The thermotectonic style and chronology of the ∼1.2 to 1.0 Ga Grenville Orogeny vary markedly along its length in North America. Southeast of the Grenville Front in western Labrador, three lithologically distinct structural decks were telescoped along marginal ductile shear zones into their present configuration during the Grenvillian Orogeny. The Gagnon and Molson Lake terranes, parautochthonous to the North American craton, are overlain by the Lac Joseph terrane which is devoid of lithologies traceable across the Grenville Front. This paper reports constraints and implications from UPb geochronology for the thermotectonic evolution of the Lac Joseph and Molson Lake terranes. Two distinct leucosomes in the supracrustal rocks in the Lac Joseph terrane have been dated at 1660_-6⁺⁸ and 1636_-5⁺⁶ Ma, respectively, and are interpreted to have formed, along with the earliest mineral fabrics, during a protracted Labradorian Orogeny. Migmatization and early folding were followed by the development of localized ductile shear zones, one of which has been dated at 1633±4 Ma. A suite of weakly deformed mafic and granitoid rocks intruded the migmatites late in the Labradorian Orogeny indicating that the majority of deformation and metamorphism in the Lac Joseph terrane is pre-Grenvillian. Granitoid rocks and the Shabogamo Gabbro, the two rock types of the adjacent, structurally lower Molson Lake terrane, have been dated at 1651±5 Ma and 1459_-22⁺²³ Ma, respectively. The Molson Lake terrane was extensively recrystallized and deformed, apparently for the first time, between 1000 and 990 Ma, during the Grenvillian Orogeny. The Lac Joseph terrane was emplaced as a pre-assembled, coherent tectonic slice along a southeastwardly dipping ductile shear system over the Molson Lake terrane at 989±12 Ma but was not itself extensively thermally or structurally affected at this time.