Geochronology of plagioclasite veins from the Lipovskoye pegmatite field (Middle Urals)

Abstract

The relevance of the research is due to the need to improve the method of chemical dating as applied to high-thorium and high-uranium accessory minerals, which are difficult to date by isotopic research methods. The purpose of the research is to study the chemical composition of monazite and uraninite from plagioclasite veins of the Lipovskoye pegmatite field and to determine their age. Research Methodology: quantitative analysis of the chemical composition of monazite and uraninite was performed on an X-ray spectral electron probe microanalyzer CAMECA SX 100. Measurement conditions: accelerating voltage 15 kV, current strength 250 nА, electron beam diameter 2 µm. Sample chamber pressure 2 ∙ 10–4 Pa. The spectra were obtained on tilted wave spectrometers, the intensity was measured using analytical lines: Th Ma, U Mb, Pb Ma, Y La, Si Ka, Ca Ka, P Ka, Ce La, La La, Pr Lb, Nd La, Sm Lb, Dy La, Gd Lb. Age calculation was carried out according to well-known methods of foreign authors in addition to their own developments. Results. According to the chemical composition, monazite belongs to the cerium variety and is characterized by high contents of ThO2 up to 13,4 wt. %, UO2 up tо 0,8 wt. % and PbO up to 0,18 wt. %. According to chemical dating, monazite-(Ce) gives an age of 266 ± 17 Ma. In undeformed plagioclasites, uraninite contains impurities of ThO2 up to 11,6 wt. % and PbO up to 3,0 wt. %, its calculated age is 258 ± 3 Ma. In tectonized plagioclasites, uraninite is characterized by impurities of ThO2 from 26 up to 45 wt. % and PbO up to 2,4 wt. %. According to chemical dating data, two ages are calculated in these uraninites: 255 ± 10 Ma in the central part of the grains, and 244 ± 4 Ma in the marginal parts of the grains. Conclusions. Microprobe data on the composition of these minerals have been obtained, and the complex geochronology of these rocks has been established by chemical dating. These plagioclasite bodies were formed after the Middle Permian quartz-potassium feldspar (topaz-beryl) granitic pegmatites, widely developed within the Lipovskoye field, and later underwent superimposed changes twice in the Upper Permian and Triassic times.