大兴安岭中段扎赉特旗地区满克头鄂博组火山岩锆石U-Pb年龄及地球化学特征

Zircon U-Pb age and geochemistry of volcanic rocks in Manketouebo Formation from Jalaid Banner,central Da Hinggan Mountains

对大兴安岭中段扎赉特旗地区满克头鄂博组火山岩进行了锆石U-Pb年龄和地球化学研究。用LA-ICP-MS测得流纹质隐爆角砾岩(TW6)和流纹岩(TW14)中锆石的^(206)Pb/^(238)U年龄分别为157.2±1.2Ma和154.8±1.1Ma,表明火山岩形成于晚侏罗世。岩石地球化学特征表明,火山岩富硅碱、贫钙镁,为过铝质钙碱性系列;(La/Yb)_N=20.24~22.68,轻、重稀土元素分馏明显;球粒陨石标准化稀土元素配分模式图呈右倾形式,Eu为正异常;原始地幔标准化蛛网图显示,大离子亲石元素Rb、K相对富集,高场强元素Nb、Ti、P强烈亏损,显示壳源岩浆的特征。综合本区火山岩的特征并结合他人研究成果,认为扎赉特旗地区满克头鄂博组火山岩的形成与蒙古-鄂霍次克洋闭合后伸展作用关系密切。

Chronologic and geochemical studies of volcanic rocks in Manketouebo Formation from Jalaid Banner area are presented in this paper. The zircon U-Pb dating results show that the ages of rhyolitic breccia （TW6） and rhyolite （TW14） are 157.2±1.2Ma （MSWD=0.33,n=22） and 154.8±1.1Ma （MSWD=0.34,n=28） respectively, indicating that the volcanic rocks were formed in Late Jurassic. Geochemical characteristics indicate that the volcanic rocks are rich in silicon and alkali but poor in calcium and magnesium,and belong to peraluminous calc-alkali series. The chondrite-normalized REE patterns indicate that the volcanic rocks are enriched in light rare earth elements with significant fractionation of HREE and LREE （（La/Yb）N=20.24-22.68）, and Eu has a positive anomaly. The primitive mantle- normalized trace element spidergram shows that the volcanic rocks contain high LILE（ Rb, K） and low HFSE （ Nb, Ti, P）,suggesting crust magma source. In combination with all the characteristics of the volcanic rocks and other research results, the authors hold that the volcanic rocks in Manketouebo Formation were closely related to the lithosphere extension after the closure of Mongol-Okhotsk Ocean.