Petrology And Geochemical Characteristics Of Jurassic Volcanic Rocks In Baita Area,western Liaoning

With the wide distribution of volcanic rocks,the Baita area in western Liaoning Province,located in the eastern section of the North China Craton the Yanshan orogenic belt.In the past,it has been regarded as yixian formation for the early cretaceous,but it is considered as middle-later Jurassic TIaojisshan formation by our group research achievement.Late Jurassic is a vital period of regional tectonic systemic transformation,Its magmatic activity not only has profound significance to the tectonic evolution in western Liaoning Province,but also play a crucial role in destruction of the North China Craton.This thesis studies on the volcanic rocks in the Baita area in western Liaoning.Based on previous studies,through Large scale basic geological survey in the field,systematic petrology,rock association,major and trace element geochemistry and zircon U-Pb ages,petrogenesis,the nature of magma source and magmatic evolution will be discussed.Furthermore,the basis for the tectonic setting of western Liaoning Province was further theoretical studied.The volcanic rocks are divided volcanic channel facies,Eruption facies,extrusive facies by volcanic facies in the field.They are mainly composed by trachyandesite,rhyolitic breccia ignimbrites,trachyandesite-pyroclastic rocks(including agglomerate and breccia).cryptoexplosive rock,trachydacite and rhyolite by indoor study.By geochronology studies of zircon U-Pb isotopic dating in volcanic rocks and theri veins and agglomerates to limit the formation of volcanic rocks.Combined our group research achievement the volcanic rocks in the study area were dememined to be 165~149 Ma.The geochemical data show that the intermediate volcanic in the study area contain high silicon,alkali-rich and low abundance of magnesium.Because of A/CNK(1.38~1.78)?1.1,It belongs to peraluminous series.Most of the intermediate volcanic rocks belong to alkaline series and a few belong to mid-high K calc-alkaline series.They are generally enriched in LILE(Rb,Sr,Ba,K,Pb)and depleted in HFSE(Ti,Nb,Ta,P),show no obvious Eu anomaly(?Eu=1.12~0.96).The intermediate rocks of Baita area of western Liaoning belong geochemically to adakitic rock,they may be result in directly partial melting of the underlying crustal basic material.The acidic volcanic contain high silicon and alkali-rich.Because of A/CNK(1.38~1.78)?1.1,It belongs to peraluminous series.They belong to alkaline series(AR:1.04~1.60).They are generally enriched in LILE(Rb,Sr,Ba,K,Pb)and depleted in HFSE(Ti,Nb,Ta,P).Eu show obvious negative anomaly(?Eu=0.49~0.63).There is resemblance between the acidic rocks of Baita area of western Liaoning and the highly fractionated I-type granites,they may be result in partial melting of the underlying crustal basic material.The intermediate volcanic rocks may be experience by fractional crystallizations of apatite,amphibole,pyroxenes,alkaline feldspar and plagioclase etc.in the Baita area of western Liaoning Province.The acidic volcanic rocks may be experience by fractional crystallizations of amphibole and feldspar.Both the field geological phenomena and geochemistry characteristics indicate that there isn't continuous evolution relationship between the intermediate and acidic volcanic rock.The volcanic rocks of Baita area were mainly caused by northwest subduction of the paleo-pacific plate in the North China plate,which causes the region to be in the uplift stage,resulting in partial melting of the underlying crustal basic material.