A precise geochemical volcano-stratigraphy of the Deccan traps

Abstract

This geochemical study is based on 69 volcanic rocks representing the entire volcano-stratigraphy of the 12 lava formations of the Deccan Traps (DT) of the Indian peninsula. The DT constitutes one of the largest continental flood basalt province (CFB) in the world. More than 90% of the lavas of this province are considered to have erupted in less than one million years by plume volcanism, coinciding with the Cretaceous-Paleogene (K-Pg) boundary, leading to the speculation that this volcanism had a major role in the end-Cretaceous mass extinction. The current study was undertaken to fully geochemically characterize the DT plume with respect to major-trace element composition as well as their correlated Sr-Nd-Pb and Hf isotope geochemistry of each lava formation in step-by-step sampling the entire Deccan vertical stratigraphy of over 60 samples mostly on the Western Ghat Escarpment as collected by Beane (1988).The upper lava formations from Poladpur to Panhala-Desur, the Wai subgroup, are sharply distinguishable geochemically by all the geochemical parameters from the lower lava formations of the Lonavala and Kalsubai subgroups. This distinction is remarkable as they are on samples in a continuous supposedly vertical sequence in space and time. The Wai subgroup shows restricted range in FeO/MgO ratios at a given MgO wt% but indicating a more Fe-rich mantle source than many other CFB (average Mg# 46.7 as opposed to the 53.3 Mg# for the lower subgroups) province; the lower subgroups show a widely variable FeO/MgO ratios, interpreted as results of partial melting of subducted eclogitic slab. This interpretation is supported by the trace elements including the rare earths, and the isotopic ratios of Nd, Sr, Pb and Hf that were all measured in the same lava samples. Our high precision Pb isotopic measurements reveal the gist of the Deccan plume characteristics as Pb content in the mantle is low in contrast with their higher abundance in the continental crust. The Wai subgroup demonstrates a relatively consistent initial 206Pb/204Pb ratios as opposed to their heterogeneous compositions in the lower formations of Lonavala and Kalsubai. In addition, the Pb isotopes in a 206Pb/204Pb vs 207Pb/204Pb plot define an age of 2604 ± 140 Ma that we interpret as the “provenance” age of the subducted slab that sourced the lower formation lavas. Interestingly, this “errorchron” is also consistent with the “Dupal anomaly” concept developed on the sub Indian Oceanic mantle derived basalts.The combined multiple isotopic volcano-stratigraphy shows the geochemical contrast more dramatically than any other previous study on the Deccan. This high precision multiple isotopic study of the Deccan is consistent with recent U-Pb and Ar-Ar geochronologic studies on the rapid eruption of the Deccan, mostly the plume head, interpreted here constituting the upper lava formations of the Wai subgroup as without much interaction with the continental crust.