Characteristics And Tectonic Significance Of Leibaoemubao Porphyry In Zanzongcuo Region,Shuanghu County,Tibet

The study area is located in the Bangong-Nujiang suture zone in Tibet.In 2016,during the regional geological mapping,the author discovered and decided to study the Leibaoemubao porphyry rock mass in the Zanzongcuo region.On the basis of geological mapping in the wild area,Leibaoemubao porphyry rock mass was discovered in the Zanzongcuo region.The study of this rock mass is of great significance for the evolution of the middle section of the Bangong-Nujiang suture zone.This dissertation relies on the project of "1:50000 Regional Geological Survey in Zanzongcuo region,Shuanghu County,Tibet" and is guided by plate tectonic theory and magmatic rock related theories.Based on the field geological survey and sample collection,the petrological,geochemical and isotopic geochronological characteristics of Late Cretaceous porphyries in the Zanzongcuo region were systematically studied,the formation mechanism of magmatic rocks and related structural significance were discussed.The research suggests that the porphyry rock group in Leibaoemubao is granite porphyry and granodiorite porphyry,and the surrounding rock is sandy siltstone of Qushenla Formation.Petrogeochemical studies have shown that the Leibaoemubao porphyries belongs to acid rock,peraluminous high-potassium calcium alkaline series rocks;The porphyries are rich in large-ion-lithophilic-elements such as K and Rb,and relatively depleted of high-field-strength-elements such as Ta and Nb.The magmatic rock formation mechanism is partial melting.The specific mechanism is consistent with the batch partial melting model and can be further divided into low degree of partial melting and high degree of partial melting of the two phases.The two phases separately formed granite porphyry and granodiorite porphyry.The zircon LA-ICP-MS U-Pb isotope dating shows that the weighted average age of granodiorite porphyry is 76.40±0.92 Ma,MSWD=0.34;According to previous data,the formation age of granite porphyry ranged from 79.6 Ma to 78.3 Ma,slightly earlier than granodiorite porphyry.Through the analysis and comparative study,it is considered that the environment during the formation of the granite porphyry and the granodiorite porphyry are collisional tectonic environments.The granodiorite porphyry better reflects the characteristics of the post-collision environment.This shows that the structural-system transition occurred from 78.3 Ma to 76.4 Ma in the Zanzongcuo region of the middle section of the Bangong-Nujiang suture zone,and it changed from the collision stage to the post-collision stage.After 76.4 Ma,the Zanzongcuo region was completely transformed into the post-collision stage.