Late Paleozoic Tectonic Evolution In The Northern Margin Of The Alxa Block

The north margin of the Alxa Block occupies a key position in the southern margin of the Central Asian Orogenic Belt(CAOB),bridging the Beishan-Tianshan orogenic belt to the west and Xing ' an-Mongolia orogenic belt to the east and records the late Paleozoic subduction and closure processes of the Paleo ? Asian Ocean(PAO).Intensive structural deformation and extensive magmatic and volcanic activities were documented in the Nuoergong-Langshan Belt(NLB),Alxa Block.Based on 1:50 000 structural geological mapping research,this paper presents systematic petrological and geochronological data on magmatic rocks,volcanic rocks,sedimentary rocks,coupled with the deformed and undeformed veins intruding the tectonic belt,to constrain the formation ages of these rocks and analyze provenance and tectonic setting of the sedimentary rocks.Besides,Zircon U-Pb dating was used for defining structural deformation ages.On this basis,the major and trace elements and Sr-Nd-Hf isotopic analysis of the Permian pluton-volcanic rock in the NLB were employed to determine their origin,petrogenesis,and tectonic setting.We also discuss the mechanism that triggered magmatic flare-up in the continental arcs and the tectonic affinity and evolution of the Alxa block in the late Paleozoic.The field geological mapping research shows that the overturned isoclinal folds with NE-and SW trending fold hinges and thrust faults with SE or NW dips are widely developed in the Langshan.Zircon U-Pb age of the folded veins showing the features of syn-tectonic deformation as well as the age data from granitic protolith within fault zone supported that the fold?thrust belt was formed in the late stage of the early Permian.In addition,the granitic vein intruding the Neoproterozoic Langshan Group yiels a zircon U-Pb age of 327±5 Ma,indicating crustal thickening also occurring before the Early Carboniferous.A large-scale dextral ductile shear deformation was identified in the Langshan for the first time.Zircon U ? Pb ages for the granitic mylonites and the undeformed intruding diorite vein in the dextral shear zone constrain the deformation age between 272 and 249 Ma.The dextral ductile shear zone is also exposed in the Tamusu on the northern margin of Alxa,and connects the BeishanTianshan to the west and the northern margin of North China to the east,forming a transcurrent dextral ductile shear zone in the southern margin of the CAOB.This large?scale dextral strike?slip throughout the southern CAOB probably resulted in eastward lateral extrusion of the thickened crust relative to the Tarim and North China cratons in the final accretion and convergence processes.Zircon U-Pb dating of intermediate-felsic volcanic rocks(andesite,dacite,and rhyolite)in the NLB reveals that the eruption age of these volcanic rocks was between272 Ma and 265 Ma.Geochemical analysis reveals that these volcanic rocks have subduction-related arc geochemical affinities,which confirm the existence of an earlymiddle Permian continental volcanic arc in the NLB.The Dahongshan Formation unconformably covers the Palaeoproterozoic Diebusige Group and Neoproterozoic Langshan Group.Detrital zircon U-Pb dating combining with the geological facts that late Permian basalts unconformably covers the Dahongshan Formation confirm that the Dahongshan Formation was deposited in the Middle and Late Permian.The Dahongshan Formation is located in the frontal sector of the fold?thrust belt and unconformably overlies the Precambrian basement,indicating a wedge?top basin in the retroarc foreland,where pyroclastic material with Carboniferous to Permian ages from the continental arc and sediments eroded from the Alxa Precambrian basements were deposited.Zircon U-Pb ages of intermediate dikes and monzogranite in the Bayannuoergong batholith,as well as diorite in the central Langshan,are clustered into a short time interval ca.279-275 Ma.Zircon U-Pb dating reveals that these dikes and host granitoids are nearly contemporaneous within analytical error,indicating that these intermediate dikes are syn-plutonic.Intermediate dikes are derived from the rapid ascent and chilled quickly of small batches of magma,and thus more likely retain their source signatures.Major and trace elements and Sr-Nd isotopic data show the intermediate dikes and host granitoids are cogenetic,but the host granitoids have undergone sufficient magma mixing and homogenization.Zircon retains the original isotopic signatures during the magma evolution process.Zircon Hf isotopes from the intermediate dikes coupled with the Zircon oxygen isotopes from the Bayannuoergong batholith,we argue that these rocks were probably generated by mixing between felsic components from the ancient continental materials and mafic magma.A large volume of the early Permian magmatic rock outcrops in the NLB was mainly formed in a short time interval of 285-270 Ma,indicting an early Permian magmatic flare-up.Compiled data together with deformation age data show that the magmatic flare-up events were simultaneous with crustal thickening in the southern CAOB,implying that the flare-ups in the continental arc are typically related to the crustal thickening.Based on statistics of the late Paleozoic geochronological and geochemical data on the northern margin of the Alxa block,combined with the temporal and spatial distribution of the late Paleozoic magmatic rocks indicate a late Paleozoic retreating accretionary orogen process of the PAO.We also proposed a synthetical late Paleozoic tectonic scenario.By comparing the magmatic,metamorphic,and deformation events on the northern margin of the Alxa Block and North China Block since the Paleozoic,combined with the results of recent stratigraphic comparisons,we argue that the Alxa Block was part of the North China Block at least before the Paleozoic.