Late Paleozoic Tectonic Relationship Between Alxa And North China Blocks:Constraints From Sedimentology And Provenance

Alxa Block is located at the northwest of North China Block,which is separated by Helan Mountains.For a long time,there are many opinions and disputes about the timing and process of amalgamation of the two.Most previous studies believed that they had been amalgamated together in the Precambrian,but in recent years,it was believed that they might last until the Late Paleozoic or even the Mesozoic.The Ordos Basin in the west of North China Block is a large energy basin.Basins such as Bayanhot and Chaoshui in eastern Alxa Block show hydrocarbon in the Upper Paleozoic and Jurassic strata.Discussing the tectonic relationship between Alxa and western North China Blocks are not only an important basic geological problem,but also an indicator for oil and gas exploration in related basins.On the basis of absorbing the achievements and knowledge of predecessors,this paper studies the Bayanhot Basin in the east of Alxa Block,the northwest margin of Ordos Basin in the west of North China Block,and the Helan Mountains area between them by combining sedimentology and provenance analysis.The sedimentary characteristics and provenance relationships of the Late Paleozoic strata in three areas are mainly compared,and the paleogeography and tectonic features of the Carboniferous-Permian study area are preliminarily restored.The study shows that the Late Paleozoic strata in the study area are widely distributed,mainly in the Carboniferous and Permian,and the Late Carboniferous-Permian strataare comparable.The Lower Carboniferous in Bayanhot Basin is mainly distributed in the southern Zhongwei-Hongshuibao area,and the Upper Carboniferous and the Permian have deposits in the studu area.Silurian to early Carboniferous strata are absent in the northwestern margin of Ordos.Late Carboniferous to Permian strata are widely distributed.Late Carboniferous-Permian strata are sporadically exposed in Helan Mountains,mainly distributed in the north-central part and east of Niushoushan and Daluoshan.The analysis of outcrop and drilling cores shows that the study area mainly accepts marinecontinental transitional facies sedimentary environments such as delta facies,tidal flat facies,coastal facies and barrier island-lagoon facies.The Carboniferous-Permian marginal facies deposits in Helan Mountains are undeveloped.It is inferred that the North China Sea and the Qilian Sea were interconnected during the Early Permian.,and then happened regional regression.Provenance analysis of heavy mineral assemblages,paleocurrent and clastic zircon dating in drilling wells and outcrops in Bayanhot Basin shows that the Alxa Block has provenance information from North China Block in Early Carboniferous,while the provenance compositions of Bayanhot and northwest Ordos in Late Carboniferous-Early Permian are similar,suggesting that the Qilian Sea and North China Sea may converge.Based on the analysis of sedimentology and provenance,the palaeogeographic features of late Paleozoic in the study area are restored.It is considered that the study area underwent the sedimentary evolution from transgression to regression from early Carboniferous to Permian.In the early Carboniferous,the paleotopography of the study area was high in the East and low in the west.The Bayanhot Basin took the lead in accepting sedimentation.The western part of the North China Block was located in the uplift area and provided material resources for the Alxa block.During the Late Carboniferous,the Bayanhot and Ordos Basins subsided as a whole.and the Qilian Sea intruded northward and eastward on a large scale.In the late Late Carboniferous,the Qilian Sea and the North China Sea began to penetrate and form a restricted epicontinental sea environment in the northern part of the Ordos Basin.During the Early Permian Taiyuan Formation,the Xingmeng trough in the north and the Qinling trough in the South subducted and collided with the North China Block,resulting in the continuous subsidence of the Ordos Basin and the further expansion of the transgression area.The Qilian Sea in the West converged with the North China Sea in the east.By analyzing the Late Paleozoic the sedimentology and provenance of the Bayanhot Basin,Helan Mountains and the northwestern margin of Ordos,it can be seen that the Alxa Block and the western North China Block had a close sedimentary relationship and underwent a general synchronous sedimentary-tectonic evolution,indicating that there was no oceanic separation between them,and the combination of the two should have been completed before the Late Paleozoic.