Subalkaline picrobasalts and plateau basalts from the Putorana plateau (Siberian continental flood basalt province): II. Melt inclusion chemistry, composition of "primary" magmas and P-T regime at the base of the superplume

We report data for melt and fluid inclusions from phenocrysts of subalkalin e picritic basalts and tholeiitic plateau basalts from the Putorana plateau , Siberian trap province. Analyses of reheated melt inclusions from the roc ks show that the magmas of subalkaline picritic basalts from the Putorana p lateau were significantly enriched in moderately incompatible (Ti and P) an d highly incompatible (K) elements, whereas their concentrations in tholeii tic flood basalts are similar to MORE (with the exception of K). Volatile c omponents played a substantial role in the magmas of subalkaline picritic b asalts, as is evidenced by the finding of fluid inclusions containing liqui d CO2 in the olivine phenocrysts, the presence of amphibole and phlogopite as daughter minerals in melt inclusions and in the matrix of the rocks, the saturation of early melts with respect to molten sulfide phases, and by el evated fluorine contents (0.15-0.35 wt %) in the reheated melts quenched in to glasses. This confirms that Siberian basaltic volcanism is a possible co ntributor to the mass extinction at the Permian-Trias-sic boundary. The var iations of the compositions of reheated melt inclusions from phenocrysts of subalkaline picritic basalts show distinct olivine control even after reca lculation of their compositions to equilibrium with the olivine of fertile mantle Iherzolites; this must be due to the loss of nonnative olivine from primary melts either because of the reequilibration with the host olivine a fter the entrapment of melts by growing crystals, or because of the interac tion with the minerals of mantle peridotites during the ascent of the origi nally produced pat tial melts through the intergranular space of surroundin g mantle rocks. The recalculated compositions of reheated melt inclusions f rom subalkaline picritic basalts with the maximum normative olivine are sim ilar to near-solidus melts in fertile mantle Iherzolites at pressures close to 7 GPa corresponding to potential temperature 270 degreesC above that fo r MORE-generating systems; this confirms that the origin of Siberian traps is related to the ascent of hot material from the inner zones of the Earth. Subalkaline picritic basalts from the Putorana plateau are not primary mag mas for less magnesian tholeiitic flood basalts from the same locality, but rather they were formed from a portion of melt generated at the base of an extensive melting column mixed in the crustal magma chamber with the preva iling volume of more depleted instantaneous melts from the overlying levels of the same column.