Mobilization of Ti Nb-Ta during subduction : Evidence from rutile-bearing dehydration segregations and veins hosted in eclogite, Tianshan, NW China
Mobilization of Ti Nb-Ta during subduction : Evidence from rutile-bearing dehydration segregations and veins hosted in eclogite, Tianshan, NW China
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DOI：
10.1016/j.gca.2007.07.027
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发表时间：
2007-10
影响因子：
5
通讯作者：
Jun Gao;T. John;R. Klemd;X. Xiong
中科院分区：
文献类型：
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作者：
Jun Gao;T. John;R. Klemd;X. Xiong
来自中国西北部西天山俯冲杂岩的野外证据表明，高场强元素Ti，Nb和Ta被动员，从而在脱水过程中从Zr和Hf中分离出来，将蓝片岩转化为榴辉岩。这两个隔离与耗尽晕，认为是代表初始动员过程中脱水，和运输静脉，指示长距离运输进行了研究。在每种情况下，厘米大小的金红石晶粒生长为针状晶体的偏析和棱柱晶体的静脉。在偏析的寄主岩石中，石榴子石和绿辉石的Ti含量，金红石和钛铁矿的模态丰度以及岩石的Ti，Nb和Ta含量向偏析方向降低。这些观察结果与Ti、Nb和Ta从主岩迁移到偏析中是一致的。榴辉岩相脉矿物的结构和地球化学数据表明，富钛铌钽流体长距离（在最小米尺度）输送过程中的地幔控制的流体流动。络合物形成配体（例如，Na-Si-Al聚合物和F−）可能增强了Ti、Nb和Ta在流体中的溶解度。流体成分的变化（例如，XCO_2）既可使金红石型又可使破碎的Ti、Nb、Ta从LILE和REE中沉淀出来。
Field evidence from the western Tianshan subduction complex in northwestern China indicates that the high field strength elements Ti, Nb, and Ta were mobilized and thereby fractionated from Zr and Hf during the dehydration process that transformed blueschist into eclogite. Both a segregation with a depletion halo, thought to represent initial mobilization during dehydration, and a transport vein, indicative of the long distance transport were investigated. In each case, centimeter-sized rutile grains grew as needle-like crystals in the segregation and as prismatic crystals in the vein. Within the host rock of the segregation, the Ti contents of garnet and omphacite, the modal abundances of rutile and titanite and the bulk rock Ti, Nb, and Ta contents decrease towards the segregation. These observations are consistent with transport of Ti, Nb, and Ta from the host rock into the segregation. Textural and geochemical data for the eclogite-facies vein minerals indicate that Ti–Nb–Ta-rich fluids were transported over long-distances (at minimum meter-scale) during fracture-controlled fluid flow. Complex forming ligands (e.g., Na–Si–Al polymers and F−) may have enhanced the solubility of Ti, Nb, and Ta in the fluid. Changes in fluid composition (e.g., XCO2) may both precipitate rutile and fractionate Ti, Nb, and Ta from LILE and REE.