Petrography, Mineral Chemistry and Geothermobarometry of the Inukurti Anorthosite Complex and Associated Rocks from the Nellore Schist Belt, Andhra Pradesh

摘要

The Inukurti Anorthosite Complex (IAC) forms a small elliptical dome (1.5 * 1.0 km) within the Archaean metamorphosed volcano-sedimentary sequences of the Nellore Schist Belt (NSB). The IAC comprises dominantly of anorthositic rocks, which are cut by mafic sills and dykes; volumetrically insignificant lenses and bands of diorite occur along its periphery. All the rocks are deformed and metamorphosed under amphibolite-facies conditions, but exhibit relict magmatic textures despite metamorphic recrystallisation and deformation. The minerals from anorthosites, diorites and amphibolites exhibit compositional characteristics distinct to each group. Garnet-hornblende-plagioclase-quartz equilibria form the IAC indicate a narrow range of metamorphic equilibration temperature between 644 and 570 ℃, but a broad range of pressure between 7.6 and 2.7 kbar. Geothermobarometry on zoned garnet porphyroblasts indicates that the rocks of the IAC were subjected to decompression of the order of about 5 kbar from 7.6 to 2.7 kbar related to rapid exhumation.