Vol 26, No 2 (2018)

The U–Pb isotope data and corresponding ages of detrital zircons from rocks of the basal complexes of the Uralides of different segments of the Ural Fold Belt are considered. It was established that complexes of ancient domains of the East European Platform (Volga-Uralia, Sarmatia, Kola, etc.) seem to have been the main provenance areas of the clastic material for the Southern, Middle, and Northern Urals. This means that there were relatively remote and local (igneous formations of the pre-Uralides) provenance areas. Rift rock associations of the Uralides of the Subpolar and Polar Urals were formed mainly through erosion of local provenance areas (predominantly, Late Riphean–Vendian island-arc and orogenic magmatic complexes of the Proto-Uralides–Timanides). Detrital zircons of Riphean age dominate in rocks of the basal complexes of the Uralides. A source for them could have been rock complexes of Svecofennian-Norwegian Orogen and Cadomides of the Scythian-Turan Plate, intraplate magmatic formations, and metamorphic complexes, as well as blocks accreted to the margin of the East European Platform in the Late Precambrian–Cambrian and later detached and displaced during the Ordovician rifting and spreading. In general, the basal complexes of Uralides were formed owing to supply of clastic material from both remote and local sources. Despite the appearance of information of a totally new level (U–Pb isotope ages of detrital zircons, their Lu–Hf systematics, and the distribution features of rare earth and trace elements), the contribution of these sources to the formation of the Late Cambrian–Early Ordovician clastic strata is hardly possible at present to evaluate.

This work presents the results of geological, geochemical, Sm–Nd isotope-geochemical studies of metasedimentary rocks of the Teploklyuchevskaya, Garmakan, and Algaja formations of the Tukuringra Terrane of the eastern part of the Mongol–Okhotsk fold belt, as well as U–Th–Pb geochronological (LA-ICP-MS) studies of detrital zircons from these rocks. It is established that the lower age boundary of formation of the protolith of metasedimentary rocks of the Teploklyuchevskaya Formation is about 243 Ma (Middle Triassic); those of the Garmakan and Algaja formations are ~175 Ma (Lower–Middle Jurassic boundary) and ~192 Ma (Lower Jurassic), respectively. This makes it possible to correlate the Teploklyuchevskaya, Garmakan, and Algaja formations with the youngest sedimentary complexes of the eastern part of the Mongol–Okhotsk fold belt. In terms of geochemistry, the protoliths of metasedimentary rocks of the above-mentioned formations are the most similar to sedimentary rocks of island arcs and active continental margins. The source terrigenous material was transported from the southern frame of the Mongol–Okhotsk fold belt. It is not improbable that Lower Mesozoic deposits of the western part of the Tukuringra Terrane, in particular, and the eastern part of the Mongol–Okhotsk fold belt, as a whole, are relics of residual basins, preserved in “gaps” in the collision zone between the southern margin of plates of the North Asian Craton and the Amur Superterrane.

It has been accepted until recently that a regionally developed gap corresponding to the boundary interval between the stages (equivalents of the Chetaites chetae and Volgidiscus lamplughi zones) exists between the upper Volgian Craspedites nodiger Zone and the Ryazanian Stage. Recently, the presence of this interval was established in Yaroslavl oblast. In this paper, we publish for the first time a detailed description of the sections of the terminal part of the upper Volgian Substage, the majority of which have not previously been published, as well as illustrations of characteristic mollusks (ammonites and bivalves) from these deposits. For this stratigraphic interval, the recognition of the Volgidiscus singularis Zone, including the V. pulcher and V. singularis biohorizons, is substantiated. In this zone, ammonites are mainly represented by the genus Volgidiscus, but the pulcher Biohorizon also contains very rare Garniericeras, whereas the glacial boulders together with V. cf. pulcher contained Shulginites. The Singularis Zone has a high correlative potential and, at an infrazonal level, directly correlates with the uppermost zones of the upper Volgian Substage of the Subpolar Urals, northern Siberia, England, and the North and Barents seas. It is most likely that the biostratigraphically confirmed gap between the upper Volgian Singularis Zone and Ryazanian stage is absent. In both the Subpolar Urals and in European Russia, the terminal part of the Volgian Stage and the lower part of the Ryazanian Stage both contain ammonites of the genus Shulginites. Bivalves are represented mainly by the genera Anopaea, Camptonectes (Camptonectes), Entolium (Entolium), and Plagiostoma. The studied sections lack Buchia, one of the most characteristic taxa of the Jurassic–Cretaceous boundary beds of the Panboreal Superrealm. It is proposed that the Volgian–Ryazanian boundary beds in the upper reaches of the Cheryomukha River (Rybinsk district, Yaroslavl oblast) are recognized as the Chudinovo Formation, the upper part of which contains numerous ammonites of the Singularis Zone.