阿尔金造山带帕夏拉依档岩体锆石U-Pb年代学、地球化学特征及地质意义

摘要

出露于阿尔金造山带帕夏拉依档沟一带的帕夏拉依档岩体主要岩性为二长花岗岩、偶含斑黑云母二长花岗岩和正长花岗岩。二长花岗岩LA-ICP-MS锆石U-Pb测年结果显示其形成年龄460．1±3．9Ma，表明帕夏拉依档岩体形成于中—晚奥陶世。地球化学结果显示，常量元素具有富硅（SiO2＝72.08％～72.27％）、富铝（Al2 O3＝14.10％～14.79％，A／CNK＝1.06～1.18）、富钾（K2O＝5.55％～6.33％，K2O／Na2O ＝1.95～2.21），低钛（TiO2＝0.19％～0.23％）、贫铁（TFeO＝1.48％～1.79％）、贫镁（MgO＝0.22％～0.28％）的特点，为弱过铝质花岗岩系列，具典型的高钾钙碱性特征。稀土元素总量较高（165.58×10－6～229.86×10－6），轻稀土元素相对富集、重稀土元素亏损，有右倾型特征和明显的负Eu异常（δEu＝0.11～0.17）。Ba、Sr、Ti等具负异常，Rb、Th、U、K等大离子亲石元素具正异常，Rb／Sr比值高（9.05～12.57），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，认为帕夏拉依档岩体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和柴达木地块间阿尔金造山带已由挤压碰撞阶段转为伸展后碰撞阶段。%The Paxialayidang plutons are located from the Paxialayidang ditch on South Altyn Tagh.The granitic rocks are mainly composed of monzonitic and porphyritic monzonitic.The U-Pb dating of zircons from the Paxialayidang plutons using LA-ICP-MS yields a group age of 460.1 ±3.9Ma ,indicating the crystallized-age of the plutons in Middle—Late Ordovician epoch.The geochemical analysis shows that major element characterized by high SiO2(72.08% ~72.27%),Al2O3 (14.10% ~14.79%),A/CNK =1.06 ~1.18 and K2O(5.55% ~6.33%)contents;low TiO2(0.19%~0.23%),MgO(0.22%~0.28%)and TFeO(1.48%~1.79%)contents, which suggest that syenogranite belong to the typical high-K calc-alkaline series with weakly peraluminous.In addition,they are enriched in total REE contents (165.58 ×10 -6 ~229.86 ×10 -6).The samples are enriched in LREE (light rare earth elements)and depleted in HREE (heavyrare earth elements)with Eu anomalies (δEu=0.1 1 ~0.17 ).The chondrite normalized REE pattern curve shows right-sloping type.The syenogranitees are riched in large ion lithophile elements of Rb,Th,K,U and depleted in high field strength elements of Ba,Sr,Ti and Rb/Sr value (9.05 ~12.57 ),with the characteristics of S-type granite,combining with the diagrams for discriminating compositions of original rocks,indicate the rock is forned by the partial melting of meta pelitic sedimentary rock from lower crust. Combined with the dataes of regional geological characteristics, the Paxialayidang plutons were formed in the transitional tectonic setting from the compressional to extensional segimes, belonging to the post-collisional granites.It shows that Altyn Tagh orogenic belt between blocks on both sides of Azhong Block with Qaidam Block suggests that it entered into transformation period from compression to extension during Middle—Late Ordovician epoch.