内蒙古中东部上侏罗统满克头鄂博组划分及时代

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...

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

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 SiO₂ 71.47%-75.96%, Ai₂O₃ 12.50%-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 element ΣREE 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-logτ diagram, 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.