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

H M Ramachandra1 and Abhinaba Roy2 1Geological Survey of India, AMSE Wing, Bangalore 560 078 2Geological Survey of India, Central Region, 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 the Sausar Group of rocks and is bordered by the fold belt in the south. The Bhandara part of the BBG belt is quite restricted, comprising a medium to coarse grained two-pyroxene granulite body that is of gabbroic composition and preserves relic igneous fabric. The main part of the belt in Arjuni-Balaghat section includes metasedimentary (quartzite, BIF, Al- and Mg-Al metapelites) and metaigneous (metaultramafic, amphibolite and two-pyroxene granulite) protoliths interbanded with charnockite and charnockitic gneiss. These rocks, occurring as small bands and enclaves within migmatitic and granitic gneisses, show polyphase deformation and metamorphism. Geochemically, basic compositions show tholeiitic trend without Fe-enrichment, non-komatitic nature, continental affinity and show evolved nature. Mineral parageneses and reaction textures in different rock com- positions indicate early prograde, dehydration melt forming reactions followed by orthopyroxene stability 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 are preserved at places. Retrogressive hydration events clearly post-date the above paths. The present study has shown that the BBG belt may form a part of the Bastar Craton and does not represent exhumed oceanic crust of the Bundelkhand Craton. It is further shown that rocks of the BBG belt have undergone an earlier high-grade granulite metamorphism at 2672  54 Ma (Sm-Nd age) and a post-peak granulite metamorphism at 1416  59 Ma (Sm-Nd age, 1380  28 Ma Rb-Sr age). These events were followed by deposition of the Sausar supracrustals and Neoproterozoic Sausar orogeny between 973  63 Ma and 800  16 Ma (Rb-Sr ages).

1. Introduction all components of complex Precambrian tectono- metamorphic belts is thus essential for building Precambrian crustal evolution is characterized by coherent regional scale models. Recent work in successive development of supracrustal belts and the Sausar Mobile Belt (SMB) in central India orogens often associated with development of high- (Bhowmik et al 1999, Roy et al 2000), has shown grade metamorphic belts. Regional and global cor- that granulite belts in the northern and south- relation of such events is an onerous task as ern parts of the SMB respectively, retain peak younger tectono-metamorphic events often over- metamorphic imprints of pre-Sausar orogeny and print (and rotate) earlier structures and result in that there is a distinct gap in metamorphic grade degradation of earlier mineral assemblages (Harley between the granulite belts and the Sausar Group 1992; Harley and Fitzsimons 1995). Study of of 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

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1A-−1D

Figure 1. Geological map of central India showing cratonal and major lithotectonic components. (1) Granulite Belt (A) Bhandara-Balaghat Belt, (B) Ramakona- Belt, (C) Kondagaon Belt (D) Bhopalpatnam Belt. (2) (a) Sukma-Bengpal Group (Bastar Craton), (b) Singhbhum Craton, (c) Dharwar Craton. (d) Bundelkhand Cra- ton (3) Bailadila Group. (4) 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. This (Roy et al 2000). The Bastar craton comprises paper presents an account of the southern mar- Palaeo- to Mesoarchaean Sukma gneiss-granitoids ginal granulite belt of the SMB represented by the and supracrustals, Bhopalpatnam and Konda- Bhandara-Balaghat granulite (BBG) belt and dis- gaon granulite belts, the Bailadila Group of Iron cusses the role of this granulite belt in the evolution formations, the Chandenar-Tulsidongar mobile of the SMB and the adjoining Bastar craton. belt (including the Bengpal Group), younger supracrustals of the Dongargarh and Kotri Super- groups (including the Dongargarh and Malanjk- 2. Geological setting hand granitoids), and cover sediments (Mishra et al 1988; Prakash Narasimha et al 1996; Ramachan- The Precambrian crust in central India is rep- dra et al 1998) (figure 1). The margins of the resented by the northern Bundelkhand and the Bastar craton show involvement in the East- southern Bastar Craton, separated by the SMB 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- mal facies supracrustals intruded by granitic rocks nies in the southwest and the Sausar mobile belt (Narayanaswamy et al 1963). Huin et al (1998), (SMB) orogeny in the north (Ramachandra et al Khan et al (1998, 1999), and Bhowmik et al (1999) 1998). The Bundelkhand craton (figure 1) com- have recorded three generations of structure from prises Palaeo- to Mesoarchaean Bundelkhand and the Sausar Group showing successive overprinting Sidhi associations, younger supracrustals of the relations and have established that the first genera- Mahakoshal Group along with intrusive granitoids, tion structure in the Sausar Group overprints early cover sequences of Bijawar and Gwalior Groups mylonitic fabric in the Tirodi gneiss-migmatite and Meso- to Neoproterozoic Betul-Chhindwara and northern Ramkona granulites. Four episodes and Bilaspur-Raigarh-Surguja mobile belts and the of metamorphism, namely, M1, M2, M3 and M4, Vindhyan cover sediments. The marginal relations have been recorded from the Sausar Group of rocks of the Bundelkhand craton at its south with the with peak upper amphibolite facies event during SMB are yet poorly understood, but are character- M2. Bhowmik et al (1999) have shown that the ized by ductile shearing and tectonic interleaving Ramkona granulites at the northern margin of the of older and younger components (Roy et al 2000). SMB comprise a sequence of mafic granulite, and The SMB separating the Bastar and the felsic migmatitic gneiss interleaved with the Sausar Bundelkhand cratons occurs as a curvilinear, Group of rocks. They have also shown that the ENE–WSW to E–W trending, almost 300 km granulites record pre-Sausar structures and meta- long and 70 km wide belt, and is comprised of morphic imprints with the peak M2 event recording three main components, including the northern ∼10.5kb pressure and 775◦ C temperature. They Ramkona granulite belt, central Sausar Group have further inferred that the granulites formed of supracrustals and southern Bhandara-Balaghat the basement for deposition of Sausar supracrustals granulite (BBG) belt (Bhowmik et al 1999; Roy and were tectonically imbricated with the latter et al 2000) (figure 2). The Sausar Group of rocks, during the Sausar orogeny correlated with the forming the central part of the SMB is represented 1000 Ma Grenvellian event. by an older metamorphic group including the Geochronological data for the SMB is meager Tirodi gneiss-migmatites and supracrustals, and a and includes the Rb/Sr whole rock isochron age of sequence of intensely deformed younger platfor- 1525  70 Ma and mineral isochron age of 860 Ma 354 H M Ramachandra and A Roy by Sarkar et al (1986) for the Tirodi gneiss, the for- to 10 cm in size occur at places. The main folia- mer considered to mark the main phase of amphi- tion trends from NE–SW to N70◦E–S70◦W with bolite facies metamorphism in the Sausar Group steep southerly dips and plagioclase and pyroxenes and the latter mineral age as representing the ter- show protomylonitic character along local sinis- minal thermal overprint of the Sausar orogeny. Lip- tral shears sub-parallel to the main foliation, as polt and Hautmann (1994) have obtained Ar/Ar evidenced by recovery and recrystallization and age of 950 Ma for cryptomelane from the Sitapur bending of twin lamellae in plagioclase. The two- mines and interpret the age to represent the clo- pyroxene granulite shows a mineral assemblage of sure of Satpura (≡Sausar orogeny) cycle amphi- Pl-Opx-Cpx-Hbl-Opq ScpGrt. bolite facies metamorphism. Recent geochronolog- The granite gneiss in the northern margin of ical data for the BBG belt include Sm-Nd dates the BBG belt is in tectonic contact with myloni- of 2672  54 Ma and 1416  59 Ma, and Rb-Sr tised low-grade metasediments of the Sausar Group date of 1380  28 Ma for charnockite and two- showing development of protomylonitic fabric in pyroxene granulite of the BBG belt, representing both metasediments and granite gneiss and a low- different ages of granulite facies metamorphism; northerly plunging stretching lineation. The gran- and Rb-Sr dates of 800  16 Ma for charnockites ite gneiss delimiting the southern margin of the and 973  63 Ma for mafic granulites in the north- BBG belt also shows mylonitic and phyllonitic ern shear zone of the BBG belt, representing devel- character. The full spectrum of rocks of the BBG opment of amphibolite facies assemblages in gran- belt is best exposed in the Arjuni-Balaghat area ulites (Abhijit Roy, personal comm.). (figure 2) and comprises a sequence of quartzite, BIF, metapelite, two-pyroxene granulite, charnock- 3. Lithology and field relations ite and charnockitic gneiss and migmatitic and granitic gneiss. These are shown in table 1 along The Bhandara-Balaghat granulite belt, forming the with their main mineral assemblages. southern margin of the SMB, extends discontin- The different rock types mentioned above occur uously for more than 100 km from northwest of interbanded mainly within migmatitic and granite Bhandara in the southwest, in , to east gneisses. Granite gneiss is mainly exposed in the of Balaghat in the northeast, in Madhya Pradesh. northern part and southern margins of the BBG It was earlier included as a part of the older belt are strongly tectonised on ENE–WSW trend- metamorphics at the base of the Sausar Group ing planes. The migmatitic gneiss includes imper- (Narayanaswamy and Venkatesh 1971). Rajurkar sistent salic-mafic bands a few mm to a few cm (1974) and Mohabey and Dekate (1984) recorded thick. The pegmatitic salic bands contain up to the presence of two-pyroxene granulite near Bhan- 10 cm long deformed K-feldspar phenocrysts. Melt dara. Jain et al (1995) showed that granulite rocks formation is indicated by the presence of nebu- of this belt occur as lensoid bodies within gneisses lous and fleck structures in the rocks, biotite grains extending discontinuously from Bhandara in the around K-feldspar phenocrysts and melt supported southwest to Ratanpur in the northeast. They also texture (e.g. tabular feldspars, granitic intergrowth reported the presence of two-pyroxene granulites, texture) in the quartzofeldspathic salic bands. orthopyroxene and garnet bearing BIF, cordierite Quartzite is rare and occurs as bands up to bearing metapelites and gneisses and have inferred 10 m in size as near Bhandi. BIF is exposed as a that the granulites represent oceanic basalt, now major folded band extending for a few 100 m near occurring, along with associated metasediments, as Larsara and as small bands near Amgaon. The exhumed blocks within migmatites. Yedekar and rock is coarse grained and contains orthopyroxene Jain (1995) have interpreted Ar/Ar age of 1300 Ma and garnet grains measuring up to 5 cm in size, obtained from hornblende in two-pyroxene gran- along with smaller grains of quartz, hornblende and ulite as representing the episode of post-granulite cummingtonite-grunerite. Metapelites are associ- retrogressive event. ated with granitic leucosomes and commonly occur The Bhandara part of the BBG is represented by as interbands less than a metre long with gneisses a large two-pyroxene granulite body elongate along and other granulites. They are best exposed in the ENE–WSW, extending along strike for about 5 km, Hurkitola, Dongargaon, Larsara and Mendki areas. with a maximum width of 1.5 km, exposed near Al-metapelites are medium grained and schistose Bhandara and is locally associated with migmatitic with sillimanite, kyanite and micas measuring from gneisses and a thin band of quartzite. The two- a few mm to 0.5 cm in size. Cordierite bearing rocks pyroxene granulite is medium to coarse grained and are finer grained and less schistose. Two-pyroxene shows uniform gabbroic composition with occa- granulites occur as long, linear bands, several hun- sional relict ophitic and sub-ophitic texture. Small dred metres in length mainly in the central part of (a few cm thick) gabbro-anorthositic gabbro lay- the BBG belt, near Lingmara, Mendki, Larsara and ers together with rare segregations of pyroxene up Dongariya. The rocks show relict ophitic and sub- Evolution of the Bhandara-Balaghat granulite belt 355 Table 1. Generalized tectonically significant lithosuccession and mineral assemblage in rocks of 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 Sph intermediate 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-Zir Banded iron formation Qz-Opx-Grt-Hbl-Cum/Grun-Mag Quartzite QzFuchChlGrt Migmatite and granite gneiss Qz-Kfs/Pth-Pl-Grt-Bt-Hbl-Apa-Sph- Zir ophitic texture, variably developed foliation and in the kinematics it is interpreted that these shear local layering due to metamorphic differentiation. zones were developed at different times and sequen- Charnockite and charnockitic gneisses are medium tially accreted different tectono-metamorphic ter- grained rocks of intermediate to acidic composition rains together. and have the typical greasy charnockitic appear- The granulites exhibit a prominent ENE–WSW ance. They are weakly or strongly foliated and are striking sub-vertical foliation/layering defined typically interbanded with two-pyroxene granulite mainly by amphiboles. This layering shows and migmatitic gneiss in Arjuni, Dongariya and mylonitic structure including S-C fabric in different Larsara areas, and also occur as small patches or rocks. At least one set of small folds is developed stocks. on this foliation plane. These folds have E–W to ENE–WSW striking sub vertical axial planes and moderate to steep (even vertical) plunge of the fold 4. Structural set up axes. Thin section study has shown that the amphi- boles defining the above layering post-date gran- The BBG belt is located in the Central Indian ulite assemblages. Further, mesoscopic structures Shear Zone (CIS) that is marked by the develop- related to granulite facies metamorphism could not ment of mylonites and comprises multiple shear be recognised. This indicates that the amphibolite zones having polycyclic histories. The present grade structures post-date those of the granulite study has shown that there are at least two promi- facies metamorphism. nent ENE–WSW trending ductile shear zones within the Central Indian Shear Zone. The first one marks the boundary between the Sausar belt 5. Petrography in the north and basement gneisses with granulites in the south. The second one delineates the bound- The two-pyroxene granulite and the charnockitic ary between granulite belt in the north and the rocks show superposition of deformation fabric over Amgaon Gneissic Complex (AGC) in the south. igneous melt supported textures, which are locally Thus, there are two prominent ductile shear zones, preserved in the form of ophitic, subophitic and which border the granulite belt, one in the north intergrowth textures. Metamorphic fabrics include and other in the south. The northern shear zone granoblastic texture, mineral reactions involving has steep northerly dip, though at places, rever- neocrystal growth of coronitic or porpyroblas- sal in the attitude has been noted. The stretching tic minerals, exsolution texture and development lineation on this steeply dipping mylonite plane is of perthite and myrmekites. Protomylonitic tex- invariably steep. The lineation data, in conjunc- ture is common along shear zones and is repre- tion with shear bands suggest predominant dip-slip sented by incipient recovery and recrystallization component with northern Sausar belt going down in quartz and feldspars, development of kink bands with reference to the granulite belt. In contrast, the and asymmetric tails in pyroxene, mica and horn- steep northerly dipping shear zone on the south of blende. the granulite belt exhibits a reverse sense of move- Metapelites show granoblastic texture and devel- ment, wherein the granulite belt has been thrust opment of sillimanite, kyanite, cordierite and over the Amgaon gneisses. Based on the difference garnet porphyroblasts in rocks of appropriate 356 H M Ramachandra and A Roy (Al- or Al-Mg) compositions. Reaction textures The plots of analyses of all the above rocks in the involving coronitic or symplectitic growth involv- SiO2 vs. total alkali diagram after Wilson (1989), ing orthopyroxene, sillimanite, cordierite, garnet, as given in figure 3 distinctly occur in two separate sphene, rutile and opaque are well preserved. Pres- clusters, in the gabbro and granite compositional ence of pegmatite, rare aplite and small bands and fields. The two-pyroxene granulites occur in a rel- pools of granitic leucosomes (neosomes) up to a atively tightly clustered group in the gabbro field few cm in dimension showing melt supported fabric whereas the granitoids and the charnockites occur indicate generation of partial melts. Mylonitic fab- in the granitic field, showing a restricted compo- ric including development of S-C fabric, ’fish’ struc- sitional spread in the acidic range from quartz- ture, kink bands etc. in orthopyroxene and mica is diorite (granodiorite) to granite. Thus a bimodal also present. relationship between the basic and acidic composi- tions is indicated. In the AFM diagram after Irvine and Baragar (1971) (figure 4) the basic and acidic 6. Geochemistry compositions clearly follow different trends and fur- ther depict the bimodal relationship between the Twenty nine major and trace analyses of pyrox- two. The basic trend is typically tholeiitic, shows ene granulites, charnockites and granitoids from an evolved nature and lacks Fe enrichment com- the BBG belt are presented mainly to study if monly observed in differentiated high-level mafic magmatic or tectono-magmatic relationships exist complexes. Normative CIPW values indicate qz- between different lithocomponents. The analyses normative and hence saturated nature of the two- were carried out at the chemical laboratories of pyroxene granulites. The charnockites and gran- the GSI, Central Region, Nagpur, by XRF meth- itoids show a mixed tholeiitic and calc-alkaline ods using suitable internal standards. Of the 29 nature, but as the granitoid sample includes a vari- samples, 15 represent different types of grani- ety of sources ranging from leucosome of para- or toids including migmatitic leucosomes and granite ortho-migmatites and granite intrusives, the trends gneisses; 10 of two-pyroxene granulites including 6 at best can be taken as suggestive. from Arjuni-Balaghat area and 4 from Bhandara; The two-pyroxene granulites in Jensen’s (as and 4 of charnockite and charnockite gneiss. The modified by Jensen and Pyke 1982) diagram (fig- analytical results are given in table 2 and CIPW ure 5) generally show tholeiitic character excepting Norms in table 3. two plots that occur in the komatiite field. But as The granitoids show a range in SiO2 composition these two rocks are characterized by relict ophitic varying from 63.53% to 74.08% with Al2O3 rang- and subophitic textures typical of tholeiites and ing from slightly above 11% to 16.98%. These rocks lack spinifex or related textures they are inferred are typically low in iron and magnesium and are to be of tholeiitic affinity only. A majority of the generally low in calcium. K2O is commonly higher other samples show high-Mg rather than high-Fe than Na2O and total alkalis range from slightly tholeiitic character. Most of the two-pyroxene gran- above 4.5% to 8.5%. Sr content (varying from 130 ulites plot in the continental basalt field of TiO2- ppm to 620 ppm) is usually higher than that of Rb P2O5-K2O diagram (figure 6) after Pearce et al (60 ppm to 490 ppm) and Zr varies from 10 ppm (1975), indicating the continental-tholeiite charac- to 420 ppm. Cr shows a variation from 10 ppm to ter of these rocks. The restricted compositional 240 ppm. The two-pyroxene granulites show a silica spread among acidic compositions is brought out range between 47.60% and 53.64% with MgO vary- in the Ab-An-Or diagram after Barker (1979) (fig- ing from 7.03% to 12%. They are moderately iron ure 7). The paucity of tonalite and trondhjemite rich and show low- to moderate TiO2 and typically possibly indicate thorough crustal reworking of low alkalis. The values of Mg # ranging from 48 to protoliths for the acidic rocks and likely influence 69 for these rocks indicates their evolved nature. of continental processes in generation of precursor Sr content is commonly higher than that of Rb in magma for charnockite rocks. CIPW norm calcula- the two-pyroxene granulites and Cr values range tions indicate that most of the acidic rocks includ- from 20 ppm to 960 ppm. Zr varies from 30 ppm to ing charnockites are corundum normative. They 280 ppm. The SiO2 in the charnockites ranges from mainly occur in the peraluminous field of SiO2 63% to 74.15% with a narrow spread in Al2O3 con- vs. A/CNK diagram (figure 8) and a larger num- tent. MgO is low with variable low- to high contents ber show S- rather than I- type character (after of Iron oxides and calcium. K2O tends to be higher Chappel and White 1974). These mixed signatures than Na2O and total alkali contents are generally and especially the high Al nature of the rocks similar. Sr and Rb contents are generally similar in compared to their alkali content reflects the vari- the charnockites with Cr varying from 20 ppm to able para- and ortho- nature of the samples. The 80 ppm. Zr shows a variation from 60 ppm to 230 plots of granitic and charnockite compositions in ppm. the Q-Ab-Or diagram (figure 9) after Anderson Evolution of the Bhandara-Balaghat granulite belt 357 G-5 & = Coarse G-12 = Gar-gneiss; G-3&4 = Fine grained granite dyke in stromatic ---- G-7 = Hbl-gneiss; = Leucosome in banded gneiss; G-2 G-11 &13 = Gar-plag gneiss; 4.801.34 6.01 1.25 3.09 1.36 2.390.15 1.10 2.88 0.41 1.18 0.82 0.84 0.28 0.66 0.25 0.28 0.20 0.20 0.15 1.030.55 0.81 0.67 1.05 0.77 2.520.25 0.27 1.47 0.20 0.23 0.94 0.15 0.32 1.05 0.29 0.20 2.73 0.14 0.01 0.34 0.10 0.07 50.75 50.86 53.64 63.00 63.09 72.14 74.15 70.0014.38 70.83 13.55 71.26 13.81 71.77 14.48 72.46 12.93 72.94 12.89 73.27 14.61 74.08 13.35 12.09 12.39 14.25 14.00 16.07 13.65 13.19 G-1 1.15 2.17 1.09 0.50 2.51 2.67 2.83 2.90 2.18 2.46 1.93 2.56 3.40 2.58 1.70 4.16 2.76 0.01 0.67 0.44 1.39 2.31 2.47 3.85 3.73 5.85 4.60 6.53 4.32 3.73 3.12 5.49 2.95 4.11 0.33 0.01 0.43 0.31 0.22 0.34 0.30 0.32 0.39 0.38 0.62 0.33 0.45 0.50 1.33 0.40 0.28 20 210 960 860 70 80 30 20 99.45 99.49 99.37 99.16 99.35 99.17 99.24 99.21 100.19 TP-7 TP-8 TP-9 TP-10 CH-1 CH-2 CH-3 CH-4 570 190 390 310 270 620 310 430 130 120240 150 20 120 230 160 160 130 10 140 150 120 10 80 10 220 10 7.2 8.320.13 7.03 0.92 12.00 0.14 9.10 0.15 2.41 0.11 1.89 0.09 0.65 0.06 0.65 0.01 0.01 0.88 0.730.04 1.02 0.04 0.54 0.01 0.51 0.01 0.66 0.04 0.43 0.01 0.38 0.05 0.47 0.01 0.35 0.01 0.01 9.90 5.22 10.08 7.92 6.30 6.12 4.32 1.98 1.98 3.42 3.064.60 3.24 2.80 1.98 1.57 2.16 1.70 0.36 2.20 3.42 1.50 0.90 1.84 0.54 2.00 1.44 1.36 1.00 — — 20 — 40 30 40 10 10 90 20 240 210 710 280 280 210 180 20 10 310 10 20 300 10 190 180 150 20107010—30203040 52.00 67.0020 51.0060 67.00 120 68.00 180 70 30 20 60 80 280 270 80 290 160 6040 150 230 30 140 30 20 60 20 50 30 40 10 140 120 30 10 220 20 20 20 — 70 10.20 16.50 10.56 6.4099.81 7.50 99.95 5.80 99.78 99.35 5.10 98.39 2.46 100.20 1.71 100.27 99.17 99.73 G-6 G-7 G-8 G-9 G-10 G-11 G-12 G-13 G-14 G-15 = Leucocratic part in migmatitic gneiss; = Coarse grained leucosomes from stromatic migmatite; G-10 G-6 = Leuco-granite gneiss. = Charnockite & charnockitic gneiss; CH G-14 & 15 = K- feldspar rich granite; 2.640.85 3.31 1.16 3.34 1.87 2.620.25 1.79 4.67 0.20 0.97 4.84 0.42 1.36 4.41 0.29 0.11 0.25 0.29 0.20 2.291.29 1.73 1.45 2.83 0.95 2.36 0.700.60 0.87 0.24 0.35 0.33 0.46 0.62 0.63 0.15 0.84 0.09 0.30 0.25 47.6016.30 47.85 14.97 48.03 11.42 48.36 14.49 49.08 13.50 49.82 13.85 50.00 12.78 63.5316.98 65.99 14.05 66.57 14.92 66.98 14.20 74.64 11.29 68.83 13.86 68.99 13.69 G-9 Major and trace element analyses of two-pyroxene granulites and charnockites from the BBG. belt 3 3 3 3 O 0.30 0.22 0.34 0.33 0.85 0.33 O 0.40 0.34 0.32 0.29 0.44 0.29 5 5 2 2 = Two-pyroxene granulite; O 1.43 1.28 0.85 1.82 1.91 1.75 O 2.04 2.97 2.06 1.76 1.82 2.32 2 2 2 2 O O O O 2 2 O 0.07 0.07 0.11 0.12 0.14 0.14 O 2.51 1.54 4.27 3.31 6.33 4.70 O O 2 2 2 2 2 2 = Leucosome from stromatic migmatite; 2 2 Table 2. SampleSiO TP-1Al TP-2FeO TP-3Fe TiO TP-4CaO 9.90 TP-5MgOK TP-6 9.00Na 12.08MnO 8.38 11.87 8.82P +H 12.30 9.36 12.6Total 0.16Mg# 11.50 8.90Nb 6.02 0.16 99.96Sr 14.05 7.26 57.00Rb 99.45 0.36Cr 8.04 61.00Zr 99.36 — 0.13Y 80 67.00 98.71 20Sample 2010302010 320 0.21 48.00 —SiO 99.69 120 G-1 40Al 64.00 10 400FeO G-2 —Fe 20 40TiO 20 G-3 80CaO — 110 3.06MgO G-4 60 250 10K 5.58 — 40 Na G-5 4.34 60MnO 130 2.10 20 2.52P 3.82+H 170 1.81 4.14 200 Total 0.07 2.64Nb 1.67 1.44 Sr 0.05 2.37 99.21Rb 2.34Cr 0.06 99.68 2.50 40Zr 0.61 140 99.27Y 0.10 280 30 98.70 120TP 260 0.01 100.28 60 50 100.19 40 440 60 30 490 420 350 170 20 10 210 50 340 140 20 10 110 100 600 170 190 30 10 160 50 migmatite; 8 leucocratic augen gneiss; 358 H M Ramachandra and A Roy 0.39 0.13 0.74 1.56 0000 0000 0000 00000 2.240 0 0 0 0 0 0.48 0 0 00000 00000 0000 00000 00000 0 0.30 0 0 0 0 0 0 0 . 3.22 0.54 1.25 2.4 2.27 14.15 2.37 25.66 2.410.33 9.05 1 15.35 22.65 2.01 23.13 34.53 0.67 37.65 0.33 0.33 0.33 7.59 7.81 7.12 6.02 3.690.67 2.52 0.33 2.65 0.33 0.33 0.33 0.82 0.33 0.17 4.23 2.37 1.88 0 4.88 0.41 0 3.89 2.78 8.34 13.34 14.45 22.79 22.24 5.08 0.1 0 1.5 0 0 0 2.566.93 2.41 8.77 2.56 4.62 2.11 3.46 2.26 4.15 0.6 1.15 0.6 0.92 1.051.61 1.2 1.15 1.47 1.61 0.45 0.46 0.45 2.07 0.6 1.38 0.6 0.92 3.92 0.02 18.34 9.43 4.19 20.96 22.53 24.1 24.62 G-7 G-8 G-9 G-10 G-11 G-12 G-13 G-14 G-15 21.43 12.72 0.58 0 2.7 0 0 0 10 5.93 7.69 10.09 6.67 8.24 7.73 6.11 4.54 29.71 21.4 27.8 32.25 20.29 24.37 11.3 7.69 26.31 32.41 29.97 34.28 34.3818.86 39.5135.02 20.96 27.24 38.95 16.24 38.92 34.68 22 25.57 39 28.82 22.24 22 18.9 32.8 14.67 17.79 35.63 24.46 23.58 TP-7 TP-8 TP-9 TP-10 CH-1 CH-2 CH-3 CH-4 0.94 0.28 0 0 0 4.59 14.43 7.01 0.41 0 1.19 0 0 0 1.25 1.51 2.6 1.4 1.3 1.7 1.1 1 1.2 0.9 13.42 6.26 10.48 29.68 23.13 4.8 4.7 1.6 1.6 3.14 3.051.66 0.85 2.22 0 0.59 0 0 0 0 0 00000000 00000000 3.27 3.459.82 3.39 8.72 2.36 7.82 3.47 16.11 3.12 2.21 5.33 9.23 2.31 1.71 6.6 0.79 4.75 0 2.92 3.23 CIPW norms for two-pyroxene granulites and charnockites of the BBG belt qzapilmmt 0an 0.67 1.66ab 1.4 0.67or 3.92 2.26 37 1cor000000000 2.09 4.85 3.47 12.05wo 2.49 34.47 12.05 4.85 3.47 0.55 0.67 27.24fs 1.87 7.33 3.92 1.81 0.55 0.33 31.41 8.46 15.72 6.03 6.93 28.07fs 2.56 0.55 0.68 9.89 16.24 3.4 29.6 6.93 0.15 0.55 13.1 14.67 0.33 fa 6.23 1.11 9.95 hm000000000 4.79rut000000000 0.82 17.36Sample 0.61 0.05 8.08 qz G-1ap G-2ilm 30.81 G-3mt 30.07an 1.34 G-4 2.41 30.89ab 0.67 G-5or 3.46 34.05 2.71 17.98 G-6 1cor 34.96 2.54 1.81 17.29 17.33wo0000 26.31 25.15 4.15 15.01 10.56 1.35 0.33 4.41 17.81fs0000 11.01 3.46 8.89 0.45 0.21 15.19 1.29 25.57 4.17 1.26fs 1.2 15.19 0.67 20.01 13.62 3.05 0.46 79.38 37.25 0.67 1.66 fa000000000 3.7 0.92 27.8 hm000000000 0rut000000000 0 0 Sample TP-1 TP-2 TP-3 TP-4 TP-5 TP-6 Di en 4.8 5.92 8.72 2.34 12.33 Dien00001.5 Ol fo 0.86 Olfo000000000 Table 3. Hy en 14.5 14.97 20.07 15.95 7.86 Hy en 5.3 4.5 4.2 5.9 0 Evolution of the Bhandara-Balaghat granulite belt 359

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. 360 H M Ramachandra and A Roy

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) show the largely continental setting of the rocks. Symbols as in figure 3. Evolution of the Bhandara-Balaghat granulite belt 361

Figure 7. Plots of gneisses and granitoids of the BBG belt in the granite classification diagram after Barker (1979) show restricted compositional spread for these rocks. Symbols as in figure 3. and Bender (1989), used to determine the depth been observed (figure 10). This may be due of magma generation, show some spread. Samples to associated with orthomigmatites plot at shallower minima, typically below 2–4 kb minima. The grani- Ti − Hbl+Qz=Grt+Opx+Cpx+Ilm+H2O toid samples from paramigmatitic association show (1) plots occurring mainly between 4 and 7 kb and 7 and 10 kb minima. However as time of melt gen- representing a prograde path involving rise eration and movement and effect of fluid during in temperature during metamorphism. Inter- melting are not constrained, further elaboration on estingly orthopyroxene1, being replaced by this theme is not possible at present. orthopyroxene2 has also been observed, and may be related to this event, probably representing the break down of high Al-pyroxene to lower Al- 7. Mineral reactions pyroxene. Mg-Al rich compositions show the presence of Petrographic study of two-pyroxene gran- porphyroblastic orthopyroxene containing inclu- ulite, charnockite and charnockite gneisses sions of cordierite, K-feldspars, perthite, phlo- and metapelitic granulites of the BBG gophitic biotite and garnet. This indicates the pres- belt has resulted in documentation of min- ence of a pre-existing cordierite-garnet assemblage eral textures and reactions distinctive of along with melt. Thus an early reaction as given polyphase granulite facies metamorphism in the below was operative: belt. Charnockites and charnockitic gneisses con- Bt + Sil + Qz = Crd + Grt + Kfs + melt (2) tain plagioclase grains showing antiperthitic character. Presence of brown hornblende ear- and may be taken to have heralded the onset of lier to porphyroblastic and coronal garnet granulite facies metamorphism. has been observed in many samples. Horn- Crd + Grt + Sil + Spl assemblage, without blende rimmed by orthopyroxene has also orthopyroxene, is characterized by an early stable 362 H M Ramachandra and A Roy

Figure 8. Gneisses and granitoids of the BBG belt in the SiO2 vs. A/CNK diagram showing mixed, but dominantly peraluminous character of the rocks. Symbols as in figure 3. assemblage of coarse garnet, sillimanite, cordierite, that aluminous metapelites first passed through perthite and opaque association with or without the kyanite stability field and with increase in tem- quartz. This assemblage, as in the case of the perature went into the sillimanite field. orthopyroxene bearing one described above, may Orthopyroxene in different lithologies is represent an early melt forming event from reac- rimmed or coronated by second garnet or

tion (2), given above. Presence of Grt + Crd + Bt garnet2 + quartz + clinopyroxene symplectite. This bearing quartzofeldspathic (granitic) veins in asso- garnet2 also shows inclusions of orthopyroxene, ciation with the above metapelites supports this cordierite and opaque. The orthopyroxene break- contention. down may have proceeded by the reaction Orthopyroxene associated with either cordierite or plagioclase (figure 11) in the above set up could High Al pyroxene = low Al pyroxene + Grt (7) have originated through reactions such as, In the BIF compositions, orthopyroxene shows Grt + Qz = Opx + Crd (3) spectacular deformation lamellae and twins and Grt+Qz=Opx+Pl (4) many grains show stretching along shear planes and ‘fish’ structure. Garnet occurs as porphyrob- Grt+Kfs+v=Opx+Crd+Bt (5) lasts or as corona around orthopyroxene. Gar- Alumina rich metapelites show that sillimanite is net bands replacing kink bands in orthopyrox- derived from both inversion of kyanite and from ene also occur. Garnet and orthopyroxene show Mus + Qz = Sil + Kfs + melt reaction (cf. Yardley grain refinement and stability along shear planes. 1989). Garnet either occurs as porphyroblasts or Bluish green hornblende or actinolite-hornblende- mantles sillimanite and K-feldspar, indicating gen- Fe amphibole intergrowth replaces orthopyroxene eration of partial melt by a reaction such as in some samples. The two pyroxene granulites show reaction tex- Bt + Sil + Qtz = Grt + Kfs + lq (6) tures around orthopyroxene and plagioclase includ- ing: fine grained clinopyroxene next to garnet- As only kyanite and sillimanite occur as aluminosil- quartz intergrowths around orthopyroxene (fig- icate phases, and as either andalusite or its pseudo- ure 12); clinopyroxene rim around orthopyroxene; morphs have not been identified, it is suggested and rarely, clinopyroxene-garnet symplectites. In Evolution of the Bhandara-Balaghat granulite belt 363

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 to and rimmed by an aggregate of intergrown orthopyroxene, clinopyroxene and ilmenite. Quartz and minor plagioclase occur in other parts of the photograph. [Charnockite, near Larsara, BBG belt.] Width of photograph= 1 mm; semicrossed nicols. 364 H M Ramachandra and A Roy

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=1 mm; semicrossed nicols. some samples, secondary garnet growth around Textural data as documented can further be earlier formed, larger garnet grains is present. Gar- interpreted as follows: net included in some orthopyroxenes may repre- • The earlier textures indicate breakdown of horn- sent garnet formed from re-equilibration of alu- blende to yield garnet, orthopyroxene, clinopy- minous pyroxene. Presence of hercynitic spinel in roxene with or without a melt. This is a pro- orthopyroxene noticed in some samples may also grade dehydration reaction involving an amphi- be due to the same reason. Garnet coronas sepa- bolitic protolith. Such a reaction has been doc- rating orthopyroxene and plagioclase, or, clinopy- umented from the mafic mineral assemblage of roxene and plagioclase also occur. These reaction banded gneisses. textures are indicative of reactions of the type, A series of reaction textures in Al-rich and Mg-Al rich metapelite involve firstly, melt gen- Opx+Pl=Grt+Cpx+Qz (8) eration in biotite + sillimanite bearing (amphi- Cpx + Pl = Grt + Qz (9) bolite facies?) protoliths, along with garnet + Opx + Pl = Grt + Qz (10) cordierite assemblage (reaction 6), going on to yield Opx + Crd + Pl assemblages (reactions 7, 8 and 9) with or without melt to com- plete the prograde path. Kfs/pth + Pl + Crd + 8. Interpretation of textural data Grt bearing granitic leucosomes occurring in close spatial association represent leucosome and The reaction textures described from lithological melanosomes derived from the above reactions. components as discussed from petrographic stud- In the next stage, coronitic and symplectitic ies in the above section are all typical of granulite garnet form over earlier minerals in lithologies of facies (e.g. Harley 1989). All of these also easily different compositions indicating onset of retro- plot well within the PT limits considered typical of grade (IBC) path. The type charnockite and two- granulite facies rocks (Bohlen 1987). The high PT pyroxene granulite in the area show the melt- equilibrium attained by these rocks is well illus- supported fabric, being directly overprinted by trated by the tholeiitic component of intermediate coronitic and symplectitic garnet (reactions 1, 2 Mg/Mg+Fe (now two-pyroxene granulite) in the and 3). This may indicate that these bodies were study area, which contain a mineral paragenesis of emplaced into the above set up at this point of cpx + gar + qz (Green and Ringwood 1967). time during the evolution of the granulite belt. Evolution of the Bhandara-Balaghat granulite belt 365

Figure 12. Photomicrograph: Coronotic garnet and clinopyroxene resulting from breakdown of orthopyroxene (center of the 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- of two-pyroxene granulite has been reported from scopic evidence in different lithologies show that the Ratanpur area. The above account indicates orthopyroxene, garnet and other high P-T min- the regional extent of the granulite belt. eralogies as formed above continued to be stable The contact between the granulite belt and the during ductile shearing events. Sausar supracrustals is tectonic in nature. Though • Bluish green or brownish green hornblende par- the lithologies of the granulite belt share later tially or completely replacing pyroxene; saus- deformation events that have affected the Sausar suritization and more commonly sericitization supracrustals, they are characteristically differ- of plagioclase; chloritization of garnet; pinitiza- ent in other aspects from the latter. The Sausar tion of cordierite; muscovite alteration of alu- supracrustals (without the Tirodi gneiss compo- minosilicates, are all retrograde events proba- nent) from the data available, show peak metamor- bly connected with juxtaposition of the granulite phic condition only up to sillimanite grade without lithologies with those of the lower grade Sausar involving formation of a melt phase. The litholo- supracrustals. gies of the granulite belt, on the other hand, show, as documented in the present study, a high grade metamorphism under granulite facies and reaction 9. Discussion: regional correlation and relations suggest the polyphase nature of metamor- tectonic evolution of the BBG belt phism. The high temperature ductile shears found in the granulite lithology are also absent from those The BBG belt flanks the southern margin of the of the Sausar supracrustals. All these point to the SMB marking the contact between the northern antiquity of the granulite facies protoliths as com- Bundelkhand and southern Bastar Cratons in the pared to Sausar supracrustals. Bhowmik and Pal Precambrian of central India. This narrow, 15 km (2000) from their study, have corroborated the wide, NE–SW to ENE–WSW trending gneiss dom- present findings, and have indicated that the BBG inated belt is in tectonic contact with the Sausar granulites record two episodes of pre-Sausar meta- supracrustals in the SMB. The continuation of morphism; an early high-T, moderate P metamor- the granulite belt further west-southwestwards phism followed by an episode of cooling, showing (from Bhandara) is uncertain as the area is under IBC P-T-t path of evolution. thick alluvial cover. East-northeastwards, granulite A sequence of metapelites including Al- and lithologies have been traced up to southeast of Bal- Mg-Al rich compositions is exposed discontinu- aghat and further in the same direction, presence ously all along the eastern margin of the Sakoli 366 H M Ramachandra and A Roy belt (figure 1). This sequence includes cordierite- 980 Ma from Ar/Ar systematics for Mn miner- K-feldspar—sillimanite—plagioclase—quartz—gar als in the Sausar supracrustals has been obtained net; cordierite—gedrite-garnet—hornblende— by Lippolt and Hautmann (1994). As the Tirodi plagioclase; gedrite-cordierite—plagioclase and gneiss is considered to be older to the Sausar quartz—sillimanite—kyanite-corundum-cordierite (Narayanaswamy et al 1963), it is possible that mineral parageneses, which are indicative of upper the younger ages mentioned above may be directly amphibolite facies metamorphic conditions. These related to the evolution of the Sausar supracrustals. rocks are closely interbanded with granitoid leuco- It is shown from this study that the lithologies somes and migmatitic gneiss containing garnetifer- in the BBG belt show both older and higher ous amphibolite restites. It is a moot point if these degree of (and polycyclic) metamorphism than the supracrustals represent protolith of the lithologies Sausar supracrustals. The continent-continent col- occurring in the granulite belt, as, continuation of lision model invoked by Jain et al (1991, 1995) lithologies from these areas to the granulite belt involves evolution of both the lithologies of the could not be traced. If such a correlation is ten- BBG belt and the Sausar supracrustals in one able, then the protoliths of the BBG belt would event. It is very unlikely to be so from data and form a part of the Bastar Craton. interpretation presented in this work. The BBG belt has been interpreted by Jain et al Sm-Nd dating of the charnockite and two- (1991, 1995) to represent exhumed oceanic crust pyroxene granulite from the BBG belt recently has of the northerly Bundelkhand protocontinent (with yielded two sets of dates at 2672  54 Ma and the Bundelkhand area as the nucleus) that was sub- 141659 Ma, as well as Rb-Sr date of 138028 Ma ducted below the southerly Deccan protocontinent (Abhijit Roy, pers. comm.). The older, Archaean (Bhandara or Central Indian Craton; Bastar Cra- age, is interpreted to represent the first attained ton as referred to in this study) during a Palaeopro- prograde granulite grade metamorphism in the terozoic continent-continent collision event. These BBG belt with charnockites retaining the early authors also considered the Sausar supracrustals peak metamorphic assemblage of Grt (porphyro (including the Tirodi gneiss) as representing the blast) +Opx+Pl. This was followed by the younger marginal basin of the northern (Bundelkhand) pro- age representing the second granulite metamor- tocontinent. They then interpreted the BBG belt phism, as seen from the development of coronal to be a suture zone. The present study has however garnet (cooling texture) in the two-pyroxene gran- shown that the magmatic rocks within the BBG ulite during Mesoproterozoic. It is postulated that, belt are intra-plate (continental) rather than being postdating the second event, the granulites were plate margin or oceanic derivatives; the ultramafic- exhumed and thrust over the basement gneisses mafic association in the belt does not represent along the southern shear zone of the BBG belt, relict ophiolites; and evidences for deep sea sed- though it has not been possible to work out the imentary protoliths are not present, (Platformal) exact timing of this shearing and exhumation quartzite-K-pelite-BIF association in fact being the event. This was followed by the development of characteristic supracrustal association in the belt. Sausar belt in the north during terminal Meso- It has also been shown that the two-pyroxene gran- proterozoic. Sausar orogeny is the most pervasive ulites in the BBG belt represent continental tholei- and widespread in CITZ, imprints of which are itic magma additions to the crust and can not be recorded in all the pre-existing rocks. The pen- taken as evidence for exhumed oceanic crust. Addi- etrative foliation defined by amphibole in rocks tionally, presence of supracrustals similar to those of the BBG belt represents the imprints of much in the BBG belt occurring in the area on the east- younger Sausar orogenic event. The development ern margin of the Sakoli belt as mentioned earlier of the northern shear zone in the BBG belt thus also goes against the hypothesis of Jain et al (1991, coincides with the Sausar tectonothermal event 1995). with the development of amphibolite facies assem- Available geochronological data indicate an Rb- blages superposed on granulites. This corresponds Sr whole-rock isochron age of 1525  70 Ma from with Rb-Sr ages of 80016 Ma in charnockites and the Tirodi gneiss (Sarkar et al 1986) and a Rb- 973  63 Ma in mafic granulites (Abhijit Roy, pers. Sr whole rock isochron age of more than 2200 Ma comm.). for Amgaon gneiss (Sarkar et al 1981). A tentative It is thus concluded that the BBG belt most retrogression age of 1300 Ma from hornblende in likely represents a part of the Bastar Craton, the the two-pyroxene granulite from BBG belt through gneiss-supracrustals sequence of which underwent Ar/Ar method has been quoted in Yedekar and polycyclic prograde granulite metamorphism prior Jain (1995- from unpublished NSA, USA data). to the evolution of the Sausar supracrustals. The A younger age of 860 Ma from K-Ar systemat- granulite event in the more southerly Bhopalpat- ics for the Tirodi gneiss has also been obtained nam granulite belt in the southwestern part of (Sarkar et al 1967). Recently, a cooling age of the Bastar Craton, as can be judged from its cor- Evolution of the Bhandara-Balaghat granulite belt 367 relation with the Karimnagar belt, occurred at Irvine T N and Baragar W R A 1971 A guide to the chemical 2600 Ma (Rajesham et al 1993). The older Sm- classification of the common volcanic rocks; Can.J.Earth Nd age of 2672  54 Ma for charnockite in the Sci. 8 523–548 Jensen L S and Pyke D R 1982 Komatiites in the Ontario BBG belt may be correlated with this event as portion of the Abitibi belt In: Komatiites. Geol. (eds) representing a peak-metamorphic event within the N T Arndt and E G Nesbitt (Allen and Unwin, London) BBG belt. It is not known if the post-peak Sm- Pp. 147–157 Nd (also granulite facies) age of 1416  59 Ma Jain S C, YedekarDBandNairKKK1991 Central Indian and Rb-Sr age of 1380  28 Ma in two-pyroxene Shear zone:Amajorprecambrian crustal boundary; J. Geol. Soc. India 37 521–531 granulites of the BBG belt can be correlated JainSC,NairKKKandYedekar D B 1995 Geology of with the younger Eastern Ghat granulite event at the Son-Narmada-Tapti lineament zone in central India; 1400–1600 Ma. In: Geoscientific studies of the Son-Narmada-Tapti linea- ment zone. Sp. Pub. 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