Nature and composition of interbedded marine basaltic pumice in the ~52a€“50 Ma Vastan lignite sequence, western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

摘要

The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels a€“one at ~5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ~52a€“50Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ?± pyroxene ?± Fea€“Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii a‰¤10 to a‰￥30 ??m), non-coalescing microvesicles (40a€“60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fea€“Mn-rich clay and hydrated alteredbasaltic glass (palagonite). The Fea€“Mn-rich clay is extremely low in SiOa??, Ala?? Oa??, TiOa?? , MgO, alkalies and REE, but very high in Fea??Oa??, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiOa??, Ala??Oa??, MgO and variable gain in Fea??Oa??, TiOa??, Ni, V, Zr, Zn and REE. Bubble initiationto growth in the ascending basaltic magma (liquidus ~1200a€“1250a—|C) may have occured in ~3 hr. Shortdistance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.