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

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We have investigated the effects of different Fe 2 O 3 bulk contents on the calculated phase equilibria of low-T/intermediate-P metasedimentary rocks. Thermodynamic modelling within the MnO-Na 2 O-chemical system of chloritoid-bearing hematite-rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe 2 O 3 is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P-XFe 2 O 3 diagrams, obtaining equilibration conditions of 475°C and 9-10 kbar related to a post-compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rockforming silicates that often yield important thermobaric information. In Fe 2 O 3 -rich compositions, garnet-and carpholite-in curves shift towards higher temperatures with respect to the Fe 2 O 3 -free systems. The presence of a ferric-iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550°C. The use of P-T-XFe 2 O 3 phase diagrams may also provide P-T information in common greenschist facies metasedimentary rocks.

We present a petrological study of phyllites from the Variscan basement at Punta Bianca (Northern Apennines). According to the literature the studied rocks are believed to be the result of Alpine and Variscan metamorphism of pre-Carboniferous pelites, which consist of Fe-chlorite, potassic white mica, quartz, minor paragonite, and accessory hematite, rutile, monazite, zircon, xenotime, and florencite. Microstructural observations indicate a main S 2 foliation overprinting an earlier S 1 foliation. The mineral assemblage can be related to both S 2 and S 1 . Large flakes of relatively Mg-rich potassic white-mica along the two foliations are probably detrital grains that preserved their composition. The calculation of isochemical phase diagrams, their contouring by various mineral chemical parameters and the deformation microstructures of quartz indicate intermediate pressure-low temperature conditions (5-7 kbar and 300-400°C). The anhedral habit and the very low Pb contents (<50 ppm) in monazite point to dissolution of this mineral probably during Tertiary times.

Strain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly-deformed basements, the loss of information on pre-kinematic phases of mylonites hinders large-scale correlations based on tectono-metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P)-temperature (T)-deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine-grained K-white mica and chlorite enveloping micafishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg-rich cores and Mg-poor rims. The amphibolite contains green amphibole+plagio-clase+garnet+quartz+ilmenite defining S 1 with a superposed mylonitic fabric localized in decimetre-to centimetre-scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P-T constraints that are linked to the reconstructed metamorphic-deformational stages. For the first time an early high-P stage at >11 kbar and 510°C was constrained, followed by a temperature peak at 550-590°C and 9-10 kbar and a retrograde stage (<475°C, <7 kbar), during which ductile shear zones developed. The inferred clockwise P-T-D path was most likely related to crustal thickening by continentcontinent collision during the Variscan orogeny. A comparison of this P-T-D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono-metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established. K E Y W O R D SHigh-P metamorphism, mylonite, Northern Apennines, P-T-D path, Variscan orogeny