ABSTRACT The Grenville Province is the largest and most structurally complex orogenic belt that formed in the Mesoproterozoic, representing the amalgamation of the supercontinent Rodinia. The Mattawa domain, part of the Algonquin domain in Ontario, Canada, hosts some of the most deeply buried metamorphic rocks of this orogen. This high-grade metamorphic terrane consists of large areas of felsic orthogneiss and kilometer-sized mafic boudins. Dark-colored metabasite cropping out near Mattawa, Ontario, Canada, contains relict mineral assemblages and decompression textures indicative of high-pressure eclogite. Garnet porphyroblasts surrounded by plagioclase coronae are hosted in fine-grained symplectic intergrowths of diopside + plagioclase ± amphibole, which compositionally reintegrated into an omphacite composition (Na/[Na + Ca] ~0.5). Phase equilibria analysis revealed an eclogite-facies peak pressure of ~2 GPa at 850 °C. This temperature is consistent with the zirconium contents of rutile inclusions in garnet (up to 1725 ppm Zr). Despite high-temperature metamorphism, garnet growth zonation is partially preserved. Diffusion modeling of representative garnet profiles yielded a time scale of <0.1 m.y. for decompression from the peak pressure to ~1.2 GPa, suggesting an average exhumation rate of several decimeters per year. Decompression was followed by fast cooling within hundreds of thousands of years. Such fast decompression and cooling rates contrast with the protracted metamorphic evolution recorded in most of the Grenville orogen and likely resulted from local extrusion of lower-crustal material in response to localized extension during the early Ottawan stage. Since very few examples of Precambrian short-duration regional metamorphism have been documented, the fast decompression documented in this study provides valuable constraints for the geodynamic transition to a modern plate-tectonic regime.