Geological,geochemical characteristics,U-Pb age of volcanic rocks of the Baiyingaolao Formation in Sanjie Aobao area,East Ujimqin Banner,and their geological significance
Objectives:Aiming at the problems such as chaotic stratigraphic division,unclear evolution background,volcanic stratigraphic sequence and weak research on volcanic mechanism of Baiyingaolao Formation,this paper takes the volcanic rocks of the Baiyingaolao Formation developed in Sanjie Aobao area,East Ujimqin Banner as the research object,based on 1∶50000 regional geological survey and the triple mapping method of"volcanic structure,lithology,lithofacies and volcanic stratigraphy".It can be divided into three lithologic segments which constitute the complete sedimentation—eruption cycle:pyroclastic sedimentary rocks,acidic volcanic rocks and neutral volcanic rocks.The magma evolved from acidic to neutral from early to late period.Volcanic sedimentary facies,cave-down accumulation facies,overflow facies and volcanic channels were developed,and six V-class volcanic structures were delineated.Based on LA-ICP-MS zircon U-Pb isotope dating,it is determined that the second member rhyolite of Baiyingaolao Formation in Sanjie Aobo area was formed in 135.0±1.0 Ma,and the age is Early Cretaceous.Geochemical characteristics show that the main element SiO2 content ranges from 69.56％to 71.11％,K2O content ranges from 3.96％to 4.39％,K2O/Na2O value ranges from 0.69 to 1.03,and A/CNK value ranges from 0.97 to 1.07.The volcanic rocks of the Baiyingaolao Formation in Sanjie Aobo area belong to the high potassium calc-alkaline series.Rb,K,Th,U are relatively enriched,Sr,Ti,Nb,Ta,P are relatively depleted,and the difference of heavy and heavy rare earths is obvious,and the former is higher than the latter,and the standardized partition curve of rare earth elements chondrites shows A right-leaning"seagull",which conforms to the geochemical characteristics of A-type granite.Combined with previous studies and data analysis,the volcanic rocks of the Baiyingaolao Formation in Sanjie Aobao should be formed in the post-orogenic extensional tectonic environment.Therefore,the determination of the volcanic rocks of the Baiyingaolao Formation in this study area provides petrological,chronological and geochemical constraints on the stratigraphic sequence and tectonic setting of the Mesozoic volcanic rocks in the eastern part of the Central Asian orogenic belt.Methods:Based on field geological characteristics,LA-ICP-MS zircon U-Pb chronology and geochemistry,the volcanic eruption rhythm,volcanic mechanism characteristics,petrogenesis and tectonic setting of the Baiyingaolao Formation volcanic rocks in Sanjie Aobao area are discussed.Results:(1)The volcanic rocks of the Baiyingaolao Formation in Sanjie Aobao area are divided into three lithologic segments which can form a complete sedimentation—eruption cycle:Member 1,Member 2 and Member 3.The early to late magmas are characterized by the reverse evolution from acidic to neutral.By using the mapping method of"volcanic tectonic—litho—lithofacies—volcanic strata",the volcanic facies,including volcanic sedimentary facies,void deposition facies,overflow phase and volcanic channel,are divided into six V-class volcanic structures.(2)The zircon U-Pb age of the Second Member of the Baiyingaolao Formation is 135.0±1.0 Ma,indicating that the rhyolite was formed in the Early Cretaceous.(3)The rhyolite of the Second Member of the Baiyingaolao Formation has the characteristics of A2-type granite,indicating that the formation of the Early Cretaceous volcanic rocks is related to the extensional tectonic environment of the closing and post-closing of the Mongolia—Okhotsk Ocean.Conclusions:It is confirmed that the volcanic rocks of the Baiyingaozao Formation in Sanjie Aobo area were formed in the Early Cretaceous(135.0±l.0 Ma),and are related to the extensional tectonic environment after the Mongolia—Okhotsk Ocean closed orogeny.Volcanic facies such as volcanic sedimentary facies,cave-down accumulation phase,overflow phase and volcanic channel were divided,and six V-level volcanic institutions.
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