Fluid-induced breakdown of monazite in medium-grade metasedimentary rocks of the Pontremoli basement (Northern Apennines, Italy)

The last (decompression) stages of the metamorphic evolution can modify monazite microstructure and composition, making it difficult to link monazite dates with pressure and temperature conditions. Monazite and its breakdown products under fluid-present conditions were studied in micaschist recovered from the cuttings of the Pontremoli 1 well, Tuscany. Coronitic microstructures around monazite consist of concentric zones of apatite + Th-silicate, allanite, and epidote. Chemistry and microstructure of the monazite grains, which preserve a wide range of chemical dates ranging from Upper Carboniferous to Tertiary times, suggest that this mineral underwent a fluid-mediated coupled dissolution-reprecipitation and crystallization processes. Consideration of the chemical zoning (major and selected trace elements) in garnet, its inclusion mineralogy (including xenotime), monazite breakdown products and phase diagram modelling allow the reaction history among accessory minerals to be linked with the reconstructed P-T evolution. The partial dissolution and the replacement by REE-accessory minerals (apatite-allanite-epidote) occurred during a fluid-present decompression at 510 ± 35 °C. These conditions represent the last stage of a metamorphic history consisting of a thermal metamorphic peak at 575 °C and 7 kbar, followed by the peak pressure stage occurring at 520 °C and 8 kbar. An anticlockwise P-T path or two clockwise P-T loops can fit the above P-T constraints. The former path may be related to a context of late-Variscan strike-slip dominated exhumation with minor Tertiary (Alpine-related) reworking and fluid infiltration, while the latter requires an Oligocene-Miocene fluid-present tectono-metamorphic overprint on the Variscan paragenesis.