We present results of new mineralogical, geochemical, geochronological, and isotope (Sm-Nd, Rb-Sr, and O) studies of the volcanic rocks of the Tyya complex in the Olokit trough. These are normal tholeiitic basalts and scarcer basaltic andesites forming a fractionated series with mg# = 45-65, medium TiO2 contents (0.73-1.62 wt.%), low P2O5 contents (0.04-0.25 wL%), and a significant domination of Na over K (Na2O/K2O = 2.1-50.0). The rocks are metamorphosed to greenschists, which are composed of chlorite, actinolite, epidote, and albite with quartz, titanite, ilmenite, and magnetite impurity. The metabasalts have is an age of 915 +/- 5 Ma (zircon U-Pb dating) and are characterized by wide variations in epsilon(Nd) (T) (-3.5 to -11.9) and Sr-87/Sr-86 (0.70602-0.70732) and high delta O-18 values (9.0-15.2960) as compared to the mantle ones. According to the isotope-geochemical characteristics, the studied metabasalts have features of both IAB and E-MORB. The Tyya metabasalts might have resulted from the melting of the lithospheric mantle with a subductional component. Comparison of the studied rocks with volcanic rocks of recent geodynamic settings shows their similarity to basalts of back-arc basins. The Tyya metabasalts might belong to a back-arc basin of the late Mesoproterozoic Nyurundukan island arc system.

The chapter gives a brief description of the geology and tectonic evolution of large geological structures in Northern Asia, such as orogenic systems of the western part of Central Asian Orogenic belt—the Urals Mountains, Sayan-Baikal mountain area, Verkhoyansk-Kolyma and orogenic structures of the Paleozoic basement of the West Siberian Plate. The history of the regional studies, data on the magmatic and sedimentary complexes as well as isotope-geochronological datings are provided.

В работе приведены результаты новых минералогических, петрогеохимических, геохронологических и изотопных (Sm-Nd, Rb-Sr, O) исследований вулканических пород тыйского вулканогенного комплекса Олокитского рифтогенного прогиба (Байкало-Муйский пояс). Вулканиты имеют возраст 915±5 млн лет (U-Pb метод, циркон), представлены низкои нормальнощелочными толеитовыми базальтами, реже пикробазальтами, образуют фракционированный ряд с вариацией значений магнезиальности (mg#) в интервале 45-65, с умеренными содержаниями TiO2 (0.75-1.54 мас. %), низкими P2O5 (0.04-0.25 мас. %) и существенным преобладанием содержаний натрия над калием (Na2O/K2O = 2.1-50.0). В результате метаморфизма превращены в зеленые сланцы. Для них характерны пониженные содержания Nb, повышенные Th и легких РЗЭ, а также отрицательные Nb-Ta, P и Ti аномалии на мультиэлементных спектрах, что свойственно островодужным базальтам. Установленные низкие отрицательные значения Nd(T) (-3.7 -12.1), ɛ аномально высокие 87Sr/86Sr (0.70737 0.70799) и δ18O (+9.0 – +15.2 ‰) указывают на древние коровые источники магматизма. Об этом же свидетельствует Nd модельный возраст протолитов (ТNd(DM) = 2.1-до 2.8 млрд лет), что сближает их по возрасту с протолитами Нюрундуканской островной дуги. Анализ полученных данных позволил сделать вывод о том, что происхождение мезопротерозойских тыйских метабазальтов связано с частичным плавлением литосферного обогащенного мантийного источника, содержащего субдукционный компонент древней океанической коры. Показано, что Олокитский рифтогенный прогиб сформировался в задуговой области Нюрундуканской островной дуги. Таким образом, изотопно-геохимические особенности тыйских метавулканитов обладают чертами двойственности, заключающейся

New structural, petrological, chemical, isotope, and paleomagnetic data have provided clues to the Late Riphean–Paleozoic history of the Uda–Vitim island arc system (UVIAS) in the Transbaikalian sector of the Paleoasian ocean, as part of the Transbaikalian zone of Paleozoids. The island arc system consists of three units corresponding to main evolution stages: (i) Upper Riphean (Late Baikalian), (ii) Vendian–Lower Paleozoic (Caledonian), and (iii) Middle–Upper Paleozoic (Hercynian). The earliest stage produced the base of the system composed of Late Riphean ophiolite (971–892 Ma, U-Pb) and volcanic (837–789 Ma, U-Pb) and sedimentary rocks (hemipelagic siliceous sediments and dolerite sills) which represent the Barguzin–Vitim oceanic basin and the Kelyana island arc. The main event of the second stage was the formation of the large UVIAS structure (over 150,000 km2) which comprised the Transbaikalian oceanic basin, the forearc and backarc basins, and the volcanic arc itself, and consisted of many volcanic-tectonic units exceeding 100 km2 in area (Eravna, Oldynda, Abaga, etc.). Lithology, stratigraphy, major–element compositions, and isotope ages of Vendian–Cambrian volcanic rocks and associated sediments indicate strong differentiation of calc-alkaline series and the origin of the island arc system upon oceanic crust, in a setting similar to that of the today’s Kuriles–Kamchatka island arc system. The Middle–Upper Paleozoic stage completed the long UVIAS history and left its imprint in sedimentary and volcanic rocks in superposed trough basins. The rocks were studied in terms of their biostratigraphic and isotope age constraints, as well as major- and trace-element compositions, and were interpreted as products of weathering and tectonic-magmatic rework of the UVIAS units.

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.

The Transbaikal orogenic belts are located in southern Siberia, in the Lake Baikal area. They include the Baikal-Patom fold-and-thrust belt, together with the Siberian platform forming the North Asian craton, and a collage of terranes to the south of the belt that were accreted to the craton in Late Riphean and Early to Late Paleozoic times. The delineated terranes are the fragments of Riphean and Paleozoic island arcs, oceanic crust, turbidite basins of the continental slope and its rise, shelves, and fragments of the craton consisting of Archean and Early Proterozoic crystalline rocks. The accretion resulted in the successive build-up of the Siberian continent from north to south, accompanied by large strike-slip displacements caused by the wedge-like shape of the southern protrusion of the North Asian craton, as well as by repeated deformations and high-temperature metamorphism of the previously accreted terranes and adjacent craton margin.