Phase relations of the REE minerals florencite, allanite and monazite in quartzitic garnet–kyanite schist of the Eclogite Zone, Tauern Window, Austria

Abstract

The three REE phases florencite-(Ce), allanite-(Ce)–Ce-rich epidote solid solution, and monazite-(Ce) are present in two of four mica schists with the assemblage garnet + chloritoid ± kyanite + muscovite + quartz from the Eclogite Zone, Tauern Window, Austria. Prograde peak conditions of ~560–600°, 20–25 kbar are associated with the reaction: chloritoid + quartz = garnet + kyanite + H2O. Florencite-(Ce) inclusions are mostly found in ganet rims and kyanite, but also in the muscovite-quartz matrix, where it has partly broken down to symplectites containing, e.g ., monazite-(Ce) and more rarely apatite. Allanite-(Ce) is mostly restricted to garnet cores and none was found in the matrix of florencite-bearing domains. Pseudosections with preliminary thermodynamic data for florencite and bulk compositions of four mica schists including REE reveal that florencite-(Ce) is stable from the low to the higher P–T regions. Increase in bulk Ca, with all other components unchanged, markedly reduces florencite stability due to a shift to higher pressure, but extends allanite stability. Increase of Al content has a similar, albeit smaller effect, whereas an increase in Fe results in a moderate reversal of this trend. A higher Fe3+/Fetot ratio shifts florencite and allanite stabilities towards higher P–T conditions. The contrasting assemblages of two compositional domains in one sample support the strong influence of Ca: a) the Ca-poor domain bears numerous florencite grains in the matrix and included in garnet, but only a single allanite inclusion in garnet, b) the Ca-rich domain contains no florencite but abundant allanite-(Ce)–epidote as inclusions in garnet and in the matrix. Fractional garnet crystallization has a strong influence and extends florencite stability closer to the “allanite out” line. Florencite stability in the present samples is mainly due to the low bulk-Ca content together with relatively high P .