Geological studies of the greenstones in the northern Hidaka Zone

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The results obtained from this research project are summarized as follows. 1) Geological and petrological studies on the Shimokawa, and Rurochi greenstone complexes of the northern Hidaka zone, the greenstones from the Makimine and Mugi Formations reveal that these greenstones are accompanied with no pelagic sediments such as chert and limestone, and extruded upon or intruded into terrigenous clastic sediments. 2) Geochemical signatures of these greenstones in terms of major and minor element chemistries exhibit that these greenstones are comparable with typical mid-ocean ridge basalts (N-MORB) and apparently different from island arc basalts. 3) Mineral chemistries of these greenstones are also similar with those of N-MORB and distinctive from those of island arc basalts and oceanic island basalts. Chemical zonings of plagioclases and clinopyroxenes of the greenstones indicate magmatic processes such as magma mixing and abrupt change of physical conditions during the crystallization of these greenstones. 4) On the basis of these lines of evidence, the greenstones are regarded to be extruded upon or intruded into unsolidified terrigenous sediments due to the collision of ocean ridge with trench.