Four eruption cycles feeding the most voluminous Deccan Eruptions

Large Igneous Provinces (LIPs) are unusual volcanic events in which massive amounts of melt (~106 km3) erupt in relatively short time periods (< 106 yrs). Many LIPs were emplaced at the former positions of currently active hot spots and are thought to have formed due to extensive decompression melting of anomalous hot mantle triggered by an initiating mantle plume. Most LIP magmas have interacted extensively with the continental crust, however the location and dimensions of the crustal magmatic plumbing systems and the processes triggering eruptions are debated. Here we present new major and trace element and radiogenic isotope data for 43 individual lava flows from a continuous 1200 m thick stratigraphic profile through the upper, most voluminous part of the Deccan LIP (Bushe to Mahalabaleshwar Formations). Eruption rates for this section are constrained by published paleomagnetic directions and absolute U-Pb ages for zircons from weathered flow tops exposed in the profile. We find four magmatic cycles each lasting ~104 to ~105 yrs during which major and trace element compositions change systematically, which are best explained by repeated sequences of recharge, fractional crystallization and eruption. The lavas at the beginning of each cycle generally have the highest MgO contents, but are the most contaminated (lowest Ce/Pb and Nb/Th), inconsistent with the combined assimilation-fractional crystallization trends usually associated with single magma bodies. Rapid changes of isotopically different contaminants suggest that the magmatic cycles were fed by several different magma reservoirs at different levels in the crust, rather than a single large reservoir.