Petrology of Early Cretaceous flood basalts and dykes along the rifted volcanic margin of eastern India

Abstract

An approximately 220-m thick sequence of Early Cretaceous flood basalts (the Rajmahal Basalt Group) crop out over some 4300 km² in Bihar, eastern India, forming the leading edge of a seaward-dipping reflector sequence emplaced during the break-up of India and Australia/East-Antarctica. Geochemical data support a division of the basalts and associated dykes into high-Ca and low-Ca magma types. High-Ca tholeiites have CaO contents >10.0 wt%, mg# 50.3–59.6 and K₂O 0.11–0.55 wt%. $\text{La}\text{Nb}$ ranges from 1.29 to 3.62. Rocks of the low-Ca magma type have ≤ 10.5 wt% CaO, mg# 52.1–70.7 and K₂O 0.26–1.1 wt%. $\text{La}\text{Nb}$ is between 1.6 and 3.29. These element abundances and ratios are similar to those of Cretaceous tholeiites from the central Kerguelen Plateau (ODP Site 120–749). Plate reconstructions indicate that the plateau lay adjacent to the Indian continental margin during Early Cretaceous times. It is shown that certain of the Rajmahal basalts (low-Ca magma type) have been contaminated by Indian upper crust, whilst others (high-Ca lavas) retain the near-flat mantle-normalized trace element patterns of oceanic plateau tholeiites.