Clastic Sources for the Upper Riphean Arkose Complex of the Southern Urals: Some Geochemical Constraints

The trace element systematics (Rb, Hf, Y, Zr, Nb, Yb, Ta, and other trace elements) is considered in sandstones and fine-grained clastic rocks (shales and argillaceous siltstones) of the Upper Riphean arkose complex of the Southern Urals (Biryan Subformation of the Zilmerdak Formation, Karatau Group). These rocks were formed during the first sedimentation cycle; therefore, it can be assumed that their lithogeochemical features largely reflect the petrogeochemical characteristics of source rocks, in this case, granitoids. An analysis of the distribution of data points of arkose and subarkose sandstones and fine-grained clastic rocks in different discriminant diagrams allows us to conclude that the main sources of clastics for the Upper Riphean arkose complex were volcanic-arc (VAG) and collisional (COLG) granites. The contribution of within-plate granites (WPG) is also possible, which is clearly seen in most of the Pearce diagrams and in the Hf–Rb–Ta and Zr–(Nb/Zr) PM diagrams. According to their petrogeochemical characteristics, these granites could belong to alkaline and magnesian varieties and were close to S-granites. This assumption is also supported by the wide spectrum of U–Th–U isotope ages of detrital zircons present in the arkose sandstones of the Biryan Subformation of the Zilmerdak Formation, which possibly reflect the age range of zircon xenocrysts inherited from the melt source(s). All these facts do not provide information on definite massifs and their distribution areas, but allows us to significantly specify information about the possible nature of granite sources for clastics of the Upper Riphean arkose complex of the Southern Urals.