Petrological study on polymetamorphism of the Oki metamorphic rocks

对奥基变质岩进行了地质和岩石学研究。根据本次研究获得的数据，OKI变质地块的构造发育过程如下：1.大约2000 Ma前形成了OKI变质岩的原始泥质沉积和玄武岩火山碎屑岩。泥质沉积物的源岩年龄大于2200 Ma。Oki地块中的这些早元古代岩石可能与中朝地块中属于前长城系的岩石相对比。原始岩石沉积后，冲绳地块至少经历了两期区域变质作用。较老的区域变质作用：该事件的时代被认为是元古代。矿物共生和矿物组成表明，变质作用发生在麻粒岩相条件下(温度大于700kb C，压力大于5kb)。同时还发生了广泛的偏头痛。较年轻的区域变质作用：该事件的年龄约为200 Ma前。在早期麻粒岩相条件下形成的矿物，在较年轻的区域变质作用的上角闪岩相条件下(温度5 5 0~6 5 0ﾟC，压力2~4Kb)进行了退行性再平衡。从次要褶皱轴线得到的现今构造走向和矿物线理大多是在这一阶段形成的。之后，冲绳变质地块遭受糜棱岩化和花岗岩类侵入，随后日本海开始扩张。本研究项目还讨论了塞舌尔群岛和非洲大陆之间的地质和岩石学相关性。

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.