Vol 32, No 1 (2024)

Based on the proposed numerical model of the stress-strain state of polymineral rocks, which describes the formation of blastomylonites in the Yenisei Regional Shear Zone (PRSZ) in the Yenisei Ridge, the possibility of local tectonic overpressure exceeding the lithostatic pressure in rocks subjected to shear deformations is shown. For tectonites of the southern (Angara-Kan block) and northern (Isakovka terrane and Garevka complex) segments of the PRSZ, estimates of the maximum overpressure were obtained from 2–3 to 4–5 kbar, which range from 25 to 50% of the lithostatic pressure. It is shown that excess pressures can be preserved in a local volume on a geological time scale sufficient for their fixation in metamorphic minerals. Model values of overlithostatic pressure in garnet-amphibole tectonites and geobarometric estimates of peak values during stress metamorphism allow us to offer new evidence of pressure inhomogeneity in natural mineral associations. Using the results of numerical modeling for the evolution of fault metabasite blastomylonites, it was established that the overpressure at the stage of syn-deformation metamorphism in the shear zone are possible at temperatures up to 600–650°C and not reaching 800°C; the presence of fluid or partial melt prevents the occurrence of overpressure. The amount of excess pressure due to shear stresses depends on the mineral composition and structure of the rock.

The eclogite-blueschists Maksyutov Complex is characterized by a complex fold-and-thrust structure that was developed during the Late Devonian collision between Baltica (East European Plate) and the Magnitogorsk Arc, that was formed during the Early Devonian intraoceanic subduction. Eclogites are the most studied rocks of the complex; their formation and exhumation are usually associated with the collisional stage of orogen development. At the same time, the origin of meta-ultramafic rocks, which together with eclogites form sheeted and lenses within metasedimentary rocks (shales and quartzites), still remains unknown. This paper presents the results of the first detailed petrological study of meta-ultramafic rocks, represented by antigorite-chlorite and magnesite-antigorite metaharzburgites, chlorite-antigorite metaorthopyroxenite. Mineral compositions and textural relationships between minerals in metaharzburgites indicate at least two stages of rock transformations. Minerals of the early mineral paragenesis (first stage) – olivine, accessory chromite and low-fluorine Ti-clinohumite – have a metamorphic genesis; ultrahigh-pressure (UHP) conditions of their formation are discussed. At the second stage, there was a partial replacement of olivine by orthopyroxene-bearing parageneses with Cr-Al antigorite and/or high-chromium chlorite. Based on the phase equilibria modeling using the Perple_X software package, it was found that the formation of antigorite-orthopyroxene paragenesis was associated with Si-Al metasomatism at: T ~ 630°С, P ~ 2 GPa, loga_SiO₂ ~ –0.6, loga_Al₂O₃ ~ – 2.5. It is important to note that the mineral paragenesis are highly sensitive to a_SiO₂: a slight decrease in lga_SiO2 relative to the above value would lead to the growth of olivine with antigorite, and an increase would lead to the growth of orthopyroxene. The latter may explain the formation of meta-orthopyroxenites, which are widely distributed among the meta-ultramafic rocks of the Maksyutov Complex. Similar calculations performed for the range of X_CO₂ = 0.01–0.05 in H₂O-CO₂ fluid showed replacing silicate minerals by magnesite under the established thermodynamic conditions. Carbonation and Si-Al metasomatism are specific features of high-pressure transformations of meta-ultramafic rocks, which have not been established in the associated eclogites, quartzites, and shales. Such selectivity of fluid influence on different rock types is interpreted as a result their different tectono-metamorphic evolution: meta-ultramafic rocks are fragments of the suprasubduction mantle, which were tectonically combined with the rocks of the subducting plate (eclogites and metasedimentary rocks).

The temperature and compositional parameters of the parental magma of the ore-bearing apophysis DV10 from the Yoko-Dovyren massif are estimated based on the results of thermodynamic modeling the equilibrium crystallization of melts of 24 samples, following the method of geochemical thermometry. Thermometric calculations were carried out using the COMAGMAT-5.3 program with a step of 0.5 mol. % to a maximum degree of crystallization 75–85% under oxygen fugacity controlled by the QFM buffer. The model order of mineral crystallization corresponds to the sequence: olivine (Ol) + + Cr-Al spinel (Spl) → plagioclase (Pl) → high-Ca pyroxene (Cpx) → orthopyroxene (Opx). Silicate-sulfide immiscibility was modeled to occur mostlybefore the onset of plagioclase crystallization, being consistent with sulfide saturation of the parental magma. The results of calculations demonstrate the convergence and intersection of the model liquid lines of descent at temperatures of about 1185°C. When applied to the average composition of the DV10 apophysis, this temperature indicates the existence of a suspension of the original crystals (52.1 wt. % cumulus olivine (Fo_83.6), 2.3 wt. % plagioclase (An_79.7), 0.24% clinopyroxene (Mg# 88.8), 1 wt. % aluminochromite (Cr# 0.62)) and about 0.2% sulfide liquid in a moderately magnesian melt (53.6 wt. % SiO₂, 7.4 wt. % MgO). At that the solubility of sulfide sulfur (SCSS) was estimated to be 0.083 wt. %. This heterogeneous system had a viscosity of 4.71 log. units (Pa · s) and an integral density of 2929 kg/m³. Such rheological properties do not contradict the possibility of the migration and emplacement of the protocumulus mush from the main Dovyren chamber. However, a more probable scenario includes a localized accumulation of olivine in the trough-like part of the DV10 subchamber, which preceded or occurred in parallel to the accumulation of segregated sulfides.