Field, geochronological, and rare earth element (REE) evidence obtained from the Fournier oceanic fragment of the Canadian Appalachians indicates that its plagiogranite component was generated by the anatexis of amphibolite and not by the fractionation of basic magma. We propose that the process occurred in two stages: first, gabbro was plastically deformed at high temperature to form dry, low-angle shear zones that subsequently evolved to amphibolite via the addition of light REE-enriched hydrothermal solutions; second, the amphibolite underwent partial melting during shear to yield a migmatite comprising bands of plagiogranite alternating with amphibolite restite. The plagiogranite locally coalesced to form pods, dikes, and lenses that injected the surrounding undeformed gabbro. We attribute the development of the Fournier plagiogranite to dynamothermal processes occurring in proximity to a spreading center due to asthenosphere-induced shear within ocean layer 3. This serves as an important illustration of the dynamic nature of metamorphism and melting that can occur in the ocean crust.