THE KEY ROLE OF µH2O GRADIENTS IN DECIPHERING MICROSTRUCTURES AND MINERAL ASSEMBLAGES OF DUCTILE SHEAR ZONES: EXAMPLES FROM THE CALABRIA POLYMETAMORPHIC TERRANE

A careful petrologic analysis of mineral assemblages of mylonites is crucial for a thorough comprehension of the rheologic behaviour of ductile shear zones active during an orogenesis. In this view, the knowledge of the way new minerals form in ductilely sheared rocks and why relict porphyroblasts are preserved in zones where mineral reactions are generally supposed to be deformation-assisted, is essential. To this goal, the role of chemical potential gradients, particularly that of the aqueous fluid (µH2O), was examined here through phase equilibrium modelling of syn-kinematic mineral assemblages developed in three distinct mylonites from the Calabria polymetamorphic terrane. Results revealed that gradients in chemical potentials are recorded in the mineral assemblages of the studied mylonites, and that new syn-kinematic minerals formed within sites where the µH2O was higher than in the surroundings. In each case study, the banded fabric of the mylonites resulted from fluids internally generated by the breakdown of OH-bearing minerals. The gradients in µH2O favoured the origin of bands enriched in hydrated minerals alternated with bands where anhydrous minerals were preserved even during exhumation. Phase equilibrium modelling shows that during the prograde stage of metamorphism, high µH2O were necessary to form new minerals while relict porphyroblasts remained stable in condition of low µH2O even during exhumation. Hence, the method used in this contribution is an in-depth investigation of the fluid-present/-deficient conditions that affected mylonites during their activity, and provides a more robust interpretation of their microstructures, finally helping to explain the rheologic behaviour of ductile shear zones.