The nature of the Archean upper crust as revealed by the geochemistry of the Proterozoic shales of the Kaladgi basin, Karnataka, Southern India

Abstract

Major, trace and rare earth element geochemical studies of the shales belonging to the ~1.8 Ga Proterozoic Kaladgi Supergroup of southern India shows that they are derived from strongly chemically weathered Archean upper crust. This has resulted in depletion of Na₂O, CaO, Sr and U in these shales. The shales of lower stratigraphic intervals are enriched in MgO, Fe₂O₃, Cr, Ni, Co and Sc relative to the upper shales. The upper shales, by contrast, show enrichment in elements such as Ba, Th and Zr. These characteristics and the barium-cobalt ratios show that the early sediments were derived from a more mafic provenance and the contribution of granitoids increased towards the later stages of Kaladgi sedimentation. Rare earth element (REE) modelling shows that the average Kaladgi shale represents a contribution from a crust with a mafic to felsic rock ratio of 60:40. The post-depositional introduction of potassium and LREE in these shales has been inferred based on the overall geochemical characteristics.