Geochemical characteristics of calcareous and non-calcareous shales from the Meso-Neoproterozoic basins of the Bastar craton, Central Indian Shield: implications for provenance, tectonic setting, paleoweathering and paleoredox conditions

Abstract

The Meso-Neoproterozoic shales of the Bastar craton, Central Indian Shield have been analyzed geochemically to study their geochemical characteristics and to evaluate paleoweathering, provenance, tectonic setting and paleoredox conditions. The shale samples have been classified into the calcareous and the non-calcareous shales on the basis of presence of calcite and CaO concentrations. In comparison to the calcareous shales, the non-calcareous shales have higher concentrations of most major elements and trace elements including rare earth elements (REEs). The Upper Continental Crust (UCC) normalized elemental ratios of the calcareous and the non-calcareous shales suggest evolved sources similar to the UCC (Upper Continental Crust). The Th/Co versus La/Sc and La-Th-Sc diagrams, chondrite normalized REE patterns and negative Eu/Eu* values of the calcareous and non-calcareous shale samples further reveal that the sediments have been derived from a felsic source and the source rocks have been identified to be the granites and the gneisses of the Bastar craton. The SiO2 versus K2O/Na2O and the SiO2/Al2O3 versus K2O/Na2O tectonic setting discrimination diagrams indicate sediments were deposited in a passive margin tectonic setting. The CIA (Chemical Index of Alteration), PIA (Plagioclase Index of Alteration), Th/U and K/Rb ratios and the K2O-Fe2O3-Al2O3 relations of both the calcareous and the non-calcareous shales indicate that source area was affected by moderate to intense weathering history. The geochemical parameters like Ce/Ce* and Mn* suggest that the calcareous shales were deposited in suboxic environment compared to the non-calcareous shales.