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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 (HP) 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 + H2O 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 HP 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 HP 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.

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