Uran-leadium age by zirkon of Kapitanivskyi massif (Middle Pobuzhzhia, Ukrainian Shild)

Abstract

The deposits of silica nickel and chromium are associated with the ultra-basic intrusions of the Kapitanivsko-Derenyukhinsky complex, common in the Middle Pobuzhzhіa. According to the current correlation chronostratigraphic scheme of the Ukrainian shield, the rocks of the complex are Neoarchean formations. The age determination of zircones from amphibolite by the local uran-leadium isotope method was carried out on the ion-microprobe SHRIMP-II at the VSEGEI isotope research center. The body of amphibolite, with a power of about 1 meter, is located in the southern part of the Central Quarry and lies among the serpentinites. Zircon crystals of amphibolite are represented mainly by two generations. The first generation has a dense brown color and forms the core on which the transparent light-pink zircon of the second generation grows. Brown zircon, in comparison with light pink is characterized by significantly higher concentrations of uranium (177–2759 ppm in comparison with 14–35 ppm in light pink), which may indicate a higher temperature crystallization of light pink zircons, and hence the progressive orientation of the endogenous processes in which crystallization of zircons occurred in amphibolite. According to the content of uranium and crystallomorphological features, light pink zircon of amphibolites is similar to “precious” zircon typical to mafic granulite (amphibole two-piroxenes crystalline sheets ) of Pobuzhzhіa. The age of early (brown) zircone is the same with light pink zirconium. The age of zircon, calculated only for concordate dates is 2024±24 million years, and at the upper intersection of concordia by the discord calculated according to all data – 2032±31 million years. This date is the upper age board of the Kapitanivska intrusion considering apomagmatic crystallization of zircon.