Spatio-temporal structure of Beishan orogenic belt and evolution of Paleo-Asian Ocean, Inner Mongolia

The Beishan orogenic belt is composed of Late Paleozoic Hongshishan-Baiheshan ophiolite mélange belt, Early Paleozoic Yueyashan-Xichangjing ophiolite mélange belt and late Paleozoic Zhangfangshan-Yushishan ophiolite mélange belt from north to south in Inner Mongolia.The "Shibanjing-Xiaohuangshan ophiolite belt" defined by previous researchers is actually an Early Paleozoic trans-crustal fault zone, across which there are intermediate-mafic rocks and minor ultramafic rocks. At the end of the Silurian, intense ductile shear deformation was superimposed upon it.The two ophiolite belts in the north reveal the two-stage evolution history of Beishan orogenic belt:The Yueyashan-Xichangjing ophiolite mélanges belt was formed from 530 Ma to 520 Ma, and there exist many intact ophiolite oceanic crust fragments along the belt(the oceanic crust structure has the characteristics of becoming younger from south to north), which, together with the Early Paleozoic Gonpoquan magmatic arc(the maturity of island arc increased from south to north), indicates the process of northward subduction of the Beishan Ocean, which included the initial subduction of 490 Ma, the peak period of subduction ranging from 450 Ma to 440 Ma, and the syn-collision stage ranging from 430 Ma to 420 Ma.The 400 Ma bimodal magmatic rock assemblage indicates the extinction of Beishan Ocean and the beginning of a new tectonic cycle.The Hongshishan-Baiheshanophiolite mélange belt is a SSZ-type ophiolite developed on the basis of the Quershan-Yuanbaoshan island arc.The formation time of the back arc cracked oceanic crust approached the earliest developed island arc magmatism in the south, which happened from 340 Ma to 320 Ma.The period of 310~290 Ma subduction caused a lot of magmatic activities that formed the Baishan magmatic arc.The existence of gabbro and granite bimodal intrusions at the end of Early Permian(275 Ma)and the Early-Middle Permian angular unconformity events of the Shuangbaotang Formation commonly reflect the closure of the Hongshishan Ocean basin.The strike-slip and thrust nappe structures since Mesozoic have transformed the Paleozoic tectonic framework, resulting in the northward left-lateral slipping of Hongshishan-Baiheshanophiolite mélange belt for more than 10 kilometers and the thrusting of Middle-Upper Proterozoic strata on the Early Paleozoic strata.The authors determined the spatial and temporal structure of the Beishan orogenic belt, which is helpful to the further research on the orogenesis and crustal accretion process of the Central Asian Orogenic Belt, and also provides research basis for the exploration of the Late Paleozoic hydrocarbon resources in the Yin'e basin.