1, metamorphic zyxwvutsrqpo Geol., zyxwvuts 1993, zyxwvutsr 11, 867-878 zyxwvuts Crustal trajectory of sapphirine-bearing granulites from Ganguvarpatti, South India: evidence for an isothermal decompression path zyxw A. MOHAN' AND B. F. WINDLEY' 7 Department of Geology, Banaras Hindu University, Varanasi-221 &IS, India 'Department of Geology, Leicester University, Leicester LEI 7RH, UK ABSTRACT zyxwvutsrqpo Ganguvarpatti is part of a Precambrian terrane characterized by granulite facies rocks, including charnockites, mafic granulites, sapphirine-bearing granulites, leptynites and gneisses. A sequence of reactions deduced from the multiphase reaction textures provide information on the metamorphic history of this area, as they formed in response to decompression during uplift. Geothermobarometiy and constraints from reaction textures define a segment of a P-T path traversed by the granulites of Ganguvarpath. Near-peak metamorphic conditions of c. 800°C and 8 kbar were succeeded by a symplectitic stage at a significantly lower pressure (c. 700" C and 4.5 kbar), documenting a nearly isothermal decompression P-T path and rapid uplift (c. 12 km) followed by cooling. The presence of many fluid inclusions of extremely low density in the charnockites is consistent with a nearly isothermal uplift path. Attainment of a maximum pressure of c. 8 kbar indicates c. 27 km depth of burial during metamorphism. This would imply a total crustal thickness of c. 65-70 km at 2.6-2.5 Ga. Such a profound crustal thickness and a clockwise decompressive P-T path is interpreted as a consequence of tectonic thickening of crust, accomplished by collision tectonics of the southern granulite terrane against the Dharwar craton along the Palghat-Cauvery shear zone via northward subduction. Key words: decompression; P -T path; reaction textures; sapphirine granulites; South India. INTRODUCTION The Southern Indian Shield is one of the classic terranes of the early crust and may be regarded as a model of Archaean deep crustal evolution. The southern granulite belt is essentially a high-grade charnockite-khondalite terrane comprising granulite facies rocks. The granulite facies metamorphism occurred at c. 2.6-2.5 Ga (Griffiths et al., 1987) and closely followed the emplacement and deposition of protoliths. This contension gained support from the recent age data of Peucat et al. (1989), indicating that the time lapse between the crust-forming event and subsequent metamorphism was short (c. 50-100 Ma). The importance of the Ganguvarpatti area, constituting a portion of the southern granulite belt (Fig. zyxwvut l), stems mainly from its abundant sapphirine-bearing assemblages (Grew, 1982; Mohan et al., 1986). Based on textural relationship and p-y diagrams, Mohan er al. (1986) proposed a retrograde metamorphic trajectory. However, in the absence of evidence for metasomatism, Hensen (1988) showed that a better interpretation of the textures was one involving decompression. Sapphirine-bearing granulites, massive charnockites, mafic granulites, gneisses and leptynites from the study area represent deep levels of the Archaean crust. The metamorphic evolution of regional granulite terranes, which are exhumed portions of ower-middle Crust, is a key problem of today (Bohlen. 1991). Application of geothermobarometry in the southern granulite belt has greatly contributed to our understanding of peak metamorphic conditions (Raith ef al., 1983, 1990; Mohan & Lal, 1985; Chacko el al., 1987; Rao et al., 1991). Nevertheless, only a comprehensive study of the record of the distinct segments of the P-T path registered in these metamorphic rocks will shed light on their evolution. The aim of this paper is to investigate the likely P-T path experienced by the Ganguvarpatti granulites when they underwent burial and exhumation during regional metamorphism. The P -T path deduced here from reaction textures is supported by petrological data, therrno- barometry, fluid inclusion studies and available geochro- nological and geophysical data. Most significant are sapphirine-bearing granulites, because they retain evidence of their early history through arrested textures (Windley el af., 1984; La1 et al., 1987; Droop, 1989). Thus, in an attempt to unravel the complex history of the lower crust, the investigation of sapphirine-bearing and associated granulites can provide key evidence for the crustal trajectory. TEXTURAL RELATIONS The Ganguvarpatti granulites display various disequi- librium reaction textures, including coronas, as well as symplecitites or resorption textures that are more 867