Isotopic dating of metamorphic and igneous complexes in stratigraphy of the Altai-Sayan folded area, south of Western Siberia

The article discusses application of absolute geochronology methods in stratigraphic studies of western part of the Altai-Sayan folded area. A research on metamorphic complexes of Mountain Altai has proven high efficiency of the modern methods of isotopic dating in justification of relative age of regional metamorphic processes. As it has been recently stated, the Late Cambrian - the Early Ordovician deep-sea trench deposits (the Mountain Altai series) have undergone regional metamorphism. Vast majority of recently published geological maps depict the Mountain Altai metamorphic formations as the Proterozoic ones. Based on geologic data and absolute dating of a number of granite massifs in the southeastern Mountain Altai, it is assumed possible to detect sub-volcanic and blanket formations of the Early Jurassic. It has been proven that in several places of the Mountain Altai, subvolcanic formations, which used to be considered as the Lower Devonian, break through the Middle and Upper Devonian deposits with marine fossils. Such facts are found in the North-Chuya ridge, valley of the Uzun-Tytugem river, near the Tashanta village (southeastern part of the area) and in the basin of the Karakol river (central Mountain Altai). In the southeastern part of the Altai Mountains, using absolute dating of granitoid massifs, the Early Jurassic Chindagatuyski (Kalgutinsky) complex was found. In the basin of the river Kalguty, these subvolcanic granites intrude into the Kalgutinsky rock mass (used to be considered the Early Devonian). Similar granites are found during the study of subvolcanic rocks in the North Chuya Ridge (basins of the rivers Jelo and Kara-gem). Granites are exposed in the deep parts of the rock mass. This indicates syngeneic nature of granites, subvolcanic bodies and possibility of the Mesozoic-Early Jurassic age for a number of subvolcanic bodies in the Mountain Altai. This was noted by the first explorers of the area in the 1930s. Introducing the Early Jurassic formations into maps of the region will allow reconsider its metallogeny and link its tungsten-molybdenum specialization with the Mesozoic tectonic activation typical for entire Ural-Mongolian mobile belt. We paid attention to the fact that by now the vast majority of boundaries of geological time scale at the epoch level have gotten reliable radiological dating. They have to be considered when dating intrusive bodies of the Altai-Sayan region. As an example, we list dating results for a number of Mountain Altay granite massifs, which yielded the Early Triassic age, although the rock masses themselves continue to be considered as the Permian-Carboniferous. It was noted that the Permian-Carboniferous boundary is not just a border between two systems, neither is a border between two erathems, but also is the largest global reorganization of its structure in the Earth's history. And if dating results yield the Mesozoic age (the Triassic), then intrusive massifs should refer to this correct age without adding Permian to it. We propose to use the absolute dating of igneous bodies more accurately, linking them to the dating of stratigraphic unit boundaries.

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