Physico-chemical control on the REE minerals in chloritoid-grade metasediments from a single outcrop (Central Alps, Switzerland)
Résumé
This study assesses the effects of fluid, whole-rock composition and oxygen fugacity, on the texture and composition of monazite, allanite, and xenotime. For this purpose, these were investigated in 13 mono-metamorphic metasediments from a single locality of the Central Alps (Switzerland), which record greenschist facies conditions (T ~ 400-450 °C). Two of the samples contain hydrothermal veins dominated by quartz and calcite + quartz, respectively. In metasediments devoid of veins, the light rare earth elements (LREE) are concentrated in allanite in all samples except for one metamarl (Ga06). Allanite formation is texturally coeval with apatite, chloritoid and xenotime, during the main tectono-metamorphic stage. Allanite formation implies significant mass transfer of Ca and P via a fluid phase, which is not clearly related to advective transport. In Ga06, elongate monazite grains have a detrital core rimmed by newly formed monazite. Significant arsenic contents are found in newly formed monazite, xenotime and apatite. Monazite texture and composition suggest (re)crystallization by pressure solution, at an oxygen fugacity sufficient to partly oxidize As, S, U, and Fe. Whether or not monazite is preserved appears to be but weakly dependent on the Ca/Al ratio and thus whole-rock composition at greenschist facies conditions. Samples with veins show peculiar features. Along carbonate-bearing veins allanite occurs as porphyroblasts overgrowing the main foliation of the host rock. As similar allanite porphyroblasts occur in calcite-bearing metamarl, their formation is attributed to environments rich in CO2-bearing fluid. A new generation of monazite is found in and along retrograde quartz veins. This study demonstrates that, at least in metasediments up to chloritoid-grade, REE minerals record fluid/rock interaction that occurred at different deformation stages. Arsenic concentrations in REE phosphates appear to reflect conditions of elevated oxygen fugacity. In cases where such conditions are not inherited from the sedimentary protolith, the oxidation reflects a hydrothermal event, the age of which may be directly datable by U/Th-Pb of As-rich monazite.