1992 Fiscal Year Final Research Report Summary
Origin and Regional Variation of the Magma in the Ryukyu Arc-Back Arc System
| Project/Area Number |
03640643
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
地質学一般
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| Research Institution | Institute for Study of the Earth's Interior |
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Principal Investigator |
HONMA Hiroji Okayama Univ., Institute for Study of the Earth's Interior, Professor, 地球内部研究センター, 教授 (70033131)
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| Co-Investigator(Kenkyū-buntansha) |
NAKAMURA Eizo Okayama Univ., Institute for Study of the Earth's Interior, Associate Professor, 地球内部研究センター, 助教授 (80201672)
KAGAMI Hiroo Okayama Univ., Institute for Study of the Earth's Interior, Associate Professor, 地球内部研究センター, 助教授 (20108179)
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| Project Period (FY) |
1991 – 1992
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| Keywords | Ryukyu arc / Volcanic rocks / Trace element / Strontium isotope ratio / Neodymium isotope ratio / Magma genesis / Two-component mixing model / Alkaline rocks |
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| Research Abstract |
Contents of Rb, Sr, Nd and Sm and ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd ratios have been analyzed for Cenozoic volcanic rocks of the Ryukyu arc system. The distinctly low Sm contents and Sm/Nd ratios of hornblende-bearing rocks and the variation of Zr/Sr in rocks of the arc side with significantly low Zr/Sr ratios in relatively basic rocks clearly indicate the effects of the hornblende- and plagioclase-dominant fractionations, respectively, which most probably are related to crystallization differentiation of the magma. The Sr-Nd isotopic characteristics are best interpreted by the paired two-component mixing models, namely the 'source-mixing' and 'crustal-mixing' : (1)Strontium and Nd in the primary magmas consist mainly of the mixture derived from the MORB source-type peridotite and subducted sedimentary rocks. (2)Contribution of sedimentary components to the primary magma is distinctly lower in the back-arc side (27-30%). (3)Often magmas of both sides have been affected by considerable degree of isotopic modification by crustal materials during their evolution in the crust. (4)The ^<87>Sr/^<86>Sr and ^<143>Nd/^<144>Nd ratios also change with changing geological settings. (5)The high magnesian andesite has the high ^<143>Nd/^<144>Nd ratio compared to its ^<87>Sr/^<86>Sr ratio, and its primary magma should have originated from the different source material. (6)Alkaline basalts of the Obisyo characteristically have the isotopic signature of the plume-type basalts.
Both the Zr-Sr and Sm/Nd-SiO_2 systematics and their accordance with each other strongly support the hypothesis that the difference in the melting degree (increase from back-arc side towards the arc side) should be the main controlling factor for the chemical difference of the primary magmas between both sides.
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