Two Stratigraphic Concerns on the Central and Northern Tianshan Tectonic Complex,Northwest China

Two tectonostratigraphic concerns on the Late Paleozoic Tianshan tectonic complex were addressed in this study:(1) the stratigraphic succession of the Bayingou ophilite melange and(2) the timing of tectonic accretion of the Siberian craton in the Aiweiergou area by integrating the relevant data of stratigraphy,palaeontology,tectonopaleogeography and tectonopaleobiogeography.In the Bayinggou area,the detailed paleontological studies have rejected the presence of the brachiopod Gigantoproductus fauna of Early Carboniferous age in the Anjihai Formation.In contrast,the brachiopod assemblage suggests a Late Devonian age for the Anhinhai Formation.Integration of fossil assemblages and paleogeographical and paleobiogeographical settings indicates that the Bayingou area belongs to the eastern Junggar tectonic complex rather than the central-south Tianshan complex.The latter is characterized by the presence of Gigantoproductus fauna during the Early Carboniferous.Thus,the Bayinggou ophiolite m lange is late Devonian in age.The top of the stratal package of the ophiolite m lange should be re-assigned to the Taoxigou Group rather than the Keguqingshan Formation in view of lithological features and tectonostratigraphic provinces.The former was established to accommodate the stratal package of volcanics,conglomerates and mudstone in the Junggar block,while the latter is poorly defined for the conglomerates exposed in the western Tianshan areas.Accordingly,the Bayinggou ophiolite m lange succession comprises the late Devonian Anjihai Formation,the Carboniferous Bayinggou and Shadawang Formations,and the Taoxigou Group of Early Permian age.Xiao et al.(2008) figured an outcrop section showing a distinct angular unconformity between the Permian turbidite and the Middle-Upper Triassic red beds at the Aiweiergou area,about 110 km south of Urumqi,the capital city of Xinjiang Province,northwest China.These authors considered that this unconformity indicates the closure of the Tianshan Ocean and final tectonic accretion might have taken place between the latest Permian and the Triassic.However,the six-decade stratigraphic studies reveal that the Upper Permian in the Aiweiergou area does not represent a turbidite sequence,but comprises the terrestrial lake facies sandstone and mudstone of the Loucaogou Formation,yielding abundant non-marine fossils of plants,bivalves and gastropods.The overlying red beds comprise the conglomerate and sandstone and belong to the Kelamayi Formation.The unconformity between these two units is widely distributed in southern margins of the Junggar Basin,indicating a further tectonic uplift event during the Late Permian.The Aiweiergou area,however,has uplifted and became terrestrial settings during the Early Permian.Thus,Xiao et al.'s(2008) observation on the Permian and Triassic sediments and inference at the Aiweiergou section are incorrect.In fact,the molasse-type sediments of the Taoxigou Group are conspicuous at the basal part of the Permian successions at the Aiweiergou area.This unit is constrained as an Early Permian age by the macrofloral assemblage.The molasse succession therefore can be interpreted as the result of tectonic uplift due to the closure of the south Tianshan Ocean prior to the Early Permian.This tectonic inference agrees well with faunal assemblages from both sides of the South Tianshan complex and the previous tectonic studies at that region.Here,we infer that the closure of the South Tienshan Ocean and tectonic accretion of the Tianshan blocks might have taken place during the Late Carboniferous to Early Permian times.