Geochemical indicators of the evolution of the ultrabasic-alkaline series of Paleozoic massifs of the Fennoscandian shield

Rare-earth element distribution in the rocks and minerals of the olivinite-clinopyroxenitemelilitolite-melteigite-ijolite-nepheline syenite series was analyzed to study the evolution trends of the alkaline-ultrabasic series of the Kola province. The contents of REE and some other trace elements were determined in olivine, melilite, clinopyroxene, nepheline, apatite, perovskite, titanite, and magnetite. It was established that distribution of most elements in the rocks of the Kovdor, Afrikanda, Vuoriyarvi, and other massifs differ from that in the Khibiny ultrabasic-alkaline series, being controlled by perovskite crystallization. Primary olivine-melanephelinite melts of the minor ultrabasic-alkaline massifs are characterized by the early crystallization of perovskite, the main REE-Nb-Ta-Th-U depository. Precipitation of perovskite simultaneously with olivine and clinopyroxene results in the depletion of residual magma in rare-earth elements and formation of low-REE- and HFSE ijolite and nepheline syenite derivatives. In contrast, the formation of the Khibiny ultrabasic-alkaline series was complicated by mixing of olivine melanephelinite magma with small batches of phonolitic melt. This led to a change in crystallization order of REE-bearing titanates and Ti-silicates and accumulation of the most incompatible elements in the late batches of the melt. As a result, the Khibiny ijolites have the highest REE contents, which are accommodated by high-REE apatite and titanite.