Petrological study on polymetamorphism of the Oki metamorphic rocks

对冲木变质岩进行了地质岩石学研究。根据本研究项目获得的资料，对奥基变质地块的构造发育进行了如下构建：冲木变质岩的原始泥质沉积物和玄武质火山碎屑形成于约2000 Ma以前。泥质沉积物烃源岩年龄在2200 Ma以上。Oki地块的这些早元古界岩石可能与中朝地块的prechangcheng系岩石相对应。原岩沉积后，奥基地块至少发生了两个阶段的区域变质作用。更古老的区域变质作用：这一事件的年龄被认为是元古代时期。矿物共生特征和矿物组成表明，变质作用发生在麻粒岩相条件下（温度大于700ºC，压力大于5ºKb）。同时也发生了广泛的哺乳化。更年轻的区域变质作用：这一事件的年龄大约在200 Ma以前。原麻粒岩相条件下形成的矿物在较年轻区域变质作用的上角闪岩相条件下（温度550 ~ 650，压力2 ~ 4 Kb）进行了退性再平衡。目前的构造走向由小褶皱轴得到，矿物线理主要形成于这一阶段。此后，冲木变质地块遭受糜棱岩化和花岗岩类侵入，日本海开始扩张。在这个研究项目中还讨论了塞舌尔群岛和非洲大陆之间的地质和岩石对比。

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.