GRANITIC MAGMA GENESIS IN OCEANIC ARC SETTING AND CONTINENTAL CRUST FORMING PROCESSES

为了评价洋岛弧体系中花岗质岩浆的生成和地壳的生长，对中新世坦桑尼亚洼洼杂岩进行了一系列低钾玄武岩成分的含水部分熔融实验和岩石学、地球化学研究。高压熔炼试验中熔体成分呈低钾钙碱性倾向，属铝质或过铝质调色石。这些特征与伊豆-波宁-马里亚纳弧中的长英质岩（如坦泽深成岩）相似。tanawa闪长岩的成岩成因与地球化学模拟结果一致，表明tanawa闪长岩（代表IBM弧的中地壳）的稀土元素特征可能是由下地壳的角闪岩部分熔融（1050 C，低于1.2GPa）产生的。坦泽深部复合体被解释为IBM弧的暴露的中地壳，在碰撞过程中被抬升。水合基性下地壳的部分熔融作用可能对IBM弧长英质中地壳的形成起重要作用。

In order to evaluate granitic magma generation and crustal growth in the oceanic island arc system, a series of hydrous partial melting experiments on low-K basalt compositions were done together with petrological and geochemical studies on the Miocene Tanzawa plutonic complex. Melt compositions in the high pressure melting experiments show low-K calc-alkalic trends, and are classified as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu-Bonin-Mariana (IBM) arc, e.g., Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrate that the REE characteristics of Tanzawa plutonic rocks (which represents the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust (-50% melting at 1050ﾟC and below 1.2GPa). The Tanzawa plutonic complex is interpreted to be exposed middle crust of the IBM arc, which was uplifted during the collision. Partial melting of hydrated mafic lower crust might play an important role in felsic middle crust formation in the IBM arc.