| 新疆欧西达坂花岗质岩体地球化学特征和锆石LA-ICP-MS定年：西南天山古生代洋盆俯冲作用过程的启示 |
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| 引用本文： | 王超,罗金海,车自成,刘良,张敬艺.新疆欧西达坂花岗质岩体地球化学特征和锆石LA-ICP-MS定年：西南天山古生代洋盆俯冲作用过程的启示[J].地质学报,2009,83(2):272-283. |
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| 作者姓名： | 王超 罗金海 车自成 刘良 张敬艺 |
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| 作者单位： | 西安地质矿产研究所;-;-;-;- |
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| 基金项目： | 国家自然科学基金项目（面上项目，重点项目，重大项目） |
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| 摘 要： | 本文对出露于南天山南缘断裂带的欧西达坂岩体进行了系统的岩石学、地球化学和锆石U-Pb年代学研究，重点讨论了其岩石成因、源区性质及其大地构造意义。岩体主要由石英闪长岩、二长花岗岩和花岗斑岩组成，并且显示了强烈的后期变形特征。LA-ICP-MS微区定年获得了两组年龄：421±3Ma，273±2Ma，前者为二长花岗岩结晶年龄，而后者则代表了南天山后碰撞伸展走滑作用的时代。地球化学特征显示，二长花岗岩的SiO2含量介于65.16%～75.50%之间，Mg#值低（0.23～0.41），A/CNK＝0.86～1.15，为过铝-强过铝质，钙碱-高钾钙碱系列花岗岩；(La/Yb)N为4.40～6.32，δEu =0.58～0.77。与二长花岗岩相比，石英闪长岩的SiO2含量较低（55.46%～58.29%），相对富钛、钙、铁、铝和MgO等；铝饱和指数A/CNK＝0.88～1.04，Mg#值高（0.45～0.50），为钙碱性系列。二长花岗岩的 (La/Yb)N为7.84～9.48，δEu=0.52～0.78。在原始地幔标准化蛛网图上，样品均明显富集Rb、Ba、Th、K等大离子亲石元素和LREE，亏损Nb、Ta、P、Ti等高场强元素。而二长花岗岩表现为更加亏损Nb、Ta、P、Ti等元素，并具有明显的Sr负异常。欧西达坂岩体中的石英闪长岩和二长花岗岩均为钠质，具有高的Na2O和CaO含量。 该岩体高的Al2O3、La/Nb、Th/Ta值和原始地幔标准化图解中Nb-Ti的负异常说明该套岩石形成于板块边缘环境或来自岛弧环境下的陆壳源区。石英闪长岩可能来源于镁铁质岩浆底侵导致的下地壳部分熔融，二长花岗岩可能是来自于中下地壳的无水熔融。欧西达坂岩体体现了南天山洋早古生代向南短暂而局部的平坦的低角度板片俯冲。综合区域地质资料分析推测在早古生代南天山洋可能存在双向俯冲，从而形成同一洋盆中的蛇绿岩在南天山南北两侧的就位；而在晚古生代南天山洋一直向北俯冲，具有长期、多阶段的特征。
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| 关 键 词： | 岛弧花岗质岩石 LA-ICP-MS锆石定年 欧西达坂 南天山 |
Geochemical characteristics and zircon U-Pb LA-ICP-MS dating of Oxidaban pluton from Xinjiang, China: implications for a Paleozoic oceanic subduction process in the southwest Tianshan |
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| wang chao,-,-,- and -.Geochemical characteristics and zircon U-Pb LA-ICP-MS dating of Oxidaban pluton from Xinjiang, China: implications for a Paleozoic oceanic subduction process in the southwest Tianshan[J].Acta Geologica Sinica,2009,83(2):272-283. |
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| Authors: | wang chao - - - and - |
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| Abstract: | Southern Tianshan is one of the regions with most complex tectonic evolution history in the Tianshan Orogenic Belt. Oxidaban pluton, located in the southwestern section of the Southern Tianshan, is carefully studied. Based on geochemistry and geochronology data, the petrogenesis, characteristics of the magma source, as well as the tectonic significanec are then discussed. The Oxidaban pluton located in the Kuche County, and emplaced into the south thrust belt of southern Tianshan. Lithologically, the pluton comprise mainly of quartz diorite, granite porphyry and monzonitic granite. The strongly deformed minerals imply the deformation they had undergone. In situ zircon U-Pb LA-ICP-MS dating on the zircons yielded two groups of age: 421±3Ma and273±2Ma, the former represents the emplacement age of the pluton and the later imply the time of the post-collision extension of south Tianshan. Geochemically, the monzonitic granite are calc-alkaline to shoshonitic series, with the A/CNK values between 0.86 and 1.15, which fall into the range of metaluminous to slightly peraluminous rocks. Compared to the monzonitic granite, the quartz diorites showing lower contents of SiO2 (55.46%~58.29%), and higher values of Ti, Ca, Fe, Al and MgO, with the A/CNK values between 0.88 and 1.04 and Mg numbers that range from 0.45~0.50. In the PM-normalized trace element pattern, all the rocks characterized by distinct enrichment in Rb, Ba, Th, K and LREE and depletion of Nb, Ta, P and Ti. However, the depletion of Nb, Ta, P, Ti are higher in the monzonitic granite than in the quartz diorites, and with obviously negative Sr anomaly. The Oxidaban pluton is rich in Na2O and CaO. The elemental ratios of the Oxidaban pluton, such as La/Nb, Th/Ta, combined with the high Al2O3 and negative Nb-Ti anomaly in the PM-normalized trace element pattern, indicate they were formed at a continental active margin or arc related crustal source. The quartz diorites most likely generated by partial melting of mafic magmas underplated in the lower crust. The dehydration melting of middle-lower crust is the likely source for the monzonitic granite. The Oxidaban pluton is product of melting events in arc-derived rocks, produced in the southward flat and dipping subduction of the southern Tianshan Ocean. Combined with the previous studies, it is concluded that the southern Tianshan Ocean underwent double-direction subduction, which induced the emplacement of southern Tianshan Ocean ophiolitic mélange along both side of southern Tianshan in early Paleozoic; whereas, the direction of the subduction was northward at Late Paleozoic, which characterized by the multistage and long-term evolution. |
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| Keywords: | Zircon LA-ICP-MS U-Pb dating volcanic arc granite Oxidaban pluton Southwest Tianshan |
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