新疆西天山异剥钙榴岩地球化学特征及其对俯冲带流体的指示意义  

Geochemical characteristics of rodingite derived from eclogite in western Tianshan,Xinjiang,China and its implication for subduction zone fluid

作　　者：申婷婷[1] 张立飞[1] 李旭平[2] 

机构地区：[1]北京大学造山带与地壳演化教育部重点实验室,北京100871 [2]山东科技大学地质科学与工程学院,青岛266510

出　　处：《岩石学报》2012年第7期2235-2249,共15页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41121062;41090371);国家973项目(2009CB825007)联合资助

摘　　要：新疆西天山长阿吾子异剥钙榴岩是目前首例发现的在俯冲带形成的原岩为榴辉岩的异剥钙榴岩,与其作用的流体为俯冲带流体。该异剥钙榴岩与大洋底变质作用形成的异剥钙榴岩相比,岩石学特征非常相似,但是元素迁移方面存在一定的差别。为了推断流体成分,我们使用了三种方法。一、测试了异剥钙榴岩化前后主要矿物相石榴石和辉石化学成分。二、在ACF图上,可以看出全岩成分从F端元向C端元演化,代表Fe减少,Ca增加的趋势,而且完全异剥钙榴岩成分落在了前人所做的异剥钙榴岩成分区间内。三、假设Al为不迁移元素,根据Grant的计算方法,做出主量元素、微量元素和REE的等浓度图。综合以上分析结果推断,俯冲带中流体的成分是复杂和变化的:部分异剥钙榴岩化阶段流体富集Ca、Si、Ti、Mn、Nb、Ta、Zr、Hf,贫乏Fe、Mg、Na、K、REE、Tb、Y,氧逸度升高,流体属于不饱和状态,因此导致不平衡结构发育;完全异剥钙榴岩阶段,流体富集Ca、Mg、LREE、Ni、Cr,贫乏Fe、Ti、Rb、Ba、MREE、HREE等元素,此时流体作用强烈,富水流体相的强烈淋滤作用,使得大部分元素发生了迁移。Rodingite in Changawuzi, Western Tianshan, China, is a new rodingite type derived from eclogite, and is the first occurrence that formed in the subduction zone. The fluid triggered rodingitization is subduction zone fluid. Compared with the rodingite formed during ocean-floor metamorphism, Changawuzi rodingite have similar petrology characteristic, but something different in elements migration. In order to infer the composition of the fluid, we tried three methods. First, we tested the chemical compositions of the main minerals garnet and pyroxene before and after rodingitization. Second, the ACF diagram, showing the bulk compostion changed from F side to C side,illustrate that Fe is decreasing and Ca is increasing. In addition, the bulk compositions of completely rodingited rocks are included in the area studied before. Third, assuming that A1 is not moved during rodingitization, based on the method of Grant, we calculated the isocon diagrams of major elements, trace elements and REE. Based on the three methods we infer that the composition of the fluid is complicated and varied during rodingitization. In the stage of partial rodingitization, the fluid is rich in Ca, Si, Ti, Mn, Nb, Ta, Zr, Hf with high oxygen fugacity and Fe, Mg, Na, K, REE, Tb, Y are depleted. The fluid in this stage is not abundant so that a lot of unbalanced structures developed. In the stage of complete rodingitization, the fluid is rich in Ca, Mg, LREE, Ni, Cr and poor in Fe, Ti, Rb, Ba, MREE, HREE. The fluid process is strong and most of the elements migrate because of intense eluviation by the water-rich Ca bearin~ fluid.

关 键 词：异剥钙榴岩 地球化学 俯冲带流体 长阿吾子 西天山 中国 

分 类 号：P588.346[天文地球—岩石学]