New data on the zircon age of Adui pluton granites

Research subject . Zircon of granites in the western contact part of the Adui massif (Middle Urals). Aim. To obtain new U-Pb dating and to clarify the geochronological model of the Adui granite massif. Methods . U-Pb dating of zircon was established using a SHRIMP-II high-resolution secondary ion microprobe (VSEGEI Research Center, St. Petersburg). The major and trace elements were determined using SRM-18, SRM-25, and VRA-30 X-ray fluorescence spectrometers and an ELAN-9000 mass spectrometer with inductively coupled plasma. Results. In terms of chemical composition, the vein granites in the western contact zone belong to the rocks of the granite-leucogranite formation that make up the massif. Their hosting gneisses possess features similar to the granitoids of the gabbro-tonalite-granodiorite-granite formation. Zircons from orthogneisses were found to have a simple structure and a close Th/U ratio (0.6) with wide variations in uranium and thorium contents. Their concordant age ranges from 290–270 million years. Such a wide age range may be due to the loss of lead during metamorphism. The zircon of granites is different. It consists of cores, often clastic in appearance, overgrown with a rim of high-uranium zircon. Zircon grain cores from early (pegmatoid) granites are similar to those from host orthogneiss and can be interpreted as captured from them. The cores of zircon grains in late granites are characterized by a rather low Th/U ratio (0.1) and wide variations in U contents; their dates form a single age cluster with an average concordant age of 270 Ma. The rims, which are characterized by high U concentrations and low Th/U ratios, mark the period of granite formation in the range of 265–250 million years. Conclusions. The interpretation of new and previously published age data consists in identifying a prolong stage of metamorphism and migmatization of the host rocks in the range from 290 to 270 million years, preceding granite formation. The completion of granite formation with the formation of the main phases of the Adui massif corresponded to a time interval of 265–250 million years.