The geochemistry of Indian bole horizons: palaeoenvironmental implications of Deccan intravolcanic palaeosurfaces

Abstract

Deccan intravolcanic bole horizons represent weathering products formed during major hiatuses of a major volcanic episode. In these quiescent periods weathering processes pervasively altered the newly formed volcanic landscape and subsequent flows covered and effectively fossilized the resulting weathered palaeosurfaces. The current work is a detailed geochemical study which examines patterns of element mobilization during these intravolcanic weathering events. The bole horizons normally rest on top of altered basaltic lavas. Both boles and altered lavas represent a comparatively early stage in weathering because the content of chemically residual elements, such as aluminium and iron, are closer to fresh basalt than laterite. Nevertheless, these is clear evidence for significant chemical alteration because the more mobile elements such as calcium and sodium have been substantially removed. A chemically distinctive nature for some boles can be demonstrated from chondrite normalized REE plots and, in these instances, strontium and neodymium isotopic compositions demonstrate that the fine-grained bole material is derived from a chemically distinct source. In addition, thin sections reveal that these fine grained portions commonly contains glass shards and occasionally fresh phenocrysts. It is therefore suggested that many Deccan boles are in fact weathered pyroclastic material, and that the pyroclastic content of the basaltic succession may be greater than previously supposed. A significant pyroclastic input during the Deccan eruptions has important palaeoenvironmental implications for the fate of late Cretaceous flora and fauna in peninsular India.