~(40)Ar/~(39)Ar dating of tholeiitic flows and dykes of Elephanta Island, Panvel flexure zone, western Deccan Traps: A five-million-year record of magmatism preceding India-Laxmi Ridge-Seychelles breakup

摘要

Elephanta Island, located near Mumbai in the western Deccan Traps, is composed of subaerially erupted tholeiitic lava flows and dykes, with no evolved or pyroclastic rocks. In contrast, Deccan volcanism in Mumbai includes subaerial and subaqueous lava flows, intrusions, and pyroclastics, of highly varied composition. The Mumbai and Elephanta sequences, being in the Panvel flexure zone along the western Indian rifted continental margin, dip prominently seaward. The bulk of the Mumbai sequence stands dated at 62.5 Ma, contemporaneous with continental breakup between the Seychelles and Laxmi Ridge-India, voluminous offshore magmatism, and Panvel flexure formation. We have obtained whole-rock Ar-40/Ar-39 ages of 66-65 Ma on six Elephanta tholeiites (representing two lava flows and four dykes) and a picrite dyke. An ankaramite dyke yields a whole-rock Ar-40/Ar-39 age of 61.6 +/- 0.4 Ma (2 sigma). These ages imply that the bulk of the Elephanta sequence was formed rapidly, well before continental breakup, and is contemporaneous with (and potentially a part of) the main Deccan sequence in the Western Ghats escarpment to the east. The younger age of the ankaramite dyke shows that the total duration of Elephanta magmatism was not 3.5 Myr and possibly as much as 6 Myr. Interestingly, the age of the Elephanta ankaramite dyke overlaps with that of the Powai ankaramite flow in Mumbai, suggesting the dyke to be a potential feeder of that flow. (C) 2019 Elsevier B.V. All rights reserved.