Chronology And Petrogenesis Of Jurassic-Cretaceous Volcanism In The Area Of Zhejiang-Fujian Boundary

The Zhejiang-Fujian boundary region in the coastal volcanic rock belt of Southeast China, occurred intense volcanism in late Mesozoic. Geological investigations and scientific researches in southeast coast are relatively inadequate due to the complex terrain and traffic inconveniencies, particularly in the systematic studies of petrogenesis and tectonic background. Therefore, the thesis selected two areas which include Qingyuan area in southwest Zhejiang province and Zhenghe area in northern Fujian province(hereinafter referred to as the study area), for researching volcanic rocks in late Mesozoic, include volcanic geology, geochemistry and zircon U-Pb chronology. This preliminary research divided the Jurassic-Cretaceous volcanic activity stage and discussed each stage of the volcanic rock and evolution of the tectonic background.Volcanic activity in Zhejiang and Fujian provinces was weak in Jurassic. It only scattered in Pankeng Formation in eastern Fujian province and Maonong Formation in Zhejiang province as interlayer in early Jurassic, and not reported as predecessor in this area. In Cretaceous, volcanic activities were strong, and they formed a large area of volcanic rock formationn the eastern of Zhejiang province, they were respectively: lower of the volcanic rocks series from bottom to top are Dashuang formation, Gaowu formation, Xishantou formation, Chawan formation and Jiuliping formation. Upper of volcanic rocks series are Guantou formation, Chaochuan formation, Xiaopingtian formation, Fangyan formation and Taishun formation. In the eastern of Fujian province, they were respectively: lower of the volcanic rocks series are Changlin formation, Nanyuan formation and Xiaoxi formation. Upper of volcanic rocks series are Huangken formation, Zhaixia formation and Shiniushan formation. Dashuang formation, Chawan formation, Fangyan formation and Shiniushan formation did not reported in this study area.In the study area of volcanic rock formation, the lithology for the lower volcanic rock series are mainly rhyolitic ignimbrite for Gaowu formation; Xishantou formation are mainly rhyolitic ignimbrite, trachyandesite with multilayer sedimentary rocks; Lower Nanyuan formation are rhyolitic ignimbrite, the top of thick layer of porphyroclastic lava; Xiaoxi formation are mainly rhyolitic ignimbrite interbedded with sand and shale. For the upper volcanic rock series, Guantou formation and Chaochuan formation are are respectively sallow tuffaceous sandstone and red mud; Xiaopingtian formation are mainly rhyolitic vitric tuff and rhyolitic ignimbrite bedded with red mud; Taishun Formation mainly rhyolitic crystal vitric ignimbrite interbedded with Dark grey mudstone; Lower Huangkeng Formation are motley tuffaceous sandstone, the upper section are grey rhyolitic ignimbrite; Lower Zhaixia Formation are red rhyolitic ignimbrite, the upper section are rhyolitic ignimbrite.Through the zircons LA-ICP-MS U-Pb dating techniques, we found the early(176.0 Ma) and middle Jurassic(169.1Ma) volcanic rock in the study area. They belong to a part of Maonong formation; and we also measured the Dashuang formation volcanic rocks in the age of 144.0Ma~139.9Ma, Huangkeng formation volcanic rocks in the age of 101.9Ma, Zhaixia formation volcanic rocks under the age of 98.7Ma~97.4Ma at the same time. Isotope chronology and route geologic profile data show that Zhejiang-Fujian border region of Mesozoic volcanic activity began in the early Jurassic(176Ma), and ended in the late Cretaceous(about 97Ma). This preliminary research can be divided into four volcanic activity time: J12-J21 (176~169Ma)?J3(162~148Ma)?K11(144~126Ma)?K12-K2(101~98Ma). This was great significance for cognition in volcanic activity time and space in the southeast coastal area.Geochemical analysis showed that the Zhejiang-Fujian border region volcanic rocks are mainly a set of the weakly peraluminous high potassium calc-alkaline series quality-rhyolitic volcanics class, general performance for ?REE volume is higher and the rich light rare earth elements, enrichment of large ion lithophile elements K, Rb, Th, obvious loss of high field strength elements Nb, Ta, Ti, in primitive mantle standardization on spider diagram with significant P negative anomaly and the Pb is abnormal, and has the feature of shell source arc magmatic rocks. Volcanic rocks were formed in the transition from extrusion to stretch tension tectonic background, the tectonic setting is associated with the subduction of the ancient Pacific plate.