Late Paleozoic Tectonic Transition And Evolution Of Paleo-oceanic Basin In Central West Junggar

The Central Asian orogenic belt?CAOB?is the largest Phanerozoic accretionary orogen on Earth and records the multiple accretion and convergence of numerous terranes within the Paleo-Asian Ocean.The West Junggar orogen occupies a crucial location in the southwestern CAOB,providing an ideal opportunity for studying the accretionary orogenesis and Phanerozoic continental growth of the CAOB.Ophiolitic mélanges represent fundamental litho-tectonic units of both accretionary and collisional orogens.They contain important evidence about formation,subduction-accretion,and final closure processes of paleo-oceans,and play a significant role in tectonic reconstruction of orogens.In this study,we conducted a comprehensive study on the Darbut and Karamay ophiolitic mélanges and adjacent turbidites in the western Karamay area of central West Junggar,including detailed litho-structural mapping,field structural analysis,geochemical and geophysical analyses,and detrital zircon U-Pb dating.Combined with previously published data,we attempt to discuss the nature of paleo-oceanic basin,reconstruct the paleotectonic regime,and constrain the process of ocean-continent transition during the Late Paleozoic.These results will also provide important insights into understanding the accretion process of the western CAOB.The Darbut and Karamay ophiolitic mélanges display similar geological features;they are in fault contact with the Carboniferous turbidites and strike NE.The ophiolitic mélanges have a relatively complete ophiolitic rocks.However,the ophiolite sequence has been totally dismembered.Serpentinized peridotite,gabbro,rodingite,pillow basalt,chert,limestone and sandstone blocks with different sizes are dispersed within an extensively sheared serpentinite matrix,displaying typical block-in-matrix fabrics.Geochemically,the mafic rocks in both the Darbut and Karamay ophiolitic mélanges are divided into two groups:group 1 consists of gabbros and tholeiitic basalts showing both MORB and arc-like geochemical affinities;and group 2 is characterized by alkaline basalts with OIB-like characteristics.Geochemical data suggest that group 1 mafic rocks are likely to have formed in a back-arc ocean basin and were derived from a depleted mantle source that had been metasomatized by slab-derived fluids.The origin of group 2mafic rocks is attributed to seamounts related to hotspots.The cherts in the mélanges appear to have formed in a restricted oceanic basin,showing a transitional depositional setting between a deep marine pelagic environment and a shallow continental margin environment.The Carboniferous strata in the western Karamay area represent a coherent sequence of thick abyssal-bathyal volcaniclastic turbidites deposited in a progressively shrinking remnant ocean basin environment,and their basement consists of trapped oceanic crust or ophiolitic mélanges.Detrital zircon U-Pb data suggest that the Carboniferous turbidites include,from bottom to top,Xibeikulasi,Baogutu and Tailegula formations,which were deposited from the Visean to Moscovian.Folds are widespread in the Carboniferous strata,and two phases of folds,F₁ and F₂,are recognized.Map-scale F₁ folds usually display sub-vertical axial planes with numerous smaller parasitic folds;F₂ folds refolded the F₁folds and are dominated by S-shaped drag folds with vertical fold axes.Detailed structural analysis show that the different litho-structural units across the western Karamay area record three stages of deformation?D₁-D₃?.D₁ deformation is only locally preserved in the Darbut ophiolitic mélanges;it is characterized by thrust-imbricated faults,which likely resulted from the subduction and accretion of paleo-oceanic crust.D₂ deformation consists of NE-SW-striking sub-vertical right-lateral shearing in both the Darbut and Karamay ophiolitic mélanges.Kinematic indicators,such as S-C structures,asymmetric blocks,slickenlines and steps,and rotated porphyroclasts,are well developed within the two ophiolitic mélanges.D₃ deformation is characterized mainly by NE-SW-striking left-lateral strike-slip faulting attributed to post-orogenic evolution from the Permian.It cross-cut all litho-structural within the western Karamay area and overprints the early structural patterns.Our structural observations suggest that the Darbut and Karamay ophiolitic mélanges were extruded into the Carboniferous strata through the D₂ right-lateral shear zones accompanied by the closure of the remnant ocean basin in the Late Carboniferous,and they cannot be regarded as subduction suture zones.Based on the structural,geochemical,geochronological,and geophysical data presented in this study,we propose a tectonic model showing Late Paleozoic evolution of paleo-oceanic basin and tectonic transition in central West Junggar.?a?Devonian back-arc basin.A back-arc ocean basin opened in the western Karamay area in response to a northwest-dipping intra-oceanic subduction system;contemporaneous hotspot-derived OIBs erupted in this basin.?b?Carboniferous remnant oceanic basin.The back-arc basin began to shrink as oceanic crust subducted in the Early Carboniferous and evolved to a remnant oceanic basin because of the bending of the Kazakhstan orocline.Numerous volcaniclastic materials were deposited in this remnant oceanic basin,forming the thick volcaniclastic turbidites.?c?Late Carboniferous ocean-continent transition.The remnant oceanic basin in central West Junggar closed accompanied by extensively folds in the Carboniferous strata in response to regional orogenic contraction,resulted from the Late Carboniferous paleo-oceanic closure and collision events in the southwestern CAOB.Meanwhile,the remnant oceanic crust beneath the Carboniferous strata were dismembered by the right-lateral shear zones,leading to the diapiric emplacement of the Darbut and Karamay ophiolitic mélanges.?d?Latest Carboniferous to Permian intra-continental evolution.Widespread and intensive post-collision magmatism intruded the whole West Junggar orogen,indicative of an intra-continental extension setting,followed by large-scale left-lateral strike-slip faulting.The transcurrent deformation overprinted the early structural patterns and directly influenced the final configuration of the West Junggar orogen.