The San José pluton in Baja California, México, comprises at least two well-defined, texturally distinct units. The northern unit was intruded by the central unit after the former had extensively crystallized at its margins. During intrusion of the central unit, the margin of the northern unit underwent brittle and crystal–plastic deformation, at least part of which occurred in the presence of residual melt. We infer that biotite grains in this rock readily deformed by slip and frictional sliding along (001) planes, which caused strain-rate and differential-stress gradients across their grain boundaries into the surrounding plagioclase framework causing it to fracture. These microfractures grew and coalesced, and became sites of localized ductile flow. Continued development of these microshear zones led to coalescence of biotite grains, mainly by mechanical entrainment, and ultimately to a pervasive mylonitic foliation. Thus, in a single deformation, these rocks passed through a brittle–ductile transition. The development of an anastomosing network of ductile microshear zones allowed the progressive partitioning of strain rates, probably over several orders of magnitude, between the microshear zones and intervening polymineralic aggregates. Numerical experiments were conducted to evaluate the process of biotite-assisted fracturing of the stress-supporting framework, and the progressive evolution of differential stress and strain rate. The results are consistent with experimental evidence that biotite is extremely weak in shear, and that phyllosilicate-bearing rocks may accommodate strain rates several orders of magnitude higher than the bulk strain rate. This study also supports previous suggestions that strain rates associated with the growth of crustal magma chambers may be several, to many, orders of magnitude greater than those normally associated with regional tectonic deformation.