The ophiolites of the Sayano-Tuvan forearc zone are an integral part of the Early Caledonian system formed during the evolution of the Tuvan-Mongolian island arc of the Paleoasiatic ocean. This segment of the Early Caledonids contains the main structural elements of the island-arc system: the forearc, island-arc and back-arc zones. The forearc zone extends in a northeasterly direction for 550 km, has a width of about 300 km and includes early Caledonian structures in the adjacent territories of Western Sayan and Tuva. The Sayano-Tuvan forearc consists of sub-zones: a) front Dzebash, b) transition Levokhemchik and Kurtushiba, and c) back Khemchik-Tapsa. The ophiolites are represented in all subzones except Dzebash. In the frame of Kurtushiba ophiolites developed sedimentary deposits V-Є₁ Chingin strata, Khemchik ophiolites – sedimentary deposits V-Є₁ Aldynbulak strata. Both strata were accumulated in the conditions of forearc basin, Chingin sediments lie in the form of tectonic plates, they are more finely fragmented compared to Aldinbulak, which are usually represented in the olistostrome, which lies in the wedge-shaped outlets of the Khemchik-Systygchem collision deflection. The geological position of the ophiolites of the Kurtushiba ridge of the Western Sayan and the Khemchik river basin in Western Tuva indicates that they are a component of the forearc Sayan-Tuva zone of the Tannuola-Khamsara island-arc system. Features of the geological structure of these ophiolites is the presence of at least two generations of dikes of different orientation, indicating the processes of scattered spreading. For us rare element compositionof the studied ophiolites correspond, on the one hand, the of island arcs tholeiites, with the other N-MORB, but it is discriminatory on the chart Y – La/Nb of points of their compositions tend to the field of forearc platform basalts. Geological and geochemical data indicate the formation of Kurtushiba and Chemchik ophiolites in areas of scattered spreading in the suprasubduction zone. At the same time, in Kurtushiba ophiolites the dyke complex which structure is characteristic only for N-MORB is fixed. The results of our studies suggest that Kurtushiba ophiolites were formed in paleogeodynamic conditions of the origin of subduction and the beginning of the formation of primitive island arcs on the oceanic lithosphere. Island-arc processes during the formation of ophiolites probably was a partial replacement of primary magmatic oceanic crust supra-subduction melts. Khemchik ophiolites (Shatsky massif), including dikes of different orientations, were also formed under conditions of scattered spreading at the stage of subduction zone origin.

Mongush Andrey Alexandrovich, Candidate of Science (Geology and Minerology), Leading Researcher, Laboratory of Geodynamics, Magmatism and Ore Formation, Tuvinian Institute for Exploration of Natural Resources SB RAS, 117a, Internatsionalnaya st., Kyzyl, 667007, Russian Federation, tel.: 8(39422)6-62-18, e-mail: amongush@inbox.ru

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