欧龙布鲁克地块西北缘青海冷湖北片麻状花岗闪长岩锆石U-Pb年龄、地球化学特征及地质意义

摘要

The gneissic granodiorite in northern Lenghu of Qinghai Province is characterized by low SiO2 and MgO,and high Al2O3, Na2O and CaO, with alumina saturation index A/CNK of 0.92~0.797. These features suggest that this gneissic granodiorite belongs to the high-K calci-alkaline peraluminous rocks and metaluminous rocks. The chondrite-normalized REE values show right-in-clined patterns with slightly negative Eu anomaly. The primitive mantle normalized trace element spidergrams are characterized by en-riched LILE(such as Rb, Th and U)and depleted HFSE(such as Nb,Ta and Zr), which is comparable with the characteristics of oro-genic magmatic rocks,suggesting a volcanic arc tectonic setting associated with subduction. Moreover, the gneissic granodiorite is characterized by rich Na2O and A12O3, high Sr, Sr/Yb and LREE, and inconspicuous differentiation of HREE. These features imply that the rocks in northern Lenghu of Qinghai were probably formed by the melting of basaltic rocks with amphibole as a residual phase. With the U-Pb isotopic age of 470.2±1.2Ma(n=19,MSWD=0.68)of granodiorite,the authors believe that north Al-tun oceanic crust subducted toward Oulongbuluke block in Early Ordovician.%欧龙布鲁克地块西北缘青海冷湖北片麻状花岗闪长岩SiO2、MgO含量较低,Al2O3、Na2O、CaO含量较高,铝饱和指数(ASI)为0.79~0.92,属高钾钙碱性系列.球粒陨石标准化稀土元素配分曲线表现为轻稀土元素富集型,轻、重稀土元素分馏明显,Eu呈弱的负异常(δEu=0.82~0.90).微量元素地球化学特征显示,岩石富集大离子亲石元素Rb、Th和U,弱亏损高场强元素Nb、Ta、Zr等,显示出与板块俯冲作用有关的造山带岩浆岩的特征.青海冷湖北片麻状花岗闪长岩为角闪岩相压力条件下变玄武质岩石部分熔融的产物.LA-ICP-MS锆石206Pb/238Pb年龄为470.2±1.2Ma(n=19,MSWD=0.68),代表片麻状花岗闪长岩的成岩年龄.结合区域地质演化特征认为,该岩体形成于火山弧构造环境,是早奥陶世北阿尔金洋壳向欧龙布鲁克地块俯冲作用的地质记录.