Origin of clays in the interbasaltic red boles of Deccan Volcanics: Pedogenesis versus palagonitization

Abstract

Red-coloured clay-rich beds (bole beds) occur at flow breaks in parts of the Deccan volcanic province (DVP). These boles in the Deccan province retain paleoenvironmental information that prevailed at the K-T boundary, including the timing of the Deccan eruption synchronous with the K-T mass extinction. This contribution assesses the origin of clays in various red boles, selected based on their field appearances, physical properties, and relation with bounding units. The saprolitic red bole, fragile red bole and grey bole capping red bole have kaolinite and smectite clay with broad 001 peaks (FWHM>0.62°). The selective dissolution of grey bole reveals the highest Alo, Sio and Feo/Fed (>3) characteristics of short-range ordered alumino-silicates and oxyhydroxide. Both red and grey boles show relative enrichment in immobile elements, indicating leaching within the incipient stage of weathering (saprolite equivalent). Clays from these boles show FeFe and AlAl segregated domains in the octahedral sheet, features characteristic of short-range ordered clay, indicative of a pedogenesis. Smectite in these boles shows Fe > 0.3 apfu with trans vacant (tv) nature, suggesting oxidizing conditions at low temperatures. The STEM-EDS of clay FIB section reveal smectite laths with multiple chemistries in each sample, indicating re-equilibrating conditions in a closed system due to fluctuating wet and dry seasons. Contrary to the above two red boles, indurated-structure less and indurated-laminated red boles have glass shards with yellowish rinds and smectite clay (FWHM<0.5°), indicating palagonitization. The cation occurrence probability in the smectite octahedral sheets of later boles has values characteristic of randomly distributed cations, suggesting a relatively higher temperature of interacting solution. We envisage two processes of clay formation in red boles: a) pedogenesis and b) palagonitization of basalt flow top and/or mafic volcanoclastic deposits.