Vendian Island-Arc Intrusive Magmatism of the Lake Zone of Western Mongolia (Geological, Geochronological, and Petrochemical Data)

—Based on new geochronological data on gabbroid and plagiogranitoid associations (Tavan-Hayrhan, East Bayan Tsagaan, Bayan Tsagaan Uul, Tungalag, Three Hills, and Shutkhuin massifs) located among the Vendian island-arc volcanic complexes of the Lake Zone of Western Mongolia, an independent stage of Vendian island-arc intrusive magmatism (560–542 Ma) is substantiated. Geochronological ages determined by xenogenic zircon from Vendian gabbroids and granitoids (716–559 Ma) indicate a wide time interval of their formation and different natures of the sources. Several such sources are assumed. The source of the first type is rocks of the late Riphean oceanic crust of the Paleoasian Ocean, on which the Vendian island arc of the Lake Zone formed later. This is evidenced by the presence of xenogenic zircon with the ages of ~716, 658–642, 613–611 Ma. The source of the second (probably main) type is rocks of the Vendian island-arc crust of the Lake Zone. This is indicated by the presence of xenogenic zircon with ages of 583–559 Ma, observed in all studied Vendian intrusive associations.

Multistage Thrust and Nappe Tectonics in the Southeastern Part of East Sayan and Its Role in the Formation of Large Gold Deposits

––Comprehensive studies of structural geology and metallogeny, taking into account the authors’ previous works started as early as the last century, have shown that the southeastern part of East Sayan formed mainly in the Neoproterozoic–early Paleozoic in the settings of multistage thrust and nappe tectonics and tectonomagmatic restructuring of autochthonous and overthrust allochthonous oceanic (ophiolitic), island arc, and ocean-marginal terranes as well as amalgamation of accretion–collision and postcollisional igneous complexes that formed during the opening and subsequent closure of the Paleoasian Ocean marginal structures. In the middle and late Paleozoic, active intraplate volcanic and plutonic processes continued in the thrust/overthrust fault setting, which led to the formation of new dome-shaped nappe structures and the redistribution of ore matter (gold etc.) in large mineral deposits. The final structure of the East Sayan region formed during the late Cenozoic as a result of mountain uplifting and volcanic eruptions, including those in the valley of the Zhombolok River.

First data on the nature and age of the protolith of high-pressure tectonites of Yenisei ridge: a link to the early stage of formation of the Paleoasian Ocean

The petrogeochemical characteristics of high-pressure mafic tectonites from the Yenisei suture zone of Yenisei Ridge indicate that N-MORB and E-MORB with an age of 701.6 ± 8.4 Ma were their protolith. The formation of N-MORB with the more primitive chemical composition occurred at the initial stages of spreading, when the upper horizons of the depleted mantle were molten. The high-Ti basalt was formed as the product of melting of the enriched mantle substrate at the later stages of spreading. These events are related to the early stage in the evolution of the Paleo-Asian Ocean on the western margin of the Siberian Craton.

The first petrological evidence of subduction at the western margin of the Siberian сraton

The discovery of relics of glaucophane schist in high-pressure tectonites of the suture zone of Yenisei Ridge has provided evidence for a Cordillera-type convergent boundary controlled by subduction of the oceanic crust beneath the continental margin on the western side of the Siberian Craton. According to the microtextural patterns of rocks and the results of geothermobarometry, two stages of metamorphism were distinguished. The formation of high-pressure tectonites indicates the early stage in the evolution of the Paleoasian Ocean and the final stage in the Neoproterozoic history of the Yenisei Ridge including completion of the accretion–subduction processes on the western margin of the Siberian Craton.