北祁连东段柏木峡—门岗峡地区蛇绿岩的识别及其区域构造意义

Identification of Ophiolites in the Baimuxia-Mengangxia Area in the Eastern Part of the North Qilian Mountains and Their Regional Tectonic Significance

为加强对北祁连早古生代多岛弧盆系蛇绿混杂岩的调查,选取柏木峡—门岗峡蛇绿岩开展岩相学、年代学和地球化学研究。柏木峡—门岗峡蛇绿岩位于青海省海东市互助县,构造上处于北祁连造山带中东段。由橄榄岩、辉长岩和基性火山岩组成较为完整的蛇绿岩单元。对辉长岩进行单颗粒锆石LA-ICP-MS U-Pb同位素测年,获得^(206)Pb/^(238)U加权平均年龄为(525.2±1.1)Ma(MSWD=0.06),代表了蛇绿岩的形成年龄,相当于早寒武世。岩石地球化学研究表明,该蛇绿岩中的基性火山岩属于拉斑系列,具有洋岛玄武岩的地球化学特征;玄武岩Th/Yb-Nb/Yb和TiO_(2)/Yb-Nb/Yb等构造环境判别图显示,该套蛇绿岩的形成环境与俯冲作用无关。结合详细的野外调查和区域对比,认为该蛇绿岩代表早古生代北祁连洋壳,与玉石沟—川刺沟等蛇绿岩共同构成了达坂山—玉石沟蛇绿岩带。

In order to strengthen the investigation of Early Paleozoic Archipelagic arc-basin-system ophiolitic mélange in North Qilian,the Baimuxia-Mengangxia ophiolite is selected for petrographic,chronological and geochemistry study.Baimuxia-Mengangxia ophiolite is located in Huzhu County,Haidong City,Qinghai Province,tectonically located in the middle-eastern section of the North Qilian orogenic belt.The ophiolite is mainly composed of peridotite,gabbro and basic volcanic rocks,making up a relatively complete ophiolite unit.Single zircon LA-ICP-MS U-Pb isotope dating of gabbro was carried out,and the weighted average age of ^(206)Pb/^(238)U was(525.2±1.1)Ma(MSWD=0.06).It represents the age of ophiolite formation,corresponding to the Early Cambrian.Petrogeochemical studies show that:The basic volcanic rocks in the ophiolite belong to tholeiite series,showing the geochemical characteristics of oceanic island basalts;in basalts Th/Yb-Nb/Yb and TiO_(2)/Yb-Nb/Yb tectonic discrimination diagrams,it shows that the formation environment of the ophiolite is not related to subduction.Through detailed field investigation and regional comparison,it is considered that the ophiolite represents the northern Qilian oceanic crust of the Early Paleozoic,and together with the ophiolites such as Yushigou-Chuancigou,it constitutes the Dabanshan-Yushigou ophiolite belt.