Research Of Characteristics,Genesis And Tectonic Setting Of Volcanic Rocks In The Baiyingaolao Formation From The Mid-southern Segment Of The Da Hinggan Mountains

Widely developed Mesozoic volcanic rocks in Da Hinggan Mountains attracts widespread concern.Previous researchers have conducted detailed studies on the volcanic rocks in the northern part of Daxinganling,but research on the Mesozoic volcanic rocks especially Baiyingaolao Formation in the Balinzuoqi area in the south-central segment are weak,and there are still great controversies about the epochal attribution,genesis and tectonic setting of volcanic rocks.The volcanic rocks from the Baiyingaolao Formation in the Balinzuoqi area of the central-south section of Daxing'anling are observed and descripted in detail.Rrepresentative rock samples are selected and detailed tests and researches on petrology,chronology,and geochemistry were carried out.The Formation consists mainly of rhyolite and its corresponding pyroclastic rocks,containing a small amount of dacite and trachytic dacite rocks.The zircon U-Pb ages range from 131 to 138 Ma and the weighted average age is 134.5 ± 2.0 Ma,i.e.,the Early Period of Early Cretaceous.The results of rock geochemical analysis show that the volcanic rocks are high-K calc-alkaline rocks with K2O/Na2O ratio is 1.20 to 1.69 and Rittman index is 1.1 to 2.9,which is a calcium alkaline series.A/CNK is near 1,A/NK=1.07-1.24,which is preliminarily aluminum-weak peraluminous rock.On the SiO2-(Na2O+K2O-CaO)diagram,it mainly falls into the A and CA regions,ie alkaline and calcineous regions.Large ionic lithophilic elements are enriched,and high field strength elements are lost.The distribution curve of rare earth elements shows that light rare earth elements are enriched and there is a negative Eu anomaly.On the 10000 × Ga/Al-Zr,10000×Ga/Al-Nb,1000×Ga/Al-Ce,and 1000×Ga/Al-Y diagrams,most of the volcanic samples fall in the I,S,and M type granite regions.In combination with FeOT/MgO-Zr+Nb+Ce+Y,10000×Ga/Al and Zr+Nb+Ce+Y diagrams and elemental geochemical analysis,the volcanic rocks in this area are considered to be the most felsic type ? volcanic rocks.It is suggested that the magma be derived from the partial melting of crustal materials.In the diagram of the Pearce series of tectonic settings,falling into the same collision and volcanic arc environment in the Nb-Y diagram,in the Ta-Yb,Rb-(Yb The+Ta)and Rb-(Y+Nb)plot data are located in the volcanic arc(VAG)region.It shows that this Early Cretaceous volcanic rock is a product of the volcanic arc's tectonic setting and is related to subduction.Combined with regional tectonic evolution,this paper believes that the Early Cretaceous volcanic rocks in this area may be affected by the joint subduction of the Mongolian-Okhotsk ocean and the paleo-Pacific,formed in the volcanic arc environment.