Magmatism in the epochs of ural paleocean closure and mobile belt development: Composition, distinctive evolutionary features, and sources

The paper reports major evolutionary trends of Paleozoic magmatism in the Urals in relation to the closure of the oceanic basin and associated subduction, obduction, and collision. An attempt is undertaken to use these data as a basis for reproducing the geodynamic and thermal regime during various stages of the regional geological evolution. Distinctive compositional features of the rocks, the discrete character of magmatism reflecting changes in the geodynamic environments, and the transition from predominantly mantle sources of magmatism (at 460-380 Ma) to mantle-crustal (at 360-310 Ma) and finally crustal (290-250 Ma) ones, alternating epochs of relatively “dry” and “hydrous” mantle magmatism at 460-430 Ma and the “hydrous” type of later magmatism make it possible to reproduce the thermal regime of the orogen. During the intense low-hydrous basite magmatism (460-430 Ma), the temperature in the magma generation region was 1300-1100°C and decreased to 1100-900°C during predominantly hydrous magmatism (425-400 Ma). The P-T parameters of hydrous anatexis (360-310 Ma) that gave rise to tonalites and granodiorites were 7-9 kbar and 800 ± 50°C; they constrain the maximum temperature in the lower crust. The next stage of the hydrous anatexis of tonalites and granodiorites resulted in intense granite-forming processes (290-250 Ma) and occurred at pressures close to 5-6 kbar and temperatures of 700 ± 50°C. In the situation of the adiabatic heat transfer with fluids, the minimum temperatures in the upper mantle in the Ural suprasubduction epioceanic mobile belt at a depth of approximately 80 km can be provisionally estimated as follows: 1300 ± 100°C at 450-425 Ma, 1100 =- 100°C at 425-400 Ma, and 900 ± 100°C at 375-250 Ma. Data reported in the paper testify that the temperature in the upper mantle and crust of the Ural Mobile Belt was 150-200°C lower than in neighboring oceanic and continental areas, and this places the Urals among orogens of the “cold” type (Maruyama et al.,2007).