The reduction of grain size is believed to play a critical role in strain localization to form shear zones. Although many mechanisms have been proposed, the source of grain size reduction remains debated. The South Armorican Shear Zone (SASZ) is a crustal-scale strike-slip fault that deforms granitoids at mid-crustal conditions. The SASZ records the transition from protolith to ultramylonite, representative of increasing ductile shear strain. To investigate the evolution of strain localization, the different states of deformation were studied using a combination of detailed microstructural, chemical and electron backscatter diffraction analyzes. Increasing strain from protolith to ultramylonite resulted in (1) grain size reduction, (2) the development of interconnected monophase layers of mica and incipient mixed-phase zones composed of phengite-quartz ± K-feldspar in protomylonite and low-strain mylonite, and (3) the formation of fine-grained mixed-phase zones composed of K-feldspar-quartz ± phengite in high-strain mylonite and ultramylonite. We propose that the causes for interconnection of mica are the formation of cracks in the protolith combined with fluid-assisted nucleation. The latter process also plays a major role in phase mixing, as attested by the precipitation of K-feldspar at triple junctions in quartz-rich layers and in fine-grained tails of inherited K-feldspar porphyroclasts in ultramylonite. The transition from quartz-rich layers and mixed-phase zones is accompanied by a strong dispersion of the quartz lattice preferred orientation. These microstructural and textural evidences suggest that phase nucleation is the major process behind phase mixing, possibly accompanied by the action of grain-boundary sliding to open cavities during deformation. Instead of a single process, we therefore highlight a succession of weakening processes in the evolution of the SASZ, starting with the crystallization of mica as a first weakening material, and then evolving with the formation of very fine-grained mixed-phase zones made principally of feldspar, quartz and phengite.