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    柴北缘查干哈克花岗伟晶岩年代学、地球化学及其成因和构造环境

    Chronology, geochemistry, petrogenesis and tectonic setting of Chaganhake granitic pegmatite in North Qaidam

      摘要
    • 摘要:
      研究目的 厘定查干哈克花岗伟晶岩的形成时代、成因和构造环境对于青藏高原北缘古特提斯洋演化与稀有金属成矿作用研究具有重要意义。
      研究方法 对花岗伟晶岩及其围岩黑云正长花岗岩分别开展了独居石和锆石U−Pb定年，以及岩石地球化学研究。
      研究结果 花岗伟晶岩独居石U−Pb年龄为250.4±0.7 Ma(MSWD=0.1, n=27)，黑云正长花岗岩锆石U−Pb年龄为374.3±8.5 Ma(MSWD=0.15，n=8)。花岗伟晶岩具有高硅、碱、铝，低钛的特征，A/CNK值为1.15~1.32，稀土元素含量很低，呈现轻稀土元素弱富集的右倾配分模式，Eu强烈亏损，δEu为0.07~0.18，具有M型稀土元素四分组效应，强烈富集Rb、K、U、P，强烈亏损Ba、Sr和Ti，中等富集高场强元素Nb、Ta、Zr、Hf；黑云母正长花岗岩富硅、钾，贫钛，A/CNK值为1.13~1.32，稀土元素含量中等，呈现轻稀土元素强烈富集的右倾配分模式，Eu亏损明显，δEu为0.19~0.32，强烈富集Rb、K、Th，强烈亏损Sr、P和Ti。
      结论 年代学和地球化学结果表明，花岗伟晶岩与黑云正长花岗岩不具成因联系。结合区域地质背景认为，柴北缘西段查干哈克花岗伟晶岩不同于宗务隆花岗伟晶岩，宗务隆花岗伟晶岩是古特提斯洋大陆边缘增生造山和碰撞造山过程中形成的花岗岩经高分异演化而成，而查干哈克花岗伟晶岩形成于早三叠世古特提斯洋俯冲环境下的柴达木地块大陆内部，是大陆内部挤压混合岩化过程中达肯大坂岩群小比例部分熔融的产物，具有铌钽矿成矿潜力。

       

      Abstract:
      Objective The determination of the formation age, petrogenesis, and tectonic environment of Chaganhake granite pegmatite is of great significance for the study of the evolution of the Paleo−Tethyan and the metallogenesis of rare metals in the northern margin of the Xizang Plateau.
      Methods In this paper, monazite U−Pb dating and zircon U−Pb dating have been carried out for granite pegmatite and its surrounding rocks, and their geochemistry has been studied.
      Results The results show that the monazite U−Pb age of granite pegmatite is 250.4±0.7 Ma(MSWD=0.1, n=27), while the zircon U−Pb age of biotitite syenogranite is 374.3±8.5 Ma(MSWD=0.15, n=8). The granite pegatite is characterized by high Si, alkali, Al and low Ti, with A/CNK value of 1.15 to 1.32 and low rare earth element contents, showing a right−leaning partition pattern with weak enrichment of light rare earth elements (LREE), strong Eu deficit, δEu of 0.07 to 0.18, showing a four−group effect of M−type rare earth elements, significant enrichment of Rb, K, U and P, strong depletion of Ba, Sr and Ti, moderate enrichment of high field strength elements(HFSE) Nb, Ta, Zr, Hf. Biotite synenite granite is rich in Si, K, and poor Ti, with A/CNK value of 1.13 to 1.32 and moderate rare earth element contents, showing a right−leaning partition pattern of strong enrichment of LREE, obvious Eu deficit, δEu of 0.19 to 0.32, strong enrichment of Rb, K, Th, and strong depletion of Sr, P and Ti.
      Conclusions Combined with the regional geological background, it is believed that the Chaganhake granite pegmatite in the western part of the North Qaidam margin is different from the Zongwulong granite pegmatite. The Zongwulong granite pegmatite evolved from the granite formed in the process of accretionary orogeny and collision orogeny of the Paleo−Tethys Ocean continental margin through high differentiation, while the Chaganhake granite pegmatite was formed in the continental interior of the Qaidam block under the Paleo−Tethys Ocean subduction environment of the Early Triassic. It is the product of small proportion partial melting of the Dakendaban rock group during the process of extrusion migmatization within the continent, and has the metallogenic potential of niobium−tantalum deposit.

       

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