Permalink : http://hdl.handle.net/2115/36751

The Yakuno ophiolite is exposed in the eastern part of the Maizuru Tectonic Belt which consists of ophiolite rocks. The Yakuno ophiolite is made up of ultramafic, mafic, intermediate, and felsic rocks which are regarded as the fragments of an ancient oceanic crust. Major element analyses (23 samples) and Sr isotope determinations (12 samples) were made for the representative rocks of the Yakuno ophiolite. The chemical features of the rocks and relict clinopyroxenes indicate that the ophiolite belongs to the tholeiitic rock series and shows a similarity to MORB in discrimination diagrams. However, the Yakuno ophiolite is much thicker than normal oceanic crust, and is associated with evolved rocks such as quartz diorite, granophyre and trondhjemite, but not with typical sheeted dyke complex. The Yakuno ophiolite is higher in normative plagioclase content and initial Sr isotope ratio (0.705092) than typical MORB. Hence, the Yakuno ophiolite resembles both ocean ridge and oceanic island basalts in magmatic nature, which suggests that it has been generated by oceanic island magmatism at an ocean ridge. The Rb-Sr isochron age of the Yakuno ophiolite was determined as 285 Ma, which is almost identical with those of other ophiolites from the Maizuru Tectonic Belt. So far as the available chemical data are concerned, the ophiolites from the Maizuru Tectonic Belt seem to have originated at various tectonic settings, i.e. oceanic island at ocean ridge, ocean ridge, island arc and marginal sea from east to west.