The Mooroton shear zone (MSZ) of the Metasedimentary Belt of the Grenville Orogen in Ontario separates the Mazinaw domain to the east from the Elzevir domain. The shear zone is composed of mafic metavolcanic, metaclastic, and marble mylonites, and is characterized by a subvertical foliation, which crosscuts the regional foliation, and a near-vertical stretching lineation. Microstructures indicate deformation occurred under crystal–plastic conditions, and shear-sense indicators give movement of east (Mazinaw) side up. The geometry of the MSZ, which follows the eastern margin of the Elzevir tonalite, implies that lithologic anisotropy guided its location. Garnet–biotite and garnet–hornblende peak temperatures are juxtaposed across the MSZ, with temperatures ranging from 460 to 530 °C in the southeastern Elzevir and 490 to 620 °C in the southwestern Mazinaw, conditions in amphibolite facies. Limiting barometers applied to rocks in both domains show no significant offset. Petrologic data within the shear zone show that deformation occurred at amphibolite-facies conditions. New and previously published hornblende 40Ar/39Ar ages around the shear zone range from 927 ± 3 to 1006 ± 3 Ma, and show no systematic offset; similarly, muscovite 40Ar/39Ar ages in the area cluster around ca. 900 Ma. These combined data indicate that displacement had ceased along the MSZ by the time the rocks cooled through the closure temperature of hornblende (i.e., before ca. 1000 Ma), and most likely before the last regional metamorphism in the area (i.e., ca. 1030 Ma). Thus the Elzevir and Mazinaw domains acted as a coherent tectonic unit during at least the later part of the Grenville orogenic cycle.