The Archean Age of Granite-Gneiss Complexes from the Kama–Vyatka Zone (the Volga-Ural Segment, East European Craton)

Abstract

The formation history of granulite complexes is fundamental significance for understanding the growth of early continental crust. The work presents the results of an isotope-geochronological study of rock samples from the main lithotectonic complexes of the Kama–Vyatka zone (the Volga–Ural segment of the East European Craton)—enderbites of the Otradnenskaya Group and quartz diorites of the Tanaisky plagiogranitoid massif. The model ages of quartz diorites of the Tanaisky plagiogranitoid massif and enderbites of the Otradnenskaya Groups calculated from Sm–Nd data are 3.2 and 3.0 Ga, respectively. Zircons from the quartz diorites of the Tanaisky plagiogranitoid massif and the enderbites of the Otradnenskaya Group were dated by U–Pb LA-ICP-MS method. Zircon from quartz diorites yielded the Archean age of protolith of the plagiogranitoids of the Tanaisky Massif. This time interval of 3.04–2.98 Ga marks the stage of the oldest granulite metamorphism immediately following the magmatic event. Zircons from weakly gneissose enderbites of the Otradnenskaya Group is subdivided into two age groups: 3.0–2.8 and 2.750–2.60 Ga. Based on the morphology, internal structure of the crystals, and their isotope-geochemical characteristics (Th and U contents, Th/U ratio), each of the indicated age groups includes several zircon generations. Within a time interval of 3.0–2.8 Ga, the identified zircon generations record the following events: the formation of primary enderbites, local partial melting under the granulite-facies conditions, and retrograde metamorphism under transitional granulite–amphibolite facies. With allowance for the model age of the enderbites, the Otradnenskaya Group of the Kama–Vyatka zone of the Volga–Ural segment was dated for the first time at 3.0 ± 0.1 Ga. In the time interval of 2.75–2.60 Ga, zircon from weakly gneissose enderbites records the peak granulite metamorphism, which spanned the entire Volga-Ural segment, and subsequent retrograde metamorphism accompanying by the input of hydrous fluid and temperature decrease.