The Paleozoic Tectonic Attribute Of The Southern Alxa Block

The Alxa Block is located among the Central Asian Orogen Blet?CAOB?to the north,the North China Craton?NCC?to the east,and the North Qilian Orogenic Belt?NQOB?to the south,The geological evolution of the Alxa Block has attracted a lot of attention due to its special tectonic position,and a certain amount of data has been accumulated.However,there are still some different views on the tectonic attributes of the Alxa Block during the Paleozoic.The tectonic affinity and the tectonic setting of the southern Alxa Block in the early Paleozoic are debatable.Moreover,the relationship between the Alxa Block,the North Qilian Orogen Belt and the western North China Craton,as well as the tectonic evolution of the Alxa Block during the Paleozoic remains debate.In the late Paleozoic,the Alxa Block was involved into the CAOB during the closure of the Paleo-Asian Ocean,while the role of the Alxa Block in the formation of the CAOB and the subsequent intraplate deformation are relatively ignored.This study conducts the research of detrital zircon ages on the early Palaeozoic strata in the southern Alxa Block to investigate the tectonic affinity of the Alxa Block in the early Paleozoic.Besides,we collect the detrital zircon ages of the Paleozoic strata from the southern Alxa Block and other tectonic units around it to restore the tectonic setting of the southern Alxa Block and its vicinity during the Paleozoic using the method of Cawood et al.?2012?.Forthermore,combined with the research of the structural deformation and the regional geological data,we will discuss the role of the Alxa Block in the formation of the CAOB and the subsequent intracontinental evolution.Paleozoic marine clastic rocks exposed along the south margin of the Alxa Block was termed the“Dahuangshan Formation”.Characteristics of the sedimentary structures,palaeocurrents,detrital zircon age distributions,and cumulative probability curves of crystallization ages for detrital zircon grains relative to the depositional ages indicate that the early Palaeozoic strata of the southern Alxa Block were sourced predominantly from Neopeotrrozoic orogenic belts in the eastern Gondwana continent,instead of the Alxa Block to the north,the North China Craton to the east and the North Qilian Orogenic Belts to the south.During the early Palaeozoic,the Alxa Block was situated to the northwest of the eastern Gondwana with its long axis trending north northwest-south southeast,and it belonged to a passive continental margin dipping to the north northwest,with the eastern margin located closer to the eastern Gondwana.The Hexi Corridor is part of the Alxa Block and a part of the same slope basin of the eastern Gondwana during the early Palaeozoic.The southern Alxa Block has experienced a complicated tectonic evolution during the Palaeozoic.The cumulative probability curves of crystallization ages for detrital zircon grains relative to the depositional ages obtained from the Palaeozoic strata of the southern Alxa Block,the North Qilian Orogen and the North China Craton demonstrate a continuous evolution of the tectonic setting of the southern Alxa Block.During the early Palaeozoic,the southern Alxa Block was located at a passive continental margin.At the end of the early Palaeozoic,the southern Alxa Block was a foreland basin of the North Qilian Orogenic Belt and subsquently underwent regional extension during the Late Devonian.During the late Palaeozoic,the southern Alxa Block was located within a back-arc extensional setting resulted from the northward subduction of the Paleo-Tethys oceanic plate under the North China Craton.During the late Palaezoic,a ductile shear zone formed along the south margin of the Alxa Block with the formation of the CAOB,and this ductile shear zone trends northwest-southeast with approximately 500km in length and about 0.15-2.5 km in width.The nearly vertical mylonitic foliation and the sub-horizontal mineral stretching lineation indicate it is a strike-slip ductile shear zone.Various kinematic indicators suggest that this ductile shear zone is a dextral ductile shear zone.The zircon U-Pb ages of the granite deformed by the shear zone and the ⁴⁰Ar/³⁹Ar age of the neo-muscovites from mylnite indicate that the strike-slip shear occurred during the late Permian?ca.253 Ma?.During the late Palaeozoic,a large number of coeval ductile shear zones developed along the south margin of the CAOB,these ductile shear zones accommodated the eastward migration of the internal orogenic segments,which resulted in the formation of the southern Alxa dextral ductile shear zone.And the formation of this ductile shear zone also indicates that the final closure of the Paleo-Asian Ocean in the northern Alxa Block was earlier than 253 Ma.