The Cambro-Ordovician age Thetford Mines Complex from the Quebec Appalachians, Canada, preserves a remarkably complete section of ophiolites at Lac de l'Est, where mafic volcanics overlie a plutonic mafic–ultramafic plate. The basaltic volcanics consist of a lower unit, representing the extrusive component of the ophiolite assemblage, and an upper unit, whose petrogenetic and tectonic relationships with the ophiolitic volcanics are problematic.The lower unit ophiolitic volcanics include high- and low-TiO2 basalts. The upper unit volcanics, of which the basal 80 m was sampled, are low-TiO2 basalts. Fractional crystallization was important in the evolution of high-TiO2 lower unit magmas but played only a minor role in the formation of other magmas. Partial melting processes were dominant, or much more important than fractional crystallization, in controlling the composition of other magmas. The parental magmas of the high-TiO2 lower unit basalts were partial melts of undepleted mantle, whereas the low-TiO2 volcanics were partial melts of residual, depleted mantle. Despite different mantle sources, the high- and low-TiO2 basalts of the lower unit are interbedded in the field.The close spatial association of chemically diverse magma types is best accounted for by generation in a back-arc or marginal basin environment. This interpretation is supported by the geochemistry of argillaceous sediments in the Lac de l'Est pile and the absence of a sheeted dike facies in the Thetford Mines ophiolites.