Monazite and zircon as major carriers of Th, U, and Y in peraluminous granites: examples from the Bohemian Massif

Abstract

The chemical compositions of zircon and monazite and the relationships between the contents of Th, U, Y, and REE in both minerals and in the bulk samples of their parental rocks were studied in three Variscan composite peraluminous granite plutons in the Bohemian Massif. It was established that granites of similar bulk composition contain zircon and monazite of significantly different chemistry. Monazite typically contains 5–13 wt% (rarely up to 28 wt%) ThO2, 0.4–2 wt% (up to 8.2 wt%) UO2, and 0.5–2 wt% (up to 5 wt%) Y2O3, whereas zircon typically contains less than 0.1 wt% (rarely up to 1.7 wt%) ThO2, less than 1 wt% UO2 (in the Plechý/Plockenstein granite, commonly, 1–2 wt% and scarcely up to 4.8 wt% UO2), and less than 1 wt% Y2O3 (in the Nejdek pluton often 2–5, maximally 7 wt% Y2O3). Monazite is an essential carrier of thorium, hosting more than 80 % of Th in all studied granites. Monazite also appears to be an important carrier of Y (typically 14–16 %, and in the Melechov pluton, up to 81 % of the total rock content) and U (typically 18–35 % and occasionally 6–60 % of the total rock budget). The importance of zircon for the rock budget of all the investigated elements in granites is lower: 4–26 % U, 5–17 % Y, and less than 5 % Th.