Petrology and Geochemistry of the Oddanchatram Anorthosite and Associated Basic Granulites, Tamil Nadu, South India

Preliminary studies of the Oddanchatram anorthosite body, the largest of the bodies lying along a 100 km linear; on the northern slopes of Patni hilI ranges, show that it has all the chemical and mineralogical characters of Proterozoic massif anorthosites. Plagioclase compositions reveal two types: a) primary plagioclase of An₆₀ and b) secondary plagioclase with An₉₄ occurring as breakdown product of garnet. Estimates based on primary pyroxenes indicate intrusive temperatures between 960°C-1000°C; pressures of 5.3 kb. based on garnet breakdown products suggest a steep geothermal gradient. The chemistry suggests that the parental magma was unusually feldspathic; primary hornblende indicates a higher H₂O content of the magma.