Late Mesozoic Temporal And Spatial Feature Of The Volcanic Events In The NE China And Its Geological Significance

The tectonic-magmatism event is widespread throughout NE China in late Mesozoic(Late Jurassic-Early Cretaeous).It is characterized by strong rifting tectonic activity,and formed a unique basin-rang structrue pattern of NE-NNE directed and large volcanic rock in late Mesozoic.These volcanic rocks are widely distributed in the basins and the Daxinganling rang.Its lithological characteristics and formation time are direct indicators to its geodynamic background.This dissertation compares the lithological characteristics and development timing of the volcanic rocks between basin and Daxinganling areas,based on the geochronological study through zircon SHRIMP U-Pb of the volcanic rock in Yingcheng Group.The main conclusions are as follows:(1) Early Cretaeous occurred two obvious magmatism activities in Songliao Basin,119～110Ma(Yingcheng Group)and 133～120Ma(Huoshiling Group).The Yingcheng Group formed in late stage of extensional rifting.Its volcanic activities began with intermediate-mafic and ended with acidic volcanic eruption.The Huoshiling Group formed in early stage of extensional rifting.Its volcanic activities began with intermediate-mafic eruption.Zircon SHRIMP U-Pb dating indicates that volcanic rocks,occurring in the 1st segment of Yingcheng Group in northen part, have comparable ages with those in the 3rd segment of Yingcheng Group in sorthern part,implying they result from the same period of magmatism,rather than two periods as previously thought.(2) Three stages of volcanism of late Mesozoic in NE China have been classified based on their age statistics,163～150 Ma,145～120 Ma and 120～96 Ma. The volcanic rocks of 163～150 Ma are mainly discoveried in the west of Daxinganling rang.Volcanic activities began with intermediate-mafic rocks.The volcanic rocks are with the same age and the volcanic rock association.The 145～120 Ma are the strongest period of volcanism in the northeast region,and its intermediate-acidic volcanic rock is widespread through NE China.The volcanic rocks of 120～96 Ma are mainly distributed Yingcheng Group in the basins and the east of Jilin and Hilongjiang region.(3) The volcanic rock in late Jurassic-early Cretaeous(163～150 Ma) maybe related to the quick closure of Mongolia-Okhotsk Ocean.However,The volcanic rocks in early Cretaeous(140～96Ma) are formed by the back-arc extensional tectonic setting of the Pacific Ocean subduction.