EARLY AND LATE ALKALI IGNEOUS PULSES AND A HIGH-HE-3 PLUME ORIGIN FORTHE DECCAN FLOOD BASALTS

Several alkalic igneous complexes of nephelinite-carbonatite affinitie s occur in extensional zones around a region of high heat flow and pos itive gravity anomaly within the continental flood basalt (CFB) provin ce of Deccan, India. Biotites from two of the complexes yield Ar-40/Ar -39 dates of 68.53 +/- 0.16 and 68.57 +/- 0.08 million years. Biotite from a third complex, which intrudes the flood basalts, yields an Ar-4 0/Ar-39 date of 64.96 +/- 0.11 million years. The complexes thus repre sent early and late magmatism with respect to the main pulse of CFB vo lcanism 65 million years ago. Rocks from the older complexes show a He -3/He-4 ratio of 14.0 times the air ratio, an initial Sr-87/Sr-86 rati o of 0.70483, and other geochemical characteristics similar to ocean i sland basalts; the later alkalic pulse shows isotopic evidence of crus tal contamination. The data document 3.5 million years of incubation o f a primitive, high-He-3 mantle plume before the rapid eruption of the Deccan CFB.