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

为了评价大洋岛弧系统中花岗岩岩浆的生成和地壳生长，对中新世田泽深成杂岩进行了一系列低钾玄武岩成分的含水部分熔融实验以及岩石学和地球化学研究。高压熔融实验中的熔体成分显示出低K钙碱性的趋势，并被归类为准铝质或过铝质英云闪长岩。这些特征类似于伊豆-小笠原-马里亚纳（IBM）弧中的长英质岩石，例如，田泽深成岩。丹泽英云闪长岩的深熔成因与地球化学模拟结果一致，表明代表IBM弧中地壳的丹泽深成岩的稀土元素特征是由下地壳角闪岩部分熔融（-50%熔融温度为1050 ℃，压力低于1.2GPa）形成的。Tanzawa深成杂岩被解释为IBM弧的暴露中地壳，在碰撞过程中被抬升。水化镁铁质下地壳的部分熔融可能对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.