Geological And Geochemical Characteristics Of The Late Jurassic Syenogranite Yezhugou,Inner Mongolia

The Yezhugou in Huoburenbudun,Inner Mongolia,is located in the coastal Pacific tectonic domain,Daxinganling magmatic belt,the magmatic tectonic activity is strong,wide range of exposure.This paper,from the field geological features,petrology,geochemistry,and the U-Pb isotopic chronology of the long-term granites of the wild boar,discussed the origin of the syenogranite in the Yezhugou and the formation environment.The syenogranite of Yezhugou intrudes into the rhyolitic breccia-bearing fused tuff of the upper Jurassic manjiatou ebo formation.The rock lithology is small porphyritic-medium grained biotite syenogranite,porphyritic medium fine grained biotite syenogranite and fine syenogranite.The main mineral components are plagioclase(15-25%),potash feldspar(>40%),quartz(20-25%)and biotite(<10%).The rock mass has an area of about 0.14km²and is produced as a rock tumor.Based on the data analysis of the main trace elements and rare earth elements in rocks,show that Yezhugou syenogranite is rich in silicon(Si O₂content of 74.64%?76.75%),alkali rich(Na₂O+K₂O of 7.12%?8.48%),low magnesium,calcium Ca O(0.70%?0.83%),Al₂O₃(12.82%?13.15%),Ti O₂(0.17%?0.25%),P₂O₅(0.037%?0.070%),Na₂O/K₂O=0.32?0.79,and A/CNK value of 1.03?1.47.Trace rare earth elements have a trace cobweb pattern,which shows the enrichment of Rb,Th,U and Pb in large ion lithophile(LILE),and the strong deficit in high field strong elements(HFSE)including Nb,Ta,Ti and P.The rare earth element partition pattern diagram is similar to the curve of the upper crust,the total rare earth content is relatively high,the light rare earth enrichment type,?eu changes at 0.54?0.80,with moderate eu negative anomaly,?ce changes at0.95?1.04,there is no ce anomaly,rare earth characteristics indicate that the magmatic source area is continental crust material,zr nb ce y is less than 350×10^-6.Based on the analysis of these chemical data,it is concluded that the syenogranites of Yezhugou are type I granites with inferior alkalinity,high potassium-calcium alkalinity,weak peraluminium-peraluminium and high differentiation.The crystallization age of Yezhugou syenogranite obtained by LA-MC-ICPMS zircon U-Pb dating method is 161.5±1.6 Ma,combined with field geological relationship,which reveals that the formation age of Yezhugou syenogranite is late Jurassic.In the discriminant diagram of tectonic environment,all the samples fall in the granite of volcanic arc,and combined with the regional tectonic position and regional tectonic evolution,the syenogranite of Yezhugou may have formed in the tectonic setting of post-orogenic extension under the Late Jurassic paleo-Pacific plate.