Vol 31, No 5 (2023)

Based on the correlation of U–Pb isotopic age of detrital zircon grains from modern fluvial deposits of the largest rivers of Western Transbaikalia and Northern Mongolia and U–Pb isotopic ages of bedrock of the Mongol-Transbaikal sector of the Central Asian Orogenic Belt, five main stages of formation and processing of the continental crust are identified in the region: (1) the Neoarchean-Paleoproterozoic stage (2.7–1.7 Ga), corresponding to the global crust-forming processes of the Precambrian cratons’ basement formation; (2) the Neoproterozoic (early and late Baikal) and (3) the Early Paleozoic (Caledonian) stages reflecting the processes of formation of juvenile crust through island-arc magmatism and accretion of island arcs, which led to the formation of folded structures of the southern margin of the Siberian craton; (4) the Late Paleozoic and (5) Early-Middle Mesozoic stages reflecting the processing of the Early Precambrian and Caledonian crust through intraplate magmatism and riftogenesis. It is shown that the peak values of the probability density distribution curve of the detrital zircon grains’ age depend on various, often unrelated factors, such as the absolute age of the rock, the area of the eroded surface, the amount of zircon in rocks of different composition, the distance of transport, etc., therefore, the statistics of age distribution cannot be considered as an indicator of the intensity of endogenous events. The absence of Cenozoic grains of detrital zircon in the fluvial deposits of the region where basalt volcanism of this time is widely manifested (Mongolia, Baikal region) indicates that endogenous events, which are mainly expressed in basalts, are not manifested in the “zircon chronicle”.

Geochronological U–Th–Pb LA-ICP-MS study of detrital zircon grains from Riphean sandstones of the Sherovichi and Belarus series of the Orsha part of the Volyn-Orsha paleotrough (boreholes Korma and By-khov, Belarus) was carried out. The received U–Th–Pb geochronological data indicate that the primary provenance of zircon was crystalline rocks of predominantly Early and Middle Riphean, as well as Early Proterozoic and Archean age. The presence of detrital zircon grains with an age of 1 Ga in the sandstones of the Sherovichi and Belarus series, which were previously considered Middle Riphean, evidences that these sequences are of Late Riphean age. A representative population of grains with an age of about 1 Ga was detected only in the rocks of the Korma borehole, whereas in sandstones from other boreholes located within the Voly-n-Orsha trough (borehole 70, borehole Vilchitsy 1 and borehole Bykhovskaya), it is absent. This indicates that a possible primary provenance of zircon could be located both within the paleotrough and at a considerable distance, within the Grenville Sweconorwegian orogen.

The results of U–Pb dating of detrital zircon from volcanoterrigenous Ediacaran–Silurian deposits, mainly from grauwacke turbidites, after accretion of which the “Caledonian” basement of the Paleozoic structure was formed in Kyzylkum and in the ridges of the Southwestern Tien Shan, including Northern Fergana, allowed us to clarify the age correlation and show the dynamics of the formation of this complex. It consists of deposits associated with the opening, filling and subsequent convergent development of three generations of oceanic basins, including the emergence and destruction of island arcs. The earliest of these basins was formed at the active volcanic margin of the eastern Rodinia/Gondwana, which included the Karakum-Tajik massif. Sandy turbidites (Besapan, etc.), whose age is not younger than the beginning of the Cambrian, were deposited here on top of siliceous (Taskazgan) Ediacaran sediments. The second generation is represented by Ordovician basins, in which radiolarian flints of the Lower Ordovician form the bathyal part of the sediments, and the main filling is turbidites of the middle Ordovician–Ruddan. The newly opened Silurian oceanic basins and their margins were filled with sediments of the third Caledonian generation, mainly graptolite aleuropelites and turbidites, starting from the end of the Llandovery. Incomplete preservation of the Ediacaran–Silurian sedimentary series is due to their partial absorption (tectonic erosion) during further subduction and collision, mainly already in the Carboniferous.

The features of sedimentation of hemipelagic deposits of the Emsian (Dzhaus Beds, Zeravshan Range, Khodzha-Kurgan valley) are reconstructed and the duration of eustatic events in this area is established on the basis of tentaculite and conodont zonality. In the studied region the Emsian Khodzha-Kurgan transgression began at the end of the pireneae zone and continued until the end of the patulus zone. The transgression maximum occurs in the Dzhaus Beds. In the Dzhaus Beds, the presence of traces of two eustatic rises, accompanied by anoxic sedimentation, and one fall of the paleobasin level were revealed. The conclusions are based on the analysis of the lithology of the Dzhaus Beds and the changes in the hydrodynamic regime in the sedimentation basin. Traces of the maximum subsidence of the sea level are recorded in the interval 29–72 m of 16-th Member (elegans tentaculite zone, inversus conodont zone). Sea level rise was recorded in the intervals of the entire 15-th Member, 0–28 and 73–102 m of 16-th Member (elegans and cancellata tentaculitic zones, inversus u serotinus conodont zones). In terms of time, the Dzhaus transgressions can be compared with the Czech Late Zlichovian and Early Dalejan eustatic events. The change in the composition of the D-zhaus cephalopod, brachiopod, and ostracod associations occurs in the elegans tentaculite zone. The information obtained from the study of the section of the Dzhaus Beds in the Khodzha-Kurgan valley makes it possible to relatively accurately date the Emsian eustatic oscillations in the South Tien Shan region. This gives the studied section a certain correlation value, since the details of the Emsian transgressions revealed in it can be used in the global correlation of eustatic events and be criteria for substantiate the position of the boundaries of the Emsian substages in the South Tien Shan.

The paper deals with the issues of the use of the new structure of the Quaternary Geological System approved recently by the International Commission on Stratigraphy in the practice of geological work in Russia, and options for possible changes in hierarchy of Quaternary subdivisions in Stratigraphic Code of Russia.