Sr-Nd-Pb isotopic systematics of the Kovdor phoscorite-carbonatite complex, Kola Alkaline Province, NW Russia: Implication for plume-related source characteristics

Abstract

Sr-Nd-Pb isotopic compositions from the Kovdor complex (380 Ma) in the Kola Alkaline Province, NW Russia, have been determined to define the possible mantle components responsible for the formation of ultramafic-alkaline-carbonatite magmas. Measured rocks plot in the depleted mantle quadrant of the Nd-Sr correlation diagram with negative eSt(t) (-5.4 to -12.2) and positive eNd(t) (1.0 to 4.7). Combined with the previous results, the fairly large range of isotopic compositions of Kovdor rocks is not in favor of a simple, closed system magmatic evolution

it suggests a complex evolution implying several magma batches derived either from an isotopically heterogeneous mantle source or from various mixing proportions of two mantle reservoirs. Two end members (KCL, by Kramme, 1993) have been suggested to explain the isotopic compositions of the Kola ultramafic-alkaline rocks and carbonatites

One is the EM I component and the other is the DM component. However, our new Pb isotopic data suggest an involvement of fairly different component from the DM, which is relatively primitive isotopic component, probably derived from a lower-mantle plume. Though the Pb isotope data are still very insufficient in the Kola Alkaline Province, the present data from the Kovdor complex suggest that one end-member of isotopically primitive component should be similar to that of 'FOZO' component, derived from the lower mantle reservoir, the so-called '5th component' in mantle plumes. Thus we propose that a Devonian equivalent of 'FOZO' component represents one of the mantle sources responsible for the formation of the ultramafic-alkaline-carbonatite complexes in the Kola Alkaline Province.