Middle Paleozoic age of metamorphism in the South Chuya complex in Gorny Altai (results of Ar-Ar, Rb-Sr, and U-Pb isotope dating)

The South Chuya metamorphic complex, Gorny Altai, lies on the northeastern margin of the Altai-Mongolian terrain. Metamorphic rocks of the kyanite-sillimanite type occur in the western part of the complex, whereas higher-temperature and shallower rocks (andalusite-sillimanite type) exist in the center and in the east. In our preceding works, polymetamorphic nature of the South Chuya complex was established, and Early Cambrian age of the first stage of metamorphism of the kyanite-sillimanite type was substantiated. This paper reports results of complex isotope dating of metamorphic rocks and migmatites of the superposed step of metamorphism of the andalusite-sillimanite type. Geochronological study was carried out at three reference localities in the central and eastern South Chuya complex. As a result, only Middle Paleozoic ages were obtained (from the Late Silurian through the Early Carboniferous). Comparison and analysis of the geochronological data obtained by different methods permit us to recognize the following stages in the history of the South Chuya complex: (1) Early Cambrian metamorphism of the kyanite-sillimanite type; (2) Late Silurian superposed high-temperature metamorphism and ultrametamorphism of the andalusite-sillimanite type; (3) completing Middle-Late Devonian and Early Carboniferous thermal events, which manifested themselves in disturbance of K-Ar isotope systems of biotites and white micas of some localities. The last thermal event was involved with the intrusion of veins of leucogranites and was accompanied by brittle and brittle-plastic deformations.