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The Rogomu massif, located in a marginal part of the central Lapland–Belomorian Belt (LBB), has a concentrically zoned structure. It is composed of a core of ultramafic rocks consisting of a peridotite core zone (PCZ) and a closely associated apopyroxenitic zone (APZ) enclosed in an apogabbroic zone (AGZ). The massif crystallized under hypabyssal conditions in the general sequence PCZ → APZ → AGZ. At the initial stage, the paragenetic association of olivine (Fo87–81) and chromian members of the spinel group crystallized and accumulated in the inner and highest-temperature zone of the magma reservoir. During the formation of the PCZ, the interstitial fluid had an elevated Cl concentration, which is usually characteristic of the lower ultramafic zones of layered intrusions. The PCZ rocks are of harzburgite type, similar to bodies of the Serpentinite Belt (SB). The average chondrite-normalized REE distribution patterns in PCZ rocks are similar to those of the Chapesvara, Khanlauta and Lotmavara massifs in the SB. These are characterized by the extremely low HREE contents, which reflects their more primitive composition with respect to the LBB massifs. The presence of complete and continuous compositional series of the spinel-group minerals and plagioclase is attributed to the unstable conditions of crystallization due to the hypabyssal setting. The Rogomu massif, especially the AGZ, recrystallized at conditions of the epidote-amphibolite and amphibolite facies. Mineralogical features of the AGZ rocks show evidence of a limited mobility of high field-strength elements (REE, Y, Th, U) in the metamorphic fluids during regional recrystallization. The REE contents progressively accumulated and formed aggregates of epidote–clinozoisite grains of the higher-REE second generation. The REE (up to 12 wt % oxides in total), dominantly cerium, substitute for Ca in zoned grains of epidote–clinozoisite, which also contain zones enriched in Cr and Cl. The following scheme of coupled substitution is suggested: (REE3+ +) + Cl– → 2Ca2+ + O2–. Thorium and uranium accumulated jointly in H2O-bearing fluids to cause the repeated crystallization of submicrometric grains of thorite. These precipitated episodically on the facets of rim-to-core growth-zoned chamosite–clinochlore at a temperature ≤770–880°C. Our observations suggest the possibility of ore zones of unconventional (REE, Y, Th, U) mineralization in the differentiated mafic-ultramafic massifs in the Lapland–Belomorian Belt and other regions.
Barkov et al. (Mon,) studied this question.