Extreme ductility of feldspars from a mylonite,Parry Sound,Canada

Single feldspar crystals in mylonites, from the Grenville structural province of Canada, have accumulated extreme strains by ductile mechanisms. The samples studied are from a deep-crustal shear zone and were collected at Parry Sound, Ontario. The mylonites were derived from granite and leucogabbro, and the feldspar crystals originated in late syntectonic pegmatite dykes.Optical and transmission electron microstructures of microperthitic alkali feldspars show evidence of syntectonic and synchronous dislocation climb (leading to recovery), Si/Al ordering, and Na/K interdiffusion. Evidence for the operation of these processes is, respectively, ubiquitous subgrains, development and coarsening of tweed texture and transition of monoclinic to triclinic K-feldspar, and exsolution domain reorganization. These processes occurred concurrently with extreme straining of the host crystals by dislocation and diffusion mechanisms.The effects of ductile deformation preserved in these crystals are quite different from those usually observed in feldspars. The unusual microstructures are thought to be due to the conditions of high temperature and confining pressure which existed during deformation, in concert with a deformation-enhancing point defect chemistry, possibly associated with incorporation into the crystals of a species of ‘water’.