A large Nd isotope data set is presented to generate a crustal formation age map for the southwest Grenville Province, placing it in the context of the province as a whole. Previous studies have shown that most crust in the central and eastern parts of the province is pre-Grenvillian, composed largely of juvenile 1.5, 1.7, 1.9, and 2.7 Ga arc terranes, partly reworked by later ensialic arcs and collision zones. In contrast, new Nd data reported here show that juvenile monocyclic crust is restricted to the SW Grenville Province, in part of the Central Metasedimentary Belt interpreted as a failed backarc rift zone. There is a clear isotopic distinction between this rift zone and the pre-Grenvillian crust on the flanks. On the flanks of the rift zone, there is evidence for two crust-forming events, comprising an earlier (1.5 Ga) juvenile Pinwarian event, and a later 1.4–1.3 Ga continental margin arc (the Elzevirian Orogeny). Based on the geographical distribution of Nd model ages in the SW Grenville Province, a revised tectonic model for the Grenville Orogeny is developed. In this model, backarc spreading created a rift zone behind a (pre-Grenvillian) Elzevirian subduction zone that obliquely straddled the edge of a Mesoproterozoic continental margin. As a result of this geometry, the rift zone died out to the north as it propagated into cold Archean crust. Elzevirian subduction was probably halted by the collision of the continental margin with an offshore arc, and later with a major continental block, giving rise respectively to the Shawinigan and the Ottawan orogenic events. These collisional events can be grouped as parts of the Grenville Orogenic Cycle.