1993 Fiscal Year Final Research Report Summary
Petrological and geochemical study lower crustal materials beneath the lnner Zone of Southwest Japan Arc.
| Project/Area Number |
03452070
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
鉱物学(含岩石・鉱床学)
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| Research Institution | Shimane University |
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Principal Investigator |
IIZUMI Shigeru Shimane University Faculty of Science Professor, 理学部, 教授 (80032639)
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| Co-Investigator(Kenkyū-buntansha) |
TAKASU Akira Shimane University Faculty of Science Associate Professor, 理学部, 助教授 (00183848)
AKASAKA Masahide Shimane University Faculty of Science Associate Professor, 理学部, 助教授 (20202509)
SAWADA Yoshihiro Shimane University Faculty of Science Associate Professor, 理学部, 助教授 (80196328)
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| Project Period (FY) |
1991 – 1993
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| Keywords | Inner Zone of SW Japan / Lower crust / Mantle / Cretaceous / Tertiary / Felsic magma / Granite / Sr isotope ratio |
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| Research Abstract |
Cretaceous-Tertiary igneous rocks, which are extensively distributed in the Inner Zone of Southwest Japan Arc, were petrochemically and geochemically examined in order to clarify the constitution of the lower crust and geochemical evolution of the lower crust and upper mantle beneath the Southwest Japan Arc. The Southwest Japan Arc is considered to have constituted a part of the eastern margin of the Eurasian Continent before the arc drifted away from the continent in the Middle Miocene, resulting in the opening of the Japan Sea.
Cretaceous l-type ilmenite-series granitoid and felsic volcanic rocks in the Ryoke and Sanyo Belts have higher Sr and lower Nd isotope ratios than those in the San'in Belt. These granitoid rocks could not be formed by melting of a Jurrasic accretinary complex, which comprise the upper part of the crust in SW Japan, but could be formed by partial melting of the lower crust, since the Jurrasic accretionary complex has much higher Sr and lower Nd isotope ratios th
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an these granitoid rocks. Cretaceous metadiabase and gabbroic rocks in the Ryoke Belt, crystallized from mantle-derived tholeiitic magmas, have also high Sr and low Nd isotope ratios comparable to the granitoid rocks, suggesting that the upper mantle asa well as the lower crust beneath the Ryoke Belt were enriched during the Late Cretaceous. On the other hand. Paleogene granitoid rocks in the San'in Belt have low Sr and high Nd isotope ratios compared with those in the Ryoke and Sanyo Belts. These data suggest that the lower crust of the southwest Japan Arc iscomposed of two or more geochemically different materials. These two lower crustal materials should have been in contact with each other beneath the Maizuru Tectonic Belt. Geochemical characteristics of the Cretaceous upper mantle beneath the Ryoke Belt had been largely changed by injection of magmas from asthenosphere during the Miocene magmatism, although the lower crus-tal materials had not been effectively affected by the magmatism.
Upper mantle and lower crust beneath the San'in Belt became more and more depleated during the Paleogene and were most depleated at the Middle Miocene. This may suggest that the upper mantle and lower crust benarth the San'in Belt had been geochemically changed by the injection of the Asthenosphere-derived more depleated magmas.
These geochemical evolution of the upper mantle and lower crust beneath the Southwest Japan Arc is supported by geochemical data of mantle- and lower crust-derived xenoliths in Cenozoic volcanic rocks in the Southwest Japan Arc. Less
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[Publications] Honma, H., Kusakabe, M., Kagami, H., Iizumi, S., Sakai, H., Kodama, Y.and kimura, M.: "Major and trace element chemistry and D/H, ^<18> O/^<16> O.^<87> Sr/^<86> Sr, ^<143> Nd/^<144> Nd ratios of rocks from the spreading center of the Okinawa Trough, a marginal back-arc basin." Geochemical Journal. 25. 121-136 (1991)
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[Publications] Iizumi, S., Ireland, T.R., Kawachi, Y., Watanabe, T., Nakai, Y., Tainosho, Y., Kagami, H.and Hayama, Y.: "SHRIMP U-Pz zircon ages of gabbroic and metadiabase from the Kinki Ryoke Belt, SW Japan Arc." (in preparation.).
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[Publications] Kagami, H., Iizumi, S., Owada, M., Tainosho, Y., Nereki, T.and Hayama, Y.: "Geochemical evolution of lower crust and upper mantle beneath SW Japan Arc : Implications to genesis of felsic and mafic magmas." (in preparation.).
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「研究成果報告書概要(欧文)」より
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