Geochronological and Geochemical Characteristics of Volcanic Rocks in the Permian Gequ Formation in East Kunlun and Their Tectonic Significance
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摘要: 东昆仑古特提斯造山带经历了从洋壳俯冲到陆-陆碰撞及后碰撞伸展的造山过程, 但对古特提斯洋闭合时间一直存在争议, 争论的焦点集中在晚二叠世格曲组沉积盆地原型及中上二叠统之间不整合关系代表的构造事件。本文对东昆仑红石山地区格曲组火山岩开展了锆石U-Pb年代学与岩石地球化学研究, 结果表明, 岩石富集Rb、Th、Ba等大离子亲石元素(LILE)而亏损Nb、Ta、Ti、P等高场强元素(HFSE), 具有大陆岩浆弧的地球化学特征。火山岩具有高Ba/Th比值(53.6)、Th/Ce比值(0.23)、Nb/Ta比值(18.35)及低Dy/Yb比值(1.42), 表明晚二叠世火山岩由枕状玄武岩和洋底沉积物等洋壳物质俯冲至60 km左右深度时熔融所形成。锆石U-Pb定年表明, 格曲组下部碎屑岩段流纹质凝灰岩和英安质沉凝灰岩206Pb/238U加权平均年龄分别为(257.5±2.5) Ma和(256.2±4.8) Ma, 上部灰岩段粗安质晶屑凝灰岩206Pb/238U加权平均年龄为(251.8±2.3) Ma。结合区域上已报道的格曲组及花岗岩研究成果, 认为古特提斯洋在晚二叠世开始向北俯冲消减, 沉积于弧前盆地的格曲组为古特提斯洋向北初始俯冲的沉积响应, 而格曲组与下伏地层之间的角度不整合关系记录了古特提斯洋晚二叠世开始向北俯冲这一构造事件。Abstract: The Paleo-Tethys orogenic belt in East Kunlun has experienced an orogenic process driven by oceanic crust subduction, continent-continent collision, and post-collision extension.To date, the closure time of the Paleo-Tethys ocean has consistently been controversial, with the focus of this debate being the Late Permian Gequ Formation as a sedimentary basin prototype, and the tectonic event represented by the nonconformity between the Middle and Upper Permian.In this study, volcanic rocks from the Gequ Formation located in the Hongshishan area of East Kunlun were studied using zircon U-Pb chronology and petrogeochemistry.These rocks are enriched in large-ion lithophilic elements, such as Rb, Th, and Ba, and depleted in high field-strength elements, such as Nb, Ta, Ti, and P, and have the geochemical characteristics of a continental magmatic arc.The volcanic rocks have high Ba/Th (53.6), Th/Ce (0.23), and Nb/Ta (18.35) ratios, and a low Dy/Yb ratio (1.42), indicating that the Late Permian volcanic rocks were formed by melted pillow basalts, ocean floor sediments, and other ocean crust materials during a subduction to a depth of approximately 60 km.Zircon U-Pb dating results show that the respective weighted average ages based on 206Pb/238U of rhyolite tuff and dacitic tuff in the clastic rocks and volcanic rocks in the lower Gequ Formation are (257.5±2.5) Ma and (256.2±4.8) Ma, respectively.The weighted average age based on 206Pb/238U of the trachyandesitic crystalline tuff in the upper limestone is (251.8±2.3) Ma.Combined with the results of studies on the Gequ Formation and granite, this data suggests that the Paleo-Tethys Ocean began subducting erosion northward in the Late Permian.The Gequ Formation deposited in the forearc basin is a sedimentary response to the initial northward subduction of the Pale-Tethys Ocean, and the angular nonconformity between the Gequ Formation and the underlying strata indicates that the tectonic event wherein the Paleo-Tethys Ocean began subducting northward occurred during the Late Permian.
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Key words:
- zircon U-Pb age /
- geochemistry /
- forearc basin /
- Gequ Formation /
- East Kunlun
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