Tertiary deep crustal ultrametamorphism in the Hidaka metamorphic belt, northern Japan

The Main Zone of the Hidaka metamorphic belt is an imbricate stack of crustal material derived from an island arc in which a sequence of units with increasing metamorphic grade from low to high structural levels is exposed. The basal part of the metamorphic sequence underwent granulite facies metamorphism with peak P–T conditions of 7kbar, 870°C. In this zone pelitic granulite includes leucosomes which consist mainly of orthopyroxene‐plagioclase‐quartz. To test whether the leucosome was derived by partial melting of the surrounding pelite, melting experiments of the pelitic granulite were carried out for water‐saturated and dry systems at 7 kbar and 850°C. The chemical composition of the leucosome produced during these runs shows a peraluminous S‐type tonalitic affinity and is located very close to the tie‐line between the average melts produced in water‐saturated systems and the average composition of the residual orthopyroxene + plagioclase. This therefore suggests that the lecosome in pelitic granulite was formed by incipient anatexis at close to the highest P–T condition of the Main Zone. The age of the crustal anatexis is determined by the Rb‐Sr whole rock isochron method for garnet‐cordierite‐biotite gneiss (host rock), garnet‐orthopyroxene‐cordierite gneiss (restite) and S‐type tonalite (melt). This gives an age of 56.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.705711. The S‐type tonalite magmas that form large intrusive masses in the Main Zone were probably generated by crustal anatexis in deeper parts of the crust at the same time (late Palaeocene).