Abstract The proposition that the Grenville Province is a remnant of a large hot long-duration collisional orogen is examined through a comparative study of its present orogenic front, the Grenville Front, and a former front, the Allochthon Boundary Thrust. Structural, metamorphic and geochronologic data for both boundaries and their hanging walls from the length of the Grenville Province are compared. Cumulative displacement across the Grenville Front was minor (10 s of km) whereas that across the Allochthon Boundary Thrust was major (100 s of km), consistent with the observation that the latter boundary separates rocks with a different age, and P – T character, of metamorphism. On an orogen scale, Grenvillian metamorphism can be subdivided into two spatially and temporally distinct orogenic phases, a relatively high T Ottawan ( c . 1090–1020 Ma) phase in the hanging wall of the Allochthon Boundary Thrust, and a relatively lower T Rigolet ( c . 1000–980 Ma) phase in the hanging wall of the Grenville Front. It is argued that the structural setting and ≥50 My duration of Ottawan metamorphism are compatible with some form of channel flow beneath an orogenic plateau, with the Allochthon Boundary Thrust forming the base of the channel. Channel flow ceased at c . 1020 Ma when the Allochthon Boundary Thrust was reworked as part of a system of normal-sense shear zones, and following a hiatus of c . 20 My the short-lived Rigolet metamorphism took place in the former foreland and involved the development of a new orogenic front, the Grenville Front. Taken together, this suggests the Grenville Orogen developed as a large hot long-duration orogen during the Ottawan orogenic phase, but following gravitational collapse of the plateau the locus of thickening migrated into the foreland and active tectonism was restricted to a subjacent small cold short-duration orogen. The foreland-ward migration of the orogenic front from the Allochthon Boundary Thrust to the Grenville Front, the contrasting P–T–t character of the metamorphic rocks in their hanging walls, and the evidence for orogenic collapse followed by renewed growth, provide insights into the complex evolution of a long-duration collisional orogen.

The Cape Caribou River allochthon is a thick Grenvillian thrust sheet composed of Paleoproterozoic orthogneiss and Mesoproterozoic mafic dikes in which penetrative syn-thrusting deformation and recrystallization are largely restricted to the 1- to 2-km-wide basal shear zone, the Grand Lake thrust system. Grenvillian mylonitic fabrics in orthogneiss in the Grand Lake thrust system are characterized by the peak high-pressure (H P ) granulite-facies assemblages garnet-clinopyroxene-plagioclase-clinoamphibole-quartz (Grt-Cpx-Pl-Cam-Qtz) and orthopyroxene-clinopyroxene-plagioclase-garnet-clinoamphibole-quartz (Opx-Cpx-Pl-Grt-Cam-Qtz) in mafic and intermediate lithologies, respectively. Evidence for the prograde reactions Cam + Pl = Cpx + Grt + Qtz + H 2 O and Opx + Pl = Grt + Cpx + Qtz is preserved in some samples. Partial retrograde replacement of the subassemblage Grt-Cpx by Cam-Pl is widespread, but retrogression was domainal on both outcrop and thin-section scales. Thermobarometry applied to the syntectonic H P granulite-facies assemblage Grt-Cpx-Pl-Qtz-Cam ± Opx in the Grand Lake thrust system shear zone and adjacent hangingwall and footwall has yielded apparent peak pressure-temperature ( P-T ) estimates of ∼14 kilobars/875 °C that are considered to closely approximate the P at maximum T experienced by rocks, despite evidence in some samples for postpeak resetting of thermometers. The rocks are inferred to have followed a clockwise P-T path that involved quasi-isothermal decompression following peak T conditions, although details of the path remain only qualitatively constrained because of the unknown extent of postpeak reequilibration. The P-T results and tectonic setting suggest that the Cape Caribou River allochthon is part of the H P belt of the Grenville Province, and that the localized Grenvillian granulite-facies recrystallization in the Grand Lake thrust system was possibly partly a result of frictional heating.