Late Paleozoic-Triassic Tectonic Evolution Of The Langshan Area In The Northeastern Margin Of The Alxa Block

Langshan area is located in the northeast margin of Alxa block in Inner Mongolia.Because of its special geography,geological location and resources enrichments,it has become one of the hot research areas in China.The Alxa block is located in the south of the central Asian orogenic belt.Previous studies on the tectonic evolution of the Alxa block mainly focus on the Enguewusu and Chaganchulu areas locating in the northern margin of the Alxa block.However,the studies on the Langshan area lying on the northeastern margin of the Alxa block are relatively weak.Previous studies have different views on the late Paleozoic structural evolution of the Langshan area.Regional tectonic environment in the Langshan region in Inner Mongolia will greatly improve our understanding of the evolution history of the Alxa block and central Asian orogenic belt in the late Paleozoic.Therefore,the pillow basalt in Langshan area has been studied.In order to accurately understand the genesis mechanism of late Paleozoic pillow basalts in the langshan area,using field investigation,petrology,geochemistry,geochronology and other research methods,we obtain the following results:(1)basalt LA-ICP-MS U-Pb dating results show that a large number of basalt zircons are trapped in the basement.According to the zircon characteristics and the age of granite intruding into the basalt,the basalt age is about 254 Ma?252 Ma,belonging to the late Permian.(2)Pillow basalt is characterized by high SiO2(51.75?54.74%),high Al2O3(13.57%-15.86%),low alkali and potassium(Na2O/K20=2.11?11.82).Basalt are depleted in Nb,Ta,Ti,P but enriched in elements such as Rb,Ba,U.REE variations are 74.28?91.29 ppm,which is obviously higher than that of N-MORB and E-MORB,but lower than that of OIB.LREE was obviously enriched but HREE was relatively deficient.LREE and IREE fractionation was relatively obvious(LaN/YbN= 5.03?6.95).Eu negative anomaly indicates that basalt experienced the separation and crystallization of plagioclase during diagenesis.Pillow basalt shows characteristics of island arc basaltAccording to the characteristics of geochemistry and chronology,we believe that the basalts in Langshan area belong to continental margin arc basalts and formed in the active continental margin arc environment.The Langshan area was an active continental margin arc formed by the subduction of the paleoasian ocean from late Permian to early Triassic.This indicate that the northern margin of the Alxa block was a continental margin arc environment in the late Permian.This provides new evidence for studying the late Paleozoic tectonic background of the northeastern margin of the Alxa block,the closure time of the Paleo-Asian ocean and the late paleo-paleozoic tectonic evolution of the central Asian orogenic belt.