Tendencies of REE Distribution in the Kivakka Olivinite–Gabbronorite Layered Intrusion (Northern Karelia)

—This work is devoted to the examination of REE behavior in rocks of the Kivakka olivinite–gabbronorite layered intrusion (Northern Karelia). The paper reports detailed petrographic characteristics of mineral assemblages throughout the massif section. It is shown that the same minerals are present as both intercumulus and cumulus phases depending on the degree of melt fractionation. Their contents which determine the petrographic characteristics and rock name are not indicative of their presence in cumulate assemblage (cumulus phase). REE contents in rocks and rock-forming minerals are analyzed, including their variations in the vertical section as well as in the critical zones of contrasting intercalation. REE in the Kivakka massif behave as incompatible elements, are accumulated in a residual melt together with U, Th, Zr, and other incompatible elements, and can be used as indicator of the degree of melt fractionation. In some cases, they could reflect different structural–genetic features such as the cumulus density in the similar cumulus assemblages. Hydrothermal alteration strongly affects the Ce content. The similar shape of REE patterns indicates the close-system crystallization differentiation of the massif.