阿尔金北缘尧勒萨依花岗岩的成因及构造意义:锆石U-Pb年龄和地球化学制约

Petrogenesis and tectonic implication of Yaolesayi granite in North Altyn-Tagh:Constraints from the zircon U-Pb ages and geochemistry

尧勒萨依花岗岩出露于阿尔金北缘西段尧勒萨依河下游至河口一带,岩性为黑云二长花岗岩。为确定尧勒萨依花岗岩形成时代、成因和构造环境,本文从岩石学、锆石U-Pb年代学和地球化学方面进行研究。通过LA-ICP-MS锆石U-Pb定年,显示年龄为240±2 Ma,形成时代为中生代早期晚三叠世。其高硅(SiO;=69.68%~70.64%)、富碱(Na;O+K;O)=9.61%~9.87%,里特曼指数σ=3.34~3.63,呈碱性;铝饱和指数(A/CNK)=0.73~0.80,属准铝质;大离子亲石元素U、Th、K、Rb等相对富集,高场强元素Ti、P、Ta、Nb等相对亏损;稀土含量较高,相对富集轻稀土且分馏明显,(La/Yb);=79.80~88.40,Eu弱亏损(δEu=0.79~0.85)。根据岩体成因类型及区域构造演化特征,认为尧勒萨依花岗岩为壳幔混合的Ⅰ型花岗岩,形成于阿尔金断裂走滑局部应力释放的伸展环境。

The granite distributed in midstream of the Yaolesayi River, the western part of North Altyn-Tagh, is mainly composed of biotite monzonitic granite. In order to find out the origin,formation age and tectonic setting of intrusive rocks in this area, studies of petrology, zircon U-Pb chronology and geochemistry were carried out. LA-ICP-MS U-Pb dating obtains the age of 240±2 Ma, corresponding to Late Triassic period of Early Mesozoic. We have learnt that the granite in research area is rich in silicon(SiO;= 69.68%~70.64%)and alkali(Na;O+K;O)=9.61%~9.87%. The Rittman index of the granite is 3.34~3.63, which shows a characteristic of alkaline series. The aluminum oversaturated index(A/CNK) is 0.73~0.80, which is a metaluminous series. The granite is rich in LILEs such as U, Th, K and Rb and depleted in HFSEs such as Ti, P, Ta and Nb. The granite is also characterized by highSREE, LREE enrichment, notable light and heavy REE fractionation(La/Yb);= 79.80~88.40 and negative Eu anomaly(δEu = 0.79~0.85). These characteristics suggest that the granite in research area is Ⅰ-type granite with a crust-mantle mixing origin. According to the regional geological data, it is considered that the Yaolesayi granite was formed in the extensional environment of local stress release of the Altyn fault.