Vol 33, No 6 (2025)

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.

The fluid composition from coeval melt and fluid inclusions in olivine phenocrysts (Fo_92–89) from meimechites of the Guli alkaline-ultrabasic carbonatite pluton was studied using gas chromatography-mass spectrometry. The melt inclusions in olivine were fine-crystallized. Daughter phases in inclusions by scanning electron microscopy were represented by diopside, phlogopite, ilmenite, Ti-bearing magnetite, titanite, nepheline, sodalite, and xenogenic chromite. According to Raman spectrometry, the fluid inclusions were low-density and contained magnesite and water. During heating experiments, after melting of the last colorless daughter phase in melt inclusions at about 1300°C, the volume of the gas phase coexisting with the melt varied from 1/4 to 2/3 of the inclusion volume, indicating a heterogeneous state of the trapped mineral-forming environment. During olivine crystallization, the fluid phase was predominantly composed of hydrocarbons (83.0 rel. %), nitrogenated (7.2 rel. %) and sulfonated (3.4 rel. %) compounds, as well as H₂O (5.9 rel. %) and CO₂ (0.3 rel. %). The hydrocarbons and sulfonated compounds contained a relatively high amount of halogenated compounds (4.0 rel. %). The species diversity of fluid components amounted to 201 chemical compounds. Among the hydrocarbons, oxygenated components prevailed (74.5 rel. %), consisting of 34.0 rel. % alcohols and 9.9 rel. % esters, 11.4 rel. % aldehydes, 6.8 rel. % ketones, 12.2 rel. % carboxylic acids. Aliphatic and cyclic hydrocarbons constituted only 4.6 and 3.7 rel. %, respectively. Olivine crystallization occurred under relatively reducing conditions at H/(H+O) = 0.87. A comparison of the obtained data with those from olivine in olivinites of the Krestovskaya intrusion, showed that meimechites could not be the parental magma for olivinites of alkaline-ultramafic carbonatite massifs.

In addition to CO₂ coming from external mantle sources, CO₂ generated by the transformation of carbonaceous material of the protolith (internal sources) is actively involved in the processes of high-grade metamorphism in the crust. One of the mechanisms of the CO₂ generation may be the oxidation of graphite from meta-sedimentary rocks with Fe³⁺ and/or H₂O released during the decomposition and/or partial melting reactions of micas, which usually contain significant amounts of Fe³⁺. Paper present the results of experiments at 500 MPa and 900°C on partial melting of plagioclase-free garnet-two mica (+ quartz, apatite, ilmenite) schist containing 0, 4.2, 10.1, 14.6 and 18.6 wt. % of graphite. Melting of graphite-free rock leads to the formation of peraluminous melts corresponding to alkali-calcic ultra-potassic granites. As the graphite content increases, the A/CNK and A/NK indices decrease and the MALI index of melts increases, and their compositions shift towards alkalic granites. The estimated content of H₂O + CO₂ in the melts decreases with an increase in the graphite content in the starting system. The peritectic phases are represented by hercynite-magnetite spinel, orthoamphibole (gedrite), sillimanite, and potassium feldspar. A decrease in the Fe³⁺/SFe ratio in Fe-Mg minerals with an increase in the graphite content in the starting mixtures manifests an increase in reducing conditions. This conclusion is confirmed by the lgfO₂ values calculated from the equilibrium of spinel, sillimanite, and quartz in the experimental products that range from ~NNO+0.5 for experiments in the absence of graphite to values less than ~NNO−1.5 for experiments in the presence of more than 14 wt. % graphite. The interaction of Fe₂O₃ and, possibly, H₂O, released as a result of peritectic melting reactions of the initial schist minerals (primarily micas) with graphite provides the formation of CO₂. Modeling of phase relations showed that along with oxygen fugacity, water activity could be an additional factor influencing phase compositions in the presence of graphite. Raman spectroscopy of quenched melts and bubbles in them demonstrates that CO₂ is not only the predominant component of the free fluid phase accompanying the melts, but is also partially dissolved in the melt as molecular CO₂ and CO₃²⁻ complexes with alkaline and alkaline earth cations. Experiments demonstrate that under conditions of high-grade metamorphism, graphite-bearing metapelites can serve as an effective internal source for CO₂ accompanying granite melts during anatexis.