Early Cambrian U/Pb (SIMS) Zircon Age from Carbonate-Silicate Apovulcanogenic Metasomatites in the Chulaksaj Formation Stratotype Section in the East of the South Urals

Abstract— In the stratotype section of the Chulaksaj Formation, in the eastern part of the Southern Urals, carbonaceous shales predominate, which are interbedded with thin horizons of andesites, andesibasalts, basalts, as well as horizons of carbonate-silicate metasomatites and argillisites formed from intermediate volcanic rocks. The composition of volcanogenic rocks suggests their suprasubduction genesis. Among carbonate-silicate metasomatites, zeolite–kaolinite–carbonate varieties predominate, from which zircon has been isolated. Zircon has a magmatic origin, with a concordant age of 532 ± 5 Ma based on the U/Pb method (SIMS). The obtained value defines the age of the volcanogenic protolith of metasomatites and suggests the Early Cambrian age of the Chulaksaj Formation, which could have formed in the back-arc zone of the active continental margin.