Abstract Bands of multi-phase mixtures with an extreme grain size reduction called ultramylonites are a common feature in shear zones from the middle crust to upper mantle conditions. Indeed, they are one of the most typical manifestations of strain localization in high-strained polymineralic rocks in the lithosphere, especially in the semi-brittle field. Yet many questions remain on their origin, and existing models either do not explain all the observations or require the introduction of physical processes that are difficult to prove, particularly for the inter-grain mixing of phases. Here we present a case study of ultramylonite bands developed in a coarse-grained granite deformed at conditions near the base of the seismogenic zone, i.e. showing coseismic deformation microstructures overprinted by thermally activated ones. We found that in the mylonite to ultramylonite transition, two deformation mechanisms coexist promoting inter-grain phase mixing through Kfs precipitation: (1) dislocation-accommodated grain boundary sliding in quartz and (2) diffusion-accommodated grain boundary sliding in feldspar. The model proposed to explain grain mixing is based on a cavitation-seal mechanism, which is strongly dependent on grain boundary sliding for the opening of transient micro-cavities and on the diffusivity of potassium feldspar for their progressive sealing.