On the Time and Conditions of Formation of the Shoksha Quartzite-Sandstones of the South Onega Depression Based on the New Data From Isotope Geochronology

Abstract

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

Paleoproterozoic Tectonics and Evolutionary Model of the Onega Synclinorium

“…Накопление терригенных отложений вепсия происходило в континентальной аллювиально-озерной обстановке в условиях аридного климата [Galdobina, Mikhailyuk, 1971;Sokolov, 1987]. Судя по ориентировке косой слоистости, знаков ряби и возрастному набору зерен детритового циркона, снос обломочного материала происходил преимущественно из областей, расположенных непосредственно севернее вепсийского бассейна [Kuznetsov et al, 2023].…”

“…Дополнительные складки обычно пологие и весьма немногочисленны. Шире распространены мелкие (0.5-3.0 м) асимметричные складки подводного оползания [Kuznetsov et al, 2023].…”

“…9, д). В бассейне накопились молассоидные и красноцветные терригенные осадки, а поставщиком обломочного материала был разветвленный седиментационный поток, дренировавший расположенные севернее части КК [Kuznetsov et al, 2023].…”

“…Для понимания механизмов возникновения и длительного развития ОС необходим комплексный анализ особенностей осадконакопления, магматизма и структурообразования. В настоящей статье с целью разработки модели эволюции ОС приведен анализ геологоструктурных данных авторов [Kolodyazhny, 1999[Kolodyazhny, , 2002[Kolodyazhny, , 2006Leonov et al, 1996Leonov et al, , 2003Poleshchuk, 2006Poleshchuk, , 2007Kuznetsov et al, 2023], а также опубликованных и фондовых материалов [Voitovich, 1971;Galdobina, Mikhailyuk, 1971;Sokolov, 1973Sokolov, , 1987Gilyarova, 1974;Sharov, 2004;Morozov, 2010;Golod et al, 1983;Krats, 1963;Korosov, 1991;Kozhevnikov, 2000;Kulikov et al, 1999Kulikov et al, , 2017а, 2017bMakarikhin et al, 1995;Negrutsa, 1984Negrutsa, , 2011Novikova, 1975;Glushanin et al, 2011;Report..., 1991;Polekhovsky, Golubev, 1989;Polekhovsky et al, 1995;Puchtel et al, 1995;Ryazantsev, 2014;Satsuk et al, 1988;Svetov, 1979;Svetov, Sviridenko, 1991;Svetov et al, 2015;Sokolov et al, 1970;Systra, 1991;Kharitonov, 1966;Heiskanen, 1990;Amelin et al, 1995;…”