EARLY PROTEROZOIC GRANITOIDS OF THE CENTRAL PART OF THE NEPA-BOTUOBA ANTECLISE AS INDICATORS OF THE FORMATION OF THE TRANS-SIBERIAN OROGENIC BELT OF THE SIBERIAN CRATON
T.V. Donskaya1, D.P. Gladkochub1, M.O. Sukneva1, U.S. Efremova1, O.M. Turkina2, A.G. Vakhromeev1, E.I. Demonterova1
1Institute of the Earth's Crust SB RAS, Irkutsk, Russia
2V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia
Keywords: Granitoids, U-Pb age, geochemistry, Early Proterozoic, orogenic belt, Siberian craton
Abstract
The paper presents studies of gneisses and granitoids from three deep boreholes located in the central part of the Nepa-Botuoba anteclise of the Siberian craton. Based on U-Pb (LA-ICP-MS) geochronological studies of zircon, the age of the granitoid protolith of amphibole-biotite gneiss from the Danilovskaya-95 borehole is obtained as 2254±4 Ma, the age of the granitoid from the Mogdinskaya-11 borehole is estimated as 1972±9 Ma, and the granitoid from the Preobrazhenskaya-14 borehole as 1981±3 Ma. The protolith of the gneiss from the Danilovskaya-95 borehole corresponds to granodiorite, close in composition to I-type granite with high contents of high-field strength elements, is characterized by the tNd(DM) = 2.7 Ga, and could have been formed as a result of melting of the Archean crustal source within a separate block. The granitoid from the Mogdinskaya-11 borehole has characteristics comparable to I-type granites with low concentrations of high-field strength elements, and reveals the tNd(DM) = 2.4 Ga. The granitoid of the Preobrazhenskaya-14 borehole, having tNd(DM) = 2.6 Ga, is close to I-type granites with high concentrations of high-field strength elements. The totality of the data, as well as close age (~2.0 Ga) previously obtained for S-type gneissogranites from one of the Danilovskaya group wells, allow us to assume that granitoids with different geochemical characteristics were formed in the central part of the Nepa-Botuoba anteclise at the time interval of 1.97–2.00 Ga, which is possible in the accretionary orogen, including, based on the isotopic characteristics, blocks with Early Proterozoic and Archean crust. The studied area is a fragment of the Early Proterozoic Trans-Siberian orogenic belt, separating the large Archean Tunguska and Anabar superterranes, the formation of which records the early stage of accretion processes and the beginning of the formation of the Siberian craton in the time interval of 1.95–2.00 Ga.
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