Petrological and geochemical (trace elements and Sr-Nd isotopes) characteristics of the Paleozoic Kovdor ultramafic, alkaline and carbonatite intrusion (Kola Peninsula, NW Russia)

The Kovdor intrusion belongs to the Paleozoic (380-360 Ma) Kola alkaline an d carbonatite province (NW Russia). It displays a complete sequence of rock s that include in order of intrusion, ultramafic rocks, melilitolites, alka line silicate rocks of the melteigite-ijolite series, phoscorites and carbo natites, and late nepheline syenite dykes (the latter were not studied). Th e ultramafic sequence (dunite-peridotite-clinopyroxenite) consists of olivi ne-clinopyroxene cumulates (Mg# = 86-70) with intercumulus phlogopite and m agnetite and late calcite. Melilitolites, with up to 35 wt.% CaO (melilite cumulates), have a magmatic rather than metasomatic origin. Rocks of the me lteigite-ijolite series are very heterogeneous (variations of grain size, m ineralogy and modal proportions) and show disequilibrium textures (core res orption and complex zoning of the clinopyroxenes) suggesting either magma m ixing or contamination. All the rocks have strong incompatible element enri chments in multi-element spidergrams, The rare earth element (REE) patterns are steep with (La/Yb)(N) > 20; they are subparallel and do not show any E u anomaly. The variations of Nb/Ta ratios and the REE distributions suggest that the carbonatites and the rocks of the melteigite-ijolite series are n ot conjugate immiscible liquids. Most rocks (ultramafics, melilitolites, ca rbonatites and phoscorites) plot in the depleted mantle quadrant of the Nd- Sr diagram with low (Sr-87/Sr-86)(i) ratios (0.70332 to 0.70377) and positi ve epsilon(Nd)(t) values (+5.2 to +0.6). The fairly large range of isotopic compositions is not in favour of a simple, closed system magmatic evolutio n; it suggests a complex evolution implying several magma batches derived e ither from an isotopically heterogeneous mantle source or from various mixi ng proportions of two mantle reservoirs. The isotopic composition of the me lteigites-ijolites requires a slightly enriched component that could be sim ilar to that of the Kandalaksha ultramafic lamprophyres and of the Tersky C oast and Arkhangelsk kimberlites. (C) 2000 Elsevier Science B.V. All rights reserved.