Zircons of fenites of Ilmeno-Vishnevogorsky Complex (Southern Urals)

Abstract

Research subject. U-Pb zircon dating, as well as a petrological and geochemical study of pyroxene-amphibole-, pyroxeneamphibole- biotite- and biotite-bearing fenites from the Central Alkaline Band Ilmeno-Vishnevogorsky Complex.Methods. The age of zircons was determined by an ion mass spectrometer (SHRIMP II, Centre of Isotopic Research VSEGEI). The content of REE and trace elements was estimated by secondary ion mass spectrometer methods (CAMECA IMS-4F, Valiev Institute of Physics and Technology RAS).Results. The mineralogical and geochemical (U, Th, REE) features of zircons, as well as fenites, reflect their polygenic-polychronous nature. Most zircon crystals have a metastable matrix and are characterized by averaged REE contents between igneous and hydrothermal types. These crystals are distinguished from magmatic zircons by high LREE contents and low values of Ce anomalies, and from hydrothermal zircons – by differentiated REE distribution spectra. Three ages of zircon were established: 2066–1686 (PR1), 425–404 (S2) and 284–266 (P1) Ma. PR1 zircons reflect the primary features and the degree of changes in the fenite substrate. S2 zircons, limited only to the biotite- bearing fenite, correspond to the age of the miaskite formation process. The P1 zircons clearly reflect the metasomatic process of fenitization initiated by late shear deformations. The temperature of the phenitization processes (based on the Ti content in zircons) was estimated at 630–670°C for S2 and ≤ 600°C for P1 fenites, respectively.Conclusions. Central Alkaline Band fenites were formed by the metasomatic process of PR1 substrate fenitization in the late stage (P1) of shear strains, which are widely expressed in the Ilmeno-Vishnevogorsky Complex.