Donggou ophiolite occurs in Zamashi area along the eastern segment of northern Qilian orogenic belt. The ophiolite is mainly composed of pyroxene peridotite, gabbro and mafic volcanic rocks, making up a relatively complete ophiolite unit. Single -grain zircon LA-ICP-MS U-Pb isotopic dating of mafic volcanic rocks yielded a weighted average age of 499.3±6.2 Ma, which represents the ophiolite formation age, equivalent to Late Cambrian. According to geochemical studies, the ophiolite mafic volcanic rocks are of tholeiitic series, the chondrite-normalized REE patterns are nearly of flat-type; the (La/Yb)N ratios are between 0.97 and 1.26; the trace element distribution patterns basically show flat curves with the exception of a few large-ion lithophile elements (Ba, Rb, U, K); Nb, Ta, Zr, Hf have not experienced loss, suggesting geochemical characteristics of mid-ocean ridge basalt (N-MORB); in Zr-Zr/Y and Ti/100-Zr-3Y tectonic discrimination diagrams, all the samples 611 into the MORB area formed in mid-ocean ridge. A comparative study of the region shows that the ophiolite and Yushigou as well as Cuancigou ophiolite jointly constitute the spreading ridge-type ocean ophiolite.%东沟蛇绿岩位于北祁连造山带中东段的扎麻什一带,主要由辉橄岩、辉长岩和基性火山岩组成较为完整的蛇绿岩单元.对基性火山岩进行单颗粒锆石LA-ICP-MS U-Pb同位素测定,获得499.3Ma±6.2Ma年龄加权平均值,代表蛇绿岩的形成年龄,相当于晚寒武世.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玄武岩系列,球粒陨石标准化稀土元素分配模式为近平坦型,(L/Yb)N在0.97～1.26之间；微量元素分配模式除个别大离子亲石元素(Ba、Rb、U、K)外基本为平坦型曲线,Nb、Ta、Zr、Hf无亏损,显示出洋中脊玄武岩(N-MORB)的地球化学特征；在Zr-Zr/Y和Ti/100-Zr-3Y等构造环境判别图中,所有样品数据点均落入MORB区域内,表明其形成于洋中脊环境.经区域对比,该蛇绿岩与玉石沟、川刺沟蛇绿岩等一起构成了大洋扩张脊型蛇缘岩带.
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