内蒙古中东部上侏罗统满克头鄂博组划分及时代

Division and Age of the Upper Jurassic Mandetou Orbo Formation in Central and Eastern Inner Mongolia

内蒙古自治区中东部满克头鄂博组以流纹质火山碎屑岩为主夹山间湖盆相陆相碎屑岩及少量流纹岩或英安岩.沉积岩中产叶肢介Magumbonia paramecia,Nestoria reticulate,N.xishunjingensis,N.cf.pissoui,Lioestheria sp.;双壳类Ferganoconcha curta,F.subcentralis,F.elongate;植物:Ginkgoites sp.,Cladophlebis sp.,Lycopodites sp.,Baiera gracilis,Phoenicopsis sp.化石.岩石化学SiO271.47%~75.96%,Ai2O312.50%~14.18%,K2O 3.84%~4.98%,Na2O 3.22%~4.14%,ALK 7.84%~9.12%,K2O>Na2O,显示高硅高碱特征;微量元素组成上以富集Rb、K、Th、U、La和LREE,亏损Ba、Nb、Sr、P和Ti为特征;稀土元素ΣREE=47.116×10^-6~176.78×10^-6,轻重稀土分溜明显,分溜系数(La/Yb)N为4.79~25.56,δEu为0.05~0.61,为中等铕负异常.4个样品在logδ-logτ图解上全部落入造山带山花岗岩区、Rb⁃Y+Nb图解上落在火山弧花岗岩区、Nb⁃Y图解上全部落入火山弧花岗岩同碰撞花岗岩区;在阿巴嘎旗−扎鲁特旗−巴林右旗一带采集了4个锆石LA⁃MC⁃IC⁃MS同位素,年龄值分别为138.89±0.93 Ma、137.70±1.40 Ma、147.70±1.20 Ma和137.27±0.96 Ma,将满克头鄂博组的时代置于晚侏罗世−早白垩世.

The volcanic rocks in Manketou’ebo Formation,located in the middle east area of Inner Mongolia,are mainly composed of rhyolitic pyroclastic rocks,mixed with intermountain lacustrine facies continental clastic rocks and a small amount of rhyolite or dacite.Sedimentary rocks contain fossils of Estherians(Magumbonia paramecia,Nestoria reticulate,N.xishunjingensis,N.cf.pissoui,Lioestheria sp.),Bivalves(Ferganoconcha curta,F.subcentralis,F.elongate)and vegetation(Ginkgoites sp.,Cladophlebis sp.,Lycopodites sp.,Baiera gracilis,Phoenicopsis sp.).Geochemical study shows that the chemical constitutes of these fossils are SiO271.47%-75.96%,Ai2O312.50%-14.18%,K2O 3.84%-4.98%,Na2O 3.22%-4.14%and ALK 7.84%-9.12%,indicating richness in silicon and alkali.In terms of trace elements,all specimens are characterized by high ratio of Rb,K,Th,U,La and LREE,and lack of Ba,Nb,Sr,P and Ti.Rare earth elementΣREE ranges from 118.71×10^-6 to 189.99×10^-6;significant fractionation of HREE and LREE((La/Yb)N=4.79−25.56)and moderate negative Eu anomalies(δEu=0.05-0.61)are detected.All 4 specimens,on the basis of the logδ-logτdiagram,are classified into orogenic belt mountain granite area.Meanwhile according to the Rb⁃Y+Nb diagram,these specimens belong to volcanic arc granite area,and the Nb⁃Y diagram suggests 4 specimens pertain to the same volcanic arc granite with collision granite area.The ages of 4 zircon LA⁃MC⁃IC⁃MS isotope samples collected from Abagaqi⁃Zarutqi⁃Bahrainyouqi are respectively 138.89±0.93 Ma,137.70±1.40 Ma,147.70±1.20 Ma and 137.27±0.96 Ma,which therefore suggests that the era of Manketou’ebo Formation lies between Late Jurassic and Early Cretaceous.