Sedimentary Background Analysis Of Late Paleozoic Basins In Ondor Sum,Inner Mongolia

The Ondor Sum area in Sunite Youqi,Inner Mongolia,is a key area for the study of Late Paleozoic tectonics on the southern side of the Xingmeng Orogenic belt.The study of sedimentary petrology,paleobiostratigraphy and isotope chronology of the Carboniferous strata in this area is helpful to understand the sedimentary age,material source and tectonic evolution of the basins in this area.In this paper,Jiujuzi Formation,Amushan Formation,"Benbatu Formation" and Sanmianjing Formation in the Late Paleozoic sedimentary strata of Ondor Sum area are selected for systematic study including profile,Paleontology and LA-ICP-MS U-Pb dating.The sedimentary time limit of the Late Paleozoic sedimentary basins in the study area was determined,and the material source and sedimentary tectonic background of the Late Paleozoic sedimentary strata in the study area were discussed.1.Sedimentary time limit of sedimentary basinLA-ICP-MS U-Pb dating of detrital zircons in the strata of Jiujuzi Formation,Amushan Formation,"Benbatu Formation" and Sanmianjing Formation in Ondor Sum Late Paleozoic sedimentary basin is carried out.Combining with regional stratigraphic contact relationship and paleobiological evidence,the sedimentary ages of Jiujuzi Formation and Amushan Formation are Late Carboniferous,and the sedimentary ages of "Benbatu Formation" and Sanmianjing Formation should be Early Permian.In terms of chronology and biology,the division of the "Benbatu Formation" in the study area does not conform to the current definition of the Benbatu Formation,which should belong to the Early Permian strata.2.Source analysis of sedimentary basinsThe quartz coarse sandstone and quartz conglomerate at the bottom of the Amushan Formation have the same mineral composition with quartz sandstone of Jiujuzi Formation and they may have the same provenance.The Jiujuzi Formation and the Amushan Formation have similar rock assemblages.The dating results show that the clastic zircons of the two groups have similar age spectrum characteristics and are very homogeneous.The youngest age is 440 Ma(+).The main provenances of the two groups of strata should come from the early Paleozoic orogenic belt around the basin,in addition to the material from the Early Paleozoic orogenic belt,there are a small amount of provenances from the North China craton basement in the southern part of the sedimentary basin.The results of detrital zircon dating of the two groups of samples from the "Benbatu Formation" in this work show that the age distribution of zircons is relatively concentrated.The youngest zircon ages are 298+6 Ma and 274+4 Ma,respectively,which may represent two periods of volcanic activity during the Early Permian.In addition,the basement age of the North China Craton during ~1850 Ma and ~2500 Ma is in both groups,but the proportion of the characteristic age of the North China Craton in TM-12 is higher than that in the sample TM-14,which indicates that the subduction of the plate causes the basin to migrate southward,and the proportion of the provenance material of the North China Craton is increasing gradually.Dating analysis of clastic zircons in silty slate samples of Sanmianjing Formation shows that they are mainly divided into two stages: Late Paleozoic(271+3Ma-294+3Ma,378+4-411+4Ma,441+4-458+4Ma)and North China craton basement(~1850Ma,~2500Ma),representing the age of volcanic magmatic events around the early Permian basin,early Paleozoic orogenic intrusive rocks or adakitic age,and the North China craton basement.3.Sedimentary environmentThe asymmetrical ripple marks and some animal and plant fossils of the Upper Carboniferous Jiujuzi Formation show lacustrine fan delta deposits,and some coal seams show marsh facies depositional environments.The upper part of the Upper Carboniferous Amushan Formation is an environment of strong turbulence of sea water.A large number of biofossils mainly collected from Lily stems in the lower part show relatively quiet shallow sea sedimentary environment,which is generally a turbulent-quiet coastal-shallow sea sedimentary environment.The occurrence of volcanic activity in the Lower Permian "Benbatu Formation" indicates that the study area may be in an environment of late orogenic compression in the Early Permian.The main deposits are littoral-shallow marine facies with pyroclastic rocks,and the sedimentary environment is turbulent.The lower Permian Sanmianjing Formation conglomerate,lithic feldspar complex sandstone and siltstone with limestone formed in an unstable littoral-shallow marine sedimentary environment.4.Evolution history of basinsThe sedimentary basins in the study area experienced sedimentary environments of early early subduction extension and late orogenic compression.In the early extensional environment,the basin deposited in a wide range and the extensional environment led to the collapse of the Early Paleozoic orogenic belt.On this basis,similar maturity and mineral composition like quartz sandstones of the Jiujuzi Formation and quartz coarse sandstonesd,quartz conglomerates at the bottom of the Amushan Formation were deposited.The two groups of strata have similar age spectrum characteristics,and the biological age belongs to the Late Carboniferous,indicating that the deposits of the two groups of strata have the characteristic of "different phases of the same era".Up to the early Permian "Benbatu Formation",volcanic activity occurred.The early Paleozoic provenances in the southern part of the basin were gradually weathered and denuded completely.Under the late compression environment,the crust uplifted and the center of the basin shifted from north to south,resulting in the gradual increase of the provenance composition of the North China Craton.In addition to the early Permian and North China craton materials,a small amount of early Paleozoic materials appeared again in the sample material composition of Sanmianjing Formation,indicating that during the sedimentary period of Sanmianjing Formation,the basin area gradually decreased due to continuous compression,and the pre-deposited early Paleozoic materials gradually emerged behind the direction of basintransfer(on the subduction side of the Paleo-Asian Ocean plate)and deposited in the basin again.