Geochronology of zircons from gneisses of Vishnevogorsky sequence of Ilmeny-Vishnevogorcky complex (South Urals)

The article presented the results of isotopic-geochronological studies of zircons from the garnet-biotite plagiogneiss of the Vishnevogorsky series of Ilmeny-Vishnevogorsky complex (southeastern shore of the Sungul Lake, Southern Urals). Isotopic analyzes (U/Pb zircon, SHRIMP II) were performed using the ion microprobe at IRC VSEGEI by standard methods. Analytical studies were preceded by mineralogical investigations. We selected sites in heterogeneous grains corresponding to an important stage in their development. The maximum age of the substrate of plagiogneiss was Neoarchean (2700 Ma). The nature of zircons corresponded to the concepts of their polygenicity and polychronicity. Rounded fragments zircon crystaled reflect sedimentary rocks involved in substrate of plagiogneiss and the structure of some of the zircons indicated their primary magmatic formation. In the time interval of 2200-1060 Ma, Paleoproterozoic (³ 1740 Ma) granulite metamorphism and two (1500 and 1090 Ma) transformation stages were recorded. Late generations of zircons (Th/U = 0.004-0.08; 460, 350-380, and 270-300 Ma) reflected the final processes of metamorphism - metasomatism. The age of plagiogneisses of the Vishnevogorsky series was older than the gneiss of the Selyankinsky block (2080 Ma).