Effects of ridge subduction along the Japan margin -- an example of the Late Cretaceous-Paleogene

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The following summary of the research results obtained.1. Tere ara three rows of strata containing in in-situ greenstones at least in the Hidaka Belt. Geochemical signatures indicate that at least some of the in-situ greenstones are similar to mafic lava generated at spreading ridges.2. Geochemical and petrological studies on the igneous rocks from the Hidaka metamorphic belt have revealed that they were emplaced in connection with a ridge subduction.3. On the basis of geochemical studies, the granitic rocks in the Kitakami Mountains, Northeast Japan, are inferred to have been emplaced in connection with a subduction of a hot oceanic plate.4. Some melange-like units in the Shimanto Supergroup, such as Naze Formation(Amami-Oshima), Makimine Formation(Kyushu), Okitsu Melange(west Shikoku), Mugi Formation(east Shikoku), Ryujin Formation(Kii Peninsula) and Inui Group(Akaishi Mountains), contain greenstones which were extruded upon and intruded into unconsolidated fine-grained terrigenous clastic sediments.5. Geochemical signatures indicate that the in-situ greenstones from the Shimanto Supergroup are similar to mafic lavas generated at spresding ridges.6. Organic metamorphism in the Shimanto and Hidaka Supergroups was studied by means of vitrinite reflectance. The study has revealed thermal history of the both supergroup.7. The relationship between ridge collision and sedimentation on the trench floor in the Shimanto Belt has been clarified.8. Ar/Ar isotope ages from the Sanbagawa metamorphic rocks have resulted in the time relationship between the exhumation of the Sanbagawa metamorphic rocks and ridge cillision.9. Petrological study of xenoliths in the granitic rocks from the Shimanto Belt revealed a thermal event during the Eocene.