In the Hidaka magmatic zone of central Hokkaido, Paleogene mafic plutons have intruded a Late Cretaceous to early Paleogene accretionary prism, resulting in high-temperature metamorphism and anatexis. Primitive basaltic rocks, which make up part of these mafic plutons, are typical normal mid-ocean ridge basalt (MORB). Because this magmatism is probably synchronous with arrival of the Kula-Pacific ridge, we propose that the Hidaka magmatism represents ridge-trench collision. Isotopic ratios vary significantly within the mafic plutons, indicating that intracrustal evolution of normal MORB was accompanied by assimilation of the accretionary prism. Thus, juvenile continental crust in the Hidaka magmatic zone formed in a near-trench setting through the interaction of a spreading ridge and an accretionary prism, in marked contrast to normal mid-ocean ridges.

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