| Co-Investigator(Kenkyū-buntansha) |
TAKASU Akira Fac. Sci & Eng. SHIMANE UNIVERSITY PROF., 総合理工学部, 教授 (00183848)
SAWADA Yoshiyuki Fac. Sci & Eng. SHIMANE UNIVERSITY PROF., 総合理工学部, 教授 (80196328)
IIZUMI Shigeru Fac. Sci & Eng. SHIMANE UNIVERSITY PROF., 総合理工学部, 教授 (80032639)
TSUCHIYA Nobutaka IWATE U.DEPT.EDUC., ASSOC.PROF., 教育学部, 助教授 (50192646)
YOSHIDA Takeyoshi TOHOKU UNIV.FAC.SCI., PROF., 理学部, 教授 (80004505)
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| Research Abstract |
We investigated metamorphic, plutonic, and igneous rocks from Southwestern Japan on their formation age and geochemistry, and examined development history of the lithosphere underneath the area. The Cretaceous and Paleogene granitoids and acid rocks predate opening of the Japan Sea. The igneous activity mostly relates with continental margin arc-type magmatism, but partly associated with adakitic rocks. Adakitic rocks are considered to be derived from slab melting but those from this area may have been derived from the partial melting of the lower crust. After the opening of the Japan Sea, subduction of the Philippine Sea Plate begun and high-magnesian andesite and alkali basalt volcanism became common. The high-magnesian andesite would show contamination of subduction sediments, whereas alkali basalts are deep depleted mantle in origin. After the Pliocene, adakitic dacite occurred in the middle of the are and is considered to be slab melt in origin. Melting of the subducting Philippine Sea Plate may have been induced by a heat source related with alkali basalt source, because the adakite and alkali basalts are coeval. Re-initiation of subduction of a younger slab could be the case of adakite-alkali basalt association of this area. All of the volcanic rocks above have evidences of crustal contamination. Overall, interaction between deep depleted lithosphere, subducting slab (MORB) and overriding crustal rocks are the source endmembers for the Neogene volcanic rocks in the Southwestem Japan arc. For the origin of melt may largely controlled by deep seated heat source and subducting MORB slab.
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