Petrology Study Of Mesozoic Volcanic Rocks In Yakeshi Region,Inner Mongolia

Yakeshi area is located in eastern Inner Mongolia,on both sides of the main ridge of DaXingAnLing Range.During Mesozoic it belonged to the Western Pacific tectonic domain,and the intense volcanic activity formatted the huge volcanic stratigraphic sequence.In this paper,my study focus on the Mesozoic volcanic rocks,by comprehensively analyzing their petrography,rock association and geochemical characteristics,research the origin and evolution process of the magma,and explore the tectonic settings of the volcanic rocks.Mesozoic volcanic activity in Yakeshi area began from Middle Jurassic and gradually ended in Early Cretaceous,concluded five volcanic activity cycles and formed five groups of volcanic stratas.Tamulangou group mainly contains a set of neutral overflow face basalts,olive basaltic trachyandesites and latites,which belong to shoshonite series,and the volcanic rocks have the geochemistry characteristics of SiO2=46.30-55.50%,Na2O+K2O=6.13-7.3%,Zr/Y>>3,Nb/Ta=13.72-16.48,Mg#=0.40-0.56,with obviously LREE to HREE fractionation and no negative Eu anomaly.Magma of Tamulangou group came from the crust-mantle boundary,with a crust-mantle mixed source attribute.Manketouebo group mainly contains a set of acid explosive face,overflow face and extrusive face rhyolites and rhyolitic ignimbrites,which belong to calc-alkaline series,and the volcanic rocks have the geochemistry characteristics of SiO2=68.70-80.11%,Mg#=0.17-0.35,with Th,U,Pb,Rb,K enrichment and Ta,Nb,P,Ti,Ba loss and obvious negative Eu anomaly.Volcanic rocks of Manketouebo group are the crustal magmatic products.Manitu group mainly contains a set of neutral-acid overflow face trachytes,trachydacites and latites,which belong to Alkaline-sub-alkaline transitional series,and the volcanic rocks have the geochemistry characteristics of SiO2=59.83-67.78%,Zr/Y>>3,Nb/Ta=1 0.23-14.50,Mg#=0.30-0.46,relatively rich in alkali,with Rb,Ba,K,Pb enrichment and Ta,Nb,Ti,P loss,without obvious negative Eu anomaly and have obviously LREE to HREE fractionation.Magma of Manitu group originated from the bottom of crust.Baiyingaolao group mainly contains a set of acid rhyolites,perlites and Rhyolitic pyroclastic rocks,which belong to calc-alkaline-alkaline series.This group developed diverse petrographic faces,and the volcanic rocks usually have geochemistry characteristics of SiO2=67.20-78.08%,mean value of Mg#is 0.24,with Th,U,Pb,Hf,Rb,K enrichment and Nb,Sr,P,Ti loss,and have negative Eu anomaly.Magama of Baiyingaolao group mainly came from crust.Meiletu group mainly contains a set of mafic overflow face olive basaltic trachyandesites and latites,which belong to calc-alkaline series,and the volcanic rocks have the geochemistry characteristics of SiO2=52.87-58.77%,Zr/Y=7.11-11.43,Th/Ta=2.06-9.89,Mg#=0.42-0.49,with Rb,Ba,K enrichment and Th,U,Nb,Ta,Ti loss,and have a high degree of LREE fractionation.Magma of Meiletu group derived from the crust-mantle boundary,and mantle component is relatively more than crustal component.Mesozoic Middle-Late Jurassic volcanic rocks formed in intraplate compressional tectonic environment.Under the influence of northern Mongolia-Okhotsk ocean basin closure and mantle-derived magma underplating,Tamulangou group and Manitu group formed in relatively slack tectonic environment,while Manketouebo group and Baiyingaolao group formed in strongly compressional tectonic environment.Till Early Cretaceous,Meiletu group gradually transformed into stretching stage after the compressive orogenesis.