摘要:阿伊山群是南祁连构造带拉脊山地区的重要岩石地层单元，其形成构造环境和时代的限定对于理解拉脊山地区构造演化过程具有重要的意义。近年来，我们通过地质填图在青海拉脊山东沟煤矿地区的阿伊山群中识别出一套安山岩、流纹岩、凝灰岩、含砾粗砂岩和粗砂岩的岩石组合，其中，火山岩呈夹层的形式出露在沉积岩中。本文在详细的野外地质调查和岩相学研究的基础上，对阿伊山群中的流纹岩夹层进行了年代学和全岩地球化学研究。LA-ICP-MS锆石U-Pb定年结果显示流纹岩的206Pb/238U加权平均年龄为447±4 Ma，属晚奥陶世。流纹岩具有较高SiO2 （69.6%～76.5%）和铝饱和指数（A/CNK = 0.85～1.10），为钙碱性—高钾碱钙性系列。岩石富集轻稀土（LREE）和Rb、Ba等大离子亲石元素，亏损重稀土元素（HREE）和Nb、Ta、Ti等高场强元素，具有高的全岩δ18O值，指示岩浆源区可能为沉积岩。结合前人研究成果，本文认为阿伊山群流纹岩是晚奥陶世陆陆碰撞过程俯冲陆壳沉积物部分熔融形成的。
关键词: 拉脊山/
阿伊山群/
流纹岩/
晚奥陶世/
全岩氧同位素
Abstract:The tectonic setting and timing of the Ayishan Group in the Lajishan area from the South Qilian tectonic belt are of great significance for understanding the evolution of this tectonic belt. In recent years, we have identified an assemblage of andesite, rhyolite, ignimbrite, sandstone and coarse sandstone in the Ayishan Group by geological mapping. These intermediate-acid volcanic rocks are interbedded in sedimentary rocks. The integrated study of field characteristic, petrography, zircon U-Pb geochronology and whole-rock geochemistry are conducted on the rhyolites from the Ayishan Group. LA-ICP-MS zircon U-Pb dating yields a Late Orodovician age of 447±4 Ma for the rhyolite. These rocks exhibit high SiO2 （69.6%～76.5%） content and aluminum saturation index （A/CNK = 0.85～1.10）, belonging to calc-alkaline to high K calc-alkaline series. They are characterized by enrichment of LREEs, LILE, such as Rb, Ba, and depletion of HREEs and HFSEs such as Nb, Ta, Ti. The extremely high whole-rock δ18O values of rhyolites indicate that their source rocks are dominated by sedimentary rocks. Combined with previous studies, it is inferred that the Ayishan rhyolites were formed by partial melting of the subducted continental sediments during Late Ordovician.
Key words:Lajishan, Ayishan Group/
Rhyolite/
Late Ordovician/
Whole-rock oxygen isotope/
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