Original paper

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

Hoschek, Gert

European Journal of Mineralogy Volume 28 Number 4 (2016), p. 735 - 750

published: Nov 1, 2016

DOI: 10.1127/ejm/2016/0028-2549

BibTeX file

ArtNo. ESP147052804002, Price: 29.00 €

Download preview PDF Buy as PDF

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 garnet 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.

Keywords

rare-earth elementstauern windowp-t pseudosectioneclogite zoneflorenciteallanite