Mylonitic gneisses from Parry Sound, Ontario, are intensely deformed and occur within a major deep crustal shear zone developed during thrusting of granulite facies rocks (800¿C, 900--1100 MPa) northward onto an amphibolite facies terrane. We have examined, by optical and electron microscopy, microstructures in perthites from these mylonites that preserve the deformation-recrystallization cycle which operated during high-ductility deformation of the crystals. The exsolution and recrystallization microstructures are intimately associated. Exsolution in the perthites occurred in two stages. A calcic plagioclase (oligoclase) exsolved first and subsequently coarsened and deformed during the single-crystal ductility phase of shear zone deformation. A progressive increase in size and boundary incoherence of oligoclase domains during deformation led to recrystallization of the host perthite crystals as discrete orthoclase and oligoclase grains. The principal mode of recrystallization was subgrain rotation, with a component of grain growth that followed the formation of mobile high-angle boundaries. Misorientation of subgrains to form grains at the recrystallization front is abrupt and occurred over one or two subgrain diameters. The second exsolution stage postdated perthite deformation and saw formation of fine albite lamellae in both host and recrystallized orthoclase. Temperatures inferred from the exsolution textures for independently determined confining pressures indicate that this deformation occurred under at least granulite facies conditions. The perthites thus record deformation from crustal depths of the order of 34--42 km. Continental megathrusts, such as the Parry Sound shear zone, provide mechanisms for both the generation of large strains observed in otherwise nonductile minerals and the preservation of ensuing microstructures. ¿ American Geophysical Union 1988