Alpine deformation in the Grimsel granodiorite (Aar massif, Central Alps) at greenschist facies conditions (6.5 ± 1 kbar for 450°C ± 25°C) is characterized by the development of a network of centimetre to decametre localized shear zones that surround lenses of undeformed granodiorite. Localization of deformation is assumed to be the result of a first stage of extreme localization on brittle precursors (nucleation stage) followed by a transition to ductile deformation and lateral propagation into the weakly deformed granodiorite (widening stage). A paradox of this model is that the development of the ductile shear zone is accompanied by the crystallization of large amounts of phyllosilicates (white mica and chlorite) that maintains a weak rheology in the localized shear zone relative to the host rock so that deformation is localized and prevents shear zone widening. We suggest that chemical processes, and more particularly, the metamorphic reactions and metasomatism occurring during re-equilibration of the metastable magmatic assemblage induced shear zone widening at these P-T-X conditions. These processes (reactions and mass transfer) were driven by the chemical potential gradients that developed between the thermodynamically metastable magmatic assemblage at the edge of the shear zone and the stable white mica and chlorite rich ultramylonite formed during the first stage of shear zone due to localized fluid infiltration metasomatism. P-T and chemical potential projections and sections show that the process of equilibration of the wall rocks (l-l path) occurs via the reactions: kf + cz + ab + bio + MgO + H 2 O = mu + q + CaO + Na 2 O and cz + ab + bio + MgO + H 2 O = chl + mu + q + CaO + Na 2 O. Computed phase diagram and mass balance calculations predict that these reactions induce relative losses of CaO and Na 2 O of $100% and $40% respectively, coupled with hydration and a gain of $140% for MgO. Intermediate rocks within the strain gradient (ultramylonite, mylonite and orthogneiss) reflect various degrees of re-equilibration and metasomatism. The softening reaction involved may have reduced the strength at the edge of the shear zone and therefore promoted shear zone widening. Chemical potential phase diagram sections also indicate that the re-equilibration process has a strong influence on equilibrium mineral compositions. For instance, the decrease in Si-content of phengite from 3.29 to 3.14 p.f.u, when white mica is in equilibrium with the chlorite-bearing assemblage, may be misinterpreted as the result of decompression during shear zone development while it is due only to syn-deformation metasomatism at the peak metamorphic condition. The results of this study suggest that it is critical to consider chemical processes in the formation of shear zones particularly when deformation affects metastable assemblages and mass transfer are involved.

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