阿尔金造山带南缘中―晚奥陶世正长花岗岩的发现及其地质意义

The discovery of Middle-Late Ordovician syenogranite on the southern margin of Altun orogenic belt and its geological significance

出露于阿尔金造山带帕夏拉依档沟一带的正长花岗岩,LA-ICP-MS锆石U-Pb测年结果显示其形成年龄为455.1±3.6Ma,属中―晚奥陶世。地球化学结果显示,主量元素具有富硅、富铝、富钾,低钛、贫钙、贫镁的特点,为强过铝质花岗岩系列,具高钾钙碱性特征。稀土元素总量较高,轻稀土元素富集、重稀土元素亏损,稀土元素球粒陨石标准化配分曲线有右倾型特征和明显的负Eu异常,与典型壳源花岗岩配分曲线一致。Ba、Sr、Ti等具负异常,Rb、Th、K等大离子亲石元素具正异常,显示S型花岗岩特征。结合原岩判别图解,推断其源区物质主要来源于上地壳变泥质沉积岩类。结合区域资料,认为正长花岗岩形成于挤压体制向拉张体制转换的构造环境,属后碰撞花岗岩类,表明在中―晚奥陶世阿中地块和柴达木地块已由挤压碰撞阶段转为伸展后碰撞阶段。

The syenogranite is located in the Paxialayidang ditch of the Altun Mountains. The U-Pb dating of zircons from the sye-nogranite using LA-ICP-MS yielded a group age of 455.1±3.6Ma, indicating that the crystallization of the intrusion occurred inMiddle- Late Ordovician period. The geochemical analysis shows that major elements are characterized by high SiO2, Al2O3 andK2O and low TiO2, CaO and MgO, which suggests that syenogranite belongs to the typical high-K calc-alkaline series with deeplyperaluminous feature. In addition,the rocks are enriched in total REE. The samples are enriched in LREE （light rare earth elements）and depleted in HREE （heavy rare earth elements） with Eu anomalies. The chondrite-normalized REE patterns show right-obliquetype. The syenogranite is enriched in large ion lithophile elements of Rb, Th, K and depleted in high field strength elements of Ba,Sr, Ti, with the characteristics of S-type granite. In combination with the diagrams for discriminating compositions of original rocks,the authors hold that the rocks were formed by the partial melting of meta-pelitic sedimentary rocks from the lower crust. Com-bined with the data of regional geological characteristics, the authors consider that the syenogranite was formed in the transitionaltectonic setting from the compressional to the extensional regime, thus belonging to the post-collisional granites. It is shown thatAzhong Block and Qaidam Block entered into a transformation period from compression to extension during Middle-Late Ordovi-cian period.