NATURE OF THE ELZEVIR-MAZINAW DOMAIN BOUNDARY, GRENVILLE OROGEN, ONTARIO

The Mooroton shear zone (MSZ) of the Metasedimentary Belt of the Grenv ille Orogen in Ontario separates the Mazinaw domain to the east from t he Elzevir domain. The shear zone is composed of mafic metavolcanic, m etaclastic, and marble mylonites, and is characterized by a subvertica l foliation, which crosscuts the regional foliation, and a near-vertic al stretching lineation. Microstructures indicate deformation occurred under crystal-plastic conditions, and shear-sense indicators give mov ement of east (Mazinaw) side up. The geometry of the MSZ, which follow s the eastern margin of the Elzevir tonalite, implies that lithologic anisotropy guided its location. Garnet-biotite and garnet-hornblende p eak temperature are juxtaposed across the MSZ, with temperatures rangi ng from 460 to 530 degrees C in the southeastern Elzevir and 490 to 62 0 degrees C in the southwestern Mazinaw, conditions in amphibolite fac ies. Limiting barometers applied to rocks in both domains show no sign ificant offset. Petrologic data within the shear zone show that deform ation occurred at amphibolite-facies conditions. New and previously pu blished hornblende Ar-40/Ar-39 ages around the shear zone range from 9 27 +/- 3 to 1006 +/- 3 Ma, and show no systematic offset; similarly, m uscovite Ar-40/Ar-39 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 hornblend e (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 Ma zinaw domains acted as a coherent tectonic unit during at least the la ter part of the Grenville orogenic cycle.