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U-Pb zircon dating and Sr isotope systematics of the Vindhyan Supergroup, India

¹ Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
² Department of Earth, Atmosphere, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
³ Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada, and Institut für Geologie, Mineralogie und Geophysik, Ruhr Universität, Bochum 44780, Germany
⁴ Department of Earth, Atmosphere, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

The Vindhyan Supergroup of central India, the focus of many paleontological studies, has been reported to contain Cambrian small shelly fossils, Ediacaran fossils, trace fossils, and Proterozoic microfossils and carbonaceous megafossils. New U-Pb zircon and ⁸⁷Sr/ ⁸⁶Sr isotopic data from the Lower Vindhyan Supergroup require that the rocks are latest Paleoproterozoic to earliest Mesoproterozoic in age. Two rhyolitic volcanic horizons from the Deonar Formation, between the Kajrahat and Rohtasgarh Limestones and below the unit containing trace fossils, yield U-Pb zircon ages of 1631 ± 5 Ma and 1631 ± 1 Ma. The Kajrahat and Rohtasgarh Limestones of the Semri Group that are below and above the reported Mesoproterozoic trace fossils have ⁸⁷Sr/⁸⁶Sr ratios of 0.70460 and 0.70479, respectively. The Bhander Limestone from the Upper Vindhyan Supergroup has an ⁸⁷Sr/ ⁸⁶Sr ratio of 0.70599, consistent with a Neoproterozoic age for this formation. These results indicate that the Kajrahat Limestone is of latest Paleoproterozoic age and the Rohtasgarh Limestone is of probable Mesoproterozoic age. These findings are in conflict with the report of Cambrian small shelly fossils and fossils of articulate brachiopods in the Rohtasgarh Limestone and argue for a Mesoproterozoic age for the formation that contains the alleged trace fossils. Reports of an Ediacaran fossil Spriggina(?) from the Lower Vindhyan Supergroup from the northern margin of the Vindhyan Basin suggest either incorrect stratigraphic correlation of units or misidentification of this fossil.

Key Words: Vindhyan Supergroup • metazoan evolution • U-Pb zircon dating • Sr isotope stratigraphy

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