Geochemistry of the Palitana flood basalt sequence and the Eastern Saurashtra dykes, Deccan Traps: clues to petrogenesis, dyke–flow relationships, and regional lava stratigraphy

摘要

Recent studies of large mafic dyke swarms in the Deccan Traps flood basalt province, India, indicate that some of the correlative lava flows reached several hundred kilometers in length. Here we present field, petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd isotopic) data on the Palitana lava sequence and nearby dykes in the Saurashtra region of the northwestern Deccan Traps. These rocks are moderately evolved, many with low-Ti-Nb characteristics. We infer that most dykes are notably (and systematically) less contaminated by ancient continental crust than the Palitana flows, but four dykes are equally or significantly more contaminated, with some of the most extreme Sr-Nd isotopic compositions seen in the entire Deccan Traps (initial εNd is as low as −18.0). A Bhimashankar-type and a Poladpur-type dyke are present several hundred kilometers from the type section of these magma types in the Western Ghats escarpment. We find no geochemical correlations between the Palitana sequence and three subsurface sequences in NE Saurashtra containing abundant picritic rocks, surface lavas previously studied from Saurashtra, or the Western Ghats sequence. Intriguingly, the Eastern Saurashtra dykes cannot have been feeders to any of these lava sequences. Feeder dykes of these sequences may be located in southwestern or central Saurashtra, or in the Dhule-Nandurbar-Dediapada areas across the Gulf of Cambay, 200–300 km east of Palitana. Our results indicate polycentric flood basalt eruptions not only on the scale of the Deccan Traps province, but also within the Saurashtra region itself.