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    青海扎巧合蛇绿混杂岩中辉长岩地球化学、锆石U-Pb年龄及对洋脊环境的限定

    Geochemical characteristics and zircon U-Pb age of gabbros in the Zhaqiaohe ophiolite mélange, and its limitation on the ocean ridge environment

    • 摘要:
      研究目的 扎巧合蛇绿岩处于中祁连和南祁连构造带的结合部位,带内蛇绿岩组分由蛇纹岩、蛇纹石化辉橄岩、辉长岩,辉绿岩、块层状蚀变玄武岩,少量枕状玄武岩、透镜状硅质岩等组成,为拉脊山蛇绿混杂岩带在青海省内的西延部分。
      研究方法 本文对扎巧合蛇绿岩中的辉长岩进行了年代学、岩石地球化学研究。
      研究结果 辉长岩以主化学成分SiO2、Na2O、K2O、(Fe2O3+FeO)及MgO等的特征判断,为拉斑系列的辉长岩。从其REE分布模式看,铕无异常或弱负异常,说明岩石在熔融过程中经历了斜长石的分离结晶作用,稀土配分曲线为一条近似平直曲线;微量元素具明显的Ta和Sr正异常特征,表明本区蛇绿岩中辉长岩具有与E-MORB类似的特征。LA-ICP-MS锆石UPb测年结果显示辉长岩结晶年龄为(524.0±2.5)Ma。
      结论 这些辉长岩起源于未发生大规模熔融的初始地幔,认为扎巧合蛇绿岩可能形成于古洋盆初始裂解阶段的洋脊环境。

       

      Abstract:
      This paper is the result of geological survey engineering.
      Objective Zhaqiaohe ophiolite is located at the junction of the Middle Qilian and South Qilian tectonic belt. The ophiolite in the belt is composed of serpentinite, serpentinized pyroxene peridotite, gabbro, diabase, block bedded altered basalt, a small amount of pillow basalt and lenticular silicite. It is the west extension of the Lachishan ophiolite melange belt in Qinghai Province.
      Methods In this paper, the study on the chronology and petrogeochemistry of gabbro in Zhaqiaohe ophiolite has been carried out.
      Results According to the characteristic judgment of the main chemical components of the gabbro, such as SiO2, Na2O, K2O, (Fe2O3 + FeO) and MgO, the gabbro is tholeiitic series, belonging to tholeiite. According to the REE distribution pattern, there is no Eu anomaly or weak negative anomaly, indicating that the rock experienced the separation and crystallization of plagioclase in the melting process, and the REE distribution curve is approximately flat; the trace elements have obvious Ta and Sr positive anomaly characteristics, indicating that gabbro in ophiolite in this area has similar characteristics with E-MORB. The results of LA-ICP-MS zircon U-Pb dating show that the gabbro crystallization age is (524.0±2.5) Ma.
      Conclusions It is suggested that these gabbros originated from the initial slowness without large-scale melting. It is suggested that the Zhaqiaohe ophiolite may have formed in the oceanic ridge environment during the initial cracking stage of the paleo ocean basin.

       

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