Disappearance of Sea Floor of the Paleo-asian Ocean: Geological Evidence from the Dong Ujimqin, Inner Mongolia, China

It is well accepted that the Central Asian Orogenic Belt (CAOB) formed by the Paleozoic collision of the Siberia and the North China plates and the simultaneous closure of the Paleo-Asian Sea, but the location of the suture zone is still very debatable. Three proposed hypotheses of where the suture zone is are: Solonker-Sonid Zuoqi-Hegenshan, Ophiolite belt along Solonker-Xar Moron River, and the Main Mongolian Lineament in Mongolia. Our mapping, geochronology and geochemistry work in Dong Ujimqin (about 240 km to the NE of Hegan Shan) support the third hypothesis the best. Paleozoic stratigraphy in the research area was studied in detail. From Ordovician to middle Permian, local stratigraphy from bottom to top includes: Ordovician echinodermmeta fossil bearing crystalline limestone, sandstone and argillite; Middle Devonian coarse grained sandstone and argillite; Lower Middle Permian interlayerred sandstone and siltstone. There is an angular unconformity between the Middle Devonian and the overlying Permian strata. The Permian strata contain fossils of Cathaysia flora which include Cladophlebis sp.1,Cladophlebis sp. 2, Pterophyllum daihoense Kawasaki, Calamites suckowiiBrongniart, Sphenopteris sp., Cladophlebis nystroemii Halle, Cladophlebis ozakii Yabe et Oishi, Emplectopteris triangularis Halle, Lepidophylloides sp., Mariopteris hallei (Stockmans et Mathieu), Cladophlebis manchuica (Kawasaki). Structurally, Ordovian strata thrust over the Middle Devonian from NW to SE, and the Middle Devonian thrust over Upper Devonian from NW to SE too. The strike of the axial plane of the folds associated with the thrusts is NE. Two Carboniferous magmatic belts were discovered in the research area. U-Pb zircon geochronology using LA-ICP-MS gave ages of 325-326Ma and 298-311Ma for the North and South belts respectively. The magamatic activities moved from NW to SE. Whole rock, trace element and REE geochemistry tell us that all these magmatic rocks are of syn-collisional characteristics. The discovery of the Cathaysia flora fossils, the NW to SE thrust faults, the syn-collisional magmatic rocks plus the shift of magmatic activities from NW to SE all indicate that the sea floor of the Paleo-Asian Ocean must located somewhere to the north of our research area, which could be the proposed Main Mongolian Lineament area.