滇西龙陵地区勐冒奥陶纪二长花岗岩LA-ICP-MS锆石U-Pb定年及其构造环境

对出露于滇西龙陵的二长花岗岩进行了系统的地球化学研究,重点讨论了奥陶纪二长花岗岩的岩石成因、成岩物质来源及其构造环境。二长花岗岩SiO2和Al2O3的含量均很高,SiO2含量变化为72.17%~76.45%,A/CNK为0.86~1.45（平均1.12）,为Si和Al过饱和类型,属典型的过铝质花岗岩。稀土元素总量（∑REE）为（138.17~383.68）×10-6,稀土元素配分曲线呈右倾型,Eu为负异常。具有富集K、Sr、Rb、Ba等大离子亲石元素,亏损Zr、Ta、Nb、Th等高场强元素的特点。岩石可能是以砂岩成分为主的沉积岩部分熔融形成的花岗质岩浆上升侵位形成的,是一种典型的壳源成因类型。LA-ICP-MS锆石U-Pb定年表明,勐冒花岗岩形成于中奥陶世（454.7Ma±1.5Ma）。奥陶纪二长花岗岩具有后碰撞岩浆活动的特征,代表冈瓦纳大陆造山运动后期岩浆活动的产物,形成于同碰撞（挤压环境）向碰撞后（伸展环境）转化阶段,为后碰撞花岗岩类,对揭示滇西地区怒江断裂带的构造属性有重要意义。

LA-ICP-MS zircon U-Pb dating of Ordovician Mengmao monzogranite in Longling area of western Yunnan Province and its tectonic setting

Mengmao monzogranite,exposed in Longling area of western Yunnan,was geochemically analyzed and discussed to probe into its genesis,ore-forming material source and tectonic setting.The results show that Mengmao granite has high SiO2（72.17%~76.45%） and Al2O3,with A/CNK ratios ranging between 0.86 and 1.45（1.12 on average）.These data suggest that Mengmao granite belongs to typical peraluminous granite,with over-saturated Si and Al.The ∑REE values range from 138.17×10-6 to 383.68×10-6,and the REE patterns show right oblique feature and obvious negative Eu anomaly.Trace elements show enrichment of LILE（large-ion lithophile elements） such as K,Sr,Rb and Ba and depletion of HFSE（high field strength elements） such as Zr,Ta,Nb and Th.Magmatic source was typical of crustal genesis and was probably formed by rising and emplacement of granitic magma,which had been partially melted by greywacke.LA-ICP-MS U-Pb dating of zircons indicates that Mengmao granite was emplaced during the Middle Ordovician（454.7±1.5Ma）,with post-collisional magmatic activity features.Mengmao granite,as post-collisional granite and a product of magmatism by late Gondwana land orogenic movement,was formed in the tectonic transition stage from syn-collision（extrusion environment） to post-collision（stretching environment）.The result of the researches on Mengmao monzogranite has significance in revealing tectonic attributes of Nujiang fault belt in western Yunnan.