Petrological study on polymetamorphism of the Oki metamorphic rocks

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Geological and petrological study was made on the Oki metamorphic rocks. Based on the data obtained from this research project, tectonic development of the Oki metamorphic massif was constructed as follows;1. Original argillaceous sediments and basaltic pyroclastics from which the Oki metamorphic rocks were derived have been formed about 2,000 Ma ago. Source rocks of the argillaceous sediments is older than 2,200 Ma in age. These early Proterozoic rocks in the Oki massif is possible to correlate with the rocks belonging to the Prechangchengian system in the Sino-Korean massif. After the sedimentation of the original rocks, at least two stages of regional metamorphism took place in the Oki massif.2. Older regional metamorphism: The age of this event is considered to be Proterozoic time. Mineral parageneses and mineral compositions indicate that the metamorphism took place under the granulite facies conditions (more than 700 ﾟC in temperature and more than 5 Kb in pressure). Extensive migmatization also occurred at the same time.3. Younger regional metamorphism: The age of this event is about 200 Ma ago. The minerals once formed under the former granulite facies conditions were retrogressively reequilibrated under the upper amphibolite facies conditions (550-650 ﾟC in temperature and 2-4 Kb in pressure) of the younger regional metamorphism. Present structural trend obtained from the minor fold axes and the mineral lineations were formed mostly at this stage.4. After that, the Oki metamorphic massif suffered mylonitization and intrusion of granitoid, and the spreading of the Japan Sea began subsequently.5. Geological and petrological correlation between the Seychelles Islands and the African Continent is also discussed in relation to this research project.