Origin and Regional Variation of the Magma in the Ryukyu Arc-Back Arc System

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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.