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

Abstract

We report data for melt and fluid inclusions from phenocrysts of subalkaline picritic basalts and tholeiitic plateau basalts from the Putorana plateau, Siberian trap province. Analyses of reheated melt inclusions from the rocks show that the magmas of subalkaline picritic basalts from the Putorana plateau were significantly enriched in moderately incompatible (Ti and P) and highly incompatible (K) elements, whereas their concentrations in tholeiitic flood basalts are similar to MORB (with the exception of K). Volatile components played a substantial role in the magmas of subalkaline picritic basalts, as is evidenced by the finding of fluid inclusions containing liquid 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 elevated fluorine contents (0.15-0.35 wt %) in the reheated melts quenched into glasses. This confirms that Siberian basaltic volcanism is a possible contributor to the mass extinction at the Permian-Triassic boundary. The variations of the compositions of reheated melt inclusions from phenocrysts of subalkaline picritic basalts show distinct olivine control even after recalculation of their compositions to equilibrium with the olivine of fertile mantle Iherzolites; this must be due to the loss of normative olivine from primary melts either because of the reequilibration with the host olivine after the entrapment of melts by growing crystals, or because of the interaction with the minerals of mantle peridotites during the ascent of the originally produced partial melts through the intergranular space of surrounding mantle rocks. The recalculated compositions of reheated melt inclusions from subalkaline picritic basalts with the maximum normative olivine are similar to near-solidus melts in fertile mantle lherzolites at pressures close to 7 GPa corresponding to potential temperature 270°C above that for MORB-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 magmas 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 prevailing volume of more depleted instantaneous melts from the overlying levels of the same column. Copyright