Природа цирконовой кластики в песчаниках рифея и венда Южного Урала

Abstract

New age dates of detrital zircons of terrigenous rocks augmented the possibilities of interpretation of their provenance. Unfortunately this interpretation is restricted by a formal comparison of age-and-composition characteristics of detrital crystals with any very distant model objects. The paper deals with a situation when the role of a source of a detritus is claimed by local objects. When comparing the age parameters of primary and detrital crystals of zircons, the data on Riphean volcanics and ancient metamorphics of the Taratash complex of the Southern Urals were used. Specifying the ideas on the nature of the zircon clastics (detritus) and its relationships with primary zircons of sources, a role of processes of mechanical abrasion is pointed out, leading to a clearing of heterogenous primary grains of defect crystals, which results in an accumulation of crystals of more homogenous appearance. The analysis of SHRIMP and TIMS-dates of zircons and U and Th concentrations in them, and also a comparison of histograms of primary zircons from Riphean volcanics and rocks of the Taratash complex on one hand and the detrital zircons from the Vendian and Riphean sandstones of the Southern Urals on the other, have shown that the age variations of both are rather comparable. It means that the age characteristics of primary zircons from the Riphean volcanics and rocks of the Taratash complex as sources of zircon clastics for the Riphean and Vendian sandstones are regulated by processes of resedimentation, and a detrital fraction of zircons is formed at the expense of local objects. The participation of very distant sources is not excluded, but in our case it is not detected.