Geochemistry of carbonate rocks of the Chilpi Group, Bastar Craton, India: Implications on ocean paleoredox conditions at the late Paleoproterozoic Era

摘要

The chemogenic sediments of the Chilpi Group (similar to 2.0-1.8 Ga) of the Bastar Craton have been analysed for the sedimentary environment, geochemistry and redox evaluation of seawater towards the end of the Paleoproterozoic Era. Fabric-retentive fine-grained micritic and oolitic carbonates suggest preservation of primary texture and precipitation from contemporaneous seawater in a shallow water tidal environment. Reducing condition of shallow sea for the Chilpi Group is also recognized by the presence of chamosite associated with Mn-ankerite. The Post-Archean Australian Shale (PAAS) normalized positive Gd and La anomalies, Lu/La, Y/Ho molar ratios and the Y anomaly indicate the preservation of pristine marine nature of the carbonates. Evaluation of multielement concentration indicates no significant effects of detrital contamination, diagenesis, metamorphism and contributions from hydrothermal sources. Positive Ce anomalies observed in all the samples, with maximum Ce/Ce* up to 2.83, and redox responsive trace element ratio patterns in association with enrichment of Mn and other redox responsive elements such as Zn, Fe, Mo and U indicate a reducing marine environment of deposition of the Chilpi carbonates. Display of positive europium anomalies (Eu/Eu* = 1.02 - 1.56) can be attributed to the reducing environment as contributions of hydrothermal fluids are ruled out for lack of LREE enrichment compared to HREE. We infer suboxic to anoxic conditions of shallow sea in the Chilpi basin towards the end of the Paleoproterozoic Era, with estimated atmospheric oxygen level of similar to 10(-3) PAL. Reducing conditions in shallow sea indicate that the Great Oxidation Event did not affect seawater composition during late Paleoproterozoic.