Discovery And Geological Significance Of The Middle Triassic Volcanic Rocks In The Middle Segment Of The Northern Margin Of The North China Plate

The study area is in the junction of Xingmeng orogenic belt,at the northern margin of the North China Craton.This area is a key area for the closure of P aleo-Asian Ocean.This area to the southwestern edge of the Horqin Sandy Land and covered heavily,which has limited geological study in this area on the tect onic evolution of the Late Paleozoic,especially the Triassic.This paper is based on former mapping project,and focuses on the newly discovered Middle Triassic volcanic rocks in Yajishan to determine its age and forming tectonic setting,for further exploration in region nearby.Yajishan volcanic rocks are exposed in the Aohan,Chifeng,Inner Mongolia.Systematic observation and sampling work had been done in field firstly,followe d by more detailed geochemical analysis.Zircon U-Pb age of these volcanic rock s is 241-245 Ma,which indicates that the Yajishan volcanic rock was formed in the Middle Triassic instead of the originally designated as Mesozoic Jurassic.The n these volcanic rocks are re-determined as the Middle Triassic Yajishan volcanic rock,a new discovery unit of Triassic volcanic rocks.Comparing data in this study with previous research,both the evolution and distribution of the Yajishan volcanic rocks are highly consistent with regional sam ples,high potassium and rich of sodium and calcium.Rare earth elements show a slight Eu negative anomaly.Trace elements also showed similar characteristics.Ba,Sr and Eu had differences indicating that plagioclase has undergone crystallizat ion and has the characteristics of homologous evolution.The ratio of Nb/Ta,Nb/La and Th/Ta of Yajishan volcanic rocks reflects that the Yajishan volcanic rocks as partially melted from the crust and mixed with ma ntle material,formed in the post-orogenic extension environment,which limits the upper limit of the early Mesozoic extended environment in the study area to 241 Ma The post-collision environment of the Yajishan volcanic rocks provides con straints for the closure time and location of the Paleo-Asian Ocean,which was c ompletely closed in the study area at about 245 Ma.