There is no consensus as to how the extreme metamorphic conditions required to form granulites are attained, or how these rocks relate to crustal growth and differentiation processes. Studying young granulites offers two advantages: the tectonic setting is likely to be well constrained, and the ambiguities that result from overprinting by younger metamorphic events are potentially avoided. We report the first ion microprobe U-Pb (zircon) ages for orthopyroxene-bearing granulites, tonalites, and gabbros of the Cenozoic Hidaka metamorphic belt (Hokkaido, Japan) to clarify the magmatic-metamorphic connection in this area. The data support a two-stage evolution for this terrane, which is attributed to episodes of supra-subduction zone magmatism (late Eocene) and back-arc extension (early Miocene). We relate granulite facies metamorphism and garnet-orthopyroxene tonalite generation to mafic magma under-accretion and lithosphere thinning due to the opening of the Japan Sea at 19 Ma. The Hidaka granulites are thus among the youngest exposed granulites on Earth, and manifest the thermal response to continental growth.