(Petrological constitution of the continental type crust upthrust in the Hidaka belt, Hokkaido.)

The authors believe that the Hidaka metamorphic belt, Hokkaido, N Japan, is the junction of two tectonic units consisting of a slab of overturned oceanic crust in the W zone, and of upthrust continental crust in the main zone. In the W zone, greenschists derived from basalts, epidote amphibolites from diabasic to gabbroic rocks, amphibolites-metagabbros from gabbros, metamorphosed layered gabbro-peridotite cumulates, and dunite-harzburgite tectonites. These occur in the sequence from W to E. A complete ophiolite sequence of 5 km maximum thickness, although faulted, can be reconstructed in this zone. On the other hand, the main zone, 23 km in maximum thickness, consists of a granulite zone, amphibolite zone, biotite gneiss zone, biotite-muscovite gneiss to hornfels zone and non-metamorphosed sediment (Hidaka group) zone, stratigraphically upwards. Spinel and plagioclase lherzolite masses derived from the upper mantle occur locally within the granulite zone. Various kinds of igneous rocks of island arc type, such as olivine gabbro, hornblende gabbro, diorite and granite are intruded into selective horizons of the main zone. The rock sequence of the main zone represents a cross-section of the continental type crust. The geothermal gradient estimated from the metamorphic conditions, nature of the igneous rocks and the thickness of the crust section may indicate that the crust is an island arc type in the initial stage of development. -I.S.