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-sil limanite type occur in the western part of the complex, whereas higher-temp erature and shallower rocks (andalusite-sillimanite type) exist in the cent er and in the east. In our preceding works, polymetamorphic nature of the S outh Chuya complex was established, and Early Cambrian age of the first sta ge of metamorphism of the kyanite-sillimanite type was substantiated. This paper reports results of complex isotope dating of metamorphic rocks and mi gmatites of the superposed step of metamorphism of the andalusite-sillimani te type. Geochronological study was carried out at three reference localiti es in the central and eastern South Chuya complex. As a result, only Middle Paleozoic ages were obtained (from the Late Silurian through the Early Car boniferous). Comparison and analysis of the geochronological data obtained by different methods permit us to recognize the following stages in the his tory of the South Chuya complex: (1) Early Cambrian metamorphism of the kya nite-sillimanite type; (2) Late Silurian superposed high-temperature metamo rphism and ultrametamorphism of the andalusite-sillimanite type; (3) comple ting Middle-Late Devonian and Early Carboniferous thermal events, which man ifested themselves in disturbance of K-Ar isotope systems of biotites and w hite micas of some localities. The last thermal event was involved with the intrusion of veins of leucogranites and was accompanied by brittle and bri ttle-plastic deformations.