Metamorphic evolution of metapelitic granulites in the south of Riangdo west Khasi hills district Meghalaya

Abstract

The present research work is an attempt towards an understanding of metamorphic evolution of the granulite facies rocks exposed around Riangdo, West Khasi Hills District, Meghalaya, comparatively a least known Proterozoic terrain in India. The chief goal of the study is to understand the metamorphic evolution of the metapelitic rocks through petrography, mineral chemistry and PT calculations based on mineral chemistry data and conventional geothermobarometric formulation. The area is characterized by interlayered and cofolded cordierite-garnet-biotite-sillimanite-gneiss, quartz sillimanite schist, massive sillimanite corundum rock, and non garnetiferous basic granulite within granite gneiss. These rocks give clear indication of multiphase deformations (D1, D2 and D3 characterized by F1, F2 and F3 foldings respectively) and polymetamorphic events. These include M2 event, the pre-S1, (peak metamorphic event) characterized by granulite facies and high temperature melting; M3, the syn-S1 fabric forming event and M4 corona forming event. The rare inclusions (fibrolite + biotite + quartz + spinel) within cordierite (M2) and rarely garnet (M2) as identified on the basis of textural study represent the earliest high grade assemblage (M1) of prograde evolutionary PT path. Our estimate of peak metamorphic temperature for (M2) mineral assemblages show a temperature ~ 8200C which is comparable with the temperature range of granulite facies metapelite as advocated by Chatterjee et al (2007). However all exchange geothermometers as for example garnet-cordierite geothermometer shows an unrealistic temperature range for M2 metamorphic episodes. This is due to the fact that M2 minerals re-equilibrated during subsequent M3 and M4 metamorphic episodes. Ti in biotite geothermometer indicates a temperature range of 7000 to 7400 C for matrix biotite (M3) assemblages. Temperature for M4 assemblage estimated from the coronitic garnet and adjacent garnet reveals a temperature range between 6200 and 6600 C. One of the important features...