evolution of the bhandara-balaghat granulite belt along

Evolution of the Bhandara-Balaghat granulite belt alongthe southern margin of the Sausar Mobile Belt of central

India

H M Ramachandra1 and Abhinaba Roy2

1Geological Survey of India, AMSE Wing, Bangalore 560 0782Geological Survey of India, Central Region, Nagpur 440 006

The Bhandara-Balaghat granulite (BBG) belt occurs as a 190 km long, detached narrow, linear,NE–SW to ENE–WSW trending belt that is in tectonic contact on its northern margin with theSausar Group of rocks and is bordered by the Sakoli fold belt in the south. The Bhandara part ofthe BBG belt is quite restricted, comprising a medium to coarse grained two-pyroxene granulitebody that is of gabbroic composition and preserves relic igneous fabric. The main part of the beltin Arjuni-Balaghat section includes metasedimentary (quartzite, BIF, Al- and Mg-Al metapelites)and metaigneous (metaultramafic, amphibolite and two-pyroxene granulite) protoliths interbandedwith charnockite and charnockitic gneiss. These rocks, occurring as small bands and enclaves withinmigmatitic and granitic gneisses, show polyphase deformation and metamorphism. Geochemically,basic compositions show tholeiitic trend without Fe-enrichment, non-komatitic nature, continentalaffinity and show evolved nature. Mineral parageneses and reaction textures in different rock com-positions indicate early prograde, dehydration melt forming reactions followed by orthopyroxenestability with or without melt. Coronitic and symplectitic garnets have formed over earlier miner-als indicating onset of retrograde IBC path. Evidences for high temperature ductile shearing arepreserved at places. Retrogressive hydration events clearly post-date the above paths. The presentstudy has shown that the BBG belt may form a part of the Bastar Craton and does not representexhumed oceanic crust of the Bundelkhand Craton. It is further shown that rocks of the BBG belthave undergone an earlier high-grade granulite metamorphism at 2672± 54 Ma (Sm-Nd age) and apost-peak granulite metamorphism at 1416 ± 59 Ma (Sm-Nd age, 1380 ± 28 Ma Rb-Sr age). Theseevents were followed by deposition of the Sausar supracrustals and Neoproterozoic Sausar orogenybetween 973 ± 63 Ma and 800 ± 16 Ma (Rb-Sr ages).

1. Introduction

Precambrian crustal evolution is characterized bysuccessive development of supracrustal belts andorogens often associated with development of high-grade metamorphic belts. Regional and global cor-relation of such events is an onerous task asyounger tectono-metamorphic events often over-print (and rotate) earlier structures and result indegradation of earlier mineral assemblages (Harley1992; Harley and Fitzsimons 1995). Study of

all components of complex Precambrian tectono-metamorphic belts is thus essential for buildingcoherent regional scale models. Recent work inthe Sausar Mobile Belt (SMB) in central India(Bhowmik et al 1999, Roy et al 2000), has shownthat granulite belts in the northern and south-ern parts of the SMB respectively, retain peakmetamorphic imprints of pre-Sausar orogeny andthat there is a distinct gap in metamorphic gradebetween the granulite belts and the Sausar Groupof rocks. These studies have thus inferred that the

Keywords. Granulite; Sausar, Bhandara; Balaghat; tholeiite; migmatite; orogeny.

Proc. Indian Acad. Sci. (Earth Planet. Sci.), 110, No. 4, December 2001, pp. 351–368© Printed in India. 351

352 H M Ramachandra and A Roy

I N D E X

1A-−1D

Figure 1. Geological map of central India showing cratonal and major lithotectonic components.(1) Granulite Belt (A) Bhandara-Balaghat Belt, (B) Ramakona-Katangi Belt, (C) Kondagaon Belt (D) BhopalpatnamBelt. (2) (a) Sukma-Bengpal Group (Bastar Craton), (b) Singhbhum Craton, (c) Dharwar Craton. (d) Bundelkhand Cra-ton (3) Bailadila Group. (4) Dongargarh Super Group. (5) Sakoli Group. (6) Chhota Nagpur Gneissic Complex. (7)Mahakoshal Group. (8) Sausar Group. (9) Abujhmar, Kairagarh. (10) Vindhyan (V), Chhatisgarh,Indravati(I), Sabari(S), Khariar (K), Albaka (A), Pakhal (P). (11) Gondwana. (12) Deccan Trap. (13) Alluvium.

evolution of the SMB was polycyclic in nature. Thispaper presents an account of the southern mar-ginal granulite belt of the SMB represented by theBhandara-Balaghat granulite (BBG) belt and dis-cusses the role of this granulite belt in the evolutionof the SMB and the adjoining Bastar craton.

2. Geological setting

The Precambrian crust in central India is rep-resented by the northern Bundelkhand and thesouthern Bastar Craton, separated by the SMB

(Roy et al 2000). The Bastar craton comprisesPalaeo- to Mesoarchaean Sukma gneiss-granitoidsand supracrustals, Bhopalpatnam and Konda-gaon granulite belts, the Bailadila Group of Ironformations, the Chandenar-Tulsidongar mobilebelt (including the Bengpal Group), youngersupracrustals of the Dongargarh and Kotri Super-groups (including the Dongargarh and Malanjk-hand granitoids), and cover sediments (Mishra et al1988; Prakash Narasimha et al 1996; Ramachan-dra et al 1998) (figure 1). The margins of theBastar craton show involvement in the East-ern Ghat mobile belt orogeny in the east and

Evolution of the Bhandara-Balaghat granulite belt 353

Figure 2. Geological map of central India showing disposition of the Bhandara- Balaghat granulite (BBG) belt.(1) Migmatite-Granite Gneiss (Tirodi & Amgaon gneiss). (2) Granulite facies rocks of BBG belt. (3) Dongargarh Super-group. (4) Sakoli fold belt. (5) Sausar belt. (6) Gondwana. (7) Deccan Trap. (8) s-s Major ductile shear zone.

southeast, the Proterozoic and younger oroge-nies in the southwest and the Sausar mobile belt(SMB) orogeny in the north (Ramachandra et al1998). The Bundelkhand craton (figure 1) com-prises Palaeo- to Mesoarchaean Bundelkhand andSidhi associations, younger supracrustals of theMahakoshal Group along with intrusive granitoids,cover sequences of Bijawar and Gwalior Groupsand Meso- to Neoproterozoic Betul-Chhindwaraand Bilaspur-Raigarh-Surguja mobile belts and theVindhyan cover sediments. The marginal relationsof the Bundelkhand craton at its south with theSMB are yet poorly understood, but are character-ized by ductile shearing and tectonic interleavingof older and younger components (Roy et al 2000).

The SMB separating the Bastar and theBundelkhand cratons occurs as a curvilinear,ENE–WSW to E–W trending, almost 300 kmlong and 70 km wide belt, and is comprised ofthree main components, including the northernRamkona granulite belt, central Sausar Groupof supracrustals and southern Bhandara-Balaghatgranulite (BBG) belt (Bhowmik et al 1999; Royet al 2000) (figure 2). The Sausar Group of rocks,forming the central part of the SMB is representedby an older metamorphic group including theTirodi gneiss-migmatites and supracrustals, and asequence of intensely deformed younger platfor-

mal facies supracrustals intruded by granitic rocks(Narayanaswamy et al 1963). Huin et al (1998),Khan et al (1998, 1999), and Bhowmik et al (1999)have recorded three generations of structure fromthe Sausar Group showing successive overprintingrelations and have established that the first genera-tion structure in the Sausar Group overprints earlymylonitic fabric in the Tirodi gneiss-migmatiteand northern Ramkona granulites. Four episodesof metamorphism, namely, M1, M2, M3 and M4,have been recorded from the Sausar Group of rockswith peak upper amphibolite facies event duringM2. Bhowmik et al (1999) have shown that theRamkona granulites at the northern margin of theSMB comprise a sequence of mafic granulite, andfelsic migmatitic gneiss interleaved with the SausarGroup of rocks. They have also shown that thegranulites record pre-Sausar structures and meta-morphic imprints with the peak M2 event recording∼10.5kb pressure and 775◦ C temperature. Theyhave further inferred that the granulites formedthe basement for deposition of Sausar supracrustalsand were tectonically imbricated with the latterduring the Sausar orogeny correlated with the1000 Ma Grenvellian event.

Geochronological data for the SMB is meagerand includes the Rb/Sr whole rock isochron age of1525 ± 70 Ma and mineral isochron age of 860 Ma


by Sarkar et al (1986) for the Tirodi gneiss, the for-mer considered to mark the main phase of amphi-bolite facies metamorphism in the Sausar Groupand the latter mineral age as representing the ter-minal thermal overprint of the Sausar orogeny. Lip-polt and Hautmann (1994) have obtained Ar/Arage of 950 Ma for cryptomelane from the Sitapurmines and interpret the age to represent the clo-sure of Satpura (≡Sausar orogeny) cycle amphi-bolite facies metamorphism. Recent geochronolog-ical data for the BBG belt include Sm-Nd datesof 2672 ± 54 Ma and 1416 ± 59 Ma, and Rb-Srdate of 1380 ± 28 Ma for charnockite and two-pyroxene granulite of the BBG belt, representingdifferent ages of granulite facies metamorphism;and Rb-Sr dates of 800 ± 16 Ma for charnockitesand 973 ± 63 Ma for mafic granulites in the north-ern shear zone of the BBG belt, representing devel-opment of amphibolite facies assemblages in gran-ulites (Abhijit Roy, personal comm.).

3. Lithology and field relations

The Bhandara-Balaghat granulite belt, forming thesouthern margin of the SMB, extends discontin-uously for more than 100 km from northwest ofBhandara in the southwest, in Maharashtra, to eastof Balaghat in the northeast, in Madhya Pradesh.It was earlier included as a part of the oldermetamorphics at the base of the Sausar Group(Narayanaswamy and Venkatesh 1971). Rajurkar(1974) and Mohabey and Dekate (1984) recordedthe presence of two-pyroxene granulite near Bhan-dara. Jain et al (1995) showed that granulite rocksof this belt occur as lensoid bodies within gneissesextending discontinuously from Bhandara in thesouthwest to Ratanpur in the northeast. They alsoreported the presence of two-pyroxene granulites,orthopyroxene and garnet bearing BIF, cordieritebearing metapelites and gneisses and have inferredthat the granulites represent oceanic basalt, nowoccurring, along with associated metasediments, asexhumed blocks within migmatites. Yedekar andJain (1995) have interpreted Ar/Ar age of 1300 Maobtained from hornblende in two-pyroxene gran-ulite as representing the episode of post-granuliteretrogressive event.

The Bhandara part of the BBG is represented bya large two-pyroxene granulite body elongate alongENE–WSW, extending along strike for about 5 km,with a maximum width of 1.5 km, exposed nearBhandara and is locally associated with migmatiticgneisses and a thin band of quartzite. The two-pyroxene granulite is medium to coarse grained andshows uniform gabbroic composition with occa-sional relict ophitic and sub-ophitic texture. Small(a few cm thick) gabbro-anorthositic gabbro lay-ers together with rare segregations of pyroxene up

to 10 cm in size occur at places. The main folia-tion trends from NE–SW to N70◦E–S70◦W withsteep southerly dips and plagioclase and pyroxenesshow protomylonitic character along local sinis-tral shears sub-parallel to the main foliation, asevidenced by recovery and recrystallization andbending of twin lamellae in plagioclase. The two-pyroxene granulite shows a mineral assemblage ofPl-Opx-Cpx-Hbl-Opq ±Scp±Grt.

The granite gneiss in the northern margin ofthe BBG belt is in tectonic contact with myloni-tised low-grade metasediments of the Sausar Groupshowing development of protomylonitic fabric inboth metasediments and granite gneiss and a low-northerly plunging stretching lineation. The gran-ite gneiss delimiting the southern margin of theBBG belt also shows mylonitic and phylloniticcharacter. The full spectrum of rocks of the BBGbelt is best exposed in the Arjuni-Balaghat area(figure 2) and comprises a sequence of quartzite,BIF, metapelite, two-pyroxene granulite, charnock-ite and charnockitic gneiss and migmatitic andgranitic gneiss. These are shown in table 1 alongwith their main mineral assemblages.

The different rock types mentioned above occurinterbanded mainly within migmatitic and granitegneisses. Granite gneiss is mainly exposed in thenorthern part and southern margins of the BBGbelt are strongly tectonised on ENE–WSW trend-ing planes. The migmatitic gneiss includes imper-sistent salic-mafic bands a few mm to a few cmthick. The pegmatitic salic bands contain up to10 cm long deformed K-feldspar phenocrysts. Meltformation is indicated by the presence of nebu-lous and fleck structures in the rocks, biotite grainsaround K-feldspar phenocrysts and melt supportedtexture (e.g. tabular feldspars, granitic intergrowthtexture) in the quartzofeldspathic salic bands.

Quartzite is rare and occurs as bands up to10 m in size as near Bhandi. BIF is exposed as amajor folded band extending for a few 100 m nearLarsara and as small bands near Amgaon. Therock is coarse grained and contains orthopyroxeneand garnet grains measuring up to 5 cm in size,along with smaller grains of quartz, hornblende andcummingtonite-grunerite. Metapelites are associ-ated with granitic leucosomes and commonly occuras interbands less than a metre long with gneissesand other granulites. They are best exposed in theHurkitola, Dongargaon, Larsara and Mendki areas.Al-metapelites are medium grained and schistosewith sillimanite, kyanite and micas measuring froma few mm to 0.5 cm in size. Cordierite bearing rocksare finer grained and less schistose. Two-pyroxenegranulites occur as long, linear bands, several hun-dred metres in length mainly in the central part ofthe BBG belt, near Lingmara, Mendki, Larsara andDongariya. The rocks show relict ophitic and sub-


Table 1. Generalized tectonically significant lithosuccession and mineral assemblage in rocksof the BBG belt.

Lithology Mineral assemblage

Charnockite and charnockitic Qz-Pl-Kfs-Pth-Opx-Cpx-Grt-Hbl-Bt-Opq-Apa- Zir-gneiss (intrusives of acid and Sphintermediate composition)Two-pryroxene granulite Qz-Pl-Opx-Cpx-Gar-Hbl-Opq-Apa-Bt(metabasics)Mg-Al and Mg-metapelites Qz-Crd-Anth/Cum/Grun-Bt-Chl-Opq-Apa

Qz-Pl-Kfs/Pth-Crd-Opx-Grt-Spl-Gph-Sil-Bt-Mus-Rt-Opq

Al-metapelites Qz-Sil-Ky-Kfs-Pl-Bt-Mus-Crn-Grt-Rt-Opq-Apa-ZirBanded iron formation Qz-Opx-Grt-Hbl-Cum/Grun-MagQuartzite Qz±Fuch±Chl±GrtMigmatite and granite gneiss Qz-Kfs/Pth-Pl-Grt-Bt-Hbl-Apa-Sph- Zir

ophitic texture, variably developed foliation andlocal layering due to metamorphic differentiation.Charnockite and charnockitic gneisses are mediumgrained rocks of intermediate to acidic compositionand have the typical greasy charnockitic appear-ance. They are weakly or strongly foliated and aretypically interbanded with two-pyroxene granuliteand migmatitic gneiss in Arjuni, Dongariya andLarsara areas, and also occur as small patches orstocks.

4. Structural set up

The BBG belt is located in the Central IndianShear Zone (CIS) that is marked by the develop-ment of mylonites and comprises multiple shearzones having polycyclic histories. The presentstudy has shown that there are at least two promi-nent ENE–WSW trending ductile shear zoneswithin the Central Indian Shear Zone. The firstone marks the boundary between the Sausar beltin the north and basement gneisses with granulitesin the south. The second one delineates the bound-ary between granulite belt in the north and theAmgaon Gneissic Complex (AGC) in the south.Thus, there are two prominent ductile shear zones,which border the granulite belt, one in the northand other in the south. The northern shear zonehas steep northerly dip, though at places, rever-sal in the attitude has been noted. The stretchinglineation on this steeply dipping mylonite plane isinvariably steep. The lineation data, in conjunc-tion with shear bands suggest predominant dip-slipcomponent with northern Sausar belt going downwith reference to the granulite belt. In contrast, thesteep northerly dipping shear zone on the south ofthe granulite belt exhibits a reverse sense of move-ment, wherein the granulite belt has been thrustover the Amgaon gneisses. Based on the difference

in the kinematics it is interpreted that these shearzones were developed at different times and sequen-tially accreted different tectono-metamorphic ter-rains together.

The granulites exhibit a prominent ENE–WSWstriking sub-vertical foliation/layering definedmainly by amphiboles. This layering showsmylonitic structure including S-C fabric in differentrocks. At least one set of small folds is developedon this foliation plane. These folds have E–W toENE–WSW striking sub vertical axial planes andmoderate to steep (even vertical) plunge of the foldaxes. Thin section study has shown that the amphi-boles defining the above layering post-date gran-ulite assemblages. Further, mesoscopic structuresrelated to granulite facies metamorphism could notbe recognised. This indicates that the amphibolitegrade structures post-date those of the granulitefacies metamorphism.

5. Petrography

The two-pyroxene granulite and the charnockiticrocks show superposition of deformation fabric overigneous melt supported textures, which are locallypreserved in the form of ophitic, subophitic andintergrowth textures. Metamorphic fabrics includegranoblastic texture, mineral reactions involvingneocrystal growth of coronitic or porpyroblas-tic minerals, exsolution texture and developmentof perthite and myrmekites. Protomylonitic tex-ture is common along shear zones and is repre-sented by incipient recovery and recrystallizationin quartz and feldspars, development of kink bandsand asymmetric tails in pyroxene, mica and horn-blende.

Metapelites show granoblastic texture and devel-opment of sillimanite, kyanite, cordierite andgarnet porphyroblasts in rocks of appropriate


(Al- or Al-Mg) compositions. Reaction texturesinvolving coronitic or symplectitic growth involv-ing orthopyroxene, sillimanite, cordierite, garnet,sphene, rutile and opaque are well preserved. Pres-ence of pegmatite, rare aplite and small bands andpools of granitic leucosomes (neosomes) up to afew cm in dimension showing melt supported fabricindicate generation of partial melts. Mylonitic fab-ric including development of S-C fabric, ’fish’ struc-ture, kink bands etc. in orthopyroxene and mica isalso present.

6. Geochemistry

Twenty nine major and trace analyses of pyrox-ene granulites, charnockites and granitoids fromthe BBG belt are presented mainly to study ifmagmatic or tectono-magmatic relationships existbetween different lithocomponents. The analyseswere carried out at the chemical laboratories ofthe GSI, Central Region, Nagpur, by XRF meth-ods using suitable internal standards. Of the 29samples, 15 represent different types of grani-toids including migmatitic leucosomes and granitegneisses; 10 of two-pyroxene granulites including 6from Arjuni-Balaghat area and 4 from Bhandara;and 4 of charnockite and charnockite gneiss. Theanalytical results are given in table 2 and CIPWNorms in table 3.

The granitoids show a range in SiO2 compositionvarying from 63.53% to 74.08% with Al2O3 rang-ing from slightly above 11% to 16.98%. These rocksare typically low in iron and magnesium and aregenerally low in calcium. K2O is commonly higherthan Na2O and total alkalis range from slightlyabove 4.5% to 8.5%. Sr content (varying from 130ppm to 620 ppm) is usually higher than that of Rb(60 ppm to 490 ppm) and Zr varies from 10 ppmto 420 ppm. Cr shows a variation from 10 ppm to240 ppm. The two-pyroxene granulites show a silicarange between 47.60% and 53.64% with MgO vary-ing from 7.03% to 12%. They are moderately ironrich and show low- to moderate TiO2 and typicallylow alkalis. The values of Mg # ranging from 48 to69 for these rocks indicates their evolved nature.Sr content is commonly higher than that of Rb inthe two-pyroxene granulites and Cr values rangefrom 20 ppm to 960 ppm. Zr varies from 30 ppm to280 ppm. The SiO2 in the charnockites ranges from63% to 74.15% with a narrow spread in Al2O3 con-tent. MgO is low with variable low- to high contentsof Iron oxides and calcium. K2O tends to be higherthan Na2O and total alkali contents are generallysimilar. Sr and Rb contents are generally similar inthe charnockites with Cr varying from 20 ppm to80 ppm. Zr shows a variation from 60 ppm to 230ppm.

The plots of analyses of all the above rocks in theSiO2 vs. total alkali diagram after Wilson (1989),as given in figure 3 distinctly occur in two separateclusters, in the gabbro and granite compositionalfields. The two-pyroxene granulites occur in a rel-atively tightly clustered group in the gabbro fieldwhereas the granitoids and the charnockites occurin the granitic field, showing a restricted compo-sitional spread in the acidic range from quartz-diorite (granodiorite) to granite. Thus a bimodalrelationship between the basic and acidic composi-tions is indicated. In the AFM diagram after Irvineand Baragar (1971) (figure 4) the basic and acidiccompositions clearly follow different trends and fur-ther depict the bimodal relationship between thetwo. The basic trend is typically tholeiitic, showsan evolved nature and lacks Fe enrichment com-monly observed in differentiated high-level maficcomplexes. Normative CIPW values indicate qz-normative and hence saturated nature of the two-pyroxene granulites. The charnockites and gran-itoids show a mixed tholeiitic and calc-alkalinenature, but as the granitoid sample includes a vari-ety of sources ranging from leucosome of para- orortho-migmatites and granite intrusives, the trendsat best can be taken as suggestive.

The two-pyroxene granulites in Jensen’s (asmodified by Jensen and Pyke 1982) diagram (fig-ure 5) generally show tholeiitic character exceptingtwo plots that occur in the komatiite field. But asthese two rocks are characterized by relict ophiticand subophitic textures typical of tholeiites andlack spinifex or related textures they are inferredto be of tholeiitic affinity only. A majority of theother samples show high-Mg rather than high-Fetholeiitic character. Most of the two-pyroxene gran-ulites plot in the continental basalt field of TiO2-P2O5-K2O diagram (figure 6) after Pearce et al(1975), indicating the continental-tholeiite charac-ter of these rocks. The restricted compositionalspread among acidic compositions is brought outin the Ab-An-Or diagram after Barker (1979) (fig-ure 7). The paucity of tonalite and trondhjemitepossibly indicate thorough crustal reworking ofprotoliths for the acidic rocks and likely influenceof continental processes in generation of precursormagma for charnockite rocks. CIPW norm calcula-tions indicate that most of the acidic rocks includ-ing charnockites are corundum normative. Theymainly occur in the peraluminous field of SiO2vs. A/CNK diagram (figure 8) and a larger num-ber show S- rather than I- type character (afterChappel and White 1974). These mixed signaturesand especially the high Al nature of the rockscompared to their alkali content reflects the vari-able para- and ortho- nature of the samples. Theplots of granitic and charnockite compositions inthe Q-Ab-Or diagram (figure 9) after Anderson

Evolution of the Bhandara-Balaghat granulite belt 357Tab

le2.

Maj

oran

dtrac

eel

emen

tan

alys

esof

two-

pyro

xene

gran

ulites

and

char

nock

ites

from

the

BBG

belt.

Sam

ple

TP

-1T

P-2

TP

-3T

P-4

TP

-5T

P-6

TP

-7T

P-8

TP

-9T

P-1

0C

H-1

CH

-2C

H-3

CH

-4Si

O2

47.6

047

.85

48.0

348

.36

49.0

849

.82

50.0

050

.75

50.8

653

.64

63.0

063

.09

72.1

474

.15

Al 2

O3

16.3

014

.97

11.4

214

.49

13.5

013

.85

12.7

812

.09

12.3

914

.25

14.0

016

.07

13.6

513

.19

FeO

9.90

9.00

8.82

12.6

6.02

9.90

5.22

10.0

87.

926.

306.

124.

321.

981.

98Fe

2O

32.

643.

313.

342.

624.

674.

844.

414.

806.

013.

092.

392.

880.

820.

66T

iO2

0.85

1.16

1.87

1.79

0.97

1.36

0.11

1.34

1.25

1.36

1.10

1.18

0.28

0.28

CaO

12.0

811

.87

12.3

08.

9014

.05

10.2

016

.50

10.5

66.

407.

505.

805.

102.

461.

71M

gO8.

389.

3611

.50

7.26

8.04

7.2

8.32

7.03

12.0

09.

102.

411.

890.

650.

65K

2O

0.07

0.07

0.11

0.12

0.14

0.14

0.01

0.67

0.44

1.39

2.31

2.47

3.85

3.73

Na 2

O1.

431.

280.

851.

821.

911.

751.

152.

171.

090.

502.

512.

672.

832.

90M

nO0.

160.

160.

360.

130.

210.

130.

920.

140.

150.

110.

090.

060.

010.

01P

2O

50.

250.

200.

420.

290.

250.

290.

200.

150.

410.

840.

250.

200.

200.

15+

H2O

0.30

0.22

0.34

0.33

0.85

0.33

0.33

0.01

0.43

0.31

0.22

0.34

0.30

0.32

Tot

al99

.96

99.4

599

.36

98.7

199

.69

99.8

199

.95

99.7

899

.35

98.3

910

0.20

100.

2799

.17

99.7

3M

g#57

.00

61.0

067

.00

48.0

064

.00

52.0

067

.00

51.0

067

.00

68.0

0-

--

-N

b—

——

——

——

20—

4030

4010

10Sr

8012

020

110

4090

2024

021

071

028

028

021

018

0R

b20

1020

1013

020

120

7020

8027

029

016

015

0C

r32

040

080

250

2020

020

210

960

860

7080

3020

Zr

4040

6060

170

6018

030

6028

080

6023

014

0Y

2010

3020

1020

1070

10—

3020

3040

Sam

ple

G-1

G-2

G-3

G-4

G-5

G-6

G-7

G-8

G-9

G-1

0G

-11

G-1

2G

-13

G-1

4G

-15

SiO

263

.53

65.9

966

.57

66.9

874

.64

68.8

368

.99

70.0

070

.83

71.2

671

.77

72.4

672

.94

73.2

774

.08

Al 2

O3

16.9

814

.05

14.9

214

.20

11.2

913

.86

13.6

914

.38

13.5

513

.81

14.4

812

.93

12.8

914

.61

13.3

5Fe

O3.

065.

582.

524.

141.

443.

423.

063.

241.

982.

160.

363.

420.

900.

541.

44Fe

2O

32.

291.

732.

832.

360.

870.

330.

631.

030.

811.

052.

521.

470.

941.

052.

73T

iO2

1.29

1.45

0.95

0.70

0.24

0.62

0.84

0.55

0.67

0.77

0.27

0.23

0.32

0.29

0.01

CaO

4.34

3.82

2.64

2.37

2.50

4.60

2.80

1.57

1.70

2.20

1.50

1.84

2.00

1.36

1.00

MgO

2.10

1.81

1.67

2.34

0.61

0.88

0.73

1.02

0.54

0.51

0.66

0.43

0.38

0.47

0.35

K2O

2.51

1.54

4.27

3.31

6.33

4.70

5.85

4.60

6.53

4.32

3.73

3.12

5.49

2.95

4.11

Na 2

O2.

042.

972.

061.

761.

822.

322.

182.

461.

932.

563.

402.

581.

704.

162.

76M

nO0.

070.

050.

060.

100.

010.

040.

040.

010.

010.

040.

010.

050.

010.

010.

01P

2O

50.

600.

350.

460.

150.

090.

300.

250.

250.

200.

150.

200.

140.

340.

100.

07+

H2O

0.40

0.34

0.32

0.29

0.44

0.29

0.39

0.38

0.62

0.33

0.45

0.50

1.33

0.40

0.28

Tot

al99

.21

99.6

899

.27

98.7

010

0.28

100.

1999

.45

99.4

999

.37

99.1

699

.35

99.1

799

.24

99.2

110

0.19

Nb

4030

4020

2040

3020

2030

1012

010

2020

Sr14

026

044

035

034

060

057

019

039

031

027

062

031

043

013

0R

b28

060

490

210

110

190

120

150

120

160

130

140

120

8022

0C

r12

060

170

140

170

160

240

2023

016

010

150

1010

10Zr

5042

050

100

1020

1031

010

2030

010

190

180

150

Y30

1010

3050

3060

5040

140

3022

020

—70

TP

=T

wo-

pyro

xene

gran

ulit

e;C

H=

Cha

rnoc

kite

&ch

arno

ckit

icgn

eiss

;G

-1=

Gar

-pla

ggn

eiss

;G

-2=

Hbl

-gne

iss;

G- 3

&4

=G

ar-g

neis

s;G

-5&

8=

Leu

coso

me

from

stro

mat

icm

igm

atit

e;G

-6=

Coa

rse

grai

ned

leuc

osom

esfr

omst

rom

atic

mig

mat

ite;

G-7

=Fin

egr

aine

dgr

anit

edy

kein

stro

mat

icm

igm

atit

e;G

-9=

K-

feld

spar

rich

gran

ite;

G-1

0=

Leu

cocr

atic

part

inm

igm

atit

icgn

eiss

;G

-11

&13

=Leu

coso

me

inba

nded

gnei

ss;G

-12

=C

oars

ele

ucoc

rati

cau

gen

gnei

ss;G

-14

&15

=Leu

co-g

rani

tegn

eiss

.

358 H M Ramachandra and A RoyTab

le3.

CIP

Wno

rms

for

two-

pyro

xene

gran

ulites

and

char

nock

ites

ofth

eBBG

belt.

Sam

ple

TP

-1T

P-2

TP

-3T

P-4

TP

-5T

P-6

TP

-7T

P-8

TP

-9T

P-1

0C

H-1

CH

-2C

H-3

CH

-4qz

01.

42.

092.

491.

876.

033.

42.

419.

0515

.35

22.6

523

.13

34.5

337

.65

ap0.

670.

671

0.67

0.33

0.68

0.33

0.33

12.

010.

670.

330.

330.

33ilm

1.66

2.26

3.47

3.47

1.81

2.56

0.15

2.56

2.41

2.56

2.11

2.26

0.6

0.6

mt

3.92

4.85

4.85

3.92

6.93

6.93

6.23

6.93

8.77

4.62

3.46

4.15

1.15

0.92

an37

34.4

727

.24

31.4

128

.07

29.6

29.7

121

.427

.832

.25

20.2

924

.37

11.3

7.69

ab12

.05

12.0

57.

3315

.72

16.2

414

.67

9.95

18.3

49.

434.

1920

.96

22.5

324

.124

.62

or0.

550.

550.

550.

551.

110.

820.

053.

892.

788.

3413

.34

14.4

522

.79

22.2

4co

r0

00

00

00

00

0.3

00.

130.

741.

56w

o8.

469.

8913

.14.

7917

.36

8.08

21.4

312

.72

0.58

02.

70

00

Di

en4.

85.

928.

722.

3412

.33

4.59

14.4

37.

010.

410

1.19

00

0fs

3.27

3.45

3.39

2.36

3.47

3.12

5.33

5.08

0.1

01.

50

00

Hy

en14

.514

.97

20.0

715

.95

7.86

13.4

26.

2610

.48

29.6

823

.13

4.8

4.7

1.6

1.6

fs9.

828.

727.

8216

.11

2.21

9.23

2.31

7.59

7.81

7.12

6.02

3.69

2.52

2.65

Ol

fo0.

860

00

00

00

00

00

00

fa0.

610

00

00

00

00

00

00

h m0

00

00

00

00

00

00

0r u

t0

00

00

00

00

00

00

0Sam

ple

G-1

G-2

G-3

G-4

G-5

G-6

G-7

G-8

G-9

G-1

0G

-11

G-1

2G

-13

G-1

4G

-15

qz30

.81

30.0

730

.89

34.0

534

.96

26.3

126

.31

32.4

129

.97

34.2

834

.38

39.5

138

.95

34.6

839

ap1.

340.

671

0.33

0.21

0.67

0.67

0.67

0.33

0.33

0.33

0.33

0.82

0.33

0.17

ilm2.

412.

711.

811.

350.

451.

21.

661.

051.

21.

470.

450.

450.

60.

60.

02m

t3.

462.

544.

153.

461.

260.

460.

921.

611.

151.

610.

462.

071.

380.

923.

92an

17.9

817

.33

10.5

611

.01

4.17

13.6

210

5.93

7.69

10.0

96.

678.

247.

736.

114.

54ab

17.2

925

.15

17.8

115

.19

15.1

979

.38

18.8

620

.96

16.2

422

28.8

222

14.6

735

.63

23.5

8or

15.0

18.

8925

.57

20.0

137

.25

27.8

35.0

227

.24

38.9

225

.57

22.2

418

.932

.817

.79

24.4

6co

r4.

411.

293.

053.

70

00

3.22

0.54

1.25

2.4

2.27

14.1

52.

3725

.66

wo

00

00

3.14

3.05

0.85

00

00

00

00

Di

e n0

00

01 .

50.

940.

280

00

00

00

0fs

00

00

1.66

2.22

0.59

00

00

00

00

Hy

en5.

34.

54.

25.

90

1.25

1.51

2.6

1.4

1.3

1.7

1.1

11.

20.

9fs

1.71

6.6

0.79

4.75

02.

923.

234.

232.

371.

880

4.88

0.41

00.

39O

lfo

00

00

00

00

00

00

00

0fa

00

00

00

00

00

00

00

0h m

00

00

00

00

00

2.24

00

00

rut

00

00

00

00

00

00

00.

480


Figure 3. Classification of rocks of BBG belt in SiO2 vs. total alkali diagram of Cox et al (1979) as modified by Wilson(1989). The plots show bimodal character of the basic and acidic compositions of the BBG belt.

Figure 4. AFM diagram after Irvine and Baragar (1971) showing bimodal relationship between basic and acidic composi-tions and tholeiitic character of basic rocks of the BBG belt. Symbols as in figure 3.


Figure 5. Plots of two-pyroxene granulites and charnockites of the BBG belt in the diagram after Jensen and Pyke (1982)showing predominantly tholeiitic and non-komatiitic nature of the rocks. Symbols as in figure 3.

Figure 6. Plots of two-pyroxene granulites and charnockites of the BBG belt in the diagram after Pearce et al (1975) showthe largely continental setting of the rocks. Symbols as in figure 3.


Figure 7. Plots of gneisses and granitoids of the BBG belt in the granite classification diagram after Barker (1979) showrestricted compositional spread for these rocks. Symbols as in figure 3.

and Bender (1989), used to determine the depthof magma generation, show some spread. Samplesassociated with orthomigmatites plot at shallowerminima, typically below 2–4 kb minima. The grani-toid samples from paramigmatitic association showplots occurring mainly between 4 and 7 kb and 7and 10 kb minima. However as time of melt gen-eration and movement and effect of fluid duringmelting are not constrained, further elaboration onthis theme is not possible at present.

7. Mineral reactions

Petrographic study of two-pyroxene gran-ulite, charnockite and charnockite gneissesand metapelitic granulites of the BBGbelt has resulted in documentation of min-eral textures and reactions distinctive ofpolyphase granulite facies metamorphism in thebelt.

Charnockites and charnockitic gneisses con-tain plagioclase grains showing antiperthiticcharacter. Presence of brown hornblende ear-lier to porphyroblastic and coronal garnethas been observed in many samples. Horn-blende rimmed by orthopyroxene has also

been observed (figure 10). This may be dueto

Ti − Hbl + Qz = Grt + Opx + Cpx + Ilm + H2O(1)

representing a prograde path involving risein temperature during metamorphism. Inter-estingly orthopyroxene1, being replaced byorthopyroxene2 has also been observed, and maybe related to this event, probably representingthe break down of high Al-pyroxene to lower Al-pyroxene.

Mg-Al rich compositions show the presence ofporphyroblastic orthopyroxene containing inclu-sions of cordierite, K-feldspars, perthite, phlo-gophitic biotite and garnet. This indicates the pres-ence of a pre-existing cordierite-garnet assemblagealong with melt. Thus an early reaction as givenbelow was operative:

Bt + Sil + Qz = Crd + Grt + Kfs + melt (2)

and may be taken to have heralded the onset ofgranulite facies metamorphism.

Crd + Grt + Sil + Spl assemblage, withoutorthopyroxene, is characterized by an early stable


Figure 8. Gneisses and granitoids of the BBG belt in the SiO2 vs. A/CNK diagram showing mixed, but dominantlyperaluminous character of the rocks. Symbols as in figure 3.

assemblage of coarse garnet, sillimanite, cordierite,perthite and opaque association with or withoutquartz. This assemblage, as in the case of theorthopyroxene bearing one described above, mayrepresent an early melt forming event from reac-tion (2), given above. Presence of Grt + Crd + Btbearing quartzofeldspathic (granitic) veins in asso-ciation with the above metapelites supports thiscontention.

Orthopyroxene associated with either cordieriteor plagioclase (figure 11) in the above set up couldhave originated through reactions such as,

Grt + Qz = Opx + Crd (3)Grt + Qz = Opx + Pl (4)

Grt + Kfs + v = Opx + Crd + Bt (5)

Alumina rich metapelites show that sillimanite isderived from both inversion of kyanite and fromMus + Qz = Sil + Kfs + melt reaction (cf. Yardley1989). Garnet either occurs as porphyroblasts ormantles sillimanite and K-feldspar, indicating gen-eration of partial melt by a reaction such as

Bt + Sil + Qtz = Grt + Kfs + lq (6)

As only kyanite and sillimanite occur as aluminosil-icate phases, and as either andalusite or its pseudo-morphs have not been identified, it is suggested

that aluminous metapelites first passed throughthe kyanite stability field and with increase in tem-perature went into the sillimanite field.

Orthopyroxene in different lithologies isrimmed or coronated by second garnet orgarnet2 +quartz+ clinopyroxene symplectite. Thisgarnet2 also shows inclusions of orthopyroxene,cordierite and opaque. The orthopyroxene break-down may have proceeded by the reaction

High Al pyroxene = low Al pyroxene + Grt (7)

In the BIF compositions, orthopyroxene showsspectacular deformation lamellae and twins andmany grains show stretching along shear planesand ‘fish’ structure. Garnet occurs as porphyrob-lasts or as corona around orthopyroxene. Gar-net bands replacing kink bands in orthopyrox-ene also occur. Garnet and orthopyroxene showgrain refinement and stability along shear planes.Bluish green hornblende or actinolite-hornblende-Fe amphibole intergrowth replaces orthopyroxenein some samples.

The two pyroxene granulites show reaction tex-tures around orthopyroxene and plagioclase includ-ing: fine grained clinopyroxene next to garnet-quartz intergrowths around orthopyroxene (fig-ure 12); clinopyroxene rim around orthopyroxene;and rarely, clinopyroxene-garnet symplectites. In


Figure 9. Gneisses and granitoids of the BBG belt in the normative Qz-Ab- Or diagram after Anderson and Bender (1989)showing varied melt generation depths for their protoliths. Symbols as in figure 3.

Figure 10. Photomicrograph: Hornblende (large, dark grain slightly above center of the photograph) breaking down toand rimmed by an aggregate of intergrown orthopyroxene, clinopyroxene and ilmenite. Quartz and minor plagioclase occurin other parts of the photograph. [Charnockite, near Larsara, BBG belt.] Width of photograph= 1 mm; semicrossed nicols.


Figure 11. Photomicrograph: Porphyroblastic garnet (dark, platy grain in central and lower parts of the photograph) break-ing down to orthopyroxene and plagioclase. [Metapelitic granulite, south of Dongargaon, BBG belt]. Width of photograph=1mm; semicrossed nicols.

some samples, secondary garnet growth aroundearlier formed, larger garnet grains is present. Gar-net included in some orthopyroxenes may repre-sent garnet formed from re-equilibration of alu-minous pyroxene. Presence of hercynitic spinel inorthopyroxene noticed in some samples may alsobe due to the same reason. Garnet coronas sepa-rating orthopyroxene and plagioclase, or, clinopy-roxene and plagioclase also occur. These reactiontextures are indicative of reactions of the type,

Opx + Pl = Grt + Cpx + Qz (8)Cpx + Pl = Grt + Qz (9)Opx + Pl = Grt + Qz (10)

8. Interpretation of textural data

The reaction textures described from lithologicalcomponents as discussed from petrographic stud-ies in the above section are all typical of granulitefacies (e.g. Harley 1989). All of these also easilyplot well within the PT limits considered typical ofgranulite facies rocks (Bohlen 1987). The high PTequilibrium attained by these rocks is well illus-trated by the tholeiitic component of intermediateMg/Mg+Fe (now two-pyroxene granulite) in thestudy area, which contain a mineral paragenesis ofcpx + gar + qz (Green and Ringwood 1967).

Textural data as documented can further beinterpreted as follows:• The earlier textures indicate breakdown of horn-

blende to yield garnet, orthopyroxene, clinopy-roxene with or without a melt. This is a pro-grade dehydration reaction involving an amphi-bolitic protolith. Such a reaction has been doc-umented from the mafic mineral assemblage ofbanded gneisses.

A series of reaction textures in Al-rich andMg-Al rich metapelite involve firstly, melt gen-eration in biotite + sillimanite bearing (amphi-bolite facies?) protoliths, along with garnet +cordierite assemblage (reaction 6), going onto yield Opx + Crd + Pl assemblages (reactions7, 8 and 9) with or without melt to com-plete the prograde path. Kfs/pth + Pl + Crd +Grt bearing granitic leucosomes occurring inclose spatial association represent leucosome andmelanosomes derived from the above reactions.

In the next stage, coronitic and symplectiticgarnet form over earlier minerals in lithologies ofdifferent compositions indicating onset of retro-grade (IBC) path. The type charnockite and two-pyroxene granulite in the area show the melt-supported fabric, being directly overprinted bycoronitic and symplectitic garnet (reactions 1, 2and 3). This may indicate that these bodies wereemplaced into the above set up at this point oftime during the evolution of the granulite belt.


Figure 12. Photomicrograph: Coronotic garnet and clinopyroxene resulting from breakdown of orthopyroxene (center ofthe photograph)[In two- pyroxene granulite, near Larsara, BBG belt]. Width of photograph= 1 mm; semicrossed nicols.

• Mesoscopic (e.g., in the BIF band) and micro-scopic evidence in different lithologies show thatorthopyroxene, garnet and other high P-T min-eralogies as formed above continued to be stableduring ductile shearing events.

• Bluish green or brownish green hornblende par-tially or completely replacing pyroxene; saus-suritization and more commonly sericitizationof plagioclase; chloritization of garnet; pinitiza-tion of cordierite; muscovite alteration of alu-minosilicates, are all retrograde events proba-bly connected with juxtaposition of the granulitelithologies with those of the lower grade Sausarsupracrustals.

9. Discussion: regional correlation andtectonic evolution of the BBG belt

The BBG belt flanks the southern margin of theSMB marking the contact between the northernBundelkhand and southern Bastar Cratons in thePrecambrian of central India. This narrow, 15 kmwide, NE–SW to ENE–WSW trending gneiss dom-inated belt is in tectonic contact with the Sausarsupracrustals in the SMB. The continuation ofthe granulite belt further west-southwestwards(from Bhandara) is uncertain as the area is underthick alluvial cover. East-northeastwards, granulitelithologies have been traced up to southeast of Bal-aghat and further in the same direction, presence

of two-pyroxene granulite has been reported fromthe Ratanpur area. The above account indicatesthe regional extent of the granulite belt.

The contact between the granulite belt and theSausar supracrustals is tectonic in nature. Thoughthe lithologies of the granulite belt share laterdeformation events that have affected the Sausarsupracrustals, they are characteristically differ-ent in other aspects from the latter. The Sausarsupracrustals (without the Tirodi gneiss compo-nent) from the data available, show peak metamor-phic condition only up to sillimanite grade withoutinvolving formation of a melt phase. The litholo-gies of the granulite belt, on the other hand, show,as documented in the present study, a high grademetamorphism under granulite facies and reactionrelations suggest the polyphase nature of metamor-phism. The high temperature ductile shears foundin the granulite lithology are also absent from thoseof the Sausar supracrustals. All these point to theantiquity of the granulite facies protoliths as com-pared to Sausar supracrustals. Bhowmik and Pal(2000) from their study, have corroborated thepresent findings, and have indicated that the BBGgranulites record two episodes of pre-Sausar meta-morphism; an early high-T, moderate P metamor-phism followed by an episode of cooling, showingIBC P-T-t path of evolution.

A sequence of metapelites including Al- andMg-Al rich compositions is exposed discontinu-ously all along the eastern margin of the Sakoli


belt (figure 1). This sequence includes cordierite-K-feldspar—sillimanite—plagioclase—quartz—garnet; c o r d i e r i t e—gedrite-garnet—hornblende—plagioclase; gedrite-cordierite—plagioclase andquartz—sillimanite—kyanite-corundum-cordieritemineral parageneses, which are indicative of upperamphibolite facies metamorphic conditions. Theserocks are closely interbanded with granitoid leuco-somes and migmatitic gneiss containing garnetifer-ous amphibolite restites. It is a moot point if thesesupracrustals represent protolith of the lithologiesoccurring in the granulite belt, as, continuation oflithologies from these areas to the granulite beltcould not be traced. If such a correlation is ten-able, then the protoliths of the BBG belt wouldform a part of the Bastar Craton.

The BBG belt has been interpreted by Jain et al(1991, 1995) to represent exhumed oceanic crustof the northerly Bundelkhand protocontinent (withthe Bundelkhand area as the nucleus) that was sub-ducted below the southerly Deccan protocontinent(Bhandara or Central Indian Craton; Bastar Cra-ton as referred to in this study) during a Palaeopro-terozoic continent-continent collision event. Theseauthors also considered the Sausar supracrustals(including the Tirodi gneiss) as representing themarginal basin of the northern (Bundelkhand) pro-tocontinent. They then interpreted the BBG beltto be a suture zone. The present study has howevershown that the magmatic rocks within the BBGbelt are intra-plate (continental) rather than beingplate margin or oceanic derivatives; the ultramafic-mafic association in the belt does not representrelict ophiolites; and evidences for deep sea sed-imentary protoliths are not present, (Platformal)quartzite-K-pelite-BIF association in fact being thecharacteristic supracrustal association in the belt.It has also been shown that the two-pyroxene gran-ulites in the BBG belt represent continental tholei-itic magma additions to the crust and can not betaken as evidence for exhumed oceanic crust. Addi-tionally, presence of supracrustals similar to thosein the BBG belt occurring in the area on the east-ern margin of the Sakoli belt as mentioned earlieralso goes against the hypothesis of Jain et al (1991,1995).

Available geochronological data indicate an Rb-Sr whole-rock isochron age of 1525 ± 70 Ma fromthe Tirodi gneiss (Sarkar et al 1986) and a Rb-Sr whole rock isochron age of more than 2200 Mafor Amgaon gneiss (Sarkar et al 1981). A tentativeretrogression age of 1300 Ma from hornblende inthe two-pyroxene granulite from BBG belt throughAr/Ar method has been quoted in Yedekar andJain (1995- from unpublished NSA, USA data).A younger age of 860 Ma from K-Ar systemat-ics for the Tirodi gneiss has also been obtained(Sarkar et al 1967). Recently, a cooling age of

980 Ma from Ar/Ar systematics for Mn miner-als in the Sausar supracrustals has been obtainedby Lippolt and Hautmann (1994). As the Tirodigneiss is considered to be older to the Sausar(Narayanaswamy et al 1963), it is possible thatthe younger ages mentioned above may be directlyrelated to the evolution of the Sausar supracrustals.It is shown from this study that the lithologiesin the BBG belt show both older and higherdegree of (and polycyclic) metamorphism than theSausar supracrustals. The continent-continent col-lision model invoked by Jain et al (1991, 1995)involves evolution of both the lithologies of theBBG belt and the Sausar supracrustals in oneevent. It is very unlikely to be so from data andinterpretation presented in this work.

Sm-Nd dating of the charnockite and two-pyroxene granulite from the BBG belt recently hasyielded two sets of dates at 2672 ± 54 Ma and1416±59 Ma, as well as Rb-Sr date of 1380±28 Ma(Abhijit Roy, pers. comm.). The older, Archaeanage, is interpreted to represent the first attainedprograde granulite grade metamorphism in theBBG belt with charnockites retaining the earlypeak metamorphic assemblage of Grt (porphyroblast) +Opx+Pl. This was followed by the youngerage representing the second granulite metamor-phism, as seen from the development of coronalgarnet (cooling texture) in the two-pyroxene gran-ulite during Mesoproterozoic. It is postulated that,postdating the second event, the granulites wereexhumed and thrust over the basement gneissesalong the southern shear zone of the BBG belt,though it has not been possible to work out theexact timing of this shearing and exhumationevent. This was followed by the development ofSausar belt in the north during terminal Meso-proterozoic. Sausar orogeny is the most pervasiveand widespread in CITZ, imprints of which arerecorded in all the pre-existing rocks. The pen-etrative foliation defined by amphibole in rocksof the BBG belt represents the imprints of muchyounger Sausar orogenic event. The developmentof the northern shear zone in the BBG belt thuscoincides with the Sausar tectonothermal eventwith the development of amphibolite facies assem-blages superposed on granulites. This correspondswith Rb-Sr ages of 800±16 Ma in charnockites and973± 63 Ma in mafic granulites (Abhijit Roy, pers.comm.).

It is thus concluded that the BBG belt mostlikely represents a part of the Bastar Craton, thegneiss-supracrustals sequence of which underwentpolycyclic prograde granulite metamorphism priorto the evolution of the Sausar supracrustals. Thegranulite event in the more southerly Bhopalpat-nam granulite belt in the southwestern part ofthe Bastar Craton, as can be judged from its cor-


relation with the Karimnagar belt, occurred at2600 Ma (Rajesham et al 1993). The older Sm-Nd age of 2672 ± 54 Ma for charnockite in theBBG belt may be correlated with this event asrepresenting a peak-metamorphic event within theBBG belt. It is not known if the post-peak Sm-Nd (also granulite facies) age of 1416 ± 59 Maand Rb-Sr age of 1380 ± 28 Ma in two-pyroxenegranulites of the BBG belt can be correlatedwith the younger Eastern Ghat granulite event at1400–1600 Ma.

Acknowledgement

This paper is published with the kind permission ofthe Director General, Geological Survey of India.Comments by Dr. Samarendra Bhattachayya,Indian Statistical Institute and two anonymousreviewers have greatly helped in improving thequality of the paper.

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