PETROGENESIS OF THE DESPEN VOLCANIC ROCKS OF THE MIDDLE-LATE ORDOVICIAN VOLCANOPLUTONIC ASSOCIATION OF THE TANNU-OLA TERRANE (southwestern Tuva)
E.V. Vetrov1, A.N. Uvarov2, N.I. Vetrova1, F.A. Letnikov3, I.A. Vishnevskaya4,5, F.I. Zhimulev1, E.S. Andreeva2, M.V. Chervyakovskaya6
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Siberian Research Institute of Geology, Geophysics and Mineral Resources, Krasnyi pr. 67, Novosibirsk, 630091, Russia
3Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia
4Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119334, Russia
5Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
6Zavaritsky Institute of Geology and Geochemistry, Uralian Branch of the Russian Academy of Sciences, ul. Akademika Vonsovskogo 15, Yekaterinburg, 620016, Russia
Keywords: Ordovician, volcanism, zircon U-Pb (SHRIMP-II) dating, petrogeochemical and isotope-geochemical (Sm-Nd) studies, Republic of Tuva
Abstract
We have studied the structure and composition of a volcanic unit in the valley of the Despen River, on the southern slope of the East Tannu-Ola Ridge. The unit was earlier assigned to the Lower Devonian Kendei Formation. The new geological and geochronological data show that it resulted from explosive volcanism at 460-450 Ma. The Despen volcanic rocks formed in association with granitoids of the Argolik complex at the end of the accretion-collision stage of evolution of the Altai-Sayan region, in particular, the Tannu-Ola terrane. These are predominantly felsic ferroan metaluminous and weakly peraluminous nappe volcanic rocks resulted from the differentiation of tholeiitic basalts. Their REE patterns, like those of the Argolik granitoids, are flat in the HREE region, show a distinct Eu anomaly, and suggest magma generation at shallow depths in the upper crust. The magmatic source was of subduction origin, as evidenced by the negative Ta-Nb anomalies in the multielement patterns and by εNd(T) = 3.1-5.6, and has a Neoproterozoic model age, TNd(DM-2st) = 0.69-0.94 Ga.
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