Geochronology and Geochemical Characteristics of the Early Ordovician Granite from Hongmiaogou Area in Northwest Margin of South Qilian and Its Geological Significance
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摘要: 笔者对南祁连西北缘红庙沟地区发育的黑云二长花岗岩体进行了精细的年代学和地球化学研究。LA-ICP-MS测定结果表明，黑云二长花岗岩加权平均年龄为（479.7±3.4）Ma（MSWD=0.047），岩体侵位时代为早奥陶世。在地球化学组成上，黑云二长花岗岩表现为过铝质（A/CNK=0.96~1.04）的Ⅰ型花岗岩特征，稀土元素总量相对较低（∑REE=67.55×10-6~83.07×10-6），轻稀土相对富集，具Eu负异常；富集Rb、Ba和K等大离子亲石元素，亏损Sr和高场强元素（Nb、Ti、P），显示弧岩浆特征。综合研究表明，本区早奥陶世黑云二长花岗岩是南祁连洋向祁连陆块俯冲过程中引起岛弧岩浆活动而形成的产物，记录了该时期重要的板块构造运动事件，为认识南祁连造山带的构造演化提供了有益资料和重要依据。Abstract: This paper makes a detailed study on the chronology and geochemistry of biotite monzonitic granite from Hongmiaogou area in the northwest margin of the South Qilian. Zircon U-Pb dating of biotite monzonitic granite yields a weighted average age of (479.7±3.4)Ma (MSWD=0.047), which indicates this granite had emplaced in Early Ordovician. Geochemical data shows that this pluton has geochemical characteristics of I-type granite and belongs to peraluminous series (A/CNK=0.9~1.04). It has relatively low total REE contents (∑REE=67.55×10-6~83.07×10-6) with LREE enrichment and Eu negative anomaly. It shows the geochemical characteristics of arc magma, with an enrichment of some large ion lithophilc elements (Rb, Ba, K) and loss of Sr and high field strength elements (Nb, Ti, P). Comprehensive research presents that Early Ordovician biotite monzonitic granite was the product of arc magma and caused by the subduction of South Qilian Ocean. It records the significant event of plate tectonics in early Ordovician and provides useful information, important basis to understand the tectonic evolution of South Qilian orogenic belt.
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Keywords:
- Zircon U-Pb dating /
- geochemistry /
- biotite monzonitic granite /
- subduction /
- South Qilian
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