From Deccan to Réunion: No trace of a mantle plume

Abstract

The widely accepted mantle plume model postulates that (1) the currently volcanically active Réunion Island in the Indian Ocean is fed by the narrow "tail" of a mantle plume that rises from the core-mantle boundary, (2) the Deccan continental flood basalt province of India originated from the "head" of the same plume during its early eruptive phase near the end of the Cretaceous, and (3) the LakshadweepChagos Ridge, an important linear volcanic ridge in the Indian Ocean, is a product of the plume. It is not generally appreciated, however, that this "classic" case of a plume contradicts the plume model in many ways. For example, there is little petrological evidence as yet that the Deccan source was "abnormally hot," and the short (~1.0-0.5 m.y.) duration claimed by some for the eruption of the Deccan is in conflict with recent Ar-Ar age data that suggest that the total duration was at least ~8 m.y. The Deccan continental flood basalts (CFB) were associated with the break-off of the Seychelles microcontinent from India. Geological and geophysical data from the Deccan provide no support for the plume model and arguably undermine it altogether. The interplay of several intersecting continental rift zones in India is apparently responsible for the roughly circular outcrop of the Deccan. The Lakshadweep-Chagos Ridge and the islands of Mauritius and Réunion are located along fracture zones, and the apparent systematic age progression along the ridge may be a result of southward crack propagation through the oceanic lithosphere. This idea avoids the problem of a 10 o paleolatitude discrepancy which the plume model can solve only with the ad hoc inclusion of mantle roll. Published Ar-Ar age data for the Lakshadweep-Chagos Ridge basalts have been seriously questioned, and geochemical data suggest that they likely represent postshield volcanism and so are unsuitable for hotspot-based plate reconstructions. "Enriched" isotopic ratios, such as values of 87 Sr/ 86 Sr higher than those for normal mid-ocean ridge basalts, which have been observed in basalts of the ridge and the Mascarene Islands, may mark the involvement of delaminated enriched continental mantle instead of a plume. High values of 3 He/ 4 He also do not represent a deep mantle component or plume. The three Mascarene islands (Mauritius, Réunion, and Rodrigues) are not related to the Deccan but reflect the recent (post-10 Ma) tectonicmagmatic development of the Africa Plate. I relate CFB volcanism to continental rifting, which often (but not always) evolves into full-fledged seafloor spreading. I ascribe the rifting itself not to mantle plume heads but to large-scale plate dynamics themselves, possibly aided by long-term thermal insulation beneath a supercontinent that may have surface effects similar to those predicted for "plume incubation" models. 477on May 26, 2015 specialpapers.gsapubs.org Downloaded from Nonplume plate tectonic models are capable of explaining the Deccan in all its greatness, and there is no trace of a mantle plume in this vast region.

Were the Deccan Flood Basalts Derived in Part from Ancient Oceanic Crust Within the Indian Continental Lithosphere?

“…It is worth exploring in detail. I conclude, as I have done previously based on many types of data (Sheth, 1999a(Sheth, , b, 2000(Sheth, , 2005, that CFB events like the Deccan do not require, and are not consistent with, the deep mantle plume model. The original plume model has ascribed CFBs such as the Deccan to high mantle temperatures alone.…”

“…Still earlier, the Rajmahal-Sylhet CFB formed at ~116-120 Ma, about 10 million years after the breakup of eastern Greater India from Australia (e.g., Kent et al, 1997). The newly formed continental margins, and the rift zones, constituted major vent areas for the Deccan CFB, as inferred from abundant mafic dyke swarms and intrusions, high heat flow, and aligned thermal springs (e.g., Sheth, 2000Sheth, , 2005.…”

“…The mainstream view is that (i) the currently active Réunion hotspot on the African plate represents the tail phase of a deep mantle plume, and (ii) the Deccan Traps originated from the head of the same plume during late Cretaceous time when the Indian subcontinent was drifting over it. This widely accepted model has been questioned on various grounds, however (Burke, 1996;Sheth, 1999aSheth, , b, 2000Sheth, , 2005.…”

“…A plume head produces voluminous flood basalts, whereas a plume tail produces hot picritic melts. The model has been widely applied to the Deccan Traps of India, one of the largest and best examples of CFBs in the world (see Sheth, 2005 and references therein). The mainstream view is that (i) the currently active Réunion hotspot on the African plate represents the tail phase of a deep mantle plume, and (ii) the Deccan Traps originated from the head of the same plume during late Cretaceous time when the Indian subcontinent was drifting over it.…”

K-T magmatism and basin tectonism in western Rajasthan, India, results from extensional tectonics and not from Réunion plume activity

“…This coincided with the separation of the Seychelles from India, large-scale Deccan volcanism, development of the Cambay rift and alkali magmatism in western Rajasthan. Crustal rifting during the K-T boundary period resulted in large-scale Deccan volcanism (Sheth, 2005a(Sheth, , 2005b) and development of the Cambay-Barmer rift system. The extensional tectonics resulted in deep fractures manifested by the development of rift basins, alkali magmatism and lamprophyre-carbonatite dykes.…”