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1.6 Ga U-Pb zircon age for the Chorhat Sandstone, lower Vindhyan, India: Possible implications for early evolution of animals

¹ Department of Geology and Geophysics, University of Western Australia, Crawley 6009, Australia
² Department of Geological Sciences, Jadavpur University, Calcutta 700 032, India
³ Department of Earth Sciences, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
⁴ Centre for Global Metallogeny, Department of Geology and Geophysics, University of Western Australia, Crawley 6009, Australia

Bedding-plane markings in the Chorhat Sandstone (lower Vindhyan), central India, were recently interpreted as burrows produced by triploblastic animals. Because the rocks were thought to be older than 1000 Ma, these structures were regarded as the oldest fossil evidence for metazoan life. However, the biological origin of the markings has been questioned, as has their age. Current age estimates are based on K-Ar, Rb-Sr, and fission- track dates, though some contentious evidence suggests that the rocks may be only 540 Ma. Here we provide the first robust age data for the lower Vindhyan by using SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon geochronology to date silicified tuffs bounding the Chorhat Sandstone. Our results show that the sediments were deposited between 1628 ± 8 Ma and 1599 ± 8 Ma. If the Chorhat markings are burrows left by worm-like animals, then our data suggest that complex metazoans had evolved before 1600 Ma, 1 b.y. before the "Cambrian explosion" when animals rapidly diversified and became ecologically dominant. However, given the doubts expressed about the origin of the bedding-plane structures, as well as the surprisingly "old" age of the host rocks, further studies are urgently required to provide supportive evidence.

Key Words: Vindhyan • geochronology • zircon • Semri Group • trace fossils • Proterozoic

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