Vol 33, No 3 (2025)

Petrological and geochronological (zircon, U-Th-Pb (LA-ICP-MS)) studies of rocks from the Poriya Guba tectonic mélange exposed on Ozerchanka and Palenyi islands were carried out to decipher composition and tectonic history of the Paleoproterozoic Lapland-Kola orogen (LKO). Tonalite-trondhjemite-granodiorite (TTG) (Grt)-Cpx-Opx gneisses dominate on the Ozerchanka island. They contain numerous bodies of mafic granulites and are intruded by syn- and post-tectonic granitoids. The TTG gneisses are Archean in age (>2.6 Ga, Т_Nd(DM) = 2.9–3.0 Ga). These are strong depleted in heavy REEs, which indicates that their parental melts of their igneous protoliths were formed in equilibrium with a garnet-bearing restite. Mafic granulite bodies vary widely in geochemical characteristics and likely represent fragments of several Paleoproterozoic mafic intrusions and dikes. Mineral assemblages in the gneisses recorded early granulite-facies (Т = 780–820°С and Р = 8.6–9.4 kbar) and later amphibolite-facies (Т = 640–650°С and Р = 6.7–7.3 kbar) metamorphic events that occurred at 1.9 Ga according to zircon ages. On the Palenyi island, Grt-Cpx-Opx gneisses are predominant and vary in composition from andesibasalts to rhyolites. The volcanic protoliths of these rocks have island-arc geochemical signatures, a Paleoproterozoic age of 1958 ± 6 Ma, and juvenile origin (ε_Nd(1960) = +1.7 ÷ +3.1; Т_Nd(DM) = 2.2–2.3 Ga). These rocks were metamorphosed under granulite facies conditions at about 1.9 Ga. Two models can explain the presence of the Archaean Ozerchanka block which occurs in the Poriya Guba tectonic mélange composed of the Paleoproterozoic juvenile island arc complexes in the core of the LKO. First, this Archean block could represent a single fragment of Archean lithosphere that was separated during Paleoproterozoic continental rifting and the opening of the Lapland-Kola ocean and subsequently was tectonically juxtaposed with Paleoproterozoic subduction complexes during the Lapland-Kola collisional orogeny. Second, the Archean Ozerchanka block may represent the edge of an adjacent Archean continent exposed in an erosional window within the Paleoproterozoic Poriya Guba tectonic mélange.

The parental larnite-normative alkaline ultramafic melt consistently forming olivinite and olivine-monticellite rocks of the Krestovskaya alkaline-ultrabasic carbonatite intrusion is enriched with hydrocarbons (HC) and their derivatives, nitrogenated, chlorinated, fluorinated, sulfonated compounds, as well as H₂O and CO₂ according to pyrolysis-free gas chromatography-mass spectrometry data (GC-MS). The aliphatic, cyclic, oxygenated compounds, and very few heterocyclic compounds are determined among the hydrocarbons. During the crystallization of olivine in olivinites, fluids are enriched in hydrocarbons (59.30 rel. %), excluding nitrogenated, chlorinated, and sulfonated derivatives and including predominant amount of oxygenated compounds (52.17 rel. %) and subordinate amount of aliphatic and cyclic compounds (6.70 rel. %). During the crystallization of perovskite in olivine-monticellite rocks, the amount of oxygenated hydrocarbons slightly decreases (34.77 rel. %) and aliphatic and cyclic compounds increases up to 10.55 rel. %. The crystallization of monticellite is accompanied by the predominance of aliphatic and cyclic hydrocarbons (59.67 rel.%) and subordinate amounts of oxygenated hydrocarbons (29.35 rel. %). The fact that the calculated H/(O + H) ratio is 0.78 and 0.77 for fluids in olivine and perovskite, respectively indicates the reducing conditions of crystallization of these minerals. On the stage of olivine crystallization of olivinite, the fluids also contain 4.1 rel. % of nitrogenated, 4.58 rel. % of sulfonated, 0.19 rel. % chlorinated, 0.12 rel. % fluorinated hydrocarbons, 0.49 rel. % CO₂, and 31.17 rel. % H₂O. The crystallization of perovskite in olivine-monticellite rocks is accompanied by further accumulation of nitrogenated compounds up to 8.95 rel. %, sulfonated (9.53 rel. %) and chlorinated (11.33 rel. %) hydrocarbons, and 16.48 rel. % CO₂. In this stage the content of H₂O in the fluids decreases to 7.66 rel. % due to its binding to cations and Al-Si-radicals of the melt into hydroxyl-bearing compounds. At the final stage of crystallization of perovskite and initial monticellite, when fluids are saturated by critical amounts of chlorinated, nitrogenated and sulfonated compounds and CO₂, they become to dissolve in the melt and react with it: most of the considered fluids, together with Ca and alkalis of the melt, form carbonate-salt compounds and the melt became silicate-salt composition. According to GC-MS analysis data, residual fluid phase of monticellite-hosted inclusions are characterized by only 2.29 rel. % nitrogenated and 1.11 rel. % sulfonated, 0.32 rel. % chlorinated, and 0.35 rel. % fluorinated hydrocarbons, 0.04 rel. % CO₂ and 6.15 rel. % H₂O with an increase in hydrocarbons up to 89.63 rel. %. During the crystallization of monticellite, silicate-salt immiscibility occurred, followed by spatial separation of the silicate and salt fractions.

The solubility of phenakite (Be₂SiO₄) in granite melts was experimentally studied at temperatures of 1000 and 1100°C and pressures of 1 and 4 kbar in dry conditions and in the presence of 10 wt. % H₂O. The starting materials were granite glasses with agpaitic coefficient of 1–2.5 and natural phenakite. It was found that the solubility of phenakite increases with increasing agpaitic coefficient (Na + K)/Al of the melt, and the solubility of BeO is higher in hydrous melts than in dry ones. The solubility of phenakite also increases with pressure. The obtained experimental data were generalized with the previous data in the form of an equation describing the solubility of BeO in alkaline-granite melts coexisting with crystalline phases of Be, depending on the agpaitic coefficient, temperature and pressure. The results of the experiments and their generalizations support the model of Be concentration in alkaline water-containing melts – products of differentiation of granite magmas.