GEOTHERMOBAROMETRY OF MAFIC GRANULITES AND METAPELITE FROM THE PALGHAT-GAP, SOUTH-INDIA - PETROLOGICAL EVIDENCE FOR ISOTHERMAL UPLIFT AND RAPID COOLING

The Palghat Gap region is located near the centre of the large souther n Indian granulite terrane, at the northern edge of the Kodaikanal cha rnockite massif. The dominant rock types in the region are hornblende- biotite +/- orthopyroxene gneisses and charnockites along with minor a mounts of intercalated mafic granulite, metapelite and calc-silicate. The P-T estimates from garnetiferous mafic granulites and metapelite s amples are generally in the range 9-10 kbar and 800-900-degrees-C usin g both conventional thermobarometric methods and the TWEEQU thermobaro metry program. These P-T estimates, which should be taken as minimum v alues, are among the highest yet reported for South Indian and Sri Lan kan granulites. The occurrence of orthopyroxene + plagioclase symplect ites around embayed garnet grains in the mafic granulites and cordieri te rims around garnet grains in metapelite suggest an isothermal decom pression-type path. Similarly, a core-rim P-T trajectory indicates c. 3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite, respectively. In both rock types, the key to the dete rmination of the retrograde P-T path was the recognition of small amou nts of second generation plagioclase with a more anorthitic compositio n than the matrix plagioclase. The preservation of high garnet-pyroxen e temperatures in the mafic granulites (despite small garnet grain siz e) suggests rapid cooling of the terrane. Calculated minimum cooling r ates range from 8 to 80-degrees-C Ma-1. Such cooling rates are more ra pid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature.