. Dobretsov et al. [1985] first described the rock complexes in Eastern Sayan as ophiolites. Ophiolites formed in Dunzhugur island arc and were obducted onto Gargan block, a Neoarchean crystalline basement of the Tuva-Mongolian Massif (TMM), as a single nappe [Khain et al., 2002;Kuzmichev, 2004]. Zircons from plagiogranite were dated at 1021±5 Ma by multigrain TIMS and 1020±1 Ma by Pb-Pb single-grains evaporation method [Khain et al., 2002]. Later [Kuzmichev, Larionov, 2013] analysed 12 grains of detrital zircons from gravelstone of the Dunzhugur formation and obtained 206 Pb/ 238 U ages from 844±8 to 1048±12 Ma. Careful examination of these data shows that 206 Pb/ 238 U ages for concordant zircons only vary from 962±11 to 1048±12 Ma. Two groups of data give Concordia ages of 974±11 and 1028±10 Ma. Rocks of the Dunzhugur complex are characterized by slightly negative to slightly positive εNd(t) values from -1.0 to +1.5 and Late Palaeoproterozoic depleted mantle Nd model ages of 1.8-1.6 Ga [Sklyarov et al., 2016]. The Dunzhugur complex was intruded by tonalite plutons of the Sumsunur complex dated at 785±11 Ma [Kuzmichev et al., 2001] and 811±7 Ma [Kovach et al., 2012]. These tonalites have strongly negative ɛNd(t) values from -13.2 to -12.3 and Nd model ages of 2.5-2.4 Ga, suggesting formation of these melts from a mixture of N.L.

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Carbonatites associated with syenites and subalkalic mafic rocks (lamprophyres) occur in the Himalayan continental collision zone, and this suggests their possible existence in other Phanerozoic collisional settings. The Early Paleozoic Ol’khon collisional system in Western Cisbaikalia is considered one of the possible occurrences. Subalkalic gabbroids as well as peculiar carbonate (brucite marbles) and calc-silicate rocks were found here, within the Tazheran massif of alkalic and nepheline syenites. Alkalic syenites, nepheline syenites, and calciphyres were dated at 471 Ma, 451–464 Ma, and 466 Ma, respectively, and their ages correspond to the main collisional events in the system. A geochemical description of igneous and carbonate rocks in the massif is provided. Close mapping showed unusual syenite and brucite marble combinations and the frequent vein or pipe-like form of carbonates and calciphyres corresponding to their magmatic intrusion. But carbonatite nature of the marble mentioned above does not fit their typical crustal geochemical features. Not ruling out the possibility of total change of geochemical signatures of mantle carbonatite in the collisional medium, we offer two other possible explanations for these facts: (1) melting of carbonate masses by syenite and mafic magmas, followed by carbonate melt intrusion into the upper crust, and (2) protrusion of carbonates into syenites and gabbroids at the late stages of the contact action of a silicate magma. In this case the above-mentioned carbonate structural features result from late recrystallization, whose mechanism is yet to be explained.

We discuss strike-slip tectonics as the key agent in the formation of the Early Paleozoic (Caledonian) collisional system of the western Baikal region. This tectonic setting implies existence of local syncompressional extension, with the ensuing conditions for mantle drainage and magmatism. Lower-middle crust collisional complexes exposed in the Olkhon area of the western Baikal region provide a record of synmetamorphic subalkaline-mafic magmatism associated with the early synorogenic collapse of the Olkhon collisional system, a part of the Central Asian collisional-accretionary belt.

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