王栋^1,2,3, 肖媛媛^1,2,3, 牛耀龄^1,2,4,5,6
1.中国科学院海洋研究所海洋地质与环境重点实验室, 山东 青岛 266071;2.青岛海洋科学与技术国家实验室, 山东 青岛 266061;3.中国科学院大学, 北京 100049;4.中国科学院海洋大科学研究中心, 山东 青岛 266071;5.杜伦大学地球科学系, 英国 杜伦DH1 3LE;6.中国地质大学地球科学与矿产资源学院, 北京 100083

摘要:
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。近年的研究发现，大洋岩石圈俯冲变质作用非常复杂。本研究以采自西南天山富含脉体的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、退变蓝片岩部分和脉体。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。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等元素相对亏损，且含量变化较大，在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，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。这些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。第一阶段中，K、Rb、Cs、Ba等水溶性元素丢失，而第二阶段中，外来流体造成水化作用改造，并使水化程度高的蓝片岩部分形成了大量白云母等含水矿物，使这些元素表现出再富集并得以保存。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等水溶性元素特征。
关键词:大洋岩石圈俯冲作用流体活动赋存矿物元素地球化学行为岛弧岩浆作用
DOI：10.11759/hykx20190123002
分类号:P595;P67
基金项目:国家自然科学基金项目（41572047，41776069）

Petrological and geochemical records of fluid activity during seafloor subduction: constraints from the (ultra) high-pressure metamorphic belt of Southwestern Tianshan, China
WANG Dong^1,2,3, XIAO Yuan-yuan^1,2,3, NIU Yao-ling^1,2,4,5,6
1.Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;2.Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China;3.University of Chinese Academy of Sciences, Beijing 100049, China;4.Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071 China;5.Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK;6.School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China
Abstract:
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 explore the influence of fluids on geochemical processes during seafloor subduction. Based on petrological characteristics, the sample was divided into three sections:the eclogite part, the blueschist part, and the vein. The vein was mainly composed of omphacite, and generally contained carbonate minerals and apatite, reflecting the sample that underwent fluid modification of CO₂, P, and halogen-rich elements under the conditions of eclogite facies metamorphism. The Lu-Hf isochron age of the sample was 332±24 Ma, which was consistent with the peak metamorphic age of the Southwestern TianShan, 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, were similar to ocean-island basalt (OIB), indicating that the original rock composition was similar to OIB. The K, Rb, Sr, and Ba of the three parts were variably 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 were 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 had undergone two stages of metamorphism, i.e., the eclogitization process and rehydration overprints with different degrees. At the first stage, water-soluble elements were lost. At the second stage, the external fluids caused rehydration, and led to the formation of abundant water-bearing minerals in the blueschist part such as muscovite, which can preserve these elements. Therefore, if the muscovite is stable, the characteristically high content of water soluble elements in the island arc magma cannot be attributed to the simple metamorphic dehydration of the subducting seafloor.
Key words:seafloor subductionfluids activityhost mineralsgeochemical behaviors of chemical elementsisland arc magmatism





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