C, O, Sr and Pb isotope systematics of carbonate sequences of the Vindhyan Supergroup, India: age, diagenesis, correlations and implications for global events

The Vindhyan Supergroup of central India records a substantial portion of Proterozoic time and contains some of the most disputed Precambrian/Cambrian fossil discoveries. Despite the significance of these “fossil” finds, many issues, including reliable geochronology, remained unresolved. Similarly, the geochemical aspects of the sedimentary sequences and their implications to the evolution of Proterozoic oceans, have been studied only at a reconnaissance level. Here we report the results of the Pb–Pb dating for the Rohtasgarh Limestone from the Lower Vindhyan Supergroup giving an age of 1601±130 Ma, an estimate that is in accord with the recent U–Pb zircon dating of the Lower Vindhyan Supergroup by Rasmussen et al. [Geology 30 (2002) 103] and Ray et al. [Geology 30 (2002) 131]. Sr isotope stratigraphy of carbonates from the Upper Vindhyan Supergroup suggests a Mid-Neoproterozoic (750–650 Ma) age for these formations. This, in turn, argues for a long hiatus between the deposition of the Lower and Upper Vindhyan Supergroups. Our results conflict with the reports of Cambrian age small shelly fossils and Ediacaran fossils from the Upper Vindhyan rocks. At the same time, they suggest that the alleged trace fossils in the Lower Vindhyan rocks must be of Paleoproterozoic age. st common δ18O values for calcitic components from limestone formations of the Supergroup range from −10 to −7‰, similar to Paleo-Neoproterozoic carbonates worldwide. The modes of δ13C in the Lower and Upper Vindhyan Supergroups are 0±2 and 3±2‰, respectively, again consistent with the worldwide dataset. The secular patterns of carbon isotope trends do not support the earlier assumptions that the carbonate sequences at the southern margin correlate with those at the western or northern margins of the Vindhyan Basin. C and Sr isotope data provide new calibration points for the seawater evolutionary curves, particularly for the Paleoproterozoic. For strontium isotopes, the data from the Lower Vindhyan Supergroup suggest that seawater departed probably less rapidly from mantle values than was previously believed.