尖閣列島魚釣島・北小島・南小島の地質

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タイトル別名
  • センカク レットウ ウオツリトウ キタコジマ ミナミコジマ ノ チシツ
  • Geology of the Uotsuri-jima, Kita-ko-jima and Minami-ko-jima in the Senkaku Retto

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As a result of writer's recent study on the geology of the Senkaku Retto (Uotsuri-jima, Kita-ko-jima and Minami-ko-jima), a summary of conclusion is led as follows. 1. The following stratigraphic division (in the decending order) is recognized in the Uotsuri-jima and its adjacent area of the Senkaku Retto. Geologic age Stratigraphic division Geologic relation Recent Alluvial bed and talus deposit Raised coral reef Miocene? Intrusive igneous rocks Uotsuri-jima formation unconformity unconformity intrusion The geologic map of the Uotsuri-jima and its adjacent area is shown in Fig. 2. 2. The Uotsuri-jima formation is mainly made of sandstone with intercalation of conglomerate, rarely with thin coal bed and siltstone. 3. The Uotsuri-jima formation shows a trend of SW-NE, EW and NW-SE strikes, and generally inclining at the angle of less than 20° N. It indicates dome structure in the Uotsuri-jima, and slight folding in other islands. 4. Intrusive igneous rocks, such as hornblende dioritic porphyrite and its associated rocks, occur as intrusive sheet in the Uotsuri-jima formation of the Uotsuri-jima and Minami-kojima. Essential constituent minerals of these rocks are hornblende, plagioclase and magnetite phenocrysts or microphenocrysts. The groundmass showing horocrystalline texture is composed of hornblende, plagioclase, quartz, magnetite and secondary minerals viz. calsite, chlorite and other clay minerals. 5. The chemical and normative mineral compositions of intrusive igneous rocks are shown in Table 2. As seen from the Table, they are generally rich in Al2O3, Fe2O3, MgO and CaO contents, while poor in FeO, total FeO, TiO2 and K2O contents. 6. The natural remanent magnetic properties of intrusive igneous rocks from the Senkaku Retto are shown in Table 3, and also their stereographic projections in Fig. 7. That indicates just four samples (no. SU-1 in the Uotsuri-jima) are magnetized reversely, while others are normally. Since intrusive igneous rocks show no signs of structural change after they were emplaced in situ, there is estimated to be at least two activity stages of them.

長崎大学教養部紀要. 自然科学. 1974, 14, p.43-73

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