海洋地质与环境重点实验室知识库

俯冲带变质作用具有非常重要的地球动力学意义，——没有俯冲带变质作用
就没有俯冲带岩浆作用。大洋岩石圈俯冲变质作用直接影响俯冲带岩浆作用和地
幔组成的不均一性。近年来岩石学和实验岩石学研究发现，大洋岩石圈俯冲变质
作用过程中元素的地球化学行为复杂，受众多因素控制，其中流体的物理、化学
性质可以极大影响元素的地球化学行为，因此，有必要对俯冲变质岩中的流体记
录及影响进行深入探讨。本研究以采自西南天山富含丰富脉体的榴辉岩相变质岩
样品为例来探讨大洋岩石圈俯冲作用过程中流体对元素地球化学行为的影响。根
据岩石学特征，将该样品分为三部分：榴辉岩部分、水化作用影响的蓝片岩部分
和脉体。脉体以绿辉石为主、并普遍含有碳酸盐矿物和磷灰石，反映该样品经历 过榴辉岩相变质条件下富 CO2、P 和卤族元素的流体改造。岩石学证据反映岩石
样品主要经历了两阶段变质作用，即榴辉岩化过程和不同程度水化作用改造。样 品的全岩 Lu-Hf 等时线年龄为 332±24 Ma，这与前人用 SHRIMP U-Pb 锆石定年 等方法得到的西天山峰期变质年龄一致。这三部分样品的全岩 Th-U-Nb-Ta-Zr-Hf 稀土元素等不易迁移元素的配分模式均类似于洋岛玄武岩（OIB）的特征，表明 其原岩与 OIB 类似。这三部分样品的 K、Rb、Cs、Ba 等元素相对 OIB 型原岩亏损，且含量变化较大，蓝片岩部分的含量明显高于榴辉岩部分，表现了这些元素 的活动性。另外，白云母中富集 Rb、Cs、Ba 元素，K 是白云母的主量元素，以 及 K 和 Rb、Cs、Ba 之间很好的相关性体现了白云母的存在对这些元素的控制作 用。第一阶段榴辉岩化作用中，K、Rb、Cs、Ba 等水溶性元素丢失，而在第二阶段由外来流体造成水化作用改造，水化程度高的蓝片岩部分中形成了大量白云母等含水矿物，从而使这些元素再富集并被保存。因此，如果白云母稳定存在， 俯冲大洋岩石圈简单变质脱水则难以解释岛弧岩浆高 K、Rb、Cs、Ba 等水溶性 元素的特征。 

Subduction zone metamorphism (SZM) has important significance for both arc magmatism and mantle compositional heterogeneity. Experimental studies and studies on natural rock have proved that the geochemical behavior of elements during the process of seafloor subduction is complex and controlled by many factors. In this paper, we studied the rocks rich in veins from the southwest Tianshan to discuss the influence of fluids on geochemical processes during the seafloor subduction. Based on petrological characteristics, the sample can be divided into three sections: the eclogite part, the blueschist part and the vein. The vein is mainly composed of omphacite， besides，some carbonate minerals and apatite is also contained , indicating the sample has experienced alteration by fluids modification of CO2, P and halogen-rich elements under the eclogite facies. The Lu-Hf isochron age of the sample is 332±24 Ma, which is consistent with the peak metamorphic age of the southwestern Tianshan by using SHRIMP U-Pb dating reported in previous studies. The partitioning patterns of trace elements for the three rock sections, such as Th-U-Nb-Ta-Zr-Hf and rare earth elements, are similar to ocean-island basalt (OIB), indicating that the original rock composition is similar to OIB. The K, Rb, Sr and Ba of the three parts are variabley depleted compared with OIB, and the content of these elements in blueschists is significantly higher than that in eclogites, indicating the mobility of these elements. In addition, Rb, Sr and Ba are significantly correlated with K. Considering K is the major element for muscovite, this correlation reflects the control of the presence of muscovite for these elements. All these lines of petrological and geochemical evidence reflect that the sample has undergone two stages of metamorphism, i.e., the eclogitization process and the rehydration overprints with different degrees. At the first stage, water-soluble elements are lost. At the second stage, the external fluids cause rehydration, and lead to the formation of abundant water-bearing minerals in the blueschist part such as muscovite, which can preserve these elements. Therefore, if muscovite is stable, the characteristically high contents of water soluble elements of the island arc magma cannot be attributed to the simple metamorphic dehydration of the subducting seafloor.