Several features of Geon 10 magmatic evolution in the Grenville Orogenic Belt is difficult to reconcile with generally accepted models of protracted (ca. 100 Myr) continent–continent collision during the Grenville Orogeny. Particularly the presence of (partly) mantle-derived magmatic rocks, some with subduction signatures, intruded during the inferred climax of the orogeny, is not well accounted for in existing models. We present new geochemical, Lu–Hf isotopic and U–Pb geochronological data from three alkaline composite plutons in Quebec, Canada, that give important clues to the tectono-magmatic evolution from ca. 1040 to 1000 Ma of the Grenville Laurentian margin. The oldest pluton, emplaced at ca. 1038 Ma, has a geochemical composition compatible with derivation in an arc setting by partial melting of subcontinental lithospheric mantle. The two youngest plutons, emplaced at ca. 1014 and 1009 Ma, respectively, have typical within-plate geochemical signatures showing no obvious influence of subduction. The new and existing data indicate that much of the Grenville Laurentian margin experienced ensialic magmatism through large parts of Geon 10, an observation calling for alternative models to the existing to explain the Geon 10 evolution of the Grenville Orogenic Belt. We propose a model where Ottawan metamorphism and magmatism resulted from accretion of terranes and continued subduction beneath Laurentia until late Geon 10. Late Geon 10 magmatism could have been related to slab breakoff prior to or at the onset of a late Geon 10 collisional event, which has also been recently proposed based on paleomagnetic arguments.