Pinpointing the Provenance of Deccan Zircons

Abstract

Recent uranium-lead dating of India’s Deccan Traps shows that its rapid eruptions of >10 km3/year temporally coincided with the end-Cretaceous mass extinction. Zircons used for their dating were found in weathered horizons between basalt flows rather than in the basalts themselves. A felsic source likely produced these zircons and deposited them through volcanic airfall. Preliminary radiometric dating of six felsic volcanic complexes on the Saurashtra Peninsula and the Phenaimata complex, both in Gujarat, India, show that they were synchronous with Deccan eruptions. I measured trace element concentrations and hafnium isotopes in 416 zircons from both regions to test the hypothesis that Gujarat volcanism produced the Deccan zircons. The Deccan samples were split into two groups by their eHf. Zircons collected from the Deccan Mahabaleshwar and upper Ambenali formations crystallized in magmas that were contaminated by crustal material, while stronger mantle inputs characterized the magmas of the lower formations’ zircons. Trace element ratios, hafnium isotopes, and Kolmogorov-Smirnov tests indicate that their provenance was not in any of these Gujarat volcanic complexes that have been dated thusfar. These results imply that other felsic sources exist in India, eastern Africa, or the western Indian Ocean that have not yet been identified.