Meta-Ultramafic Rocks of the Maksyutov Complex, Southern Urals: High-Pressure Si–Al Metasomatism and Carbonatization at the Crust–Mantle Interface in the Subduction Zone

Abstract—The Maksyutov eclogite–blueschist Complex is characterized by a complicated fold-and-thrust structure that has been formed during the Late Devonian collision between the subducting Baltica margin (East European Plate) and the Magnitogorsk island arc. Eclogites are the most studied rocks of the Complex; their formation and exhumation are usually associated with the collisional stage of the orogen development. At the same time, the origin of meta-ultramafic rocks, which together with eclogites form sheets and boudins within metasedimentary rocks (schists 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 meta-harzburgites, and chlorite−antigorite metaorthopyroxenite. Mineral compositions and textural relationships between minerals in the meta-harzburgites 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. Partial replacement of olivine by orthopyroxene-bearing parageneses with Cr−Al antigorite and/or high-chromium chlorite is established for the second stage. The phase equilibria modeling using the Perple_X software package demonstrates that formation of antigorite−orthopyroxene paragenesis was associated with Si−Al metasomatism at: T ~ 630°С, P ~ 2 GPa, log \({{a}_{{{\text{Si}}{{{\text{O}}}_{{\text{2}}}}}}}\)   ~ −0.6, log \({{a}_{{{\text{A}}{{{\text{l}}}_{2}}{{{\text{O}}}_{3}}}}}\) ~ −2.5. It is important to note that the mineral paragenesis is highly sensitive to \({{a}_{{{\text{Si}}{{{\text{O}}}_{{\text{2}}}}}}}\) : a slight decrease in log  \({{a}_{{{\text{Si}}{{{\text{O}}}_{{\text{2}}}}}}}\) 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 \({{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}\) = 0.01–0.05 in H₂O–CO₂ fluid showed the replacement of 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 selective fluid influence on different rock types is interpreted as a result of their different tectono-metamorphic evolution: meta-ultramafic rocks are fragments of the suprasubduction mantle, which were tectonically juxtoposed with the rocks of the subducting plate (eclogites and metasedimentary rocks).