Neoproterozoic Age of Collisional Metamorphism in the Transangara Region of the Yenisei Ridge (Based on ~(40)Ar/~(39)Ar Data)

摘要

In terms of geodynamics, the Yenisei Ridge is one of the most interesting regions in the southwestern folded framing of the Siberian Craton. Regional structures of the Transangara region of the Yenisei Ridge are traditionally depicted as a NW-extending system of tectonic sheets separated by faults located near the zone of collision of blocks. The collision was responsible for the regional process of heterogeneous (with respect to pressure) metamorphism expressed in the juxtaposition of two (low- and moderate-pressure) facies series. The moderate-pressure collisional metamorphism of the presumably younger stage is locally overprinted on low-pressure metamorphic rocks. Therefore, andalusite is progressively replaced with kyanite, resulting in the formation of new mineral assemblages and deformation structures. The progressive replacement of andalusite with kyanite recorded in the Yenisei Ridge is a rare process, because the stationary continental geo-therm usually does not intersect the andalusite-kyanite equilibrium line. Such replacements are commonly attributed to the retrograde phase of metamorphism. However, this interpretation contradicts the geological situation observed in the study region. Progressive transformations of andalusite into kyanite have only been reported from some regions (northwestern Cordillera, United States and Canada; Dalredian, Scotland; central and northwestern Appalachians, United States; Kola Peninsula and Yenisei Ridge, Russia), where these processes are attributed to pressure growth as the result of thrusting or magmatic loading with various PT trends. Both the reconstruction of the PT evolution of metamorphic complexes and the isotope datings of rocks are essential for geodynamic modeling of regions with manifestations of collisional and contact metamorphism. Correct interpretation of these data provides insight into the interrelation and age correlation between processes of metamorphism, tectonics, and magmatic activity. Therefore, we carried out special geochronological investigations of metapelites related to collisional metamorphism. The results obtained are discussed in this paper.