Petrogenesis And Geological Significance Of Mesozoic Bainuyangpan Formation Volcanic Rocks In Yinshan Area,Inner Mongolia

Zhuozi county is located in the southeastern of Yinshan,Inner Mongolia.A large of Mesozoic volcanic of Bainuyangpan formation rocks are occurrence in Yinshan region.Based on the detailed of field geological works,and by studying the petrography,field characteristics,geochemistry of the Bainuyangpan volcanic rocks in Dayushu village,We analyzed and discussed the petrogenesis,magma evolution of volcanic rocks and its tectonic implication.Also,the zircon U-Pb age of volcanic rocks was dated by LA-ICP-MS.Compared with the volcanic rocks from other area in Yinshan,dynamic of the Bainuyangpan volcanic rocks was discussed.Main conclusions reached as the following:1.The Bainuyangpan formation volcanic rocks are charactered by bimodal volcanic rocks in Zhuozi county.The volcanic rocks can be divided into three kinds according their lithological differences.Which are basalts lava,rhyolitic rocks and trachytic rocks,respectively.And constitute a volcanic cycle by basalts,rhyolite and trachyte.The activity of volcano charactered by intermittenteruption.2.The basalts of the volcanic rocks are of alkalic basalt series rocks,which are rich of alkaline,depleted of Mg,also have low Cr,Ni content and low Th/Ce,Th/La ratio,with weak negative Eu anomalies.The magma of basalts were experienced Crystallization separation and limited contamination when they are in the chamber and the magmatic vent.Both of the trachytic and rhyolitic volcanic rocks belong to the peraluminous alalkalic basalt series rocks.They are rich of alkaline,Fe and depleted of Mg,also have distinct negative of Ti,Eu,Sr,P anomalies,and postive Pb anomalies,which present the characteristic of crust-derived magma.3.The basaltic,trachytic lava and rhyolitic volcanic rocks of Bainuyangpan formation have different magma source.The basalts should be low partial melting of enriched lithospheric mantle modified by earlier subduction events.Rhyolitic volcanic rocks are formed by partial melting of lower crust that heated by the underplating basalts.The trachytic volcanic rocks should be formed by the partial melting of underplated basalts with the material from partial melting of the crust.4.Zircon LA–ICP–MS U–Pb mean age of 137 Ma indicate that the volcanic rocks formed in the early Cretaceous.The geochemical characteristics of the Bainuyangpan formation volcanic rocks indicate that the volcanic rocks formed in continental rift setting.The basalt with the characteristics of intraplate alkaline basalt formed in a continental rift regime.Then the felsic rocks with thetypical characteristics of A1-type granite identified by their geochemistry,formed in extensional tectonic environment.5.The volcanic rocks of Bainuyangpan formation form diffeient areas have similar geochemical characteristics,which indicate that their magma might be formed in the same tectonic setting and the similar dynamics process.The generation of the Bainuyangpan formation bimodal volcanic rocks were rather caused by a series of tectonic events.Which related to the subduction plate of Paleoasian ocean that modified the lithosphere beneath north margin of the North China Plate,tectothermal event occurred at the late Triassic,and the effects of Mongol-Okhotsk Ocean closed.Causing the upwelling of asthenosphere,then heated the lithosphere beneath the North China craton made partial melting of the mantle lithosphere and generated basaltic magma.One hand,The basaltic magma rising and erupted to the surface,and on the other hand,the basaltic magma underplating beneath the crust,which induced partial melting of the crust,forming rhyolitic magma.