The Cretaceous-Tertiary Deccan Volcanics

 The Cretaceous-Paleogene (K/Pg) Deccan Traps of India is one of the largest continental flood basalt provinces on earth covering an area of more than >=500,000 square km in western and central India and a large additional area offshore of western India. The Deccan basalts are comprised of a nearly-horizontal, sub-aerially erupted lava sequence with a maximum thickness of 3.4 km, and an estimated volume exceeding 512,000 cubic km. The Deccan eruptions were catastrophic and have been implicated as a major cause for the K/Pg mass extinctions. The Deccan province is located at the northern end of a hot spot track originating at Reunion Islands in the Indian Ocean, where a volcano is still active. Along the hot spot track, the basalts show a northward age progression 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.

Phase equilibrium constraints indicate that the Deccan tholeiitic basalt magmas in the western part of the province were multiply saturated with olivine + plagioclase + clinopyroxene (Ol+Pl+Cpx) at <=6 kb (<=23 km depth) that crystallized in the upper crust by olivine gabbro (Ol+Pl+Cpx) fractionation. Calculations involving incremental addition of Ol+Pl+Cpx and Ol+Pl in equilibrium with the tholeiitic melts show that their primary magmas (magma in equilibrium with mantle peridotite) last equilibrated with spinel lherzolite at ~8-13 kb (~30-49 km depths, the Moho or crust-mantle boundary) and ~1268-1332 oC. The primary magmas are high in Mg (Mg# or 100xMg/[Mg+Fe] 70-74) and are also tholeiitic in composition (SiO2 48-50 wt%, MgO 9.8-11.8 wt%, FeO 7.2-7.9 wt%). The groundmass and olivine-hosted melt inclusions of the Powai (Mumbai) ankaramite that originated by intrusion of tholeiitic melt into an Ol+Cpx+Opx cumulate pile also yield similar pressures and temperatures of equilibration. Trace element considerations indicate <=15% batch melting of the mantle. The minimum depth of equilibration of the primary magmas (~30 km) is shallower than the present-day Moho in the Mumbai area based on seismological data, indicating localized mantle upwelling and lower crustal interactions.

Heat flow and gravity data indicate a large anomalous mantle perturbation north of Mumbai that may have been generated by a deep mantle plume during continental break-up or plate reorganization. Two and three-dimensional gravity modeling indicates the presence of an upper crustal, north-south elongated high-density mafic-ultramafic body south of the mantle perturbation along the western continental margin rift, and smaller high-density mafic bodies along the intra-plate east-west Narmada-Tapti rift. Incompatible trace element variations in the lower basaltic dikes and flows indicate variable degrees of partial melting of mantle and fractional crystallization accompanied by little crustal assimilation near the center of mantle perturbation, but significant crustal assimilation (AFC) away from the center. 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. Thus, the upper crustal mafic bodies indicate migration and emplacement of the mantle-derived magma in the upper lithosphere along the western continental margin rift and eastward channeling along the Narmada-Tapti rift.

The felsic volcanics of Saurashtra originated by almost complete melting of the upper continental crust with heat from the intruding Deccan basaltic magma. Zircon and monazite saturation thermometry indicates that the Saurashtra crust may have been heated to approximately 900 oC. Available crustal melting models indicate that a 1-2 million year emplacement time for the Deccan Traps may be appropriate for the crustal melting characteristics observed in the Saurashtra region.


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