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 form orthopyroxene during the metamorphic overprint. An oxygen isotope traverse along a metapelite enclave-charnockite contact within the massif shows a distinct variation in δ¹⁸O values, with the charnockite away from the contact having the lowest δ¹⁸O value (+8·9() similar to that of typical metaluminous granitic rocks. The charnockite at the contact shows an intermediate δ¹⁸O value (+10·3() and the surrounding metapelite has the highest δ¹⁸O value (+12(); the intermediate δ¹⁸O value indicates partial assimilation of metapelite with a high δ¹⁸O value into the charnockite with a low δ¹⁸O 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.