New data on the age of migmatites and granitogneisses of the chuya series ( Northern Prebaikalia )

Products of the subalkaline granitization on the southern Chuya Upland are dated by three geochronologic methods. The Pb – Pb isochronous method after K-Celdspars gave the most ancient age 2096±8 Ma. Based on the Rb-to-Sr ratios in bulk samples, two isochrons dated by 1848±81 Ma and 1295±101 Ma with the crust ratio (87Sr/86Sr)0 = 0.7631±0.0589 and 0.7839±0.0221 are plotted. These values are close to the estimates of zircon age by the Pb – Pb thermoisochronous method; from 1910–1820 Ma in the core to 1360 Ma in the outer Zones. Thus, according to estimations by the three methods, the main step of subalkaliue granitization is of Lower-Proterozoic age, close to the age of the Primorskii complex granites. However, approximately 500 Ma later, the strata were warmed up again along the Abchada Fault, with Rb – Sr isotope systems being opened in the rocks and Pb – Pb in zircons. These events are correlated with the contemporaneous formation of the Baikal volcanic-plutonic belt whose geochemical rocks are identical to the Chuya Series migmatites.