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|Title:||Plume-rift interaction in the Deccan volcanic province|
|Keywords:||Cretaceous Tertiary Boundary|
Continental Flood Volcanism
|Publisher:||ELSEVIER SCIENCE BV|
|Citation:||PHYSICS OF THE EARTH AND PLANETARY INTERIORS, 99(3-4), 179-187|
|Abstract:||It is widely accepted that the grand volcanic episode of the Deccan in India was a consequence of the passage of the northerly drifting Indian subcontinent over the Reunion starting plume in the Late Cretaceous. This plume also produced the three-rift Cambay triple junction, the three arms being the West Coast graben belt, the Narmada-Tapi rift zone and the Cambay rift. Deccan-related alkalic magmatism both preceded and followed the main tholeiitic phase of similar to 65 Ma ago by about 3 MY. The uniformly tholeiitic thick basalt sequence of the Western Chats was derived from direct melting of the plume head, but the tholeiitic-alkalic magmatism of the three rift belts was due to melting of the hydrous lithospheric mantle due to variable amounts of rifting. Any lithospheric loading mechanism for generating post-tholeiite alkalic lavas does not seem feasible for the Deccan; nor can it explain the pre-tholeiite alkaline magmatism in the Deccan. Similarly, conductive heating of the lithosphere by the plume cannot have generated the alkalic melts. Some amount of actual rifting did occur. Direct melt supply from the plume is not necessary for magmatism in the rifts, though it seems to have occurred in some cases. Our documentation of indigenous magmatism for the three rift belts calls into question any attempts at inferring the size of the plume head from synchroneity in geochronologic data alone, and we negate the possibility or necessity of large-distance transport of magmas from the plume through the lithosphere, to feed the rifts. Recent evidence indicates that alkalic magmatism north of the main Deccan outcrop preceded the main tholeiite phase by similar to 3 MY. The Cambay graben was active 3.5 MY before the main tholeiite phase. As the alkalic complexes at the northern end of this graben are 68.5 Ma in age (so far the oldest well-proven Deccan-related rocks), they are the first expression of the Reunion plume, and the borehole picrites from the triple junction area, until now conventionally regarded as the oldest, can no longer be considered so. Accurate radiometric dating of these picrites is therefore much needed.|
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