THE ARCHEAN AGE OF GRANITE-GNEISS COMPLEXES FROM THE KAMA-VYATKA ZONE (THE VOLGA-URAL SEGMENT, EAST EUROPEAN CRATON)

The formation history of granulite complexes is fundamental to understanding the processes of early continental crust origins. The work presents the results of an isotope-geochronological study of rock samples from the main complexes of the Kama-Vyatka zone (the Volga-Ural segment, the East European craton) — enderbites of the Otradnensky series and quartz diorites of the Tanai plagiogranitoid massif. The Sm-Nd isotopic data were used to calculate model ages of quartz diorites of the Tanaysky plagiogranitoid massif and enderbites of the Otdnenskaya series — 3.2 and 3.0 Ga, respectively. The U–Pb isotope system of zircon was investigated by LA-ICP-MS. The zircon of quartz diorites indicated the Archean age of protolith formation of the plagiogranitoids of the Tanaysky Massif. This time interval — 3.04–2.98 Ga — includes the stage of the earliest granulite metamorphism immediately following the episode of magmatism. There are several generations of zircon grains from a sample of weakly gneissed enderbites of the Otradnensky series in the 3.0–2.8 Ga age interval, which record the following events: the formation of primary enderbites, locally manifested partial melting under granulite facies conditions and regressive stage of metamorphism in the transitional conditions of granulite to amphibolite facies. Considering to model age of enderbites, for the first time, an estimate of the age of the Otradnensky Group of the Kama-Vyatka zone of the Volga-Ural segment was obtained — 3.0 ± 0.1 Ga. In the 2.75–2.60 Ga age interval, zircon from a sample of weakly gneissed enderbites records the most significance episode of granulite metamorphism, widely manifested throughout the Volga-Ural segment and, replacing it, regressive metamorphism with the input of water-bearing fluid and temperature decreasing.