青海南山地区角孔变质岩地质特征、年代学及其地质意义

    Geological characteristics and geochronology of Jiaokong metamorphic rocks from the Nanshan area, Qinghai Province, and their geological implications

    • 摘要: 青海南山达不祖乎山北部一带的角孔变质岩系原划为早中三叠世隆务河组。通过野外实测地质剖面和路线地质调查,查明该套地层单元野外产出状态及岩石组合特征,并对其物质来源和形成时代进行分析,对进一步研究该区晚古生代-早中生代的构造演化具有重要意义。该套变质岩系为一套长石石英岩、石英岩、大理岩和黑云石英片岩组合,由下向上划分为3个岩段,叠置厚度大于3186.3m。根据碎屑锆石年龄分布特征,可大致划分为5个年龄组段,分别为250~304Ma、405~546Ma、649~1077Ma、1402~1620Ma和1861~2990Ma,其中250~304Ma可进一步划分为250~269Ma和289~304Ma两个亚组。综合前人研究资料与区域构造岩浆活动,将青海南山地区角孔变质岩的沉积时代限定为二叠纪,物源主要来自祁连造山带加里东期和晋宁期岩浆弧,柴北缘构造带海西期岩浆弧也提供了部分物质,晚古生代—早中生代经历了陆内裂陷、洋盆拉张及俯冲碰撞的构造演化过程。

       

      Abstract: The Jiaokong metamorphic rocks in northern Dabuzuhu Mountain of Nanshan area, Qinghai Province, were originally assigned to Early-Middle Triassic Longwuhe Formation. Through detailed field work, the authors examined the output state and rock assemblage of the Jiaokong metamorphic rocks. Then the provenance and the formation age of this metamorphic suite were studied, with the purpose of investigating the evolution of the study area during Late Paleozoic to Early Mesozoic period. This metamorphic suite was composed of arkosic sandstone, quartzite, marble, and mica-quartz schist. The suite has a total thickness of over 3186.3m, and can be divided into 3 sections from bottom upward. The detrital zircon ages can be divided into 5 age clusters, i.e., 250~304Ma, 405~546Ma, 649~1077Ma, 1402~1620Ma and 1861~1990Ma. The 250~304Ma age cluster can be further divided into two sub-clusters of 250~269Ma and 289~304Ma. Taking into account of previous study results and regional tectonic-magmatic activities, the authors confined the depositional time limit of the Jiaokong metamorphic rocks to Permian, and its sedimentary sources might have mainly come from Caledonian and Jinningian magmatic arc in Qilian orogenic belt, with Hercynian magmatic arc in the structural belt of North Qaidam basin also providing some sedimentary materials. Nanshan area of Qinghai experienced a complete process from intracontinental rifting, basin extension and oceanic subduction to continental collision in the Late Paleozoic-Early Mesozoic period.

       

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