Petrogenesis And Tectonic Setting Of Volcanic Rocks From Baiyingaolao Formation In Suo'lun Area,central Great Xing'an Range

This thesis studies the well-developed Baiyingaolao Formation volcanic rocks of Suo'lun area in the major ridge of the middle section of Great Xing'an Range.Based on previous studies,the rock assemblage,distribution rules and geochemistry of Baiyingaolao Formation volcanic rocks will be described,and the nature of magma source,petrogenesis and tectonic settings will be discussed in this paper,which will provide new information to researches of the volcanism and tectonic evolution history in Late Jurassic to Early Cretaceous in mid-southern Great Xing'an Range area.Baiyingaolao Formation volcanic rocks,which are composed of rhyolite,rhyolite lava,tuff lava,ignimbrite and dicate tuff lava with little intermediate-acidic volcanic rocks and sporadic sedimentary tuff and sedimentary rocks,are found at the study area.The results of LA-ICP-MS ziron U-Pb from Baiyingaolao Formation volcanic rocks dating are 126.7±1.5Ma?127.8±2.8Ma?124.6±2.2Ma?123.1±1.1Ma,revealing that Baiyingaolao Formation volcanic rocks are mainly formed between 128~123Ma in Early Cretaceous.The results of Geochemistry characteristic from Baiyingaolao Formation volcanic rocks,the Si O2 content is between 69.63 and 79.94%,and Na2O+K2O is between 6.42 and 9.35%.The ratio of Na2O/K2 O is 0.12-1.33,indicating the difference of K and Na is small and relatively rich in K.Rittmann index are less than 3.The samples are all fall on the alkaline rhyolite except two samples in the TAS diagram.In SiO2-K2 O diagram,most of the samples are belong to calc-alkaline-high-K calc-alkaline series.The value of A/CNK is 1.02-1.29,the samples are belong to peraluminous series.The volcanic rocks are also characterized by high silicon,full of alkali,full of aluminum,short of calcium and magnesium and belong to high-K calc-alkaline peraluminous rock.The chondrite-normalized REE paterns indicate that the Baiyingaolao Formation volcanic rocks are enriched in light rare earth elements and all strong negative Eu anomaly.?REE =61×10-6~208×10-6,?Eu=0.21-0.75,and the value of(La/Yb)N ranges from 3.57 to 13.27.The primitive mantle-normalized trace element spider diagrams shows right-dipping feature,which are similar with the downer crust mostly,revealing that the volcanic rocks enrich LILE(e.g.,Rb,U,K)and deplete HFSE(e.g.,Nb,Ta,Ti).These characteristics are almost in accord with the low Ba-Sr rhyolite in Greater Xing'an Range.The characteristics of enrichment are similar to crustal magma.The positive values of ?Hf(t)and TMD2(828~439 Ma)suggest the primary magma were derived from depleted mantle or basic lower-crustal re-melted materials,implying the source region may be the young crust accreted from depleted mantle in Early Paleozoic to NeoproterozoicPhanerozoic.The generation of the Early Cretaceous volcanic rocks from the Baiyingaolao Formation was related to the extensional environment caused by the subductions of the Paleo-Pacific plate beneath the Eurasian continent.