| Реферат: | eng: To carry out U-Pb isotope dating of relict zircons, 31 grains were isolated from a composite sample of restitogenic ultramafic rocks of the Shaman massif ( Eastern Transbaikalia) weighing about 4 kg. All of them were characterized by a rounded shape, rough surface and micro-fracturing, a very low intensity of cathodoluminescent glow up to its complete absence, disturbed oscillatory zoning, as well as very wide variations in the isotopic age (3050- 502 Ma). The study of the entire collection of zircons by the electron probe method showed that 20 of them contain suitable size (from the first microns to 10 mu m, in some cases - up to 100 mu m) for the analysis by this method of morphologically different microinclusions of minerals that are not characteristic of ultramafic restites (rel. %): quartz (52.3), mica (15.2), chlorite (13), calcite (6.6), plagioclase (andesine) (4.3), rutile (4.3), xenotime (4.3). The chemical composition of minerals from microinclusions and their phase composition were studied based on the results of 68 analyzes performed by the above method. Quartz microinclusions were predominantly irregular, sub-isometric, or vein-like; segregations of other minerals sometimes had crystallographic faceting. Some microimputiries of quartz were in the form of linear-streaky accumulations or epitaxic intergrowths located on the rounded surface of zircon grains. Microinclusions were located both in the inner and outer zones of the zircon grains. Studies of the composition and properties of minerals from microimputiries allowed us to conclude that they are of epigenetic nature. Their formation was due to the infiltration of fluids released by later granitoid melts. Fluids introduced into ultramafic rocks and relict zircons in them, both some mineral-forming elements and trace elements, such as REE, U, Th and others, which were deposited in microimputiries and other deformation defects of zircon grains. The uneven distribution of these impurities has led to the geochemical heterogeneity of zircons. The U introduced in this case caused the disturbance of its initial balance with the radiogenic Pb isotopes in zircons, which is supposed to be one of the reasons for the observed wide variations in the values of their isotopic age.
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