青海格尔木哈希牙地区中基性岩墙群地球化学特征与LA-ICP-MS锆石U-Pb年龄

摘要

位于东昆仑造山带上的东昆北哈希牙地区发育中基性岩墙群。其岩性主要为辉绿岩、闪斜煌斑岩和闪长玢岩，地球化学数据显示其具有高钾钙碱性玄武岩系列特征。轻、重稀土元素存在较高程度分馏，LREE/HREE值为4.72~8.66，平均值为6.57。δEu值介于0.81~0.98之间，平均值为0.91，显示出微弱的负Eu异常，表明斜长石的分离结晶作用不明显，富集Rb、Ba、Pb、K等大离子亲石元素，贫高场强元素Ta、Nb、Zr、Hf等地球化学特征，主体显示板内玄武岩特征，但有来自俯冲带的部分信息。采用LA-ICP-MS技术对锆石进行了U-Th-Pb同位素测定，结果显示锆石年龄较为分散，其中8颗岩浆锆石给出的206Pb/238U年龄加权平均值为411.5±7.5Ma，代表了格尔木哈希牙地区辉绿岩墙的结晶年龄，反映东昆北地块于晚志留世-早泥盆世已由地体碰撞的挤压状态转为碰撞后伸展状态，标志着该地区加里东期构造旋回的结束。%Intermediate to mafic dyke swarms are developed in Haxiya area which is located in the north of the Eastern Kunlun oro-genic belt. The rocks are mainly composed of diabase, dioriteic lamprophyre and diorite porphyrite, which have K calc-alkaline ba-salt series features as shown by geochemical analyses. The REE distribution patterns suggest strong fractionation between LREE and HREE and weak negative Eu anomaly, with LREE/HREE being 4.72~8.66 (6.57 on average), and δEu being 0.81~0.98 (0.91 on average), indicating indistinct fractional crystallization of plagioclase. The intermediate to mafic dyke swarms in this area are enriched in LILEs (K, Rb, Cs, Ba, Sr) and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), showing the characteristics of intraplate basalts with some information of the subduction zone. LA-ICP-MS U-Th-Pb dating shows that zircon ages are relatively scattered, and eight mag- matic zircons have yielded a weighted mean 206Pb/238U age of 411.5±7.5Ma, representing the crystallization age of the intermediate to mafic dyke swarms in Haxiya area of Golmud. That age suggests that the tectonic setting of the north of the Eastern Kunlun Block had accomplished the environment conversion from compression of colliding terranes to extension after collision, and eventually en-tered the intracontinental orogenic stage during Late Silurian-Early Devonian.