The Feeder System of the Deccan Traps (India): Insights from Dike Geochemistry

Abstract

Three large dike systems are exposed in the Deccan Traps flood basalt province of India: the dominantly north–south-trending west coast swarm, the east–west-trending Narmada–Tapi swarm in the north–central Deccan, and the Nasik–Pune swarm in the central western Deccan. Combined major and trace element and Sr–Nd–Pb isotope data reveal that probable feeder dikes for the three main lava formations in the upper part of the lava pile (Poladpur, Ambenali and Mahabaleshwar Formations) are abundantly represented in the Nasik–Pune and coastal swarms. As a group, these dikes have no clear preferred trend. Among the highly oriented dikes of the Narmada–Tapi and west coastal areas, some have affinities with the lower part of the lava pile (Jawhar, Igatpuri, Thakurvadi and Bushe Formations) and these appear to have been intruded under the influence of regional north–south and east–west extension, respectively. Other dikes in the Narmada–Tapi swarm have the high-²⁰⁶Pb/²⁰⁴Pb characteristic of flows in the far northeastern Deccan. These data suggest that Deccan lava flows could have reached as much as 700 km in length. Directed lithospheric extension appears to have been an important control on the emplacement of feeder dikes for the lower and middle formations. In contrast, emplacement of the voluminous upper formations, which span the Cretaceous–Tertiary boundary and 29R–N magnetic reversal and are estimated to make up ≥50% of Deccan lava volume, was not controlled by directed regional extension. This conclusion contradicts predictions of rifting-based models for Deccan volcanism. Finally, isotopically distinct, north–south-trending dikes cut upper-formation flows and dikes along the coast; these dikes represent minor magmatism linked to early Paleocene east–west extension following the main phase of volcanism, in association with rifting of the Seychelles Bank from India.