The diversity of geodynamic settings which are typical of granitoid magmatism, wide variations in geochemical and isotope characteristics make it possible to use granitoids as a key towards deciphering processes of continental crust formation and dating of crust-formation processes. The data on the granitoid composition can be used as indicators of geodynamic settings, common to orogenic and post-orogenic stages in formation of fold belts within continents. The Central-Asian orogenic belt and its major constituent i.e. the Mongol-Okhotsk Orogenic Belt represent structures where different geochemical types of granitoids were produced in different geodynamic settings through the Paleozoic and Mesozoic. Therefore, the Mongol-Okhotsk Orogenic Belt can be used as a reference to study interaction between compositional characteristics and geodynamic settings of granitoids. The Paleozoic granitoid Intrusions of the Olekma complex (Eastern Transbaikalia) mark the northern boundary of the Mongol-Okhotsk Orogenic belt within the West-Stanovoy terrane and record a large-scale endogenous event on the margin of the Mongol-Okhotsk paleo-ocean. The age of those rocks is conventionally taken as the Paleozoic and is not supported by geochronology data. Therefore, it is impossible to recognize their place in the geodynamic history of the Mongol-Okhotsk Orogenic Belt. In this study, we perform U-Pb LA-ICP MS geochronological studies of zircons from granite-leucogranite rocks of this complex. The new geochronological data indicate that the Paleozoic Olekma intrusive complex of East Transbaikalia contains a significant amount of Mesozoic granitoids (219–186 Ma), which represent an independent magmatic episode in the formation of the Mongol-Okhotsk orogenic belt, associated with extensional tectonic regime along the northern boundary of the suture zone. These granitoids have to be regarded as an independent intrusive complex, which is supported by Sm-Nd isotope data indicating a less radiogenic source compared to the collision granitoids of the Mongol-Okhotsk orogenic belt. The isotope Sm – Nd characteristics of granitoids of both massifs are similar to those of the Neoproterozoic continental crust with values of eNd (219 Ma) = – 0,9 (-2,1) and TNd (DM-2) = 1086 1164 Ma, which correspond to the evolution region of the Nd isotope composition of the Neoproterozoic crust within the Central Asian orogenic belt.

Dril Sergey Igorevich, Candidate of Sciences (Geology and Mineralogy), Senior Researcher, Head, Laboratory of Isotope Geochemistry, Vinogradov Institute of Geochemistry SB RAS, 1A, Favorsky st., Irkutsk, 664033, Russian Federation, e-mail: sdril@igc.irk.ru 

Noskova Yulia Vladimirovna, Junior Research Scientist, Vinogradov Institute of Geochemistry SB RAS, 1A, Favorsky st., Irkutsk, 664033, Russian Federation, e-mail: j.noskova@igc.irk.ru 

Kovach Viktor Petrovich, Candidate of Sciences (Geology and Mineralogy), Leading Research Scientist, Institute of Precambrian Geology and Geochronology RAS, 2, Makarov st., Saint Petersburg, 199034, Russian Federation, e-mail: v.p.kovach@gmail.ru 

Sasim Sergey Aleksandrovich, Candidate of Sciences (Geology and Mineralogy), Head, Department of Mineral Deposits, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, e-mail: sasimserg@mail.ru

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