The Late Paleozoic Extending Processes Of Xing'an-Mongolia Orogenic Belt(XMOB)

The extensional tectonic regime of Xing'an-Mongolia Orogenic Belt(XMOB)take place in late Paleozoic.In other words,XMOB transferred from early Paleozoic subduction and collisional orogen to strongly extensional regime after the end of early Paleozoic.This process formed the special tectonics regime,magmatism(e.g.,bimodal igneous rocks,A-type granites)and sedimentary basins(e.g.,Middle-Late Devonian molasses basin,Carboniferous continental and epeiric sea basin,Early-Middle Permian continental rift basin)in central Inner Mongolia,NE China.Named Xing'an-Mongolia Intracontinent Orogenic Belt(XMIOB).In this study,sedimentary facies analysis,zircon U-Pb dating and Hf isotopes analysis were combined to indicate the,sedimentary sequences,environment and provenance of the Carboniferous-Permian sedimentary basin and further reveal the late Paleozoic evolutionary history of the XMOB.Our new LA-MC-ICP-MS zircon U-Pb dating shows that the minimum ages of magmatic zircons from three tuff samples at the bottom of the basin were 298 Ma,302 Ma and 303 Ma,respectively,implying that the Benbatu basin was formed during 303?298 Ma,i.e.,late carboniferous-early Permian.According to the regional stratigraphic division,we suggest the formation should be a part of the Gegenaobao Formation.The estimated sedimentary rate is similar to that of the extensional rift basin.Additionally,zircon ?Hf(t)and TDM2 of tuff samples in Carboniferous-Permian sedimentary basin are ranging from 3.06?+14.13,and 461?1486Ma,respectively.These data suggest the origin from depleted mantle or juvenile crust.By contrast,the captured zircons,show lower ?Hf(t)values(-4.83?+3.63)and ancient TDM2(1490?2298 Ma),implying the major contribution of relatively ancient materials in the source.,In the south of basin,the detrital zircon ?Hf(t)and TDM2 for the sandstone(404?462 Ma)in the Niqiuhe Formation are-4.83?+3.63 and 1490?2298 Ma,respectively.They share the same Hf isotope characteristics with the coeval captured zircons in the above tuff samples,indicating the Carboniferous-Permian volcanic-sedimentary rocks in the basin were most likely originated on the Devonian basement(i.e.,Niqiuhe Formation).Furthermore,the increase of zircon ?Hf(t)values with the ages ranging from ?400 to 300 Ma,suggest the increasing involvement of juvenile crust in late Paleozoic,which was probably caused by the progressively extensional process.Taken together with the previously regional researches,we can conlude that the continuous extension ocurred in the XMOB during the late Paleozoic,and the evolutionary history can be as least divided into four stages: 1)the formation of Niuqiuhe molasse formation;2)the generation of Erenhot-Hegenshan basin along the northern part of the XMOB in the Carboniferous,formed a series of ultramafic-mafic and felsic igneous rocks;3)the late Carboniferous Erenhot-Hegenshan basin developed calstic and carbonate rocks in Benbatu and Amushan formations,which represented stable littoral and neritic marine environment;4)the Late Carboniferous-Early Permian rift valley basin formed and deposed massive black mudstone-sandstone-gravel stripe sandstone.