A structural and isotopic dating study of white micas from mylonites in the Ruby Gap duplex off central Australia shows that mylonitic deformation occurred during the Paleozoic Alice Springs orogeny. Deformation of the white micas took place under greenschist facies conditions, in the approximate temperature range 250-350 °C. Two distinct populations of white mica have been found: (1) porphyroclasts of original muscovite and (2) neocrystallized phengites that define the fabric associated with deformation. 40Ar/39Ar age spectra of neocrystallized phengites yield plateau-like spectra of Paleozoic age. In contrast, 40Ar/39Ar age spectra of muscovite porphyroclasts exhibit Proterozoic apparent ages, indicating that white micas were not open to significant argon diffusion during formation of the Ruby Gap duplex. We conclude that the ages measured on neocrystallized phengites record the cessation of ductile deformation and migration of tectonic activity toward the internal zones of the thrust system.