Petrogenesis And Geodynamics Of Late Triassic To Early Jurassic Volcanic Rocks In Yanbian-Dongning Area

This thesis carried out zircon U-Pb-O isotopic,whole-rock elemental,and Sr-Nd isotopic compositions of the Late Triassic to Early Jurassic volcanic rocks from the Yanbian-Dongning area,eastern Jilin and Heilongjiang(EJH)provinces,to reveal the nature of the magma source,tectonic setting,and dynamic mechanism underneath EJH during Late Triassic to Early Jurassic.Early Mesozoic volcanic rocks in the region were formed at two stages:the Late Triassic(221?206 Ma)and the Early Jurassic(?181 Ma).Late Triassic volcanic rocks collected from Tianqiaoling and Luoquanzhan formations are composed of dacite,rhyolite,and rhyolitic tuff,and which display high Si O₂ and total alkalis contents,as well as high Ga/Al ratios,similar to A-type granite.These volcanic rocks are also characterized by depleted Sr-Nd isotopic signatures[⁸⁷Sr/⁸⁶Sr_(i)range of 0.7029?0.7051,?_Nd(t)range of+1.90?+5.96]and zircon?¹⁸O values range of 3.80‰to 6.14‰(average values).The low zircon?¹⁸O values imply the existence of high-temperature altered oceanic crust component.Early Jurassic volcanic rocks collected from Nancun Formation are composed of basalt,basaltic andesite,andesite,and dacite.They were enriched in light rare earth elements(LREEs)and large ion lithophile elements(LILEs),relatively depleted in high field strength elements(HFSEs,such as Nb,Ta,and Ti),indicate their parental magmas are calc-alkaline with typical arc-type trace element features.The depleted Sr-Nd isotopic compositions[⁸⁷Sr/⁸⁶Sr_(i)=0.7035?0.7067,?_Nd(t)=+1.81?+6.40]and low zircon?¹⁸O value of 4.74‰(average value)indicating that the Early Jurassic volcanic rocks were formed by partial melting of the depleted mantle wedge metasomatized by sediment melt and fluid from the oceanic slab.Combined with the research before,the Late Triassic volcanic rocks formed in a back-arc extension environment,and the Early Jurassic volcanic rocks generated under an active continental margin environment,the formation of both were associated with the subduction westward of the Paleo-Pacific plate.