Geological Characteristics And Tectonic Significance Of Devonian Volcanic Rocks In The Northern Margin Of East Junggar

The Central Asian orogenic belt is one of the largest acreage orogens in the Phanerozoic,which has been one of the hot topics of geologists.The northern margin of East Junggar is located in the northeastern part of Xinjiang.The complex and variable magmatic activity process,including subduction,closure and plate collision,occurred in the study area during the Paleozoic.The Devonian magmatism was quite active,which was also the key period of the Late Paleozoic.The Devonian volcanic rocks distributed in this region can well record the magmatic activity of this period and the tectonic evolution process of the Paleozoic.A detailed analysis of this region can better interpret the tectonic history of the northern margin of East Junggar.In this paper,geochemistry,petrology and zircon U-Pb geochronology of the Devonian volcanic rocks in the northern margin of East Junggar,Xinjiang are carefully studied and discussed.Combined with previous comprehensive studies,the framework of volcanic geochronology in the study area is further defined.The geological characteristics and tectonic evolution of the Devonian volcanic rocks in the northern margin of East Junggar are deeply discussed.The important understandings are as follows:(1)The zircon U-Pb concordant ages of the almond basalt,trachyte,altered pyroxene andesite and rhyolite from the Tuoranggekuduke Formation are 412±2Ma,408±3Ma,404±4Ma and 402±4Ma using SHRIMP method,and belong to the Early Devonian.LA-ICP-MS zircon U-Pb dating of basaltic andesite in Beitashan Formation is obtained with an average age of 404±6Ma.Considering the large thickness of volcanic rocks in the Beitashan Formation in this area,the lower volcanic rocks may have entered the Early Devonian.Spinatrypa sp.,Xinjiang Golites sp.and Thamnopora sp.are found in the upper sedimentary rocks,belonging to the Middle Devonian.(2)The Early Devonian Tuoranggekuduke Formation belongs to low K-high Ca series,with high Na2 O and Ti O2,and is enriched in large ion lithophile elements Rb and Ba,enriched in light rare earth elements,and depleted in high field strength elements Nb,Ta,Y and Th.Devonian in north hill group volcanic rocks belong to low potassium-alkaline series,rich poor potassium,sodium and enrichment of large ion lithophile elements Ba,Sr,K and enrichment of light rare earth elements,loss of high field strength elements Th,Ti,Nb and aggregates are most kara group volcanic rocks belong to calcalkaline series and high in potassium calcalkaline series,sodium potassium rich poor,has the characteristics of high Ti O2,And enrichment of large ion lithophile elements Ba and K and light rare earth elements,and depletion of high field strength elements Th and Ti.The Late Devonian volcanic rocks belong to calcalkaline series and low K series,rich in sodium and poor in K,high in Al2O3 and Ti O2,rich in large ion lithophile elements Ba,K and light rare earth elements,and low in high field strength elements Zr,Ti and Th.(3)According to the discrimination of tectonic environment in the study area,the Early Devonian,Middle Devonian and Late Devonian volcanic rocks formed in the volcanic arc tectonic environment of subduction subduction zone.(4)The Paleozoic tectonic evolution in the northern margin of East Junggar can be divided into:Middle Cambrian to early ordovician marginal tension stage,late Early Ordovician silurian oceanland transition stage,the late the early devonian erqis oceanland transformation and backarc basin expansion phase,the middle devonian-ocean-land transformation stage,the early carboniferous to the early late carboniferous-Permian intraplate extension stage,after the late Permian and overlap intracontinental orogenic stage.