Prismatine and Sapphirine Bearing Assemblages from Rajapalaiyam Area, Tamil Nadu: Origin and Metamorphic History

Sapphirine and kornerupine bearing assemblages in Mg-AI- rich pelites have been reported in and around the Kodaikanal hills and in the Kambam valley, forming part of the 550 Ma old Southern Granulite Terrain (SGT). Prismatine and sapphirine are found in a new association with garnet-orthopyroxene-cordierite-spinel-plagioclase-perthite bearing pelitic enclaves in granitic bodies intruding the charnockites at Rajapalaiyam, located on the southeastern slopes of the Varushanad hills and east of the Kambam valley. Prismatine with boron > 0.5 apfu is either associated with orthopyroxene or armoured by cordierite, suggesting its early formation. Association of prismatine with orthopyroxene and quartz is significant. Boron for the formation of prismatine must have come from the sedimentary protoliths, and fixed in the prismatine during fluid activity connected with earlier granulite grade metamorphism. Sapphirine bearing assemblages occur as mm-sized specks in garnetiferous granite close to the pelitic bands. This type of sapphirine is different from the general occurrence of sapphirine-orthopyroxene-cordierite forming as a breakdown product of garnet in SGT. Sapphirine bearing assemblages of SGT generally have formed during high grade, granulite facies metamorphism, with peak at 850-1000°C at P of 8.2-8.8 Kbar and with a dominant isothermal decompression path as shown by the assemblages orthopyroxene-cordierite+-plagioclaswe hich give T of 650°C at P of 6.5 Kbar. Orthopyroxene-cordierite association after garnet gives T of 900°C at P of 7.5 Kbar in Rajapalaiyam area.