| There have been controversies about the closure method and time limit of the Late Paleozoic ocean basin in northern West Junggar.The Tarbagtay Mountain in West Junggar is a key area for studying the tectonic evolution of the Late Paleozoic in West Junggar,but the predecessors mostly focused on the study of magmatism in this area.In this paper,based on the systematic study of the petrology,sedimentology,zircon U-Pb chronology,whole-rock geochemistry from the Carboniferous-Permian volcanic-sedimentary strata in the eastern section of the Tarbagtay Mountain,we studied to provide the structural evolution of the area Important evidence of sedimentary strata.Through detailed field surveys and field surveys in Wushishui and South Hujier areas in the eastern part of the Tarbagtay Mountain,the typical volcanic-sedimentary strata exposed in this area were discovered,and three main sedimentary units were identified:Early Carboniferous delta front facies conglomerate-sandstone intercalated with tuffaceous siltstone-tuff combination,with the maximum deposition age of 345 ± 12 Ma,good sorting,single source of clastic material,reflecting the depositional characteristics of the near source within the arc;The andesitic tuff in South Hujier volcano-sedimentary strata is calc-alkaline arc magma,with an age of 335 ± 3 Ma,and the maximum deposition age of the overlying sandstone is 320 ± 7 Ma;Late Carboniferous continental shelf sea-terrestrial interfacies tuffaceous mudstone-siltstone interbed assemblage,which the maximum deposition age is 315 ± 5 Ma,with the addition of Silurian(430 Ma)clastic materials,reflects the superimposition of near-source sedimentation and long-distance transport;Early Permian inland continental facies tuffaceous sedimentary unit is dominated by a combination of tuffaceous siltstone and argillaceous tuff.The maximum sedimentation age is 275 ± 3 Ma,with poor sortability,and the age spectrum of detrital zircon presents multiple age groups(peak age is 356 Ma),reflecting the further superposition of multiple provenances.The geochemical characteristics of the Carboniferous-Permian sedimentary rock samples indicate that they belong to a continental arc sedimentary environment,also indicating that they were formed in situ accumulation of continental magmatic arc volcanic clastics.The Late Carboniferous sedimentary rock samples have high La/Yb ratio(5.83～12.41),REE content(78.0～113.8 ppm)and low Cr/Zr ratio(0.08～0.22),implying that the material is mainly derived from the upper crust.Felsic mother rock.From the Late Devonian to the Early Carboniferous,Wushishui area was located on the ancient suture zone in the southern part of the Tarbagtay Mountain.The sea water has not yet receded,and the magmatic activity has been violent,forming a series of volcanic sedimentary structures,mainly accepting Zharma-Saur near-source deposition.With the closure of the Irtysh-Zaysan Ocean in the Late Carboniferous,the entire West Junggar region entered into post-collision extensional environment.The sediment source was still dominated by near-source deposits of Zharma-Saur arc,forming a set of The interbedded sea-terrestrial tuffaceous mudstone-siltstone reflects the continuous change of sea depth.At the same time,the thinning of the crust in the extensional environment brings the underlying residual ophiolite oceanic crust to the near surface.In the Early Permian,seawater basically retreated,and the entire West Junggar region entered into continental orogenic process.The ’north-south compression began to become intense,and a series of large anticlines and syncline structures appeared,and even small faults appeared on both sides of the fold.At the same time,large-scale uplifting activities made the stratigraphic age chaotic and different in occurrence.The north-south stratigraphic age showed an old-new-old distribution,forming today’s stratigraphic pattern. |
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