Staircase 40Ar/39Ar spectra of 2 to 6 μm white micas in ductile carbonate mylonites (Morcles-Doldenhorn nappe, Switzerland) record ages between 32 and 13 Ma. These ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold-thrust nappe. We interpret these data to record protracted white mica neocrystallization and growth during low-temperature ductile deformation, such that the differences between the maximum and minimum ages in the 40Ar/39Ar spectra record the time span over which mica formed and grew. We have modeled the argon concentrations in these micas, taking into account the thermal history of the nappe, mica growth, and diffusional loss of argon. Our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite, and that the retentivity of argon in white mica is greater than generally thought.