Petrological and Geochronological Constraints on the Evolution of Charnockitic Rocks in the Massifs of Cauvery Shear Zone, Southern Granulite Terrain, India

Abstract

Cauvery shear zone of the southern granulite terrane, India, is sandwiched between the amphibolite to granulite transition zone in the north and the Neoproterozoic granulite belt in the south. Two massifs of this shear zone consist of charnockitic rocks of tonalitic to granodioritic composition and include hornblende-mafic granulite enclaves. Petrology and geochemistry clearly indicate dehydration partial melting of mafic rocks as the mode of origin of the charnockitic rocks. However, high MgO, Ni and Cr contents and negative Nb anomalies suggest some interaction with the mantle in a subduction zone environment. U-Pb and Lu-Hf isotopic data of zircons in mafic granulite enclave and charnockite-enderbite host rocks indicate ca. 2.5 Ga granulite-cum-anatectic event. Juvenile mafic magmatism at ca. 2.7 Ga could represent the precursor of the mafic granulites. With the chemical and isotopic evidence of subduction zone environment, the Cauvery shear zone could be appropriately described as a suture along which Archaean-Paleoproterozoic granulite terrain and Neoproterozoic granulite terrain had collided.