Ree and Sr-Nd Isotope Compositions of Clinopyroxenites, Phoscorites and Carbonatites of the Seblyavr Massif, Kola Peninsula, Russia

Clinopyroxenites, phoscorites and carbonatites from the Devonian Seblyavr intrusion (Kola Peninsula, Russia) have petrographic characteristics indicating that they are accumulative in origin. Their geochemical (major and rare earth elements) compositions can be accounted for by mixtures of their major rock-forming minerals and accessory phases, i.e. they reflect the record of mineral accumulation. All of the analysed Seblyavr rocks are strongly LREE-enriched with (La/Yb)_N mostly ranging from 38 to 189. However, a dolomite carbonatite with hydrothermal LREE-Sr mineralization has an extreme (La/Yb)_N value of 1659. Such late-stage dolomite carbonatites were formed by hydrothermal (rather than magmatic) processes. Whole-rock samples of representative magmatic lithologies from Seblyavr have initial ⁸⁷Sr/⁸⁶Sr and ε_Nd that fall in a very narrow range from 0.7031 to 0.7033 and +4.9 to +5.9, respectively. We therefore conclude that clinopyroxenites, phoscorites and carbonatites were formed by differentiation and crystallization of a single batch of melt. The parental melt was derived from a depleted upper mantle source that had been meta-somatised prior to melting.