A geochemical perspective on the petrogenesis of charnockites from the western parts of the Palghat-Cauvery Shear Zone, southern India: implications for collisional geodynamics

The Earth is a dynamic planet that has been evolving ever since it was formed. The formation of protocontinents and their amalgamation to supercontinents and later dispersals are one of the fascinating geologic events during the course of the evolution of Earth. Studies on the assembly and dispersals, therefore, provide insights into the mechanisms of extraction of mantle materials at different time periods, the formation of mountain belts, the recycling of crustal materials, magmatism, metamorphism, etc. The recent supercontinent assembly, namely "Gondwanaland," took place during one of the most dynamic periods of the earth's history, and almost all of the existing continental fragments have records of this great geological event. The Southern Granulite Terrane (SGT) of South India is made up of a variety of crustal blocks and collisional sutures/shears that developed during the period of multiple orogenic cycles from the Mesoarchean to the late Neoproterozoic-Cambrian, including that of Gondwana period. Among this, the Palghat Cauvery Shear Zone (PCSZ) marks a major Neoproterozoic structure of crustal accretion, and it is considered the extension of major terrain boundaries identified in Madagascar and Sri Lanka in the final stages of the Gondwana assembly. Even though there have been plenty of studies carried out to understand the nature of the lower crust, terrain assembly, and shear sense indicators along the PCSZ, most of them are concentrated on the eastern side of the shear zone, and only a few have been carried out in the high-grade western terrain; therefore, unequivocal evidence showing collisional orogenesis is lacking from this terrain. The present study attempts to infer the geochemical characteristics of charnockites from the western parts of the PCSZ in terms of accretionary and/or collision tectonics. The geochemistry suggests that the charnockites are tonalitic to granodioritic in composition and have calc-alkaline affinity, indicating an origin related to collision tectonics. These are the products of granulite-facies metamorphism, most probably of an I-type granitic magma, with a low Rb/Sr ratio and a high Ba/Rb ratio suggesting resemblance with Archaean tonalites, and as a product of the remelting of protoliths of tonalite–trondhjemite–granodiorite (TTG) composition. The whole-rock major and trace element compositions indicate that charnockites are formed as the product of partial melting of garnet amphibolite or eclogite-facies basaltic crust during granulite-grade metamorphism at a lower crustal level during a collisional event.