We propose a model of the geodynamic evolution of the Dzhida island-arc system of the Paleoasian Ocean margin which records transformation of an oceanic basin into an accretion-collision orogenic belt. The system includes several Vendian-Paleozoic complexes that represent a mature oceanic island arc with an accretionary prism, oceanic islands, marginal and remnant seas, and Early Ordovician collisional granitoids. We have revealed a number of subunits (sedimentary sequences and igneous complexes) in the complexes and reconstructed their geodynamic settings. The tectonic evolution of the Dzhida island-arc system comprises five stages: (1) ocean opening (Late Riphean); (2) subduction and initiation of an island arc (Vendian-Early Cambrian); (3) subduction and development of a mature island arc (Middle-Late Cambrian); (4) accretion and formation of local collision zones and remnant basins (Early Ordovician-Devonian); and (5) postcollisional strike-slip faulting (Carboniferous-Permian).
According to geological, petrological, geochemical, and geochronological studies, the Haraa Gol terrane in the western Hentiyn Mts. is dominated by two rock assemblages of different ages, associated with the initiation and development of the island arcs and marginal spreading seas of the Mongol–Okhotsk Ocean. The Late Cambrian, Early Ordovician, and Middle Ordovician were marked by the effusion of basalt and basaltic andesite and the formation of gabbro and gabbro-dolerite in back-arc spreading basins. In the Late Silurian–Devonian, after a short pause, tectonomagmatic processes were activated, with the formation of differentiated island-arc volcanics, gabbro, and granitoids. Their absolute 40Ar–39Ar age is given in the paper. The model age of the TNd(DM) protolith of the Haraa Gol igneous rocks corresponds to the composition of the Mesoproterozoic juvenile crust.
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