青海阿木尼克山地区牦牛山组火山岩地球化学特征、锆石U-Pb年龄及其地质意义

Zircon U-Pb dating and geochemistry of the volcanic rocks from Maoniushan Formation in Amunike area, Qinghai Province, and its geological implications

利用岩石地球化学分析和LA-ICP-MS锆石U-Pb定年方法,对阿木尼克山地区牦牛组火山岩进行了系统的地球化学特征和锆石U-Pb年代学研究。其地球化学特征为,SiO_2含量介于53.94%~64.69%之间,全碱含量介于5.38%~9.86%之间,范围较大,里特曼指数介于1.45~4.54之间,属于碱性岩和钙碱性岩,铝饱和指数A/CNK均大于1.1,具有S型花岗岩的特征;稀土元素总量为68×10^(-6)~372×10^(-6),LREE/HREE值介于4.51~18.25之间,轻稀土元素富集,重稀土元素亏损,稀土元素配分曲线右倾;微量元素中不相容元素Rb、Ba、Th富集,Ta、Nb亏损明显,蛛网曲线呈现峰谷迭起的曲线形式。锆石CL图像和U-Pb同位素分析结果显示,牦牛山组火山岩锆石自形程度好,均有岩浆振荡环带的特征,Th/U值介于0.4~1.3之间,属于岩浆成因锆石。测得该火山岩中2件样品的锆石^(206)Pb/^(238)U年龄介于371~399Ma之间,年龄加权平均值分别为374.8±3.1Ma和392.4±3.3Ma,表明火山岩形成时代为中—晚泥盆世。结合区域特征,认为牦牛山组火山岩的形成与柴达木盆地北缘的演化有关,其是后碰撞阶段伸展作用的产物。

Geochemical analysis and LA-ICP-MS zircon U-Pb dating were conducted for zircon grains and geochemical characteristics of the volcanic rocks from Maoniushan Formation in Amunike area. The geochemical characteristics show that the volcanic rocks form Maoniushan Formation have the following data: SiO_2=53.94%~64.69%, total alkali content(ALK(Na_2O+K_2O))=5.38%~9.86%, varying in a large range, Rittmann Index(δ)=1.45~4.54, belonging to alkaline rocks and calc-alkaline rocks, and all of aluminum saturation indexes A/CNK are higher than 1.1, exhibiting characteristics of S type granite. Rare earth element analyses reveal that ΣREE=68×10^(-6)~372×10^(-6),and LREE/HREE=4.51~18.25, exhibiting right-dipping REE patterns strongly enriched in LREE and depleted in HREE. Trace elements analyses reveal strong enrichment of incompatible elements Rb, Ba, Th and conspicuous depletion of Ta, Nb, assuming the curve form that cobweb curve appears frequently in peak and valley. CL investigations show that zircon grains from Maoniushan Formation are euhedral in shape and display fine-scale oscillatory zoning as well as Th/U ratio of 0.4~1.3, implying their magmatic origin. Zircon dating of two samples indicates^(206)Pb/^(238)U age of 371~399Ma, weighted mean^(206)Pb/^(238)U ages of 374.8±8.1Ma and 392.8±3.3Ma, which are interpreted to be the time of the formation of volcanic rocks. Therefore, it is suggested that the Maoniushan Formation should occur in the Middle-Late Devonian period. Combined with regional characteristics, it is concluded that the formation of volcanic rocks in Maoniushan Formation was related to the evolution of the northern area of the Qaidam Basin, and volcanic rocks were probably formed by lithospheric extension at the post-collision stage.