The deformation and recrystallization of biotite in the Woodroffe Thrust mylonite zone

The deformation and recrystallization microstructures in biotite from the Woodroffe Thrust mylonites are described and interpreted. The degree of strain causing recrystallization and the nucleation mechanisms differ across the mylonite zone. These differences are associated with the contrast in water content between the granulite and amphibolite facies felsic gneisses on either side of the zone. p]In moderately mylonitized granulite facies felsic gneisses (0.1–0.6% H₂O) subgrains form in intensely deformed host biotite and recrystallization mechanisms involve subgrain rotation both on host grain boundaries and associated with kink band bulge. In the amphibolite facies felsic gneisses (0.9–1.2% H₂O) the biotite recrystallizes by a mechanism involving localized internal kinking of the host and subsequent migration of high angle boundaries generated on the kink limbs. This combined with rotation due to the concurrent deformation generates high angle grain boundaries around the entire original kink limb and thus a new grain.