U-Pb Dating, Palaeontological and Lithological Characteristics of the Lower Devonian Salairka Formation

The Salairka Formation clearly comprises two main rock types: carbonate rocks, which formed in calm shallow-marine shelf environments, as indicated not only by their lithology but also by the presence of remains of carbonate reef-building fauna such as tabulates and rugosans. The second type is terrigenous rocks, which at certain levels replace carbonate sedimentation. Palaeontologically, the section is characterized by various faunal groups—tabulates, rugosa, trilobites, brachiopods, ostracods, and conodonts, among which the following taxa were identified: Pandorinellina philipi (Klapper), Pand. exigua exigua (Philip), Caudicriodus amplus (Izokh), Caudicriodus vicinus (Izokh) and Polygnathus kitabicus Yolkin, Weddige, Izokh et Erina, which characterize the Pragian–Emsian interval. The level of the Pragian–Emsian boundary is unequivocally indicated by the appearance of conodonts of the species Po. kitabicus, which occurs near the base of the upper part of the Salairka Fm. Terrigenous rocks of the Salairka Fm are represented by pale grey argillites and greywackes of various compositions. Petrographic study of thin sections of the greywackes showed that the clastic fraction is characterized by a high content of volcanic and carbonate rock fragments, as well as angular grains of quartz and feldspars. A characteristic feature is the abundance of clay-carbonate material, which often acts as cement. The alternation of these lithotypes indicates instability of the depositional environment: periods of calm carbonate formation were replaced by stages of active input of clastic material, while its poor sorting indicates proximity to the source area. U-Pb geochronological studies (LA-ICP-MS) were carried out on zircons from an argillite unit at the base of the section. The study showed that for most detrital zircon grains (31 grains), the age falls within the range of 440–415 Ma. At the same time, the main zircon population (29 grains) is of Silurian age, with the main peak at 424 Ma. Individual zircon grains (3 grains) have an Early Devonian age in the interval 415–418 Ma. The age of the oldest zircon grain is 495 Ma. Since the studied zircon sample is small, the geochronological data require further investigation. Since the Salair Block is composed predominantly of Cambrian mafic volcanic rocks, the predominantly Silurian age obtained for the detrital zircons cannot be attributed to these rocks. Study of the geochemical composition of these rocks showed that the contents of the major oxides (SiO2, Al2O3, Na2O, K2O, TiO2) are similar to an andesi-basaltic composition, which also excludes the influence of mafic rocks on the formation of this unit. According to published data, Silurian magmatic formations within Salair have been established only for biotite-bearing leucogranites constituting the main part of the Ulanovka Massif, dated at 426.0 ± 1.8 Ma. The Ulanovka diorite-granite massif is located in the northern part of the Salair Block, approximately 100 km from the studied area. The mechanism of formation of this massif has not yet been fully determined, but the similarity in chemical composition and the age boundaries of rock formation may indicate a genetic relationship between the deposits of the Salairka Fm and the Ulanovka Massif. The occurrence of one grain with an older age suggests a possible minor contribution of the Cambrian basement of the Salair Block to the formation of these rocks.