THE KOVDOR ULTRAMAFIC, CARBONATITIC AND ALKALINE COMPLEX (KOLA PENINSULA, RUSSIA): EVIDENCE FOR MULTI-SOURCE EVOLUTION

Abstract

The Kovdor intrusion belongs to the Paleozoic 380–360 Ma Kola alkaline and carbonatite province NW Russia . It displays a complete sequence of rocks that include in order of intrusion, ultramafic rocks, melilitolites, alkaline silicate rocks of the melteigite–ijolite series, phoscorites and carbonatites, and late nepheline syenite dykes the latter were not studied . The ultramafic sequence dunite–peridotite–clinopyroxenite consists of olivine–clinopyroxene cumulates Mgas86–70 with intercumulus phlogopite and magnetite and late calcite. Melilitolites, with up to 35 wt.% CaO melilite cumulates, have a magmatic rather than metasomatic origin. Rocks of the melteigite–ijolite series are very heterogeneous variations of grain size, mineralogy and modal proportions and show disequilibrium textures core resorption and complex zoning of the clinopyroxenes suggesting either magma mixing or contamination. All the rocks have strong incompatible element enrichments in multi-element spidergrams. The rare earth element REE patterns are steep with LarYb )20; they are N subparallel and do not show any Eu anomaly. The variations of NbrTa ratios and the REE distributions suggest that the carbonatites and the rocks of the melteigite–ijolite series are not conjugate immiscible liquids. Most rocks ultramafics, 87 86 . melilitolites, carbonatites and phoscorites plot in the depleted mantle quadrant of the Nd–Sr diagram with low Srr Sr i ratios 0.70332 to 0.70377 and positive « t values q5.2 to q0.6 . The fairly large range of isotopic compositions is Nd not in favour 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. The isotopic composition of the melteigites–ijolites requires a slightly enriched component that could be similar to that of the Kandalaksha ultramafic lamprophyres and of the Tersky Coast and Arkhangelsk kimberlites.