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Abstract

Igneous complexes containing Ultramafic, Alkaline, melilite-bearing, and Carbonatite rocks (UAC massifs) are unusual in the assemblage of rocks found together and may be extremely rich in critical materials. Major questions remain concerning the origins of these massifs, including initial melt composition and melt evolution. Kovdor is a typical and representative UAC massif in morphology, size, and constituent lithologies notable for the pristine unreplaced nature of its mafic and ultramafic rocks. This study of the 380 Ma Kovdor Massif from the Kola Alkaline Province (Northern Europe) seeks to constrain the number, type, and potential sources of melts that contributed to the massif. The work comprises petrographic analysis, electron microprobe, and Laser Ablation Inductively-Coupled Mass-Spectromtery analyses of minerals, and analyses of Pb-Sr-Nd isotopic compositions. The sample suite includes olivinites, clinopyroxenites, melilitolites, syenites, carbonatites, phoscorites, and metasomatic rocks resulting from their interactions. Geological and textural relationships among the rock types, along with clinopyroxene and amphibole chemistry and Sr-Nd isotopic data, indicate the involvement of three distinct melt batches: (1) melt crystallized an early assemblage of olivinite, clinopyroxenite, and minor carbonatite; (2) alkaline silicate melts, including melilitolites and ijolites; and (3) melts that formed late-stage phoscorites and carbonatites. The Sr-Nd isotopic systematics point to a refertilized depleted mantle source, coupled with a plume-like component compositionally similar to bulk silicate Earth. Notably, the isotopic signatures of the Kovdor rocks do not require the involvement of a HIMU mantle reservoir. This is unusual among carbonatite complexes.