The lower crust of the North Arm Mountain massif (Bay of Islands ophiolite) is composed of interfolded, porphyroclastic-textured peridotite, pyroxenite, and gabbro, belonging to several differentiation series. Low-Ti pyroxenes, high An/Fo, abundant orthopyroxene, and high spinel Cr/Al, suggest arc affinities. Discordant contacts and inclusion relationships show that many peridotites are synkinematic sills. Some host rocks were altered to sulphide-bearing, greenschist-grade assemblages prior to intrusion. Most feldspathic peridotites and melagabbros are hybrids formed by disaggregation of host gabbros into primitive intrusions. Many pyroxenites and chromitites are by-products of incongruent dissolution reactions. Much of the pyroxene component of peridotites and olivine pyroxenites is xenolithic. Synmagmatic shear augmented the efficiency and extent of hybridization and assimilation reactions. Syntexis (the melting and dissolution of xenocrysts) was as important as fractional crystallization in generating petrological diversity and controlling melt evolution. Synmagmatic extension at spreading ridges could produce similar results, and so account for many enigmatic features of mid-ocean ridge basalts (xenocrysts of anorthite and aluminous spinel, excess chlorine in basalts, and the pyroxene paradox). If syntexis is an important ridge-axis process, then assuming that fractional crystallization alone controls melt evolution may generate incorrect calculated parental melt compositions, and so lead to erroneous conclusions about mantle sources and processes. ¿ American Geophysical Union 1996