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

为了评价大洋岛弧体系中花岗岩浆的形成和地壳生长，进行了一系列低钾玄武岩成分的水合部分熔融实验，并对中新世坦泽深成杂岩进行了岩石学和地球化学研究。高压熔融实验中的熔体成分呈低钾钙碱性趋势，属准铝质或过铝质英云闪长岩。这些特征类似于Izu-Bonin-Mariana(IBM)弧中的长英质岩石，例如坦泽深成岩。坦泽英云闪长岩深熔成因与地球化学模拟一致，表明坦泽深成深成岩(代表IBM弧中壳)的稀土元素特征可由下地壳角闪岩部分熔融(1050ﾟC以下-50%熔融)产生。坦塔泽深成杂岩被解释为暴露在IBM弧的中地壳，在碰撞期间被抬升。水化镁铁质下地壳的部分熔融可能在IBM弧长英质中壳的形成过程中起重要作用。

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.