40Ar-39Ar dating of pegmatites from Murzinka-Adui region (Middle Urals): Results and their geological interpretation

Research subject. The pegmatites of Murzinka-Adui region (Ethtern sector of the Middle Urals), represented by three types of veins miarolitic pegmatites with topaz and beryl (mines Mokrusha, Kazennitsa, Semenovskaia), miarolitic rubillite-lepidolite pegmatites (Lipovka veins field) and rare metal pegmatites with Ta-Nb-Be mineralization (deposits Kvartal’noe and Lipovyi Log).Materials and methods. Isotope dating was carried out via the 40Ar-39Ar method by a mass spectrometer Micromass 5400 at the Institute of Geology and Mineralogy SB RAS for mica of different composition (muscovites, biotites and lepidolites).Results. 40Ar/39Ar ages for mica from dated pegmatite veins of the topaz-beryl type coincide within the measurement accuracy: mine Kazennitsa – 252.3 Ma, Mokrusha – 253.7 and Semenovskaia – 250.3 Ma. The average age of the two series of mica from rubillite-lepidolite pegmatites was found to be 254.1 Ma. The average age of four samples of muscovites from rare metal Ta-Nb-Be pegmatites equals 252.6 Ma.Conclusions. The closure of the isotope system of micas in pegmatites of all three studied types (topaz-beryllium, rubillite-lepidolite and rare-metal with Ta-Nb-Be mineralization) occurred almost simultaneously during the timeframe of 254 to 250 Ma years ago. The obtained age values, however, cannot be identified with the time of formation of pegmatites. The analysis of the available data suggests that the closure of the micas K-Ar isotope system of the studied pegmatites as well as previously dated schists and blastomylonites of the Bazhenovo suture zone and granites of the Murzinka-Adui block marks an important episode in the geological history of the eastern margin of the Urals and the basement of the adjacent part of the West Siberian plate: the change of the transpression regime prevailing during the collision stage of the region’s development to the regime of limited postcollision extension.