THE CHEMICAL COMPOSITION AND DATING OF ACCESSORY ZIRCON FROM GRANITIC PEGMATITES IN THE NORTH-EASTERN PART OF THE ADUISKY MASSIF

This work is made relevant by the necessity to improve chemical dating methods, when applied to high atomic and thorium zircons, for which isotopic methods cannot be used. The purpose of the work is to study the chemical composition of the accessory zircon (cyrtolite) from granitic pegmatites in the north-eastern part of the Aduisky massif (in the Middle Urals) and determine how best to date it. Methodology. The study comprised quantitative analysis of the chemical composition of the zircon by using a CAMECA SX 100 X-ray electron probe micro-analyser (with an electron beam diameter from 1 μm, BSE, SE, Cat, and determination of elements from beryllium to uranium). To measure the intensity of elements, we have selected the following analytical lines: Y Lα, Si Kα, Zr Lα, Hf Mα (analysing crystal TAP), U Mβ, Pb Mα, Ca Kα, Th Mα (analysing crystal PET), Yb Lα, Er Lα, Lu Lα (analysing crystal LiF). Calculation of the age of the zircon was carried out acя/cording to well-known, existing methods in addition to those developed by the authors. Results. According to the microprobe analysis, the impurity content of ThO2, UO2 and PbO in the zircon varied significantly, within the ranges 0.13 to 2.69, 1.59 to 15.42 and 0.05 to 0.57 wt.%, respectively. The dating calculation was carried out for each mineral (in which the analysis took place). Their age was found to be between 280 and 219 Ma. At the same time, the weighted mean was 254 ± 6 Ma (with the Mean Square of Weighted Deviates being 0.17) and the isochron showed 255 ± 7 Ma. The values of the ages found for the zircon from the pegmatites “Mys-2” agree with the isotopic data. The period of formation of the Aduisky granite massive has been estimated to be between 291 ± 8.0 Ma and 256 ± 0.6 Ma (according to zircon and monazite dating, respectively) or within the range 255 to 241 Ma (according to mica dating). Conclusion. We have studied the accessory zircon (cyrtolite) from granite pegmatites from the “Mys-2” vein, in the north-eastern part of the Aduisky massif. We have obtained the chemical composition and calculated the age to be 255 ± 7 Ma. Dating calculations show that veined pegmatites and host granites were formed almost simultaneously (at least, in this part of the Aduisky massif). This situation justifies microprobe dating of the U-Th zircon content because the minerals are usually in a metamict state and not suitable for accurate age determination