Cordierite: An indicator of rapid exhumation of Mesoproterozoic Gneissic Complex of Meghalaya, NE Indian

Exhumation rate of deeper crustal rocks helps to understand the tectonic scenario and weathering processes of crustal development (Burbank and Anderson, 2001). Rate of exhumation is calculated from the change in the lithostatic pressure over a period of time. Cordierites from the metapelites of Meghalaya Genissic Complex of NE India are used to calculate the rate of Mid Proterozoic crustal exhumation. Cordierite is the most common mineral within the metasediments (cordierite gneiss) of basement gneissic complex of Meghalaya. In this rock it is associated with garnet, sillimanite, hercynite, k-feldspar, plagioclase and quartz. Cordierites of two modes are identified as product of prograde and retrograde metamorphism. The subidioblastic prograde cordierites grew at the expense of biotite, sillimanite and quartz and the late cordierite is formed from the breakdown of early garnet porphyroblasts and it is also reflected by two different compositional clustering at different bivariate plots. Stabilisation of garnet-cordierite-sillimanite-spinel and quartz records the peak metamorphic assemblage which correspond to granulite facies metamorphism. Temperature68 pressure calculated from garnet-cordierite pair records 805 to 9250C and 6.1-7.4 Kbar. P-T condition of garnet breakdown to coriderite records 626 to 6560C temperature and 3.3 to 3.6 Kbar of pressure. The drop of pressure from ~7 to 3.5 Kbar can be correlated to the crustal uplift of ~10 km. The rate of change of pressure (~3.5Kbar) over the temperature (~2000C) is relatively high and corresponds to decompression. Chaterjee et al., 2007 has reported three thermal peaks ca. 1500 Ma, 1000Ma and 500Ma from the Meghalaya gneissic Complex and also suggested a post peak decompression after 1.5 Ga. Peak metamorphism at a depth of ~20km might have occurred in a collisional orogen by crustal thickening and exhumation of the crust to ~10 km depth was likely caused by movement along thrusts at a rapid rate. The two events have time equivalence with Grenvillian Orogeny.