Polychronous zircons of volcanics of the Navysh complex of the Lower Riphean Ai Formation (Southern Urals)

Abstract

The volcanics of the Navysh complex of the Lower Riphean Ai Formation in the Southern Urals are well studied petrochemically and dated by several methods. In 2013 zircons from a trachybasalt porphyrite (sample 2152) gave a concordant SHRIMP date 1752±11 Ma, which was used as a fundamental for the lower boundary of the Riphean with no special arguments against it. The later attempts to repeat this date for the Navysh volcanics were not successful: the collected zircons were either more ancient (> 2500 Ma), or more young (< 500 Ma). From the beginning, the zircons with such ages were regarded as xenogenic or secondary metasomatic, or belonging to paleozoic dykes intruding the Riphean volcanics. However, the clearly expressed mineralogical properties of the Paleozoic zircons and their frequent presence in volcanics, not dykes, led to a conclusion that the zircons and Navysh volcanics, containing them, and exposed within the area of development of the Ai Formation, are polychronous. To support this conclusion, the authors studied in more detail the zircons of the Navysh trachybasalts, developed in the Ai Formation. The main conclusion, obtained from this new data, was that the volcanics attributed to the Navysh complex, form a polychronous system, including both the Lower Riphean (1750 Ma) and Paleozoic (450 Ma) rocks. The zircons of these age groups differ in their mineralogical and geochemical properties supporting the idea that they belong to different primary sources which may be due to repeating plume processes, which partly reanimated – heated and melted-rocks of the previous cycle and/or created new sources of melts.