High-pressure metamorphism in the area of the Main Ural fault in the Severnaya Sosva River basin (Northern Urals)

Research subject. Metamorphic rocks of the Khomasya formation of the Lower-Middle Ordovician in the Northern Urals (the basin of the Severnaya Sosva River) in the area of the Main Ural fault (GUR) were studied. Aim. Reconstruction of the formation environment of methamorphic rocks. Materials and Methods . Geochemical and petrological-mineralogical studies were performed, along with geothermobarometry coupled with mineral formation modeling. Results. The geochemical features of the basalts of the Khomasya formation rocks indicate an enriched mantle source. The spatial association of metabasalts with arkose, quartz sandstones, and siltstones suggests that the accumulation of strata occurred during the transition from continental rifting to oceanic spreading, and that the structure under study is a fragment of an Early Paleozoic passive volcanic continental margin. Against the background of widespread metamorphism of the facies of green shales, zones of glaucophane-containing rocks are mapped near the GUR, and garnet-containing chlorite-epidote-amphibole-muscovite-quartz-albite shales are observed in tectonic lenses. Amphiboles in the GUR area are represented by actinolites, vinchites, barroisites, glaucophanes, and magnesian hornblende; garnets demonstrate a progressive (direct) zonality. White micas are represented by phengites, and sometimes there are relics of paragonite in the inner zones of garnet crystals. Garnet-containing rocks were formed at pressures not lower than 7–8 kbar and at temperatures up to ~ 600°C. Conclusions. It is assumed that the mineral composition of rocks and the thermodynamic regime of their formation correspond to the geodynamic regime of subduction (granite-free glaucophane shales) followed by a transition to the collision regime (paragenesis with garnet, actinolite, and hornblende), which agrees with the results of isotope dating of rocks and the revealed P-T parameters of their metamorphism.