The Volcanic-Sedimentary Sequences And Tectonic Significance Of The Shaerbuerti Mountains In Northern West Junngar

The tectonic model and evolution of the Early Paleozoic in the northern West Junggar has long been controversial.An in-depth study of the Early Paleozoic volcano-sedimentary sequences in the West Junggar is important for understanding the tectonic evolution of the West Junggar and the accretionary orogeny of the Central Asian orogenic belt.In this paper,the Middle Ordovician to Late Devonian volcanic-sedimentary sequences in the Shaerbuerti Mountains of West Junggar were selected as the object of study.Through detailed field geological investigations,combined with petrographic,paragenetic U-Pb chronology,petrography and other comprehensive analyses,the formation era,material source characteristics,and sedimentary-tectonic background were accurately defined,providing a key basis for the complete recovery of the Early Paleozoic tectonic framework and evolution of the northern West Junggar.The Middle Ordovician-Upper Devonian is widely exposed in the Shaerbuerti Mountains area,and the stratigraphic relationships are integrated contacts,with an overall near EW-NWW trending spreading and synclinorium,which is modified by the late near east-west reverse faults.According to the lithological and sedimentary characteristics,the stratigraphy of the study area can be divided into the Bulongguoer and Shaerbuerti area.The Bulongguoer area is located at the southwest of the Shaerbuerti Mountains,exposing the Upper Ordovician-Lower Silurian,and the two are in integrated contact,and the lithology is mainly conglomerate,sandstone and tuffaceous siltstone from the bottom to the top,with tuff lenses and slickensides at the bottom,and no volcanic interlayer.The sedimentary debris particle size is coarse overall,and the horizontal lamination is not developed,showing slope facies,which is a typical turbidite construction.The Shaerbuerti Mountains exposed continuous Middle Ordovician-Upper Devonian.The Middle Ordovician is dominated by tuffs with tuffaceous siltstone interlayer,which is intruded by early Devonian andesite and is in contact with Hongguleleng ophiolite fault.The Upper Ordovician-Upper Devonian is mainly tuffaceous siltstone and sandstone rhyolite interbeds,containing a small amount of tuff and volcanic rocks,with fine debris size and developed horizontal lamination.The basaltic and andesitic tuffs in the Silurian-Lower Devonian are more abundant,and a few volcanic rocks are exposed in the Lower Devonian in the south.The depositional environment of the Shaerbuerti area gradually changed from Middle Ordovician shelf facies to Silurian shallow Marine facies,with weak current dynamics.After entering the Early Devonian,it transformed into the coastal plain facies,with shallower water bodies,abundant fossils,and obvious laminations and wave marks,indicating its coastal plain facies and coastal carbonate facies.Zircon U-Pb chronology of volcanic rocks,volcanic clastic rocks,and clastic rocks in the Shaerbuerti Mountains area was conducted to precisely define their magmatic-sedimentary age.The maximum deposition age of tuffaceous sandstone of the Lower Silurian Bulong Formation in Bulongguoer area is 443 Ma,which is determined to have formed in the Early Silurian.The maximum sedimentary age of the sandstone previously thought to be the Middle Silurian Shaerbuert Formation in the southwest of the Shaerbuerti Mountains is 438 Ma,indicating that it was actually formed in the Early Silurian.The maximum sedimentary ages of tuffaceous sandstones,sandstones and tuffaceous intercalations previously believed to be Middle Silurian Shaerbuer Formation are 402 Ma,400 Ma and 404 Ma,respectively,indicating that they were actually formed in the Early Devonian.The maximum sedimentary ages of the pebbled sandstone and tuffaceous siltstone in the Kekexiongkuduke Formation of the Upper Silurian are 380 Ma and 377 Ma,respectively,indicating that they were actually formed in the Late Devonian.The zircon U-Pb age of basalt previously believed to be in Lower Carboniferous Heishantou Formation is 397±4 Ma,and the age of andesite tuffs of the intruded Middle Ordovician Bulukeqi Formation is 403±6 Ma,which proves the existence of Early Devonian magmatism in Shalburti Mountain.Sandstones and tuffaceous siltstones from the Lower Silurian Bulong Formation,Lower Devonian Hebukesaier Formations and Upper Devonian Zhulumute Formation show similar whole-rock geochemical characteristics,enriched in light rare earths elements and Sr,depleted in heavy rare earths elements,high field strength elements such as Nb and Ta,with slight anomalies in Eu and weak fractionation of light and heavy rare earths,with(La/Yb)N of 3.77.3.The lower rare earth element content(56.33-108.40ppm)with high Na2O/K2O ratios suggest that the clastic material is mainly from mantle-derived basal rocks of the magmatic arc.The basic volcanic rocks collected from the Lower Devonian Hebukesaier Formation and the andesitic porphyrite intruding into the Middle Ordovician show similar whole rock geochemical characteristics,the whole shows calc alkaline characteristics,the Mg value is 32.4-56.2,(La/Yb)N is 3.3-9.3,enriched in LREE,depleted in HREE and strong anomaly of Nb,Ta,Ti and Sr.May result in mantle melting with fluid metasomatism.The clastic zircon age spectra from the Lower Silurian Bulong Formation,Lower Devonian Hebukesaier Formations show a single peak(peak calculated ages of 452 Ma and 438 Ma,respectively),and the clastic apatite age spectra show a linear relationship indicating a single source,poor clastic rounding,and the peak ages of clastic apatite(450 Ma and 422 Ma,respectively)and clastic zircon in each sample group are very similar,indicating that the transport distance of clastic material after denudation is extremely short;the age spectrum of clastic zircon from the Upper Devonian Zhumulute Formation shows a single-peak feature,and the poor rounding of clastic indicates that the clastic is short-transport.The geochemical characteristics of the clastic rocks,enriched in light rare earths elements and Sr,depleted in heavy rare earths elements,high field strength elements such as Nb and Ta,with slight anomalies in Eu,from the Bulong,Hebukesaier and Zhulumute Formations indicate that the clastic provenance is Boshchekul-Chingz Arc..The overall lithology of the Shaerbuerti Mountains from Middle Ordovician to Late Devonian is tuff,volcanic clastic rocks and a small amount of volcanic interlayer,horizontal lamination is developed,and the depositional environment is stable,while the Middle Ordovician tuffs on the west side of the mountains are intruded by Early Devonian andesite,the Silurian tuff clastic component increases,and the stratification and rhythm are clear.Late Silurian diabase prophyrite and early Devonian basaltic andesite are exposed on the north slope of the mountain area,and early Devonian basaltic rocks are exposed in the southwest of the mountain area.Combining the stable sedimentary environment of each period and the formation age and outcrop location of igneous rocks,it is inferred that the Shaerbuerti Mountains were in the inter-arc basin from the Middle Ordovician to the Late Devonian.Bulonggoer area outcropped turbidite and storm breccia in Upper Ordovician.The debris with large grain size,sharp angles and disordered arrangement indicated that Bulonggoer was located in forearc trench basin from Late Ordovician to Early Silurian.The Ordovician-Devonian volcanicsedimentary strata of the Shaerbuerti Mountains area were formed in the northward subduction environment of the Junggar Ocean.