Micas and potassium feldspars from the South Cyclades Shear Zone, Ios, Cyclades, Greece, yield varied and complex 40Ar/39Ar apparent age spectra. A correlation exists between 40Ar/39Ar apparent ages and the relative timing of different episodes of recrystallization and grain growth, as indicated by fabric and microstructural analysis. The 40Ar/39Ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth, and partial to complete resetting of argon systematics in potassium-bearing minerals during Hercynian (M0), Alpine (M1) and late Oligocene-early Miocene (M2) metamorphism. Deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals. Modeling suggests the Oligo-Miocene thermal events were of insufficient magnitude and/or duration to completely reset the isotopic systematics in these samples. Our data lead to the concept of the argon partial retention and resetting zone (PRZ), defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium-bearing minerals. Within the PRZ, some radiogenic argon in preexisting potassium-bearing minerals will be outgassed and only partially retained. Tectonic exhumation of the PRZ involves movement on crustal-scale ductile shear zones, accompanied by strongly partitioned deformation and localized recrystallization. Recrystallization leads to resetting of argon systematics, and thus will result in heterogeneous 40Ar/39Ar age distributions within these ductile shear zones (e.g., in the South Cyclades Shear Zone). ¿ 1998 American Geophysical Union