PETROGENESIS AND TIMING OF VOLCANISM IN THE RAJMAHAL FLOOD-BASALT PROVINCE, NORTHEASTERN INDIA

A suite of rocks from the Rajmahal-Bengal-Sylhet Traps of northeastern India has been analyzed to ascertain the timing and duration of volcanism and elucidate their petrogenesis. Ar-40/Ar-39 step-heating studies identified specimens that suffered post-crystallization loss of Ar-40* and indicate the Rajmahal Province was extruded in similar to 2 Ma around 117 Ma ago. Trace- and rare-earth-element data suggest the existence of three different types of magmas. Rajmahal quartz tholeiites were formed from primary melts, following considerable gabbroic fractionation. Bengal Trap olivine tholeiites represent lavas formed by large partial melting of mantle material, leaving garnet in the residue. Alkali basalts in the Bengal Traps appear to represent partial melts of mantle containing LILE-enriched sections, rather than very small (<2%) melts of a garnet Iherzolite source. Whole-rock delta(18)O-values for slightly altered tholeiites fall in the range +5.9 to +6.6 parts per thousand, indicating mantle-derived melts that have suffered minor crustal contamination; two alkali basalts, formed following considerable crystal fractionation of primary magmas, yield values of similar to+7.2 parts per thousand. Sr-Nd isotopic analyses show two different contamination trends, overlapping those observed in an earlier study of surface Rajmahal quartz tholeiites, with the most primitive material showing Sr-87/Sr-86 similar to 0.70400, Nd-143/Nd-144 similar to 0.51280 at 117 Ma ago. The Bengal Trap olivive tholeiites were formed following assimilation of high-Sr-87/Sr-86 (granulitic?) material. The main contamination trend includes quartz tholeiites from the Rajmahal Traps and alkali basalts from the Bengal Traps. Tholeiites, showing considerable isotopic modification, suggest ingestion of a high-Sr component, unlikely to be upper-crustal material; for the alkali basalts, with high Sr (similar to 1000 ppm) and Nd (similar to 55 ppm) contents, incorporation of a few percent of ''exotic'' material (in the source region?) is indicated. Carbonatite is the probable contaminant, strengthening the postulated link between flood basalt volcanism and carbonatite-lamproites in this area. The occurrence of two lavas with reversed magnetic polarity, in association with the Ar-40/Ar-39 ages reported herein, suggests the ISEA reversed event is displayed in the lavas of the Rajmahal Traps.