U–Pb Age of Zircons from Gneisses of the Mariinskaya Formation, Southern Urals, and Problems Related to the Precambrian in the Trans-Urals

Abstract The paper presents results of U–Pb SHRIMP-II dating and describes some features of zircon grains from unmigmatized plagiogneisses of the Mariinsky Formation, Trans-Urals. It is shown that the zircon consists of rims, which grew as a result of granulite-facies metamorphism no older than the Late Riphean, and cores of detrital origin, dated within the range from the Archean–Proterozoic boundary to Late Riphean. The data suggest that before the onset of migmatization, the Mariinsky gneisses were formed independently of the Paleozoic Ural formations.