Microstructure and geochemistry of plagioclase and microcline in naturally deformed granite

Naturally deformed feldspars from foliated granites in a shear zone in Newfoundland exhibit transitional brittle-ductile behaviour. Brittle failure is subordinate to dynamic recrystallization, microcracking, strain enhanced diffusion and reaction enhanced ductility during the deformation. Both plagioclase (An₂₈) and K-feldspar are transformed to albite with increasing strain. Interaction of metamorphic and structural processes at the grain scale is emphasised. This is illustrated with examples of quartz-filled veins (segregation bands) in plagioclase and recrystallized polycrystalline aggregates in plagioclase and K-feldspar. The role of microcracking in plagioclase and of pre-existing internal growth structures in the formation of initially coarse grained recrystallized aggregates from large single crystals is suggested.