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    新疆伊犁地块北缘哈尔达坂中奥陶世花岗闪长岩岩石成因及其对构造演化的约束

    Petrogenesis of Middle Ordovician granodiorite in the Haerdaban of the northern margin of Yili Block of Xinjiang and its constrain on the tectonic evolution

      摘要
    • 摘要:
      研究目的 伊犁地块北缘温泉地区奥陶纪侵入岩是准噶尔洋南向俯冲演化的重要物质记录，主要侵位于温泉岩群中。哈尔达坂一带新元古界莫合西萨依岩组中识别出奥陶纪花岗闪长岩侵入体，早泥盆世火山沉积地层与岩体为沉积接触关系，查明其形成时代和构造属性，对进一步约束伊犁地块北缘早古生代构造演化具有重要意义。
      研究方法 对该岩体进行了岩石学、锆石U−Pb年代学、岩石地球化学和Sr−Nd同位素分析。
      研究结果 花岗闪长岩锆石U−Pb年龄为469±6 Ma（MSWD=1.9），形成于中奥陶世；其SiO2含量为60.6％~62.2％，全碱含量为6.79％~7.20％，K2O/Na2O值为0.56~0.75，A/CNK值为0.83~1.04；稀土元素含量中等且具有较弱的正Eu异常（δEu=1.04~1.08），富集K、Rb、Sr、Ba、Pb等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素。岩石具有准铝质高钾钙碱性系列I型花岗岩的特征，形成于陆缘弧环境。岩石Nb/Ta值为13.3，接近上地壳，Mg#值为49.3~53.4， Nd同位素研究显示其εNd(t)值为（−0.57~−0.41），二阶段模式年龄TDM2为1249~1236 Ma，表明花岗岩源区主要为壳源，并有少量幔源物质加入，具有壳幔混源岩浆的特征。
      结论 结合区域资料，哈尔达坂花岗闪长岩形成于中奥陶世的陆缘弧环境，是准噶尔洋向伊犁地块北缘俯冲的岩浆作用记录。

       

      Abstract:
      Objective The Wenquan Ordovician intrusive rocks in the northern margin of Yili Block serve as crucial material, recording southward subduction of the Junggar Ocean. These intrusions were primarily emplaced into the Wenquan Group. The newly identified Haerdaban Ordovician granodiorite intruded into the Neoproterozoic Mohexisayi Formation and was overlain by the Early Devonian volcanic sedimentary strata with nonconformity. Determining its age and tectonic affinity is of significant importance for further constraining the Early Paleozoic tectonic evolution of the northern margin of the Yili Block.
      Methods We conducted U−Pb chronology, Sr−Nd isotope and geochemical analysis for the Haerdaban granodiorite.
      Results The zircon U−Pb age of the granodiorite is 469±6 Ma (MSWD=1.9), indicating its formation during the Middle Ordovician. It exhibits SiO2 contents of 60.6%~62.2%, total alkali contents of 6.79%~7.20%, K2O/Na2O ratios of 0.56~0.75, and A/CNK values of 0.83~1.04. The rock shows moderate rare earth element (REE) concentrations with a weakly positive Eu anomaly (δEu=1.04~1.08). It is enriched in large−ion lithophile elements (LILEs) such as K, Rb, Sr, Ba, and Pb, and depleted in high−field−strength elements (HFSEs) such as Nb, Ta, and Ti. These characteristics suggest that the rocks belong to the metaluminous high−K calc−alkaline series of I−type granite and were formed in a continental margin arc setting. The rock has an Nb/Ta ratio of 13.3, which is close to that of the upper crust, and Mg# values ranging from 49.3 to 53.4. Nd isotope studies reveal εNd(t) values of –0.57 to –0.41 and two−stage model ages (TDM2) of 1249~1236 Ma, suggesting that the source was predominantly crust−derived with a minor mantle contribution, reflecting a mixed crust−mantle magmatic origin.
      Conclusions Combined with regional geological data, the Haerdaban granodiorite formed in a Middle Ordovician continental margin arc environment and represents a magmatic record of the southward subduction of the Junggar Ocean beneath the northern margin of the Yili Block.

       

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