| Project/Area Number |
08404029
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| Research Category |
Grant-in-Aid for Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Geology
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| Research Institution | KYOTO UNIVERSITY |
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Principal Investigator |
TATSUMI Yoshiyuki Kyoto University, Graduate School of Science, Professor, 大学院・理学研究科, 教授 (40171722)
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| Co-Investigator(Kenkyū-buntansha) |
ISHIKAWA Naoto Kyoto University, Faculty of Integrated Human Studies, Assistant, 総合人間学部, 助手 (30202964)
FURUKAWA Yoshitsugu Kyoto University, Graduate School of Science, Associate Professor, 大学院・理学研究科, 助教授 (80222272)
ISHIZAKA Kyoichi Kyoto University, Graduate School of Human and Environmental Studies, Professor, 大学院・人間・環境学研究科, 教授 (90025362)
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| Project Period (FY) |
1996 – 1998
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| Project Status |
Completed (Fiscal Year 1998)
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| Budget Amount *help |
¥31,900,000 (Direct Cost: ¥31,900,000)
Fiscal Year 1998: ¥2,300,000 (Direct Cost: ¥2,300,000)
Fiscal Year 1997: ¥5,600,000 (Direct Cost: ¥5,600,000)
Fiscal Year 1996: ¥24,000,000 (Direct Cost: ¥24,000,000)
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| Keywords | Setouchi volcanic belt / Rotation of SW Japan arc / Thermal simulation / Slab melting / Sr-Nd-Pb isotopes / High-Mg andesite magma / 流紋岩質マグマ / 高Mg安山岩 / 瀬戸内地域 / 堆積物の融解 / 西南日本弧 / 中新世 / テクトニクス / 異常高温 / 日本海拡大 / プレート融解 / 同位体比 |
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| Research Abstract |
9.1 Timing of activity of Setouchi volcanic belt and the rotation of the SW Japan arc
K-Ar dating and paleomagnetic investigations were conducted on lavas from the Setouchi volcanic rocks in SW Japan, in order to understand the tectonics of characteristic high-Mg andesite (HMA) magmatism. New 53 K-Ar ages confirm that the Setouchi volcanism occurred at 15-12 Ma, especially the volcanism of intermediate to mafic rocks including HMAs took place at 13 Ma. A paleomagnetic direction of 13.9 Ma was determined for the volcanic rocks in NE Shikoku area. This data and previous ones with the ages in this study indicates that NE Shikoku area rotated CW about 19。 between 13.9 and 12.8 Ma after the area experienced CW rotation at about 15 Ma as a part of SW Japan. Such additional CW rotations are inferred from paleomagnetic data in the east Shikoku and Osaka areas. The additional rotations possibly imply that rotations of faulted blocks occurred in the main part of SW Japan, or that a part of SW Jap
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an including the eastern Shikoku and Osaka areas rotated CW between about 15 and 12.8 Ma, for which rotation angle was larger than that for the CW rotation of the hole SW Japan at about 15 Ma
9.2 Thermal structure of the upper mantle and the melting of the subducting slab
Characteristic high-Mg andesite magmas were produced in the SW Japan arc at 13-15 Ma that was synchronous with the commencement of subduction of a very young (<11 m.y.) lithosphere of the Shikoku Basin. Numerical simulation <deleted> suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust. High-Mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge. HMA magmas may be commonly produced in the Archean subduction zones under relatively high mantle temperature conditions, contributing to making continental crusts.
9.3 The slab-melting and Setouchi magma genesis
In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb-Nd-Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H_2O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments. Less
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