Ultramafic–Alkaline–Carbonatite Complexes as a Result of Two-Stage Melting of a Mantle Plume: Evidence from the Mid-Paleoproterozoic Tiksheozero Intrusion, Northern Karelia, Russia

Resumo

The Tiksheozero ultramafic–alkaline–carbonatite intrusive complex, like numerous carbonatite-bearing complexes of similar composition, is a part of a large igneous province related to the ascent of a thermochemical mantle plume. The geochemical and isotopic data indicate that the formation of the ultramafic and alkaline rocks was related to crystallization differentiation of a primary alkali picritic melt, whereas carbonatite magmas were derived from an independent mantle source. We suggest that the origin of parental magmas of the Tiksheozero Complex, as well as other ultramafic–alkaline–carbonatite complexes, was provided by two-stage melting of the mantle-plume head: (1) adiabatic melting of its inner part generated moderately alkaline picrites, the subsequent fractional crystallization of which led to the appearance of alkaline magmas, and (2) incongruent melting of the upper cooled margin of the plume head under the influence of CO₂-rich fluids, which arrived from underlying adiabatic melting zone, gave rise to carbonatite magmas.