Petrogeochemistry of Late Paleozoic Gabbroic Rocks from Tekes County in West Tianshan Mountains
朱志敏(Zhi-Min Zhu)；赵振华(Zhen-Hua Zhao)；熊小林(Xiao-Lin Xiong)；韩江伟(Jiang-Wei Han)
西天山 ； 辉长岩 ； 锆石U-Pb定年 ； 岩石地球化学 ； West Tianshan ； gabbro ； zircon U-Pb dating ； petrogeochemistry
|Volume or Term/Year and Month of Publication||
29卷6期（2010 / 11 / 25）
675 - 690
西天山特克斯县东北部的哈拉达拉岩体是西天山出露规模最大的层状基性－超基性侵入岩体，主要由橄长岩、橄榄辉长岩、角闪辉长岩和辉长岩组成。本文获得该辉长质岩体锆石SIMS U-Pb年龄为306.2±2.7 Ma，属于晚石炭世。该岩体SiO2含量范围为44.49%~48.46%，K2O、Na2O含量低，Mg、Fe含量较高，Mg(上标 #)平均值达71.4。岩体具有拉斑玄武岩系列特征，且有从低钾拉斑系列向钙碱性系列和高钾钙碱性系列演化的趋势。部分样品的MgO、Al2O3和CaO含量偏高，可能为橄榄石和斜长石的堆晶所致。岩体明显富集Rb、Ba、K、Pb、Sr等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素，La/Nb比值大于1.4，与流体交代地幔楔熔融的弧火成岩浆特征类似。岩体具有Th负异常，暗示岩浆活动过程中可能混染了下地壳成分。岩体稀土元素总量较低，轻，重稀土元素分异较弱，由于斜长石堆晶作用，部分样品具有明显的正Eu异常。区域构造演化和岩石学资料等显示西天山在晚石炭世－早二叠世大地构造格架演化发生了转变，特克斯地区处于俯冲碰撞阶段向碰撞后伸展阶段变化的过渡环境中，特克斯晚古生代辉长岩体很可能是在这种构造体制的转变过程中形成的，指示了西天山大地构造演化从弧环境下的挤压碰撞到陆内伸展环境的转变。
The Haladala pluton in northeast Tekes County is the largest basic-ultrabasic layered pluton in West Tianshan area; nevertheless, little work has been done on its petrogenesis. In this paper, the authors conducted a detailed study of its geochemistry and geochronology. With the data obtained in combination of regional tectonic evolution, this paper discusses its petrogenesis, tectonic setting and geological significance. This pluton consists of a series of gabbroic rocks such as troctolite, olivine gabbro, hornblende gabbro and gabbro. Secondary ion microprobe (SIMS) zircon U-Pb dating indicates that the gabbroic rocks crystallized at the age of 306.2±2.7 Ma, belonging to the Late Carboniferous. These gabbroic rocks have SiO2 values ranging from 44.49% to 48.46% with low content of K2O and Na2O and high average Mg(superscript #) value of 71.4. Chemical compositions of these rocks are tholeiitic, but show a variation trend with the magmatic evolution from low-K tholeiite series to calc-alkaline series and even high-K calc-alkaline series. Some samples show abnormally high MgO, Al2O3 and CaO content: which may be due to the accumulation of olivine and plagioclase. They are also significantly enriched in LILE such as Rb, Ba, K, Pb and Sr, but depleted in HFSE such as Nb, Ta and Ti, with La/Nb＞1.4. All these data indicate characteristics of arc magmas derived from fluid metasomatized mantle wedge. Their Th negative anomaly suggests that the magmas may have been contaminated by the lower crust. These rocks have low content of total REE with weak differcntiation of LREE from HREE. Due to the role of plagioclase accumulation, some samples havc significant positive Eu anomalies. Compared with the mafic plutons of different ages exposed in adjacent areas in North Xinjiang, the Haladala pluton has unique petrological and geochemical characteristics, which may imply that they contain both arc-like magma components and components from extensional environment magmatism. The compositional characteristics suggest that the rocks might have been formed during the change of the tectonic framework from compression to extension. Regional tectonic history indicates that there was a shift (from compression to extension) in the tectonic framework of West Tianshan region during Late Carboniferous to Early Permian. The authors made an overall analysis of various rocks concerning their petrologic and geochronologic features at the northern and southern edge of the Yili-Central Tianshan plate, and found out that the Yili plate underwent a transition from the subduction and collision to post-collision circumstances in 293~313 Ma. Based on geochronological and geochemical characteristics of the Haladata pluton, the authors believe that the Tekesi area was at the stage of this transition of tectonic settings. It is suggested that the late Paleozoic gabbroic rocks (the Haladala pluton) in Tekes County might also have been formed in this transition stage. Hence, these rocks may indicate a change of tectonic framework in the West Tianshan area and provide evidence for the presence of magmatism in this region during the tectonic frame change.