineralization.

The rocks have been affected by several phases of deformation. During an early (D,) deformation under amphibolite facies condition S, planar structure defined by biotite-sillimanite-quartz was produced. S, is now preserved only as inclusions in porphyroblastic garnet in khondalite and spinel granulite (Dasgupta et al., 1995). D, is responsible for the development of gneissic banding (S,) in the paragneisses and is possibly syn-M, peak metamorphism (discussed later). D,and its attendant planar structures and folds are syn- to post-M, (discussed later). A late shear foliation defined by hydrous minerals in para- and orthogneisses is the manifestation of D,, which is also characterised by emplacement of pegmatites in shear zones. Enderbitic gneisses were emplaced during syn-MI. However, some varieties of massive and pegmatoidal enderbites are post MI and D,, but pre-D,.

RamDachodavaram 12 km 81'50'

Fig. 3. Geological map of the area north of Gokavaram.

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SUPERPOSED METAMORPHISM, EASTERN GHATS 23 I

Petrological history study area. Different mineral associations present in the spinel-sapphirine granulites and key mineral reactions

Geothermobarometric data from different rock vpes from the study area are given in Table 2 ( data from DasgLTta et ale, 1995, and new computations). DasguPta et (1995)

Dasgupta et a1.(1995) constructed a semi-quantitative

the help of topological constraints (Hensen and Harley1990) and natural rock data. They considered the metamorphic evolution of spinel-sapphirine granulites from the present

petrogenetic grid in the system KFMASH at high fO, with deduced by Dasgupta et are summarised in

Table 1. Summary of mineral compositions and deduced mineral reactions in spinel-sappliirine granulites (from Dasgupta et al., 1995). Mineral abbreviations used here and the text are after Kretz (1983)

Mineral Association I I1 I11 IV V VI

Porphyroblastic garnet (Grt,) and perthite are present in all associations Spl-Qtz-Grt-Crd Spl-Crd-Opx-Sil Spl-Qtz-Spr-Grt-Sil Spl-Qtz-Spr-Grt Ilm-Hem-Crd- Mineral Assemblage Spl-Qk-Crd ~'

Mineral Reactions Prograde

-0px-Sil

Retrograde (ix) Spl + Qtz + Crd

Opx-Sil

(i) Bt + Sil + Qtz + P1 W Spl +Crd + (Ilm-Hem) + M (ii) Spl + Bt + Qtz (E Crd + Grt + M (iii) Spl + Qtz -$ (viii) Fez03 iii Sil +Grt Opx + Sil SPr --f Spr + Grt Fe-Ti oxide + (x) Grt + Qtz + Crd +Opx + Sil + 0, Opx + Crd (xi) Grt + Sil + Qtz + Crd

(iv) Spl + Crd + (v) Spl + Sil + Qtz -+ (vii) Spl + Qtz

(vi) Spl + Spr + Qtz -+ Sil + Grt

Table 2.

Metamorphism Rock type Thermobarometer P (Kbar) T ("C)

P-T conditions of metamorphism from thermobarometric data

Spinel -

granulite M, (Peak) sappliirine -

Enderbite

MI, Mafic granulite

Spinel - sapphirine granulite

M3 Enderbite

GAPQ (NH)

GAPQ (KN)

GSpSQ (Bh) GSapSQ (Be) GO (LG) GOPS (PC) Two pyroxene

(K, Ca-Mg) (K, Fe - Mg)

GO (LG) GOPS (PC)

GOPS (B) GC (N)

SCOS (HH) SPC (N)

GB (D) GH (GP)

Two feldsoar (SW)

9.2 (Core) 8.9 (Rim) 9.4 (Core) 9.1 (Rim) 9.4 9.2

950 (assumed)

790 - 800 8.3 - 8.4

890 875

4.75 (assumed) 750 p, 7.6

4.75 P,,, 6.3 (Core)

P,, 7.3 (Core) (assumed)

PMR 4.75

5.1 (Rim) 750

5.9 (Rim) 4.6

670-724 580-640 440-500

Abbreviations used are the thermobarometric formulations used in this study and taken from Dasgupta et al. (1995) GAPQ (NH) : Garnet-Al,SiO,-plagioclase- quartz barometer (Newton and Haselton, 1981); GAPQ (KN) : Garnet-AlzSiO,-plagioclase- quartz barometer (Koziol and Newton, 1988); GSpSQ (Bh) : Garnet-spinel-sillimanite-quartz barometer (Bohlen et al., 1986); GSapSQ (Be) : Garnet-sapphirine-sillimanite- quartz barometer GO (LG) : Garnet-ortliopyroxene thermometer (Lee and Ganguly, 1988); GOPS (PC) : Garnet-orthopyroxene-plagioclase-quartz barometer (Perkins and Chiperra, 1985); Two pyroxene (K) : Two pyroxene thermometer (Kretz, 1982); GOPS (B) : Garnet-orthopyroxene-plagioclase-quartz barometer (Bohlen et al., 1983); GC (N) : Garnet-cordierite barometer (Nichols et al., 1992); SpC (N) : Spinel-cordierite barometer (Nichols et al., 1992); SCOS (HH) : Spinel-cordierite-orthopyroxene-sillimanite barometer; GH (GP) : Garnet-hornblende thermometer (Graham and Powell, 1984); GB (D) : Garnet-biotite thermometer (Dasgupta et al., 1991); Two feldspar (SW) : Two feldspar thermometer (Stormer and Whitney, 1977).

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232 S. DASGUPTA ET AL.

concluded that the rocks are polymetamorphic. During an early phase (M,), the rocks were metamorphosed along a high dT/dP prograde path at low pressures reaching PMax- TMax of .. 9.2-9.4 kbar, > 950°C, which was followed by near- isobaric cooling down to - 850°C (M,,) (Fig. 4). The deduced nature of the prograde path of M, attests to metamorphism along an anti-clockwise trajectory. The rocks were subsequently reworked by M, metamorphism which is characterised by nearly isothermal decompression down to 5-6 kbar (M,,> (Fig. 4). The most notable metamorphic imprint of M, was the development of second generation cordierite in the spinel granulites (reactions (ix) to (xi), Table 1). Another cordierite-forming reaction was later identified in the spinel granulite. This is : orthopyroxene + sillimanite + quartz = cordierite. This reaction is clearly correlatable with the second generation cordierite-forming stage (MJ.

The characterisation of the petrological evolution of the calc-silicate granulites from this area followed the publication of our work on spinel-sapphirine granulites (Dasgupta et al., 1995). Textural features suggest that scapolite, wollastonite, clinopyroxene, quartz and calcite/ plagioclase constitute the peak M, assemblage in the rocks. Coronal garnet appeared subsequently via the reactions:

Wo + P1 -+ Grt +Qtz Scp + Wo -+ Grt + Cal + Qtz

*..(l) ...( 2)

Finally, scapolite broke down to a vermicular intergrowth of calcite and plagioclase according to the reaction

Scp -+ Cal + P1 ...( 3) We have calculated the positions of these three fluid-

absent reactions using the program THERMOCALC (Holland and Powell, 1990) and a-X relationships of the phases as given in Sengupta et a1.(1997). We obtained a reasonably good intersection at 5.5 -1.0.5 kbar and 700 f 25°C (Fig. 4). This would indicate formation of coronal garnet and breakdown of scapolite due to cooling subsequent to decompression (M2J This is also corroborated by the fact that garnet does not show any evidence of decompression. We would, therefore, argue for a second cooling event at distinctly lower pressures (5-6 kbar) (Fig. 4). Such low pressure cooling leading to the formation of coronal garnet in calc-silicate granulites has been demonstrated by Sengupta et a1.(1997) from an adjoining area. Near isobaric cooling subsequent to decompression is a possible result of thermal relaxation of the crust in the same tectonothermal event (Harley, 1989). M, and M, phases in all the rocks were later subjected to hydration accompanying a renewed phase of localised deformation (D,) producing phases that are aligned parallel to Sh, . Important mineral reactions in this context are :

Grt + Kfs + H,O -+ Bt + Qtz

Opx + P1+ Ilm + H,O -+ Amph + Qtz

.*.(4)

...( 5) (in leptynite and khondalite)

Opx + Grt + Ilm + P1+ H,O -+ Amph + Qtz ...( 6) (both in enderbite)

10

9 -

8 - h L 0

u 5 7 - a

6 -

5 - OM3 ?

Fig. 4. P-T path deduced from the granulites of the study area. Stippled path taken from Dasgupta et al. (1995). Path ornamented with bars is from this study. Solid P-T path within inset box is of Stillwell Hills (Hensen & Harley, 1990). Abbreviations as in text.

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SUPERPOSED METAMORPHISM, EASTERN GHATS 233

Table 3. Tectonothermal history of the studied rocks

Deformation Planar structure Metamorphism Metamorphic and magmatic episodes

D4 Sh, (Shear foliation) M3 Hydration, Weak amphibolite facies metamorphic overprint Near isothermal decompression followed by near isobaric cooling Emplacement of massive and pegmatoidal enderbite Ultra-high temperature metamorphism, Emplacement of enderbitic gneiss

internal schistositv S

D, M2,

D* s2 MI

D, Sl Pre-MI Amphibolite facies metamorphism. S, preserved as

Further, the massive variety of ertderbite has been extensively biotitised along Sh, and a mylonitic fabric has developed in the shears. This is attributed to the reaction,

Opx + Kfs + H,O -+ Bt + Qtz ...( 7) Reactions 4 through 7 clearly indicate hydrous fluid flux,

possibly accompanying cooling, along Sh, during D,. Geothermornetry (Table 2) shows that hydration occurred at cn. 600-650°C. Similar late stage hydration reactions have been documented from many other areas in the Eastern Ghats Belt (reviewed in Dasgupta, 1995). We tentatively correlate it to a terminal amphibolite facies overprint (M,) on the granulitic assemblages. Correlation of structural

elements with the metamorphic history indicates that formation of pegmatite is also related to D,. D, is clearly pre-MI, D, is possibly synchronous with MI, while D, is related to M,. The deduced tectonothermal history of the studied rocks is summarised in Table 3.

Comparison with P-T Trajectories Deduced from Other Areas and Geochronological Data

Figure 5 shows a compilation of P-’I’ trajectories deduced by different workers from the Eastern Ghats Belt. In the SEGB (Southern Eastern Ghats Belt) data are available from

11.

10.

9

8

- 7 z 2 Q - 6

5

4

3 I I

I I I I 700 800 T(’C) 900 1000 1100 600

Fig. 5. Compilatioii of P-T trajectories deduced from different areas in the Eastern Ghats Belt. Numbers same as those in Fig. 1. Workers : 1 : Sen et al. (1995), 2 : Shaw and Arima (1997), 3 : Mukhopadhyay and Bhattacharya (1997), 4 : Dasgupta et al. (1992), 5 : Kamineni and Rao (1988), 6 : Seiigupta et al., 1990 Anantagiri; Sengupta et al., 1991 Araku), 8 : La1 et al. (1987), 8a : Pal and Bose (1997), 9 : Mohan et al. (1997), 10 : Dasgupta et al. (1994), 11 : Dasgupta et al. (1995), 12 : Sengupta et al. (1998), 13 : Dasgupta et al. (1997).

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234 S. DASGUPTA ET AL.

only two places, 12 and 13, both showing essentially heating to extreme (-1000°C) temperatures and cooling trajectories, but at different pressures. In the SEGB, there is no evidence of M, as deduced in this work. From the NEGB, data are available from a number of areas and practically all of them attest to polymetamorphic character of the granulites. A notable exception is path 9, which conceives a single retrograde trajectory comprising both cooling and decompression. This conclusion is in contrast with textural description from the same paper, where the authors distinguished at least three stages of evolution of the mineral assemblages. Hence, the path shown by them is an oversimplification. Prograde path for MI has been deduced from four areas in the NEGB (Fig. 5, paths 3,6,8a and 11) all defining an anticlockwise trajectory of evolution. Peak metamorphic conditions for MI are deduced to be 8-9 kbar and > 900"C, possibly even 1000°C. The retrograde path of M, is one of near isobaric cooling down to 75O-80OQC, excepting the paths 8 and 1, which predict near isothermal decompression at high temperatures. Pal and Bose (1997) showed that the P-T trajectory deduced by La1 et al. (1987) (8) is erroneous and the problem has arisen because of application of a wrong petrogenetic grid in the system FMAS, The corrected path (8a) attests to near isobaric cooling following the MI peak, thereby, corroborating evidences from other areas. Dasgupta and Sengupta (1998) have shown that the high temperature decompression deduced by Sen et al. (1995) has no textural support. Path 10 also shows initial decompression. The prograde path is not preserved in this area and the peak temperature is considerably lower than TMax for M,. Some amount of decompression following peak metamorphism can also be achieved on an anticlockwise trajectory (Harley, 1989), and does not necessarily indicate a clockwise path. In none of the areas the prograde path of the M,metamorphism could be deciphered in the absence of suitable mineral associations. The retrograde path of M,consists of significant decompression (Fig. 5). The M,? decompression has been correlated with the 950-1000 Ma tectonothermal event in the Eastern Ghats Belt (Grew and Manton, 1986; Mezger and Cosca,1998). Cooling subsequent to this M,, decompression has been documented from some areas including this work. In practically all the areas a still later hydration and amphibolite facies metamorphism have been recorded (not shown in Fig.5). Isotopic data on the NEGB granulites show a Pan-African overprint at 500-550 Ma, which has reset some of the sphenes in calc-silicate granulites (Mezger et al., 1996; Mezger and Cosca,1998). This has also produced some zircon bearing pegmatites near Visakhapatnam (Kovach et al., 1997; Mezger and Cosca,1998). Since it is necessary to have temperatures in excess of 650°C to reset the U-Pb system in sphene completely (Mezger et al., 1991), it has been argued that the

Pan-African thermal overprint did not reach temperatures above 650"C, but was more than 600°C (Mezger and Cosca, 1998). This estimate is in conformity with the temperature for M, deduced here. Mezger and Cosca (1998) showed that the effect of Pan-African overprint was stronger in the area near Visakhapatnam and immediately south of it (Fig.1). The U-Pb cooling age of sphene (863+8 Ma) from the study area (Mezger and Cosca, 1998) merits further discussion in this context. It has been argued by Mezger and Cosca (1998) that discordant ages of sphenes are most likely due to thermal overprint at 550 Ma caused by diffusional loss of Pb from pre-existing sphenes. If this conclusion is valid, the age given by sphene from the present study area is a reflection of partial resetting of the U-Pb systematics in older sphenes due to hydration and amphibolite facies metamorphism at cn. 550 Ma. Pan-African ages from sphene are also obtained from an area about 120 km north of Gokavaram (Mezger and Cosca, 1998) which attest to imprints of Pan-African metamorphism.

Concluding Remarks

The P-T evolutionary history of the rocks occurring along the Kemp Coast is, therefore, remarkably similar to that of the present study area (see comparative P-T loops in Fig. 4). In the absence of isotopic data we are, however, uncertain about the age of MI metamorphism, but a probable Archaean age can be postulated. One of the major tasks in the coming years will be to date M, through a veil of cn,950-1000 Ma old granulite facies M, metamorphism. We have earlier postulated that the development of retrograde shear zones and emplacement of pegmatites containing tungsten and graphite in the study area could be correlated with Pan- African orogeny, hydration and weak amphibolite fades metamorphism at cn. 550 Ma, This is also responsible for the discordant age of sphene from the calc-silicate granulite in the study area. The present work, therefore, strengthens the contention that the SEGB and NEGB are equivalents of Napier and Rayner Complexes respectively in east Antarctica (Sengupta et al., 1998).

Acknowledgements

The first three authors are thankful to the Department of Science and Technology for financial support through a research project. We thank Professor A .B. Roy for inviting us to write this paper. We are also thankful to Professor P. K. Bhattacharya for his constructive criticism. This paper is a contribution to IGCP Project 368.

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