Résumé
Equilibrium phase diagrams are forward models that show phase relations as a function of environmental and chemical variables. In metamorphic petrology, this tool is extensively used to estimate the P-T conditions of equilibrium. If a rock preserves a suite of mineral reactions and/or mineral chemical zoning, then portions of the P-T path might be determined. However, the application of phase diagrams in metamorphic petrology is not restricted to these two classical inverse petrological problems. Phase diagram are also essential for understanding metamorphic processes involving mass transfer (open systems). For instance the effect of melt loss/gain, dehydration/decarbonation on phase relations and its consequences on the preservation of peak metamorphic assemblages have been extensively studied this past decade. Another important and broader application of phase diagram, and more particularly phase diagram sections, is that they can provide mineralogical and physical rock properties, like density or seismic velocities. These data can be incorporated into geophysical models to investigate the influence phase transitions on the seismic structure of the crust or mantle. The goal of this contribution is to present another example of application of phase diagram for understanding the processes of strain localization. In many cases, shear zones are developed in metastable host-rock and are associated with metasomatism. Therefore to quantify the evolution of the mineralogical and physical properties it is critical to determine the sequence of metamorphic reactions involved at the new P-T-X conditions of the deformation. Three alpine shear zones developed in variscan granitic rocks from the Alps (Aar massif, Gotthard massif and Suretta nappe) were studied . We have performed a thermodynamic modeling using petrogenetic grids and pseudosections that consider variations in chemical potential (mu ) of the mobile components (MgO, CaO, and Na2O) at fixed P and T. In all cases, we show that the sequence of mineralogical and geochemical changes involved during process of equilibration of the host-rock, induce a strong weakening of the host rock and therefore drives the widening of the shear zone.