Mineralogical and Geochemical Features, Structure and Stages of Formation of the Rogomu Concentrically Zoned Massif (Lapland-Belomorian Belt, Kola Peninsula)

The Rogomu massif, investigated in a marginal area of the central Lapland-Belomorian Belt (LBB), has a concentrically zoned structure. It is composed of a core of ultrabasic rocks consisting of a core zone of peridotites (CZP) and a closely associated apopyroxenitic zone (APZ) enclosed in an apogabbroic zone (AGZ). Crystallization proceeded under hypabyssal conditions in the general sequence CZP→APZ→AGZ. At the initial stage, the paragenetic association of olivine (Fo_87–81) and chromian members of the spinel group crystallized and accumulated at a high temperature in the inner zone of the magma reservoir. During the formation of the CZP, elevated levels of Cl were attained in interstitial fluids, as is characteristic of lower ultramafic zones of layered intrusions. The CZP rocks are of harzburgite type close to bodies in the Serpentinite Belt (SB). The average REE spectra normalized to chondrite are similar in rocks of the CZP to those in the related massifs at Chapesvara, Khanlauta and Lotmvara in the SB, which are characterized by extremely low contents of the HREE as a reflectance of their primitivity with respect to massifs in the LBB. Complete and continuous series of compositions recorded in the spinel-group minerals and plagioclase are attributed to unstable conditions of crystallization in a hypabyssal setting. The Rogomu massif, especially the AGZ, recrystallized at conditions of the epidote–amphibolite and amphibolite facies metamorphism. There is evidence of a limited mobility of high field-strength elements (REE, Y, Th, U) in metamorphic fluids during regional recrystallization of the AGZ. Contents of REE progressively accumulated and formed aggregates of epidote–clinozoisite grains that belong to a second generation, having greater REE contents. High levels of REE (up to 12 wt % oxides in total), dominantly cerium, substitute for Ca in zoned grains of epidote–clinozoisite, in which zones of Cr and Cl enrichment are developed. The following scheme of coupled substitution is suggested: (REE³⁺ + *) + Cl^– → 2Ca²⁺ + O^2–. Thorium and U accumulated jointly in H₂O-bearing fluids to cause the repeated crystallization of submicrometer-sized grains of thorite deposited episodically at the edge of zoned grains of chamosite–clinochlore, growing internally, at a temperature ≤770–880°C. Our observations lead to a proposal of the hypothetical formation of ore zones of unconventional (REE, Y, Th, U) mineralization in relation to differentiated massifs in the LBB and other regions.