| Project/Area Number |
05402020
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| Research Category |
Grant-in-Aid for General Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Research Field |
Geology
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| Research Institution | Shimane University |
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Principal Investigator |
SAWADA Yoshihiro Dept.Geosci., Fac.Sci.& Eng., Shimane Univ., Professor, 総合理工学部, 教授 (80196328)
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| Co-Investigator(Kenkyū-buntansha) |
AKASAKA Masahide Dept.Geosci., Fac.Sci.& Eng., Shimane Univ., Professor, 総合理工学部, 教授 (20202509)
TAKASU Akira Dept.Geosci., Fac.Sci.& Eng., Shimane Univ., Assoc.Professor, 総合理工学部, 助教授 (00183848)
IIZUMI Shigeru Dept.Geosci., Fac.Sci.& Eng., Shimane Univ., Professor, 総合理工学部, 教授 (80032639)
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| Project Period (FY) |
1993 – 1995
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| Keywords | lithosphere / island arc / lower crust / change of lithosphere / formation of Japan Sea / Southwest Japan / Mesozoic / Cenozoic |
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| Research Abstract |
Research results are summarized as follows : 1)Petrographic and bulk geochemical and Sr-Nd isotopes features of igneous rocks in the Chugoku District change drastically between the Cretaceousearly Paleogene to late Paleogene, and the late Paleogene to early Miocene. The changes result from chemical and physical changes of the upper mantle and lower crust during a transition in tectonic setting from active continental margina to island arc, due to opening of the Japan Sea. 2)Cretaceous and early Paleogene felsic igneous rocks are divided into at least two terranes on the basis of their Sr isotope initial ratios (high ratio and low ratio). Varied assemblages of volcanic rocks, including basalts and high Mg andesites, are distributed along the boundary zone between the two Sr-isotope terranes in the northern end of the Maizuru Belt. These assemblages comprise discrete and different terranes. 3)Prior to voluminous Cretaceous felsic igneous activity, basic igneous activity including high Mg andesite occurred, suggesting that the basic igneous activity playd an importanta role as the trigger for genesis of felsic magmas. 4)Gneiss xenoliths including kyanite and sillimanite have been found in late Cenozoic basalt in the Masuda area, west San'in region. Partial melting of gneiss with alkaline rhyolitic composition had already taken place prior to injection of basalt magma. This demonstrates the potential for generation of felsic magma beneath the west San'in region in late Cenozoic time. 5)On the basis of petrography and mineral chemistry of xenoliths, subducted sediments underwent amphibolite to granulite facies metamorphism and mylonitic deformation, and partial melting occurred in the deeper part of the Shimanto accretionary prism. Geochemistry and Sr-Nd isotopes of xenoliths and granitoids indicate that S-type granodiorite was formed by mixing of high Mg andesite magma and partial melt phases of migmatites.
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