| Late Mesozoic volcanism was active in Qingyuan area of southern Zhejiang,which formed an important part of China southeast coastal volcanic rock belt.Due to the complicated terrain and inconvenient transportation,the geological survey and scientific research on volcanic rocks in this area are still in development,especially with the lack of systematic studies of chronology,stratigraphy and petrogenesis.This in turn severely hampered the understanding of late Mesozoic volcanism and geological evolution of the south-east coastline.This thesis will focus on the study of Late Mesozoic volcanic rocks of Xishantou and gaowu formations in Qingyuan area,and investigate in the systematic studies of field geological,zircon U-Pb chronology,petrography,and geochemistry.Therefore,able to analyze the age of volcanic strata,discuss the petrogenesis of volcanic rocks and evolution of the tectonic background based on the above studies.The Late Mesozoic volcanic strata in Qingyuan area are divided into Moshishan group and Yongkang group.The Moshishan group can be divided into Gaowu formation,Xishantou formation,Chawan formation,Jiuliping formation.While the Yongkang group can be divided into Guantou formation,Chaochuan formation,Xiaopingtian formation and Taishun formation arranged in chronological order.In the research area of volcanic rock formation,the lithology of Gaowu formation are mainly rhyolitic ignimbrite,Xishantou formation are mainly rhyolitic ignimbrite trachyandesite with multilayer sedimentary rocks.The zircon U-Pb dating technique indicates that late Mesozoic volcanic activity in qingyuan area can be divided into two stages:late Jurassic（153Ma～148Ma）and early cretaceous（142Ma～133Ma）.Combined with previous studies,the Mesozoic volcanic activity in qingyuan area was initially divided into four volcanic activity period:J12～J21（176Ma～169Ma）,J3（153Ma～148Ma）,K1（142Ma～133Ma）,K12～K2（101Ma～98Ma）.Geochemical analysis showed that the Late Jurassic and Early Cretaceous volcanic rocks in Qingyuan area have similar geochemical characteristics,and they belong to the weakly peraluminous high potassium alkaline series quality-rhyolitic volcanics class.The volcanic rocks of the two periods general performance for REE volume is higher and the rich light rare earth elements,contained of large ion lithophile elements K,Rb,Th,with obvious loss of high field strength elements Nb,Ta,Ti,in primitive mantle standardization on spider diagram with significant P and Sr negative anomalies,and has the feature of shell source arc magmatic rocks.Based on the comprehensive analysis,rhyolite should come from the remelting of the ancient lower crust and involve mantle material in the magmatic evolution process.It was formed in the lithospheric extension stage after the peak of subduction and compression of the ancient Pacific plate.At this time,the collision and compression were relatively weakened,and the deep plate was squeezed while the shallow plate was stretched.The subduction and collision of the paleo-pacific plate became heat source,resulting in the upwelling of mantle material and partial melting of the ancient lower crust,accompanied by the mantle source material and the transmission of mixing,separation,and crystallization,resulting in the formation of large-area I-type rhyolitic volcanic rock in the region. |
