Urukul-Pokrovsky rhyolite-trachyrhyolite complex (Middle Urals): New geochemical data

Research subject. Rocks of the Urukul-Pokrovsky rhyolite-trachyrhyolite complex in the Alapaevsk-Adamovsky structural-formational zone of the East Ural megazone form necco and dike-like bodies among early-middle Devonian volcanic and volcanic-sedimentary deposits. Aim. To determine the geodynamic regime of formation of these deposits based on their isotope-geochemical characteristics. Methods. The contents of petrogenic oxides were determined by X-ray fluorescence analysis using SRM-18 and EDX-900HS (Na2O) devices; the contents of rare elements were determined by the ICP-MS method using an ELAN 9000 analyzer. The isotope ratios of Rb and Sr were measured by a Neptun Plus multicollector inductively coupled plasma mass spectrometer and a Triton thermal ionization mass spectrometer. Results. The rock complex was found to comprise rare-porphyry trachyrhyodacites and rhyolites. These are high-potassium deposits with a potassium-sodium type of alkalinity, belonging to the ferruginous alkali-calcic series. They are characterized by high values of the alumina saturation index (A/CNK). The rocks are enriched in Rb, U, Th, Pb, Zr, and Y, exhibiting low concentrations of Sr and lanthanides. In terms of chemical composition (higher alkalinity, increased concentrations of Al and K, increased contents of Cs, Rb, Ba, Hf, Ta, Zr), the rocks differ from the associated abundant porphyry rhyolites of the Rudyanka Formation. In geodynamic diagrams, the points of rock compositions are located in the field of post-collisional formations, and the Y/ Nb value may indicate their plume-dependent nature. Conclusions. The Urukul-Pokrovsky complex might have formed under the conditions of local extension at the stage of rigid collision in the Early Permian

rhyolite, trachyrhyolite, Urukul-Pokrovsky complex, Urals, geochemistry, strontium isotopes, geodynamic setting