The Nagercoil Charnockite: a Magnesian, Calcic to Calc-alkalic Granitoid Dehydrated during a Granulite-facies Metamorphic Event

The Paleoproterozoic Nagercoil charnockite massif in southern India consists of garnet-absent and garnet-bearing varieties, with garnet occurring only in those areas where the charnockite incorporates supracrustal lithologies. There is a late Neoproterozoic-Cambrian granulite-facies metamorphic overprint on the charnockite massif This study examines whether orthopyroxene in the charnockites crystallized during the original magmatic stage or formed during the later high-grade metamorphic event. The textural association of orthopyroxene and relic garnet grains, and reaction intergrowths of garnet and/or biotite with orthopyroxene, similar to those reported in dehydration zones within the gneisses surrounding the charnockite massif, show that orthopyroxene in the charnockite massif formed during the late high-grade metamorphic event from dehydration reactions and is not part of the original igneous assemblage. Metasomatic textures such as thin K-feldspar rims, together with the presence of CO2-rich fluid inclusions in these rocks; suggest that CO2 metasomatism, widely prevalent in the gneisses surrounding the massif, probably resulted in the low water activity necessary to firm orthopyroxene during the metamorphic overprint. An oxygen isotope traverse along a metapelite enclave charnockite contact within the massif shows a distinct variation in delta O-18 values, with the charnockite away from the contact having the lowest delta O-18 value (+8.9 parts per thousand) similar to that of typical metaluminous granitic rocks. The charnockite at the contact shows an intermediate 8180 value (+10.3 parts per thousand) and the surrounding metapelite has the highest delta O-18 value (+12 parts per thousand), the intermediate delta O-18 value indicates partial assimilation of metapelite with a high delta O-18 value into the charnockite with a low delta O-18 value. Both the garnet-absent and garnet-bearing charnockites have similar compositional ranges to tonalite-granodiorite-granite (TTG) suites. Irrespective of whether garnet is present or not, the Sr contents of the charnockites show a bimodal distribution: a high-Sr group, which is magnesian, calcic to calc-alkalic, metaluminous, showing TTG affinities, and a low-Sr group, which is dominantly ferroan, calcic to calc-alkalic, metaluminous, showing similarities to the orthopyroxene-bearing dehydration zones. Petrogenetic characterization suggests that the protoliths of the Nagercoil charnockites were high-temperature melts of hydrous basalt.