Ultra-high temperature metamorphism of metapelitic granulites from Kondapalle, Eastern Ghats Belt: implications for the Indo-Antarctic correlation

Abstract

A suite of quartz- and corundum-bearing metapelitic granulites, intruded by layered gabbronorite-pyroxenite-anorthosite at Kondapalle, Eastern Ghats Belt, preserves a multitude of reaction textures involving oxide and silicate minerals that attest to several prograde and retrograde reactions. In the quartz-bearing associations, the reactions are: (a) biotite + sillimanite + quartz →garnet + liquid; (b) garnet + sillimanite →spinel (+ magnetite) + quartz; (c) Fe₂TiO₄ + O₂ →ferrian ilmenite + magnetite; (d) reversal of reaction (b); (e) Fe₂O₃-rich ilmenite + plagioclase + quartz →Fe₂O₃-poor ilmenite + garnet + O₂. Reactions in the corundum-bearing associations are: (f) spinel + biotite + sillimanite →garnet + liquid; (g) biotite + sillimanite →garnet + Ti-rich spinel + corundum + liquid; (h) biotite + sillimanite →garnet + corundum + liquid; (i) Fe₂TiO₄ + FeAl₂O₄ + O₂ →ferrian ilmenite + Fe₃O₄ + Al₂O₃ (in ilmenite); (j) garnet + corundum →spinel + sillimanite. To examine the paragenetic evolution of the metapelitic granulites, a petrogenetic grid for the KFMASH system at high temperatures and pressures, involving quartz and corundum, was constructed. The sequence of inferred reactions documents an anticlockwise heating-cooling path. Reintegrated compositions of spinel (with >10 mol % Fe₂TiO₄) and feldspars indicate ultra-high temperature (UHT) of metamorphism (>1000°C), comparable with the liquidus temperature of the enclosing magmatic rocks. Crystallization pressures inferred for the magmatic rocks and the pressure constraints imposed by the petrogenetic grid on the metapelite assemblages indicate that the emplacement of the igneous suite and the accompanying UHT metamorphism occurred in the lower crust (>8 kbar). Reported U-Pb cooling ages of monazite and allanite from a late pegmatite suggest the UHT metamorphism to be older than 1600 Ma. The deduced P-T history and the absence of Grenvillian high-grade metamorphism in the study area provide important constraints on the configuration of East Gondwana, in particular on the continuation of the Napier-Rayner terrane boundary into the Eastern Ghats Belt.