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

Kuznetsov, N.B.; Kolodyazhnyi, S.Y.; Romanyuk, T.V.; Strashko, A.V.; Baluev, A.S.; Terekhov, E.N.; Mezhelovskaya, S.V.; Dubensky, A.S.; Sheshukov, V.S.

Авторы: Kuznetsov N.B., Kolodyazhnyi S.Yu, Romanyuk T.V., Strashko A.V., Baluev A.S., Terekhov E.N., Mezhelovskaya S.V., Dubensky A.S., Sheshukov V.S.
Журнал: Geodynamics & Tectonophysics
Том: 14
Номер: 1
Год издания: 2023
Издательство: Федеральное государственное бюджетное учреждение науки Институт земной коры Сибирского отделения РАН
Местоположение издательства: Иркутск
Номер статьи: 1
DOI: 10.5800/gt-2023-14-1-0685
Аннотация: 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 (Vepsia) 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 South Onega Trough in the southwestern Cis-OnegaLake. The isolated from this sample 79 dZr grains were studied at the Center for Collective Use of the Chemistry-Isotopic Analytic Laboratory of GIN RAS.The youngest ages are 189911 Ma (D1=0.4%, D2=0.7%).190411 (D1=0.0%, D2=0.05%) Ma and 191312 Ma (D1=2.9%, D2=6.3%). The weighted average of the three youngest U-Pb isotope dates for dZr grains from sample KL-555 is 1906 ± 13 Ma. This, taking into account the known isotopic dating of gabbro-dololeites from the Ropruchei sill, which cuts through the Shoksha Formation, makes it possible to constrain the time of the Shoksha Formation accumulation by an age interval of 1.90–1.75 Ga.The set of obtained dates for dZr grains has been compared with the known ages of the Archean-Early Proterozoic crystalline complexes of the Baltic Shield and the Fennoscandian part of the basement of the East European Platform. For the dZr groups with age intervals of 2.1–1.98 Ga (6 dates), 2.05–2.08 Ga (3 dates), and 1.96–1.90 Ga (18 dates), the most probable primary sources were areas of the Ludicovian igneous rocks (2.1–1.92 Ga) of the North Onega synclinorium. The ages of six dZr grains are Late Archean in the time span of 2.8–2.5 Ga. These dates are in good agreement with the age estimates of granitoids drilled by the Onega parametric borehole at the bottom of the same name structure. At the same time, no Early Archean grains were found among the dZr from sample KL-555, the primary source of which could be tonalites of the nearby Vodlozero block with ages of 3.2–3.9 Ga.Bright frequency peaks on the Density Probability Plot (DPP) been statistically supported by more than 5 measurements, correspond to the values of 1920 and 2006 Ma. A minor peak at 2078 Ma is supported by 3 measurements. The age sets of dZr grains from quartzite sandstones of the Shoksha Formation and rocks of the Ladoga Group are very similar (KS similarity coefficient, 0.27) and characterize mainly tectonic–magmatic events that immediately preceded the manifestation of the Svecofennian orogeny (1.9–1.87 Ga). Therefore, the rocks of the Ladoga Series developed along the margin of the Svecofennian accretionary orogen could be with a high degree of probability a secondary source for the Shoksha quartzites.Based on a comparative analysis of thorium-uranium ratios (Th/U) in dZr grains from sample KL-555, it was concluded that some of the studied dZr grains originate from Ludicovian mafic rocks (dZr grains at high Th/U>1.5) and from ultra-high-pressure formations, such as eclogites known in the Salma, Kuru-Vaara and Gridino regions (dZr grains with Th/U<0.1).A significant part of the obtained analyzes has 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 impact that disturbed the U-Pb isotope system of these zircon grains in the Phanerozoic.Based on the available data, 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 sedimentary flow in the direction from north and northwest to south and south-east.
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ON THE TIME AND CONDITIONS OF FORMATION OF THE SHOKSHA QUARTZITE-SANDSTONES OF THE SOUTH ONEGA DEPRESSION BASED ON THE NEW DATA FROM ISOTOPE GEOCHRONOLOGYстатья

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