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

par Verhulst, Anne ;Balaganskaya, Elena;Kirnarsky, Y.;Demaiffe, Daniel
Référence Lithos, 51, 1-2, page (1-25)
Publication Publié, 2000-03

Article révisé par les pairs

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Résumé :

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 (Mg = 86-70) 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 (La/Yb)(N) > 20; they are subparallel and do not show any Eu anomaly. The variations of Nb/Ta 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, melilitolites, carbonatites and phoscorites) plot in the depleted mantle quadrant of the Nd-Sr diagram with low (87Sr/86Sr)(i) ratios (0.70332 to 0.70377) and positive ε(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 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. (C) 2000 Elsevier Science B.V. All rights reserved.