Early and Late Alkali Igneous Pulses and a High-³He 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 3 He/ 4 He ratio of 14.0 times the air ratio, an initial 87 Sr/ 86 Sr 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- 3 He mantle plume before the rapid eruption of the Deccan CFB.