<p>Post-collisional magmatism has played a critical role in modifying the continental crust since the Archaean, 3 billion years ago. The dual mantle–crust signature of calc-alkaline post-collisional magmas requires a hybrid source involving both reservoirs, the formation of which is not well understood. Here we use an integrated thermomechanical and magmatic experimental approach to constrain the geodynamic causes of mantle–crust hybridization and post-collisional magmatism, which reveals their association with subduction of continental lithosphere. Our numerical models predict pervasive relamination of deeply subducted continental crust onto the base of the overriding plate, owing to the decoupling of the buoyant silica-rich crust from the subducting continental lithosphere. In this setting, post-collisional magmatism is sourced from a hybrid domain following efficient mechanical mixing between the relaminated crust and mantle peridotite. Our laboratory melting experiments demonstrate that the magmatic products from this hybrid source reproduce the natural compositional trend of post-collisional igneous rocks. This mechanical–chemical interaction between relaminated crust and mantle lithosphere is recorded in magmatic isotopic signatures throughout Earth’s history and, given the similarity between Phanerozoic post-collisional calc-alkaline rocks and Archaean sanukitoids, implies that crust–mantle hybridization has occurred since Precambrian plate tectonics.</p>