The southeast margin of Laurentia was a very long lived active continental margin, part of whose history is recorded in the Grenville Province of the Canadian Shield. Within this province, Nd-isotope mapping can be used to define the boundaries between terranes with a variety of crustal formation ages and can also distinguish between crustal growth by oceanic and continental-arc magmatism. The former gives rise to large terranes with homogeneous Nd-isotope signatures and well-defined boundaries, whereas the latter leads to areas with heterogeneous Nd-isotope signatures. One of the best examples of continental-arc magmatism in the Grenville Province is provided by the region northwest of Lac St.-Jean, Quebec. Eighty new Nd-isotope analyses are used (along with aeromagnetic data) to divide this area into three blocks, bounded by abrupt changes in Nd model age. The western block consists almost exclusively of tonalitic grey gneisses with Archean model ages. The eastern block is composed almost exclusively of gneisses with Nd model ages of 1.6–1.5 Ga and tonalite–trondhjemite–granodiorite-type chemistry. In contrast, the central block has a wide range of Nd-isotope signatures and more alkaline major element chemistry characteristic of an ensialic arc. The ϵNd values in this block correlate with distance southeast of the Allochthon Boundary Thrust, suggesting that ensialic arc magmas suffered diminishing contamination in a southeastward direction by old Laurentian crust. A subduction-flip model is proposed, whereby north-dipping subduction under the continental margin followed the accretion of a Mesoproterozoic arc terrane to the Laurentian craton.