Ages of deformation from K/Ar and ⁴⁰ Ar/ ³⁹ Ar dating of white micas

A structural and isotopic dating study of white micas from mylonites in the Ruby Gap duplex of 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. ⁴⁰ Ar/ ³⁹ Ar age spectra of neocrystallized phengites yield plateau-like spectra of Paleozoic age. In contrast, ⁴⁰ Ar/ ³⁹ Ar 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. It is concluded 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. -Authors