Geobarometry, Geothermometry, and Late Archean Geotherms from the Granulite Facies Terrain of South India

Geobarometry and geothermometry of garnet-bearing chamockitic and metapelitic assemblages from the high-grade terrain from South India indicate three fields of equilibration. High pressure granulites ($$P = 8.3 \pm 1.0 kb, T = 760 \pm 40^{\circ}C$$) are restricted to the charnockites and mafic granulites of the north slopes of the Nilgiris. These place a minimum constraint of ~30 km on the maximum thickness of late Archean crust. Medium pressure granulites ($$P = 6.4 \pm 1.0 kb, T = 735 \pm 40^{\circ}C$$) characterize the charnockites of the central Nilgiris and the Madras granulites to the east. Low pressure granulites ($$P = 5.0 \pm 1.0 kb, T = 700 \pm 20^{\circ}C$$) characterize the charnockites and metapelites south of the Bhavani shear zone, although migmatites from the Kodaikanal massif may reflect somewhat higher temperatures. All three granulite fields lie on a metamorphic geotherm (piezothermic array) which is strongly convex to the temperature axis and characteristic of convective heat transfer. The P/T fields encountered in the high grade terrain are therefore consistent with metamorphism induced by a $$CO_{2}$$-rich vapor phase rising from the mantle.