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

分析了琉球弧系新生代火山岩的Rb、Sr、Nd、Sm含量和^&lt；87&gt；sr/^&lt；86&gt；sr和^&lt；143&gt；Nd/^&lt；144&gt；Nd值。含角闪石岩的Sm含量和Sm/Nd值明显偏低，而相对基性岩中相对较低的锆锶比值较低的弧侧岩石中的锆锶比变化明显，分别反映了角闪石和斜长石主导的分馏作用，这很可能与岩浆的结晶分异有关。本区原生岩浆中的锶、钕同位素特征主要由MORb源型橄榄岩和俯冲沉积岩形成的混合岩组成。(2)弧后侧沉积组分对原生岩浆的贡献明显偏低(27-30%)。(3)在地壳演化过程中，两侧岩浆往往受到地壳物质相当程度的同位素改造的影响。(4)随着地质环境的变化，^&lt；87&gt；sr/^&lt；86&gt；sr和^&lt；143&gt；Nd/^&lt；144&gt；ND比值也发生了变化。(5)高镁安山岩具有较高的^&lt；143；Nd/^&lt；144&gt；Nd比值，其原生岩浆可能来自不同的物质来源。(6)奥比约碱性玄武岩具有羽状玄武岩的同位素特征，其锆锶和Sm/Nd-SiO_2体系及其一致性都有力地支持了熔融程度的差异(从弧后侧向弧侧增大)应是两侧原生岩浆化学差异的主要控制因素的假说。

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.