Zirconology of Granite Porphyries from the Eastern Slope of the Middle Urals

摘要

The eastern slope of the Urals hosts numerous granite porphyry bodies (up to 100 m thick) distributed from the Neiva River in the north to the Kumak River in the south. Most of them are characterized by peculiar yellowish gray or bluish coloration and standard assemblage of rock-forming minerals. Phenocrysts, which constitute up to 25-30% of the rock, are 0.3 to 2.5 mm across. They are represented by plagioclase (usually PI no. 4 or 5) and less common quartz, biotite, and musco-vite. Plagioclases are frequently polysynthetic and twinned segregations locally with signs of replacement by sericite and zoisite. Quartz phenocrysts are characterized by resorption of their marginal parts. In terms of the composition of petrogenic elements, granite porphyries correspond to normal K-Na granites. Three types of relations are observed between granite porphyries and host rocks: sills, dikes, and intrusive bodies confined to tectonic fractures (normal faults). Detailed information on the composition and geological position of granite porphyries is given in [1-3]. Geochronologi-cal data are almost absent for these rocks. Single K-Ar datings (150-250 Ma) are insufficient, while information on zircons from granite porphyries is missing thus far. This communication presents the first SHRIMP II microprobe datings of single zircon crystals from these rocks carried out at the Karpinskii All-Russia Research Institute of Geology (St. Petersburg) in line with the known technique [4].