东乌旗桑杰敖包白音高老组火山岩地质、地球化学特征,U-Pb年龄及地质意义

Geological,geochemical characteristics,U-Pb age of volcanic rocks of the Baiyingaolao Formation in Sanjie Aobao area,East Ujimqin Banner,and their geological significance

针对白音高老组地层划分混乱、演化背景不清、火山岩地层层序及火山机构研究薄弱等问题,笔者等以内蒙古东乌珠穆沁旗桑杰敖包地区发育的白音高老组火山岩为研究对象,根据1:5万区域地质调查和“火山构造—岩性岩相—火山地层”三重填图方法,将其划分为构成完整沉积—喷发旋回的3个岩性段,分别为:火山碎屑沉积岩、酸性火山岩、中性火山岩。岩浆从早期至晚期具有由酸性向中性演化的特征。发育火山沉积相、空落堆积相、溢流相、火山通道相等火山岩相,圈定出6个V级火山机构。根据LA-ICP-MS锆石U-Pb同位素定年,确定了桑杰敖包白音高老组二段流纹岩形成于135.0±1.0 Ma,时代为早白垩世。地球化学特征显示,主量元素SiO_(2)含量介于69.56%~71.11%,K_(2)O含量介于3.96%~4.39%,K_(2)O/Na_(2)O值在0.69~1.03之间,A/CNK值为0.97~1.07,桑杰敖包白音高老组火山岩属高钾钙碱性系列,Rb、K、Th、U等相对富集,Sr、Ti、Nb、Ta、P等相对亏损,轻重稀土分异明显,且前者富集程度高于后者,稀土元素球粒陨石标准化配分曲线呈右倾“海鸥式”,符合A型花岗岩的地球化学特征。结合前人研究及数据分析,桑杰敖包白音高老组火山岩应形成于造山后伸展构造环境。因此,本研究区白音高老组火山岩的厘定对中亚造山带东段大兴安岭中生代火山岩地层层序及构造背景提供了岩石学、年代学及地球化学的约束。

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 SiO_(2) content ranges from 69.56%to 71.11%,K_(2)O content ranges from 3.96%to 4.39%,K_(2)O/Na_(2)O 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±1.0 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.