Geochemistry of shales from the Paleoproterozoic to Neoproterozoic Vindhyan Supergroup: Implications on provenance, tectonics and paleoweathering

摘要

Major, trace and rare earth element (REE) concentrations were measured for eighty eight shale samples from the Paleoproterozoic to Neoproterozoic Vindhyan Supergroup in central India. The major and trace element data of the Vindhyan shales are compared with North American Shale Composite, upper crust, mid-oceanic ridge basalts and many other shale standards of Archean, Proterozoic and Phanerozoic age. REE concentrations of the Vindhyan shales are comparable to the upper continental crust. Various discriminant diagrams were used to reveal the inferred provenance, tectonics and paleoweathering conditions. A predominantly granitic source rock is inferred for the Lower Vindhyan shales, whereas partial contribution from basaltic source is evident during deposition of the Upper Vindhyan shales. Existing paleocurrent data and the present geochemical investigations together suggest Archean-Paleoproterozoic basement rocks, exposed beyond the southern basin margin of the Vindhyan outcrops as the possible source rocks. The deposition of the Vindhyan shales took place in an intracratonic basin where sediments were primarily derived from stable continental areas. Moderate climatic conditions are inferred during the deposition of the shales. The Upper Vindhyan shales, however, reflect more intense weathering compared to the Lower Vindhyan shales, which may be attributed to relative tectonic stability and more humid conditions during deposition of Upper Vindhyan shales.