Age of rocks of the Shurmak Formation according to U-Pb dating of zircon by LA-ICP-MS method (south-eastern Tuva)

Abstract

In southeastern Tuva, early Proterozoic volcanogenic-sedimentary deposits were studied, including conglomerates from the Shurmak Formation. As a result of U-Pb zircon dating from tuff with andesitic composition, conglomerate matrix and volcanicomictic sandstones from this formation, it was established that it was accumulated at around 500 Ma. The dating results for zircons from tuff are a geochronological date indicating the time of manifestation of andesitic volcanism synchronous with accumulation of Shurmak Formation deposits. Given the close age of zircons of the main populations in both tuff and clastic rocks, one can assume that the formation of the latter is related directly to the manifestations of volcanism on this territory at the time. Zircons with Neoproterozoic and Paleoproterozoic ages are present in matrix of conglomerates and volcanicomictic sandstones of Shurmak Formation in the form of rock fragments and in tuffas in the form of xenogenic minerals. This, together with a result from Sm-Nd isotopic dating indicated that these vocanic and sedimentary processes in the early Cambrian time took place within the range of the Precembrian block of earfth’s crust. The main supplier of clastic material during the formation of the Early Cambrian Shurmak Formation were volcanic events synchronous with sedimentation. Other sources of clastic material entering the sedimentation basin were Neoproterozoic granitoids and, less commonly, Paleoproterozoic and Neoarchean rocks. Combination with the underlying rocks of the Mugur Formation of the Erza metamorphic complex occurred as a result of tectonic processes in the post-Cambrian time, since the sedimentation of the Shurmak Formation deposits and the processes of granulite metamorphism in the rocks of the metamorphic complex occurred almost simultaneously.