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

Several alkalic igneous complexes of nephelinite-carbonatite affinities occur in extensional zones around a region of high heat flow and positive gravity anomaly within the continental flood basalt (CFB) province of Deccan, India. Biotites from two of the complexes yield 40Ar/39Ar 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 40Ar/39Ar date of 64.96 +/- 0.11 million years. The complexes thus represent early and late magmatism with respect to the main pulse of CFB volcanism 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 ratio of 0.70483, and other geochemical characteristics similar to ocean island basalts; the later alkalic pulse shows isotopic evidence of crustal contamination. The data document 3.5 million years of incubation of a primitive, high-He-3 mantle plume before the rapid eruption of the Deccan CFB.