The Cretaceous-Tertiary Deccan Volcanics

The Deccan Traps of India is one of the largest known continental flood basalt provinces on earth. Comprising a succession of more than 1.5 km thick sub-aerially erupted basaltic flows in western and central India, the Deccan flood basalts cover an area of more than 500,000 square km with an estimated volume exceeding 512,000 cubic km. The catastrophic Deccan volcanic event has been implicated as a major cause for mass extinctions at the Cretaceous-Tertiary (K/T) boundary. The Deccan province is located at the northern end of a hot spot track originating at Reunion Islands in the Indian Ocean. A volcano on Reunion Island is still active and the erupted basalts show an age progression northward with the ~ 65 Ma Deccan Traps being the oldest. Basalts of similar composition and age also occur on the submarine Mascarene Plateau and the Seychelles Islands in the Indian Ocean, indicating that these regions were adjacent to the Indian west coast at the time of the Deccan eruptions and have rifted apart since then. The aim of our study is to understand the mechanism, timing, and duration of the Deccan flood basalt event through an integration of geological, geophysical, geochemical and geochronological data from various parts of the Deccan province.

Heat flow and gravity data indicate a large anomalous mantle perturbation north of Mumbai that may have been generated by a deep mantle plume or hot spot during continental break-up or plate reorganization. Two and three-dimensional modeling of gravity data indicates a sub-surface, north-south elongated, high-density mafic-ultramafic body south of the mantle perturbation along the western continental margin rift. Several small and isolated, convex-upward, high-density mafic bodies also occur along the intra-plate east-west Narmada-Tapti rift. The disposition of these upper crustal bodies suggests migration and concentration of high density Deccan magma in the upper lithosphere dominantly along the western continental margin rift and channeling of the Deccan magma into the Narmada-Tapti rift region.

Geochemical modeling indicates that the mafic magma was emplaced in 6-8 km deep upper crustal magma chambers. Incompatible trace element variations in lower basaltic dikes and flows indicate variable degrees of partial melting of mantle, and fractional crystallization accompanied by significant crustal assimilation (AFC) away from the mantle perturbation (postulated as the plume center). AFC is not evident near the plume center. The Deccan parental magmas evidently acquired their crustal isotopic signatures before the occurrence of AFC in the crustal magma chambers. In the Narmada-Tapti rift region, AFC may be related to longer and greater magma-wall rock interaction in shallow crustal magma chambers due to crustal extension-related enlargement of the magma chambers, recharge with fresh, hot magma and convective mixing. Felsic volcanics in Saurashtra (part of Gujarat ) present further evidence of upper crustal contamination. Zircon and monazite in the felsic rocks show inherited crustal cores on which rims crystallized from the Deccan felsic melt. Temperatures estimated from zircon and monazite saturation thermometry indicate that the Saurashtra crust may have been heated to approximately 900oC by the intruding Deccan magma. Available crustal melting models indicate that a 1-2 million year emplacement time for the Deccan Traps may be appropriate for crustal melting characteristics observed in the Saurashtra region.