Genesis And Tectonic Significance Of The Early Cretaceous Rhyolite In Namucuo Area,Northen Gangdese

The Gangdese belt,as the leading edge of the direct collision between India and the Asian continent,has been widely concerned.However,due to the low recognition of the chronology and geochemical properties of the early Cretaceous magmatic rocks in the Gangdese belt,the geological evolution before the Cenozoic has not been well constrained.The study area is located in the middle part of the North Gangdese structural belt.Geographically,it is located in the transitional part of Bangong Lake-Nujiang Tethys ocean which gradually completed the final closure from east to west in the late Early Cretaceous.In this paper,the rhyolites developed in Namucuo area are taken as the research object,and their genesis,forming environment and stratigraphic attribution are studied.The rhyolite formation of Namucuo in the study area has a relatively good ancient volcanic structure,especially the Jiadapo volcano.Volcanic facies are mainly explosive facies,overflow facies and effusion facies,followed by pyroclastic flow facies and subvolcanic facies,with a small amount of eruption sedimentary facies locally,which mainly develop rhyolitic volcanic fragmentary rock and rhyolitic volcanic rock.The characteristics of major elements show that the Namucuo rhyolite has the petrochemical characteristics of high SiO₂ and K₂O,rich Al₂O₃ and alkali?K₂O+Na₂O?,poor Ti O₂,Mg O and Ca O.A/CNK=0.94?1.91,with an average of 1.16,which belongs to the peraluminous rocks of the high potassium calc alkaline-potassic basalt series.?La/Yb?_N=1.49?11.08,?Eu=0.12?0.55,which shows slightly right V-shaped distribution pattern in chondrite standardization rare earth elements;trace elements Rb,K,Th,Ta,Ce,Zr,Hf are relatively enriched,elements Nb are relatively deficient,elements Ba,Sr,P,Ti are strongly depleted,rocks have the characteristics of crust source,Namucuo rhyolite has the characteristics of the same collision type volcano,and has the affinity of arc volcanic rocks But it does not have the characteristics of traditional island arc environment.The LA-ICP-MS zircon U-Pb ages of the three rhyolite samples are 120?112Ma,of which?120.1±1.9 Ma?and?119.6±1.3 Ma?represent the age of rhyolite formed by the early volcanic activity of Namucuo,and the another?112.6±1.3 Ma?represents the age of rhyolite formed at the end of the activity of Namucuo volcano.It is preliminarily believed that in the middle of Early Cretaceous?about 120 Ma?,a major structural system transformation event occurred in North Gangdese,Namely,the subduction of Bangong Lake-Nujiang ocean basin ended in the middle of early Cretaceous,and land-land collision began between Gangdese belt and Qiangtang block.During the collision,the lower crust thickened and partially melted.Namucuo rhyolite is the product of partial melting during the collision,It can be compared with the acid volcanic rocks of the Doni formation.According to the characteristics of petrology,petrochemistry,geochemistry and chronology,compared with Linzizong group,Doni formation,Zenong group and Deqing area,Namucuo rhyolite is different in rock combination,forming environment and forming time,and can not be regarded as the same lithostratigraphic unit.Therefore,this paper considers that the name of woronggou formation can be restored and the woronggou formation can be redefined.The volcanic rocks in Deqing area may be the product of the South subduction event of ocean basin represented by Shiquanhe-laguocuo-Yongzhu-Namucuo-Jiali ophiolite melange belt,which can be compared with the buerga?stratotype profile?Zenong group near dongzenong,Shenzha county.It is more reasonable that the volcanic sedimentary strata in Deqing area belong to Zenong group.