Vol 32, No 4 (2024)

The paper presents the results of U–Pb (ID-TIMS) geochronologic, geochemical, Sm–Nd isotopic-geochemical studies of metavolcanic rocks of the Djagdagle formation, which are among the key elements in the structure of the Bureya continental massif. It was established that the age of metavolcanic rocks of the Djagdagle formation is 217 ± 7 Ma and corresponds to the Late Triassic. This fact contradicts the traditional ideas, according to which the Early Precambrian age is attributed to the Djagdagle formation. The results of Sm–Nd isotope studies of the considered metavolcanic rocks indicate the melting of rocks of continental crust with Paleoproterozoic model ages during the formation of their initial melts. The new geochronologic data and previously published data allow us to distinguish at least two stages of magmatic activity in the Triassic within the northwestern part of the Bureya massif ~243 and 219–201 Ma. On the basis of synchronous manifestation of Neoproterozoic, Early Paleozoic, Late Paleozoic and Early Mesozoic magmatic events in the history of geologic development of the Bureya and Songnen– Zhangguangcai Range massifs, an assumption about their common geological history at least since the Late Neoproterozoic has been put forward. The close spatial position of metavolcanic rocks of the Djagdagle formation with Mongol–Okhotsk orogenic belt, their Late Triassic age (217 ± 7 Ma) and geochemical features allow us to link the formation of initial melts of metavolcanic rocks of the Djagdagle formation with within-plate magmatism in the rear zone of subduction of the Mongol–Okhotsk Ocean beneath the northern (in modern coordinates) margin of the Bureya continental massif.

The research was focused on the Mara–Kamenka–Uvat interfluve of the Biryusinsky ledge of the Siberian Platform, where more than half a century ago, during prospecting works for manganese, the Marа paleovolcano was identified. However, specific volcanogenic-sedimentary rocks were considered as a part of the Karagas sedimentary series of the Late Riphean. Our mineralogical and petrographic studies have allowed us to establish the wide distribution of high-potassium pyroclastics, ignimbrites, and trachybasalts, indicating a subaerial explosive volcanic nature of the Mara volcano. The age of high-potassium volcanism has been determined based on U–Th–Pb zircon dating as 640 Ma. Lu–Hf isotope systematics of zircon indicate a relation of these volcanic rocks with mantle-derived magmas. The composition and time of formation of the studied rocks do not allow us to correlate them, as previously thought, with sedimentary Late Riphean quartz and quartz-feldspathic sandstones of the Karagas sedimentary series and dolerites of the Nersa intrusive complex. The specific mineralogical and petrographic features of the studied rocks make them suitable as a regional stratigraphic mark.

Nannofossils and planktonic foraminifers derived from the Lower Miocene Alkun Formation in the central and western Ciscaucasia were studied in the following sections: the stratotype section on the Alkunka River, the Republic of Ingushetia; stratotype of the Caucasian regional stage on the Kuban River, Karachay-Cherkess Republic; Lower Miocene reference section on the Belaya River, Adygea; and in the sections of the Alkun Formation stratotype area on the Fiagdon and Mairamadag rivers, North Ossetia. Planktonic foraminifers in the Alkun Formation were studied for the first time. The available results made it possible to reveal the composition and structure of nannofossil and planktonic foraminiferal assemblages from the Alkun sediments in the Ciscaucasia, to refine their stratigraphic and correlation potential and to recognize bionomic conditions of the Alkun basin in the studied area. As a result of the nannoplankton investigation the Cyclicargolithus floridanus Beds were recognized in the Alkun Formation and, taking into account the occurrence of the zonal Triquetrorhabdulus carinatus along with Tr. milowii, they are correlated with the upper (Miocene) part of the Aquitanian Zone NN1. Planktonic foraminiferal data permitted to recognize the Streptochilus pristinum Beds.

The biostratigraphic material accumulated to date on the subdivision of the Phanerozoic marine strata allows us to decipher the features of the evolution of not only low biota taxa, but also paleocommunities (assemblages), which can be considered as biotic groupings historically formed under certain conditions. Examples of the stages of their evolution in various marine ecosystems of Geomerida are given. The opinion is expressed about the need to intensify research on this topic with the involvement of both geologists and biologists.