Microstructure of mylonites and their descriptive terminology

The microstructural transition from amphibolite facies, gneissic country rock, with increasing deformation, to a fine-grained phyllitic mylonite is described in detail. All microstructures (e.g. polygonization, subgrains, serrated grain boundaries, kink bands, grain aggregates) are attributed to ductile deformation, recovery and recrystallization processes. The similarity of these structures to other described mylonites suggests that brittle deformation is not widespread in mylonitic rocks, and that terminology with brittle connotations should be discontinued. A new definition for the term 'mylonite' is proposed.