Long lived high-grade metamorphism in southern India: Constraints from charnockites and sapphirine bearing semipelitic granulites from the Madurai Block

The granulite terrane of southern India is a collage of Mesoarchean-Neoproterozoic crustal blocks that underwent high-grade metamorphism associated with the final assembly of Gondwana supercontinent during late Neoproterozoic-Cambrian. Here, we investigate the charnockites and associated sapphirine-bearing semipelitic granulites from the eastern part of Madurai Block. We present new petrographic, mineral chemistry and geochronological data to constrain the P-T-t evolution of the block and unravel the time scale and source of heat for the ultrahigh-temperature metamorphism. Both the rock types contain coarse grained porphyroblastic garnet and orthopyroxene yielding peak P-T conditions of 950±40°C at 9.5±0.8 kbar, using conventional thermobarometry. Peak ultrahigh temperatures are further supported by high Al content in the orthopyroxene and feldspar thermometry of the mesoperthites in the semipelite, yielding 945±20°C. Subsequent decompression-cooling has led to formation of coronal orthopyroxene and plagioclase in the charnockite and a series of symplectic assemblages (Opx2 + Pl2, Spr + Pl2, Opx2+Crd) in the semipelite, yielding P-T range of 920-820°C and 8.8-7.5 kbar. Based on the obtained P-T estimates, preserved reaction textures and phase equilibria modelling in the MnNCKFMASHTO system, a clockwise P-T evolution with decompression- cooling in the suprasolidus conditions is inferred. Texturally controlled in-situ monazite dating and rare-earth element (REE) pattern suggests Th-poor and extremely HREE depleted cores of matrix monazite, having an age range from 630-580 Ma, grew during the prograde evolution. On the other hand, Th-rich and HREE depleted rims of matrix monazite, recording an age range of 570-535 Ma, dates the peak UHT metamorphism. Compositionally homogenous HREE+Y enriched monazite in the symplectite having an age range of 570-450 Ma, date the subsequent decompression. Our results are suggestive of a long-lived hot orogeny in the Madurai Block, where the UHT conditions were sustained for at least 50 Ma. The UHT conditions were likely achieved by conductive heating through decay of radioactive elements at the core of a long-lived hot orogenic plateau. Subsequently, these rocks were rapidly uplifted to shallower crustal depth during late Cambrian to early Ordovician.