Multistage metamorphic evolution of the Trivandrum Granulite Block, southern India

The Trivandrum Granulite Block (TGB), southern India records evidence for three distinct stages of evolution (M1-M3) during the Pan-African high grade metamorphism with possible temperature gradient from north to south of the terrain as detected from mineral phase equilibria thermobarometry in three classic localities, namely Nuliyam, Kunnanpara and Nellikkala. The charnockites, both incipient and massive, were formed during the First stage (M1) at temperatures higher than their host rocks, and at appreciably lower pressures. Charnockite formation was dominantly controlled by an increase in partial pressure of CO,, along structural locales during subisothermal decompression, although an increase of potash activity could have also been an important factor in this process. The charnockites at Nellikkala in the northern margin of TGB were formed under appreciably more H2O-rich conditions (X-H2O = 0.53 +/- 0.03) than those at Nuliyam (X-H2O = 0.25 +/- 0.02) in the southern margin. It is inferred that during the period between the metamorphic stages M1 M2, the terrain experienced subisobaric cooling. Comparison of results from thermobarometry with data on absolute age determinations from geochronology of the metamorphic rocks in TGB allows the interpretation that the M1 metamorphic event took place during 540-600 Ma, M2 at about 530 Ma and M3 in the interval of 440-470 Ma. Mineralogic and thermobarometric evidence for earlier high-grade metamorphic processes, if any, have been erased from these rocks. The processes of charnockite formation and post-peak retrograde metamorphism in the TGB took place under high geothermal gradients (40-150 degrees/km). This probably testifies to the existence of a local heat source, either magmas at depth or mantle (plume) beneath the region. The general metamorphic cycle in the TGB is estimated to be ca. 100-160 Ma, which is much shorter in time span than that in the other regions of southern Peninsular India such as the Kamataka Craton and the Eastern Chats Mobile Belt. During this period, the terrain experienced rapid exhumation of approximately 6-7 cm/year, with the total amplitude of vertical movements estimated to be about 16-17 km.