Reaction textures and P–T conditions of Opx-Crd gneisses from Karimnagar, Andhra Pradesh, India

Abstract

The Karimnagar granulite terrain is an integral part of the Eastern Dharwar Craton (EDC). It has received much interest because of the only reported granulite facies rocks in the EDC. These granulites contain quartz-free sapphirine-spinel-bearing granulites, kornerupine – bearing granulites, mafic granulites, orthopyroxene-cordierite gneisses, charnockites, amphibolites, dolerite dykes, granite gneisses, quartzites and banded magnetite quartzite. The orthopyroxene-cordierite gneisses occur as enclaves within granite-gneiss in association with banded magnetite quartzites, charnockites and amphibolites. The observed reaction textures, spectacular as they are, have an extraordinary information content within a tiny domain. Coronas, symplectites and resorption textures are of particular interest as they reflect discontinuous or continuous reactions under changing physical conditions. The main mineral assemblages encountered in these gneisses are orthopyroxene – cordierite – biotite – plagioclase – perthite – quartz and garnet – orthopyroxene – cordierite – biotite – quartz – plagioclase – perthite ± sillimanite. Multiphase reaction textures in conjunction with mineral chemical data in the KFMASH system indicate the following reactions: \({\rm Bt} + {\rm Sil}+{\rm Qtz} = {\rm Grt} + {\rm Crd} + {\rm Kfs} + {\rm V}\) \({\rm Grt} + {\rm Qtz} = {\rm Opx} + {\rm Crd}\) \({\rm Opx} + {\rm Sil} + {\rm Qtz} = {\rm Crd}\) \({\rm Crd} + {\rm Opx} + {\rm Kfs} + {\rm V}= {\rm Bt} +{\rm Qtz}\)