Reaction and Geometrical Softening in Granitoid Mylonites

The microstructures of granitoid mylonites from Locquirec, N. Brittany; Ben Arnaboll, NW Scotland; and Cap de Creus, NE Spain show a similarity of development as a response to both geometric and reaction softening. A combination of these two softening processes leads to a concentration of deformation in the respective shear zones. Geometric softening leads to a rotation of quartz lattices for easy slip and is accompanied by dynamic recrystallization. Metamorphic reactions involving the breakdown of orthoclase and plagioclase feldspar to fine aggregates of muscovite plus quartz provide a secondary softening process which further concentrates deformation. These reactions indicate fluid mobility in the shear zones during deformation processes.