GEOCHEMICAL CHARACTERISTICS OF CARBONATES OF THE CHILPI GROUP, INDIA: IMPLICATIONS FOR PALEOPROTEROZOIC DEPOSITIONAL ENVIRONMENTS

The Chilpi Group in the Bastar Craton of India overlies meta-volcanics of the Nandgaon Group (2100 Ma). Deformation and metamorphism in the region at 1600 Ma constrain the age of the Chilpi Group between 2100 and 1600 Ma. The Chilpi Group is composed of inter-banded quartzite, volcano-sedimentary rocks and shale with or without carbonate and iron ore bands. We analyzed major, trace and rare earth elements in the carbonate unit of the Chilpi Group for evaluation of palaeo-redox conditions and depositional environment. The carbonates show no influence of terrestrial detritus as total REE (ΣREE) against the concentrations of Ti, Al and Th lack significant correlation.The ratios of different REE like Gd/Gd*, La/La*, and Lu_sn/La_sn suggest pristine marine nature of the carbonates. A super chondritic Y/Ho ratio is interpreted to be the result of fresh water mixing in estuarine environment. Positive cerium anomalies (Ce/Ce* up to 2.83) along with enrichment of redox sensitive elements indicate deposition in a reducing / anoxic environment. Presence of rhodochrosite corroborates low oxygen content in the basin of deposition. Further, carbonates in this study display positive europium anomalies (Eu/Eu* = 1.025 – 1.562) which can be attributed to reducing environment; contributions of hydrothermal fluids are ruled out for lack of LREE enrichment compared to HREE. Reducing conditions in shallow sea indicate that the Great Oxidation Event did not affect seawater composition during Paleoproterozoic.