Metamorphism of uni suite as an indicator of early precambrian collision processes in the vyatka belt, basement of the volga-uralia

In the central part of the Vyatka belt, sandwiched between the Archean blocks in the northeast of the Volga-Ural segment of the East European craton, according to the core data from two deep wells, the following mineral parageneses were established in the metapelites of the Uni suite: (1) Pl + Ms + Bt + Qz + Kfs + And ± Chl and (2) Pl + Bt + Qz + Kfs + Grt + Sil ± Ms ± Chl (without staurolite and cordierite), corresponding to moderate pressure metamorphism of the amphibolite facies. To determine the P-T conditions for these parageneses a set of methods was used, including classical (Grt-Bt, GASP, Ti-in-Bt, Ms-Bt) and multi-equilibrium thermobarometry (winTWQ 2.34), as well as the isopleth intersection method on pseudosection diagrams (GeoPS 3.2.2.128). The calculated P-T values of metamorphism are 520–650°C (or up to 690°C according to the isopleth method) and 2–5.4 kbar. Paragenesis (2) rock was accompanied by anatexis in the water-saturated system. Zoning can be distinguished in the metamorphism of the rocks, it belongs to a relatively shallow type, to the andalusite-sillimanite subfacies.