Abstract
The uranium-lead isotopic system of monazites from leucocratic gneiss (Pryazovsky megablock) was studied and the age of their formation was determined. Monazites from leucocratic gneiss are characterized by significant discordance, which is obviously due to the radiogenic component of impurity lead. In our case, the reverse discordance is to a large extent obviously due to the addition of radiogenic lead, which was captured by monazite during crystallization and is a component of impurity (abnormal composition) of lead. Calculating the age of monazite using a concordia diagram (Arens-Wetherill method) gave the following results: at the upper intersection of the regression line, the age was 2029 ± 145 and 1106 ± 337 Ma, at the lower one MSWD = 35. Taking into account the rather large contents of ordinary lead in the monazite samples studied and the significant dependence of the isotopic age determination results on the adequately introduced correction for ordinary (impurity lead), the isotopic composition of which may not correspond to the isotopic composition of ordinary lead according to Stacey and Kramers, calculated the relationship between numerical age values obtained from the 207Pb/206Pb isotopic ratio and the 206Pb/204Pb ratio, which sufficiently characterizes the content of ordinary lead. Therefore, the numerical values of the age, calculated using different models, according to the obtained analytical data, are quite different: 2027 Ma, 2037 ± 18, 2029 ± 145, and 2099 Ma. In our opinion, the isotopic date, 2037 ± 18 Ma, calculated by the isotopic composition of lead, taking into account the errors in the determination of lead isotopic ratios, corresponds to the time of monazite crystallization to a greater extent. Within the margin of error, it coincides with the date of 2029 ± 145 Ma, obtained from the upper intersection of the discordia with the concordia in the coordinates of the Ahrens-Weatherill method.
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