The first results of U-Pb isotope dating of detrital zircons (dZr) from red-colored quartzitic-sandstones of the Shoksha formation (Shoksha horizon) are presented. The Shoksha formation completes the Vepsian sub-horizon (Vepsian) of the Lower Proterozoic of Karelia and is distributed within the South Onega trough. A sample (KL-555) of red-colored quartzitic sandstones was taken from the lower part of the section of the Shoksha formation in the same name deposit within the southwestern Cis-Onega Lake region. The 79 dZr grains isolated from this sample were analyzed by the staff of the Chemistry-Isotopic Analytic Laboratory of the GIN RAS using the equipment of the Shared Research Facilities of the GIN RAS. The weighted average of the three youngest U-Pb isotope dates for dZr grains is 1906±13 Ma. Taking into account the known isotopic dates of gabbro-dolerites from the Ropruchei sill, that cuts through the Shoksha formation, it makes possible to constrain the time of the Shoksha formation accumulation by ~1.90–1.75 Ga. A significant part of the carried out analyzes has yielded a high degree of discordance of the dates. The features of the distribution of the figurative points of these analyzes in the diagram with concordia suggest that the rocks of the studied section of the Shoksha formation were subjected to the alteration that disturbed the U-Pb isotope system of these zircon grains in the Phanerozoic.The set of obtained dates for dZr grains has been compared with the known ages of the crystalline complexes of the basement of the East European Platform. The age sets of dZr grains from sample KL-555 and rocks of the Ladoga group, developed along the margin of the Svecofennian accretionary orogen, are very similar (p similarity coefficient in Kolmogorov – Smirnov test is 0.27) and characterize mainly tectonic–magmatic events that had immediately preceded the manifestation of the Svecofennian orogeny (1.9–1.87 Ga). Therefore, the rocks of the Ladoga group could highly probably be a secondary source for the Shoksha quartzites. Based on a comparative analysis of ages and thorium-uranium ratios (Th/U) in dZr grains from sample KL-555, it was concluded that some of the studied dZr grains with high Th/U>1.5 originate from Ludicovian mafic rocks, but those with low Th/U<0.1 originate from ultra-high-pressure formations, such as eclogites known in the Salma, Kuru-Vaara and Gridino.A paleo-geographic scheme for the Late Vepsian is proposed, showing that the highly mature Shoksha sandstones were generated under continental conditions in a local basin due to the accumulation of clastic material carried by an extensive and branched
The article presents the results of U-Pb isotope dating (SHRIMP-II, VSEGEI, Saint Petersburg) of zircon crystals extracted from plagiorhyolites of the Monakh Cliff in the area of Cape Fiolent in the western part of the Mountainous Crimea (southern suburb of Sevastopol). a concordant age estimate of 168.3±1.3 Ma was obtained from 20 zircon crystals. It exactly corresponds to the Bajocian/Bathonian boundary of the Middle Jurassic according to the International Chronostratigraphic Chart (February 2022 version). The available results of isotope dating of igneous rocks from the Mountainous Crimea, as well as their geochemical typification are synthesised. The plagiorhyolites of the Monakh Cliff in the area of Cape Fiolent are spatially, and most likely paragenetically, associated with the wallrock (Cape Vinogradny) and ore (Heraclea Plateau on the cognominal peninsula) massive sulphide formations, as well as pillow basalts, gabbroids, and serpentinized hyperbasites, combined into the ophiolite association of Cape Fiolent. The obtained dating is the upper age limit for the entire ophiolite association of Cape Fiolent.
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