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

Major, trace and rare earth element geochemical studies of the shales belon ging to the similar to 1.8 Ga Proterozoic Kaladgi Supergroup of southern In dia 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, F e2O3, 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 characte ristics and the barium-cobalt ratios show that the early sediments were der ived from a more mafic provenance and the contribution of granitoids increa sed towards the later stages of Kaladgi sedimentation. Rare earth element ( REE) modelling shows that the average Kaladgi shale represents a contributi on from a crust with a mafic to felsic rock ratio of 60:40. The post-deposi tional introduction of potassium and LREE in these shales has been inferred based on the overall geochemical characteristics. (C) 1999 Elsevier Scienc e B.V. All rights reserved.