Decompressional P-T history in sapphirine-bearing granulites from Kodaikanal, southern India

Abstract

The sapphirine-bearing granulites exposed around the Kodaikanal region of the Madurai block, southern India, present a variety of mineral parageneses involving garnet, spinel, sillimanite, cordierite, orthopyroxene, phlogopite, potash feldspar and plagioclase. interpretation of multiphase reaction textures in conjunction with mineral chemical data and topology in the (FM)AS system is consistent with the main sapphirine-forming reactions: orthopyroxene + sillimanite = sapphirine + cordierite phlogopite + sillimanite = sapphirine + cordierite + K-feldspar + vapour Mg-tschermak = sapphirine + cordierite. The P-T evolution of these sapphirine granulites has been constrained through the use of conventional geothermobarometry, internally consistent TWEEQU programme and thermodynamically calibrated MAS equilibria. The P-T estimates define a retrograde trajectory with substantial decompression of c.4 kbar from P-T max of c.8 kbar at c.800°C. On the basis of the available evidence for a Pan-African granulite-facies event in the Madurai and Trivandrum blocks, the emerging concept of East Gondwana assembly is endorsed.