Mesoarchean charnockites from the Coorg Block, Southern India: Petrology, geochemistry and tectonic implications

Abstract

• The charnockites from the Coorg Block formed from arc magmatic rocks. • The rocks were metamorphosed under granulite facies at P-T conditions of 760-800°C and 7.1-8.4 kbar. • A fertile mafic magma similar to hydrous basalt as source. • Charnockites represent vestiges of Mesoarchean arc magmatism and crustal building. Charnockites constitute the major rock type in the crustal blocks of Southern Granulite Terrain (SGT) in India. Here we present petrology, mineral chemistry, and geochemistry of the oldest charnockites in the SGT from the Coorg Block. The charnockites from this block are dominantly composed of orthopyroxene, clinopyroxene, hornblende, biotite, K-feldspar, plagioclase, ilmenite, and minor garnet. Geochemical features of the charnockites indicate granitic to granodioritic protolith. In terms of aluminum saturation index, these rocks are metaluminous and represent magnesian, calc-alkaline to calcic series similar to fractionated I-type granites. They are characterized by high Mg#, enrichment of L.I.L.E. (Rb, Ba, Th, and K) and LREE (La, Ce, Nd), depletion of HFSE (Nb, Ta, Zr, Sr, P, and Ti), typical of volcanic arc granitoid. Their geochemical characteristics are consistent with crystal fractionation of the parental magma and chemically evolved magma. Mineral chemistry of orthopyroxene shows the trend of igneous pyroxene. The rocks were subjected to granulite facies metamorphism with estimated pressure-temperature conditions of 760-800°C and 7.1-8.4 kbar. The Coorg charnockites represent vestiges of Mesoarchean arc magmatism and crust building.