Late Carboniferous - Early Permian Volcano-sedimentary Sequence And Its Deformation In Hanwula Area,Inner Mongolia

The Central Asian Orogenic Belt(CAOB),which is bounded by the Siberian,Baltic,Tarim and North China cratons,is one of the largest Phanerozoic accretionary orogenic belt,resulting from the oceanic plate subduction and closure of the Paleo-Asian Ocean(PAO).The final closure of the PAO occurred along the southern CAOB with a Tianshan-Beishan suture in the west and a Solonker suture in the east.The Solonker suture,which is situated to the northern margin of North China Craton,gaves a criptic nature with a NW-SE-550-km width,multiple accretionary processes,and a “soft collsion”.Due to its criptic nature,the tectonic evolution of Solonker suture is still controversial.One key point is whether the PAO existed or not during the Devonian to Permian.Addtionally,few reasearches focous on the collsion-related deformation,and there is only limited knowledge regarding the mechanism of “soft collsion” deformation.The Late Carboniferous-Permian sedimentary sequence widely occurs across the Solonker suture and records the late evolution of the Solonker suture.Here we conducted an integrated study of the Late Carboniferous – Early Permian volcano-sedimentary sequence in the Hanwula area,Inner Mongolia,including the sedimentological features,depositional age,and detrital zircon populations of the lithostratigraphy,and stages,ages and styles of structural deformation.Intergating our results and other data,we discuss whether the PAO existed or not during the Devonian to Permian,and discuss the collsion-related deformation and the mechanism of soft collsion deformation.The results are showed as follows.1.The Late Carboniferous-Early Permian sequence in the Hanwula area includes the Shoushangou and Dashizhai formations.The Shoushangou Formation was deposited in ca.320-280 Ma,overlain by vocanic rocks of the Dashizhai Formation with ages of ca.280-274 Ma.Its stratigraphic reconstruction shows a coarsening-upward marine sequence,which is characterized by sandstone turbidites with a Bo?ma sequence and the numerous tuff involvements in the lower strata,showing a deep marine environment and synchronous volcanism.The 330 to 290 Ma detrital ages almost serve as the entire age component of this formation,indicating a single source.These ages are consistent with the depositional age(320-280 Ma),which suggests a synchronous volcanic source.2.Along with the reported data,detrital zircon ages from the Carboniferous-Early Permian strata in the northern side of the Solonker suture includes the Greenville-age,with the absence of the Archean to Paleoproterozoic basement ages of the North China Craton,indicating that the Carboniferous-Early Permian strata could not accept the detritus from the North China Craton.Together with the flysh deposition documented in the Hanwula area in this study and the reported synchronous arc-related volcanic rocks,these clues indicate that the PAO existed during the Carboniferous-Early Permian,and an active continental margin with a northward-dipping subduction during this period in the north side of the PAO.3.Integrating published data and our results,with a gradually deepening water depth and a geographic variation,detrital zircon age spectra from the Late Carboniferous-Early Permian sequence show distinct differences along the Hengenshan-Hanwula,and a marked shift of multiple sources to a single source occurred during the Late Carboniferous-Early Permian.We further attribute such a source variation to the spreading of a marine back-arc basin after the amalgamation of the Uliastai Active Continental Margin with the Northern Accretionary Orogen.The Shoushangou Formation was the direct depositional response to such a back-arc basin in the north side of the Solonker suture.4.Structural analyses reaveal that the Late Carboniferous-Early Permian volcano-sedimentary sequence in Hanwula area underwent two stages of deformation(D1 and D2)during the period of Middle Permian to Middle Trassic.The D1 stage produced the NE-striking flexural folds and thrusts,indicative of a NW-SE contraction,with the deformation age at ca.274-240 Ma.The D2 stage developed the normal faults and the metamorphic core complex,indicative of an extentional event,with the deformation age of the metamorphic core complex at ca.235-229 Ma.5.Intergrating our results and other structural data,both D1 and D2 stages of deformation widely occurred across the Solonker suture,resulting from a collsional contracional event of the final colosure of the PAO and a post-collsional extensional event,respectively.There exist obvious disctinct structural lineations between the D1 stage and the early W-E to NEE-striking accretionary tectonic units,suggesting an oblique collision.The temporal and spatial distribution of conllsional deformation and high Sr/Y granites indicate that the crustal shortening with a more than 250-km width and thickening with a thickness of ca.40-50 km widely occurred in the accretionary tectonic units,not restricted to the narrow and linear suturing zone along the Solonker-Xar Moron River.6.The thermal-soften and flexural juvenile crust,which is easily to form wide crustal shortening and thicking by a thrust-fold interaction during the collisional period,is one of the deformational mechanism of the soft collsion along the Solonker suture zone.In summary,our results suggest that the PAO had not closed until the early Permian,and a back-arc basin occurred resulting from the the northward subduction of the oceanic palte.In the Late Permian to middle Triassic,an oblique collision along the Solonker suture contributes to wide crustal shortening(>250 km)and thicking(ca.40-50 km)in the juvenile crust,which is the formation mechanism of the “soft collision”.