Monazite dissolution-reprecipitation in medium-grade metasedimentary rocks from the northern Apennines, Italy

Fluid-induced dissolution of parent phases and precipitation of products is a fundamental process in metamorphism (Putnis & Austrheim, 2010). Here we present the dissolution-reprecipitation behaviour of monazite, one of the main rare-earth element (REE)-bearing accessory minerals useful for obtaining geochronological and geochemical information from a wide range of Ca-poor crustal rocks. The reactions involving monazite were studied in a micaschist recovered from the Pontremoli 1 well (Tuscany) and the implications of fluid-controlled monazite breakdown on age dating were discussed (Lo Pò et al., 2016). The micaschist belongs to the Variscan basement of the Northern Apennines. In this rock monazite, either disseminated in the matrix or included in white mica and chlorite partially replacing garnet, is surrounded by coronitic microstructures consisting of concentric shells of apatite + Th-silicate, allanite, epidote. Studies of the element partitioning between garnet and accessory minerals and the garnet inclusion mineralogy suggest that monazite was partially dissolved and replaced by apatite, allanite and epidote during garnet breakdown to chlorite and muscovite. This stage was associated with the retrograde and decompressional evolution of the micaschist at fluidpresent conditions. Through thermodynamic modelling, we determined the P-T conditions of the monazite partial dissolution and replacement process at 510 ± 35°C, during a nearly isothermal decompression path from 8 kbar to 2-3 kbar. Fluid-induced alteration of monazite also resulted in a partial resetting of the monazite ages, which were determined to be between 294 and 19 Ma. The maximum extent of the alteration process occurred in monazite located within retrogressed garnet rims. In this microstructural site, Pb in the Variscan monazite was lost. Lo Pò, D., Braga, R., Massonne, H.-J., Molli, G., Montanini, A., Theye, T. (2016): Fluid-induced breakdown of monazite in mediumgrade metasedimentary rocks of the Pontremoli basement (Northern Apennines, Italy). J. Metamorph. Geol., 34, 63-84. Putnis, A. & Austrheim, H. (2010): Fluid-induced processes: metasomatism and metamorphism. Geofluids, 10, 254-269.