Zoning patterns in a plagioclase porphyroblast in a blastomylonite derived from Lewisian gneiss near Loch Eriboll, northwest Scotland, suggest that the crystal grew during deformation. Discontinuities in the concentrations of CaO, K2O, and FeO mark the boundaries of overgrowths and define a core-overgrowth structure. The simultaneous changes in concentration of CaO and K2O are used to construct a qualitative chemical potential diagram that reflects changes in aCa++/a2H+ and aK+/aH+ in the coexisting fluid during growth. Increases in these activity ratios are believed to have caused supersaturation of the grain boundary fluids with respect to plagioclase. The repeated periods of growth indicate repeated overstepping of equilibrium, which could have been caused by ion exchange on surfaces newly exposed during deformation. These data fit a model that relates cyclic variations of strain rate in fault zones with cyclic periods of crystal growth, which in turn could have a major influence on fault rock rheology.