I.V. Gordienko

Composition and U-Pb isotopic age determinations (SHRIMP II) of the ophiolitic assemblage from the Shaman paleospreading zone and the conditions of its formation (North Transbaikalia)

The Upper Riphean Shaman ophiolitic assemblage was first distinguished and described in the territory of North Transbaikalia. Ophiolites found within a narrow suture (Shaman paleospreading zone) are represented by serpentinized ultrabasites with numerous plagiogranite veins having a U-Pb age of 971 ± 14 Ma, gabbros (939 ± 11 Ma), and basalts (892 ± 16 Ma). The ophiolite section also contains dikes of diabases and gabbro-diabases, siliceous-terrigenous stratum (black shale) of Upper Riphean age. The fragments of island-arc complexes (differentiated volcanites, gabbro-diorites, granites) of the Kelyan island-arc system are also found within the Shaman zone. The presence of Upper Riphean ophiolites in Baikalides of North Transbaikalia testifies to the formation of oceanic crust of the marginal spreading basins in the Precambrian Paleo-Pacific Ocean and the emerging Paleoasian Ocean.

New data on the Early Paleozoic gabbroid and granitoid magmatism age within the Dzhida zone of Caledonides (Southwestern Transbaikalia, North Mongolia)

Dealing with the problem of the formation and evolution of oceanic and continental crust is one of the fundamental issues in the study of folded areas. The Central Asian fold belt (CAFB), which includes the Dzhida zone of Caledonides, refers to an area where according to some researchers opinions [1–3, etc.], crust forming processes occurred from the Late Riph� ean to the beginning of the Mezozoic. Upon that, as

Geology and paleomagnetism of the Vendian and lower Cambrian deposits from Argun terrane (Eastern Transbaikalia)

We present new data on the geologic structure and paleomagnetism of the Vendian and Lower Cambrian deposits of the Argun River region in East Transbaikalia. We discuss the issues related to the recognition of the Argun microcontinent within the composite blocks of the south-eastern part of the Central Asian belt, which are traditionally combined in the Amuria superterrane. Our paleomagnetic determinations help us justify the equatorial position of the sedimentary basin, in which the deposits of the Argun terrane Beletuy and Bystrin formations formed 560 and 525 Ma, close to or directly on the margin of the Siberian paleocontinent. This data is essential for the reconstruction of the geodynamic evolution of Siberia and Central Asia at the Precambrian-Paleozoic boundary.

Hentei-Dauria fold system of the Mongolia-Okhotsk belt: magmatism, sedimentogenesis, and geodynamics

The geostructural, petrological, geochemical, geochronological and biostratigraphic studies were conducted in the Hentei-Dauria fold system of the Mongolia-Okhotsk orogenic belt. This Paleozoic system is composed mainly of three heterochronous rock associations related to the onset and development of oceanic basins and active margins in the conjugation zone of the Siberian continent and the Mongolia-Okhotsk ocean. This region developed in three stages: (1) Late Caledonian (Ordovician – Early Silurian), (2) Early Hercynian (Late Silurian – Devonian), and (3) Late Hercynian (Carboniferous–Permian). In the Late Caledonian, oceanic seafloor spreading was initiated, deep-sea siliceous deposits were formed, basaltic and andesitic pillow lavas were erupted, and layered and cumulative gabbros, gabbro-dolerite dykes and subduction zones with island-arc magmatism were formed. After a short quiescence period, new zones of spreading and subduction occurred at the active margins of the Mongolia-Okhotsk ocean in the Early Hercynian. In the Late Hercynian, large back-arc sedimentary basins, accretionary prisms and connecting intraplate magmatic complexes were formed in all structures of the Hentei-Dauria fold system. As a result of our studies, we propose a comprehensive model showing the geodynamic development of the Hentei-Dauria fold system that occurred in the area of the Mongolia-Okhotsk Ocean and its margins.

U–Pb Age and Geochemical Characteristics of Ultramafic–Mafic Rocks of the Dzhida Zone Ophiolite Association (Southwestern Transbaikalia)

This article presents new geochronological and isotope-geochemical data on ultramafic–mafic rocks of the banded complex of the Dzhida zone of the Caledonides ophiolite association.