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Mineralogy and Petrology (2007) 90: 175197
DOI 10.1007/s00710-006-0174-0Printed in The Netherlands
I. N. Sharma and D. Prakash
Department of Geology, Banaras Hindu University, Varanasi, India
Received February 28, 2006; revised version accepted October 30, 2006 Published online January 30, 2007; # Springer-Verlag 2007 Editorial handling: R. Abart
Summary
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, mac granulites, orthopyroxenecordierite 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 reect 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:
Bt Sil Qtz Grt Crd Kfs V
Grt Qtz Opx CrdOpx Sil Qtz CrdCrd Opx Kfs V Bt Qtz
Reaction textures and PT conditions of Opx-Crd gneisses from Karimnagar, Andhra Pradesh, India
176 I. N. Sharma and D. Prakash
Based on chemographic relationships and petrogenetic grids in the K2OFeOMgO Al2O3SiO2H2O (KFMASH) system, a sequence of prograde (early stage), isothermal decompression (middle stage) and retrograde (late stage) reactions (back reactions and hydration reactions) are inferred. Relatively lower PT estimates (0.35 GPa=550750 C) obtained from the different geothermobarometers are attributed to late FeMg reequilibration during cooling. Therefore, the convergence method has been applied to retrieve simultaneously the PT conditions of the thermal peak of metamorphism. The near thermal peak condition of metamorphism estimated by the convergence method are 850 C=0.62 GPa. The PT estimates dene a retrograde trajectory with substantial decompression.
Introduction
The Karimnagar granulite belt, about 150 km NNE of Hyderabad, Andhra Pradesh, is situated in the north-eastern part of the Eastern Dharwar Craton (Fig. 1a, b). Enclaves of mac granulites, cordierite-garnet gneisses ( orthopyroxene or sillimanite) and quartz-absent...