Inner Mongolia Yimin Area Jurassic Rhyolite Geochemistry And Its Geological Significance

This project is based on the "Survey of Four 1: 50000 Regional Mineral Geologies including the State-owned Yiminhe Ranch Located in Inner Mongolia Hulun Buir City",selecting the rhyolite in the Jurassic manketou'ebo,Yimin District,Inner Mongolia as the research object,conducting study on petrology,zircon U-Pb dating and geochemistry to determine the formation age of this rhyolite,summarize the petrological and geochemical characteristics of the rhyolite,and discuss the causes of the rhyolite magma,thereby analyzing the tectonic background of its formation.Through the zircon U-Pb dating of the rhyolite breccia,the results show that the surface age of ²⁰⁶Pb/²³⁸U is between 170.46 and 179.54 Ma,and the weighted average age of ²⁰⁶Pb/²³⁸U is 175.7 + 1.1Ma?MSWD=0.34?.The age represents the effusion age of the rhyolite breccia,which shows that this rhyolite is a top product of the early Jurassic era.The overall characteristics of the rhyolite are high silica?SiO2=76.68%⁷7.81%?,alkali?ALK=7.59% 8.25%?and peraluminous?A/CNK=1.06 ¹.18?,close to the average components of Type A granite.In the Harker diagram,the main oxide has obvious linear evolution relationship,and with the increase of SiO₂ contents,the oxide shows a negative linear relationship,which means that they are evolved from the same primary magma.In the lignt-heavy RE elements fractionation of the rhyolite,light RE is slightly rich.?Eu=0.14⁰.22 shows that Eu is obviously negative abnormal,which is associated with the fractional crystallization of plagioclase.These features are consistent with the geochemical characteristics of most Type A granitic rocks and RE curve of typical continental rift rhyolitic rocks.The rocks are relatively enriched with large ion lithophile elements of K,Rb,Th,but they are short of Ba,Sr,Ta,Ti,P and other high field strength elements,among which Ba and Sr is strongly negative abnaormal,similar to spider diagram of continental crust trace element.This phenomenon shows that the volcano rocks may originate from the partial melting of crustal material;The geochemical characteristics of rhyolite breccia in the Jurassic manketou'ebo formation are similar to Type A granitic rocks and low Ba-Sr rhyolite in Greater Khingan Range definite by Zhang Jiheng,so in the genesis of granite discrimination diagrams,the author thinks they all belong to A2 granite rocks.According to the research results of rhyolite breccia in the Jurassic manketou'ebo formation,in combination with related data in this region,the author makes a comprehensive analysis and thinks that the rhyolite magma is formed by the melting of the crust,and developed in the post orogenic extensional environment.