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Identification of the tectonic setting for metamorphism is often extremely difficult in complex polymetamorphic terranes where individual tectonothermal events are obscured by later thermal and structural reorganizations. The traditional approach is to use mineral parageneses to outline the nature of the pressure-temperature-time path, but assigning an age to that path remains a challenge. In this case study, pressure-temperature data show that garnet in pelites of the polymetamorphic Moine Supergroup of northwest Scotland grew during compressional tectonics. In addition, the Sm-Nd systematics of these garnets demonstrate that growth occurred in the interval 820–790 Ma and was thus coeval with crustal melting documented as ca. 800 Ma. The heat source for the latter event has previously been postulated to be related to extension, but here we provide the first substantive evidence for a collisional orogeny …