ISOTOPIC AND TRACE-ELEMENT CONSTRAINTS ON MANTLE AND CRUSTAL CONTRIBUTIONS TO SIBERIAN CONTINENTAL FLOOD BASALTS, NORILSK AREA, SIBERIA

We present a tightly controlled and comprehensive set of analytical da ta for the 250-Ma Siberian flood-basalt province. Consideration of maj or- and trace-element compositions, along with strontium, lead, and ne odymium isotopic compositions, strongly supports earlier Russian subdi vision of this magmatism into three magmatic cycles, giving rise to th ree assemblages of eleven basalt suites in the ascending order Ivakins ky-Gudchikhinsky, Khakanchansky-Nadezhdinsky, and Morongovsky-Samoedsk y. Geochemical and isotopic discontinuities of varying magnitude chara cterize most of the boundaries between the eleven recognized basalt su ites in the Noril'sk area. Although we conclude that the dominant volu me of erupted magma originated from an asthenospheric mantle plume, no ne of the lavas is interpreted to directly represent asthenospheric me lts, which would have been far more magnesian. On the basis of thermal considerations, we consider it unlikely that vast volumes of basaltic melt were produced directly from the continental lithospheric mantle beneath the Siberian craton. Moreover, there is little evidence from m antle xenoliths that the geochemical signatures of such melts would co rrespond to those of the Siberian flood basalts. Studies of melt migra tion lead us to conclude that transport of asthenospheric melt through the lithospheric mantle would be rapid, by fracture propagation. Lava s from the Gudchikhinsky suite have negligible Ta-Nb anomalies and pos itive epsilon(Nd) values, and their parental magmas presumably interac ted little with the continental lithospheric mantle or crust. All othe r lavas have negative Ta-Nb anomalies and lower epsilon(Nd) values tha t we attribute to interaction with continental crust. The model that w e have developed requires discrete contributions from the plume and co mplex processing of all erupted magmas in the continental crust. The e arliest magmas represent small percentages of melt formed in equilibri um with garnet. Over time, the percentage of melting in the source reg ion and the volume of magma produced increased, and garnet was no long er stable in the plume source. All of the plume-derived melts initiall y contained more than 20 wt% MgO and became less Mg rich by fractionat ion of olivine as they traversed the lithospheric mantle. We conclude, however, that the most significant control on the geochemical and iso topic compositions of all the erupted lavas was processing of mantle-d erived magma in crustal reservoirs during periodic replenishment, peri odic tapping, continuous crystal fractionation, and wallrock assimilat ion. Rapid eruption of an extremely large volume of processed magma th at varied little in chemical and isotopic composition produced the seq uence of relatively monotonous tholeiitic basalts that constitute the 2,300-m-thick third assemblage of the Siberian flood-basalt province n ear Noril'sk.