JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 PAGES 387±420 2003 Garnetiferous Metabasites from the Sausar Mobile Belt: Petrology, P±T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone SANTANU KUMAR BHOWMIK* AND ABHINABA ROY REGIONAL PETROLOGY LABORATORY, GEOLOGICAL SURVEY OF INDIA, NAGPUR-440 006, INDIA RECEIVED JUNE 20, 2001; REVISED TYPESCRIPT ACCEPTED AUGUST 29, 2002 A suite of garnetiferous amphibolites and mafic granulites occur of extension, marked by mafic dyke emplacement. The combined as small boudins within layered felsic migmatite gneiss in the P±T path of evolution has a clockwise sense and provides northern part of the Sausar Mobile Belt (SMB), the latter evidence for a major phase of early continental subduction in constituting the southern component of the Proterozoic Central parts of the CITZ. This was followed by a later continent± Indian Tectonic Zone (CITZ). Although the two types of meta- continent collision event during which granulites of the first basites are in various stages of retrogression, textural, composi- phase became tectonically interleaved with younger lithological tional and phase equilibria studies attest to four distinct units. This tectonothermal event, of possibly Grenvillian age, metamorphic episodes. The early prograde stage (Mo) is repre- marks the final amalgamation of the North and the South Indian sented by an inclusion assemblage of hornblende1 ilmenite1 Blocks along the CITZ to produce the Indian subcontinent. plagioclase1 Æ quartz and growth zoning preserved in garnet. The peak assemblage (M1) consists of porphyroblastic garnet clinopyroxene Æ quartz Æ rutile Æ hornblende in mafic KEY WORDS: Central Indian Tectonic Zone; clockwise P±T path; granulites and garnet quartz hornblende in amphibolites continental collision; metabasite and stabilized at pressure±temperature conditions of 9±10 kbar and 750±800C and 8 kbar and 675 C, respectively. This was followed by near-isothermal decompression (M2), and post- decompression cooling (M3) events. In mafic granulites, the INTRODUCTION former resulted in the development of early clinopyroxene2A± In recent reconstructions of East Gondwanaland, the hornblende2A±plagioclase2A symplectites at 8 kbar and 775 C Central Indian Tectonic Zone (CITZ; Radhakrishna, (M2A stage), synchronous with D2 and later anhydrous clin- 1989) has been recognized as an important Proterozoic opyroxene2B±plagioclase2B±ilmenite2B symplectites and coronal collisional zone (Yedekar et al., 1990; Jain et al., 1991; assemblages at 7 kbar, 750 C(M2B stage) and post-dating D2. Mishra et al., 2000). Existing models predict amal- In amphibolites, ilmenite plagioclase quartz Æ horn- gamation of the South Indian Block [SIB; terminol- blende symplectites appeared during M2 at 6Á4 kbar and 700 C. ogy after Eriksson et al. (1999); comprising the During M3, coronal garnet clinopyroxene quartz Æ Singhbhum, Bastar and Dharwar Provinces] and the hornblende-bearing symplectites in metabasic dykes and North Indian Block (NIB; comprising the Aravalli± hornblende3±plagioclase3 symplectites embaying garnet in Bundelkhand Provinces) during the Palaeoproterozoic mafic granulites were formed. P±T estimates show near-iso- by southerly subduction of the NIB below the SIB baric cooling from 7 kbar and 750C to 6 kbar and 650C during along the Central Indian Suture (CIS) to form the M3. It is argued that the decompression in the mafic granulites Indian subcontinent (Yedekar et al., 1990; Jain et al., is not continuous, being punctuated by a distinct heating 1991; Eriksson et al., 1999; Mishra et al., 2000). The (prograde?) event. The latter is also coincident with a period timing and the location of collision have been modelled *Corresponding author. Present address: Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 Journal of Petrology 44(3) # Oxford University Press 2003; all rights 302, India. E-mail: [email protected] reserved. JOURNAL OF PETROLOGY VOLUME 44 NUMBER 3 MARCH 2003 with its transcontinental status (Harris, 1993), has obvious global significance in understanding the geodynamic processes of crustal assembly in major parts of East Gondwanaland. This study focuses on a suite of regionally distributed mafic granulites and associated garnetiferous amphi- bolites from the northern periphery of the Sausar Mobile Belt (SMB), the latter constituting the south- ernmost Meso- to Neoproterozoic component of the CITZ (Acharyya & Roy, 2000). The metabasites occur as small boudins and tectonic lenses within inten- sely tectonized felsic gneisses and bear imprints of polyphase deformation and a strong amphibolite-facies overprint. Although polymetamorphic histories and P±T paths have been extensively documented from rare residual domains of high Mg±Al bulk composition in granulites (Currie & Gittins, 1988; Droop, 1989; Harley et al., 1990; Dasgupta et al., 1995), similar polyphase histories have seldom been retrieved from mafic protoliths (e.g. Harley, 1985). The latter are often restricted to high-P granulite (Mengel & Rivers, 1991; Thost et al., 1991; Zhao et al., 2000, 2001) and eclogite-facies domains (O'Brien, 1997; Dirks & Sithole, 1999). In this study, we present detailed textural, mineral- chemical and P±T data for mafic granulites, garneti- ferous amphibolites and associated metabasic dykes to constrain the metamorphic history of the southern seg- ment of the CITZ. Despite pervasive amphibolite- facies tectonothermal reworking, these data document an early high-grade history and define a complex P±T trajectory. The results not only place important con- Fig. 1. Distribution of different lithotectonic components in the Central Indian Tectonic Zone (CITZ) between the North Indian straints on the models for lower crustal geodynamic Block (NIB) and the South Indian Block (SIB) (see text for termino- processes in the SMB but also provide insights into logy). The CITZ is demarcated by the Son±Narmada North Fault multistage exhumation histories of deep crustal rocks. (SNNF) to the north and the Central Indian Shear (CIS) to the south. The box shows the location of the Sausar Mobile belt (SMB) in the CITZ (detailed map shown in Fig. 2). The inset shows the location of CITZ in India. SNSF, Son±Narmada North Fault; BBG, Bhandara±Balaghat granulite. GEOLOGICAL SETTING The CITZ encompasses the terrain between the Son± Narmada North Fault (SNNF) and the Central Indian only on the basis of limited geological and geophysical Shear Zone (Acharyya & Roy, 2000) (Fig. 1). The data from the mobile belt (Yedekar et al., 1990; Jain SMB is a collage of at least three lithotectonic et al., 1991; Mishra et al., 2000). Detailed metamorphic components (Bhowmik & Pal, 2000): (1) the southern characterization of the rocks of the major lithotectonic Bhandara±Balaghat granulite (BBG) domain; (2) the units of the CITZ, without which any tectonic model central domain of the Sausar Group (SG) of rocks; for the mobile belt remains incomplete, has not been (3) the northern Ramakona±Katangi granulite done. Recent studies have revealed the composite (RKG) domain (Fig. 2). These domains differ in litho- character of the CITZ, comprising three distinct logical assemblage, mineralogy and metamorphic supracrustal belts, Mahakoshal, Betul and Sausar history. (Fig. 1), which have a tectono-magmatic history, The BBG domain is bounded to the north by the low- from the Palaeoproterozoic to the Neoproterozoic grade Sausar Group of rocks and to the south by (Acharyya & Roy, 2000). This implies a more complex the cratonic domain of felsic gneisses of the Amgaon evolutionary history for the CITZ, compared with gneissic complex. The BBG domain is lensoidal in existing monocyclic models. Nevertheless, the CITZ, shape, tapering towards both east and west (Fig. 2). 388 BHOWMIK AND ROY METABASITES FROM SAUSAR MOBILE BELT Fig. 2. Geological map of the Sausar Mobile Belt in central India, showing the different lithotectonic domains (after Bhowmik & Pal, 2000). The components south of the Central Indian Shear Zone (CIS) constitute the cratonic domain (compare the South Indian Block) with respect to the SMB. Also shown are the locations of the granulite occurrences in the Ramakona±Katangi granulite (RKG) domain (see text). The mafic granulites occur in the Khawasa (location 2), Pindrai (location 4) and Katangi areas (location 5) and the garnetiferous amphibolites are located in the Katangi area. The granulite suite includes cordierite gneiss (Crd Gn), greenschist±amphibolite facies transition zone has iron formation granulite, quartzite, charnockitic been recognized along the southern part of the Sausar gneiss, two-pyroxene granulite, pyroxenite and ender- supracrustals, in close proximity to the granulites of bitic granulite; they occur as pods and lenses within the BBG domain; an upper amphibolite- to upper intensely tectonized felsic gneiss, which is predomi- amphibolite±granulite-facies transition zone has been nantly mylonitic, with strong development of down- recorded in the northern and northwestern margin of dip stretching lineations. The granulites of the BBG the central domain and also from the RKG domain. domain record an early high-temperature granulite In the RKG domain, along the northern periphery metamorphism (T 900C) and strong retrograde of the SMB, a distinct lithological assemblage, includ- metamorphism dominated by both cooling and decom- ing mafic granulite, cordierite granulite and porphyri- pression textures (Bhowmik & Pal,