西天山HP-UHP脉体中的锆石和金红石：流体活动和变质演化

The fluid regime during plate subduction and exhumation is one of the most important concerns in the study of metamorphism, magmatism and mineralization in collisional orogens. Metamorphic fluids can exert significant influences not only on geodynamic processes but also on crust-mantle interaction and synsubduction arc magmatism that occurred in collisional orogens. The study of geochmical changes of subduction rocks and the role of various fluid/melt on behavior and cycling of elements in subduction zones is a frontier of earth science, which is called metamorphic geochemical dynamiscs. Metamorphic veins enclosed in HP-UHP rocks provide a direct record of fluid flow in subduction-zone processes. Thus metamorphic veins provid an unique opportunity to trace fluid activity and elements behavious during subduction. In dealing with the nature and action of metamorphic fluid in veins in fossil subduction zones, it is essential to decipher the temporal and formation condition of veins. The blueschist-eclogite belt in Western Tianshan in China has been proved to be a typical oceanic subduction zone, and there are plenty of various metamorphic veins eclosed in HP-UHP rocks. These veins provide us an advantaged natural laboratory for investigating geochemical transport and fluid action during subduction and exhumation of oceanic crust. However, the temporal and formation condition of veins, which is one of the fundamental requirements for discussion of the nature and action of metamorphic fluid, behavior and cycling of elements in subduction zone, is poor studied. In addition, the progade and retrograde ages of UHP metamorphism in Western Tianshan have still no constraints. Zircon and rutile in metamorphic veins are critical minerals in solving questions above. In this proposal, we choose zircon and rutile in vein and wall rock as main objects of study, based on detailed field observation, petrography and petrochemistry investigation, multiple in situ microbeam techniques will be used integrative to analyze mineralogical, trace elemental and isotopic features of zircon, rutile and their paragenetic minerals. The research contents will focus on the following: (1) the formation episodes and time of the metamorphic veins, (2) the metamorphic growth of zircon in vein, the fluid source and nature of various veins, (3) the formation period of rutile in vein and wall rocks and the metamorphic condition of vein, (4) the genesis of rutile with abundant tiny zircon inclusions. By means of revealing the growth history of zircon and rutile in vein and wall rock, the metamorphic evolutionary process of the blueschist-eclogite belt and the formation episodes and exact timing of veining will be revealed, the nature of fluid/melt of various veins will be identified. The results will provide significant support on the study of subduction and exhumation mechanism of oceanic UHP metamorphic rocks, and metamorphic chemical geodynamics.

变质流体的活动时间和变质条件是研究俯冲带流体活动和变质演化的重要内容。本项目选择中国西天山洋壳俯冲带的变质脉体为主要研究对象，以其中的锆石和金红石作为关键矿物，在详细的野外观测、岩相学和岩石化学分析基础上，联合使用多种微区原位测试技术，对锆石和金红石及其共生矿物进行详细的矿物学、微量元素和同位素地球化学分析，重点对变质脉体的形成期次和具体时代、脉体中变质锆石的形成过程及成脉流体的活动特征、脉体及围岩中金红石的形成期次及脉体形成的变质条件、富含微细锆石包体的金红石成因等几个方面开展研究，以脉体和围岩中变质锆石和金红石的生长变化为线索，揭示西天山俯冲带变质流体的活动特征和变质演化过程，识别不同阶段、不同类型脉体的流体/熔体性质，探讨HFSE在俯冲变质带的地球化学行为，结果将为洋壳岩石俯冲-折返动力学及变质化学地球动力学的研究提供重要支持。

项目对西天山洋壳俯冲带的高压-超高压变质岩和变质脉体中的锆石和金红石及其共生矿物进行详细的矿物学、微量元素和同位素地球化学分析，获得成果包括：（1）西天山变质脉体的形成期次和具体时代。脉体及围岩中变质锆石的形成常与流体活动有关，不同年龄的变质锆石（或锆石微区）可以反映流体活动的期次/幕次和时间。西天山哈布腾苏地区脉体锆石纪录了榴辉岩相变质时代（312-318Ma）及多期进变质（~355Ma和~336Ma）和退变质时代（~282Ma）。另外，获得榴辉岩和变质脉体中白云母的Ar-Ar年龄均在312~315Ma之间，应代表西天山高压-超高压变质岩的榴辉岩相变质年龄或退变质早期年龄，也进一步限定退变质脉体的形成时代。（2）脉体中变质锆石的形成过程及成脉流体的活动特征。脉体锆石的年龄和微量元素特征指示，西天山榴辉岩-蓝片岩带在洋壳俯冲-折返过程中经历了至少4次幕式流体活动。且流体系统可能为开放体系。变质锆石从绿帘角闪岩相和角闪岩相进变质阶段就开始形成，榴辉岩相阶段和退变质阶段也分别有变质锆石生成。脉体金红石核部-边部的剖面分析显示Nb-Ta元素含量有向边部降低的趋势，而Nb/Ta比值有升高的趋势。这种现象可能指示高压脉体中的金红石结晶于变化的流体环境。寄主榴辉岩中金红石向榍石的转变可能是导致脉体金红石中Nb、Ta分异的有效（主要）机制。（3）脉体及围岩中金红石的形成期次及脉体形成的变质条件。不同脉体金红石的微量元素研究表明，西天山高压-超高压变质带应该至少有两期退变质脉体，一期形成于绿帘角闪岩相退变质阶段，形成温度约为530℃，其围岩为榴辉岩和云母片岩；另一期可能形成于更晚期的退变质阶段，形成温度约为490℃，其围岩主要为云母片岩；脉体金红石的微量元素与其物质来源有关，以榴辉岩为围岩的脉体金红石Nb/Ta比值变化较大，平均值相对较高，以云母片岩为围岩的脉体金红石Nb/Ta比值变化较小，平均值相对较低。