Detailed field mapping in marbles from the Central Metasedimentary belt (CMB) near Bancroft, Ontario, shows that isolated exposures of marble mylonite are part of a regionally extensive shear zone, the Bancroft shear zone. Shear-sense indicators in this southeast-dipping zone give movement of the hanging wall toward the southeast, indicating extension. Local crosscutting relationships indicate that the extension is younger than the regional thrusting event that produced the predominant structural fabric. On a regional scale, however, the extensional and compressional fabrics are essentially parallel. The marbles preserve a variety of textures ranging from light-colored, coarse samples to light-colored, weakly foliated samples with remnant coarse calcite grains to banded fine-grained samples to dark, extremely fine-grained samples. These various textures represent progressive stages in the development of marble mylonite, from protolith to ultramylonite. We conclude that the Bancroft shear zone marks the Bancroft-Elzevir subdomain boundary of the CMB and that late extension is responsible for the observed differences in lithology and metamorphic grade of these Grenville subdomains.