| The Chengkou-Western Hubei aulacogen is located in the northern margin of the Middle Yangtze in South China.The aulacogen occurred initially in the Middle Permian,expanded and developed further in the Late Permian,and formed a basic pattern at the Permian-Triassic Boundary(PTB),and was filled up and completed in the Early Triassic.As a structural unit very similar to the Kaijiang-Liangping aulacogen,a series of oil and gas exploration achievements have been made in the latter ring aulacogen margin area,but the oil and gas exploration degree in the study area is still in its initial stage.With regard to the controversy about dynamics of Chengkou-Western Hubei aulacogen at present,this paper starts from the perspective of petrotectonics,Permian bedded cherts and volcanic ash interbeds in the aulacogen are selected as the research objects.By systematically analyzing the sedimentary environment,genesis and source of cherts,the volcanic ash source area and tectonic properties were determined at the same time,and the formation mechanism of aulacogen was discussed finally.First by collecting previous literature data,all Middle Permian Gufeng Formation bedded cherts profiles in the northern margin of the middle-lower Yangtze area which includes the study area were integrated.Via taking the main and rare earth elements of cherts as variables,the contents of the same element in all samples of each section were averaged respectively to redefine their origin systematically,the previous research methods on cherts were sorted out and improved at the same time.Secondly through literature investigation and field geological survey,cherts and volcanic ash interbeds of PTB Dalong Formation and Daye Formation from Mao'ershan and Tielukou sections in Western Hubei Enshi area were collected,and then the experimental analysis of the main-trace elements characteristics of the cherts and volcanic ashes whole rock main-trace elements,zircon U-Pb ages,trace elements and Hf isotopic characteristics was implemented to determine the genesis of cherts,the source area of volcanic ashes and the siliceous contribution of volcanic ashes to cherts.Finally,based on the analysis of the main geological events of the Permian in South China,the above conclusions are tested and compared based on the tectonic background,so as to discuss the dynamic background of the aulacogen.The results show that the bedded cherts of the Gufeng Formation in the northern margin of the middle-lower Yangtze area which includs the study area,are mainly of non-hydrothermal or biological origin and deposited in the deep water environment of the passive continental margin.However,some areas are affected by volcanic activities and faults and have a transition to hydrothermal origin.The sedimentation and hydrolysis of PTB volcanic ashs have provided siliceous contribution to the deposition of PTB cherts.The PTB volcanic ashes in South China has a felsic nature.Zircons of the ashes are continental zircons with U-Pb ages ranging from 255.4±1.4 Ma to 248.3±1.3 Ma and?Hf(t)ranges from-8.6 to 1.1,and possibly from the Jiangda-Weixi volcanic belt in the Sanjiang region of Southeast Asia.In combination with the Permian regional tectonic evolution of South China,the east Palaeotethys region exists mantle plume tectonic setting,besides the cherts in the study area and its volcanic ash interbeds indicate the extensive eruption of Emeishan basalts and the closure of the eastern Paleotethys ocean on the north and south sides of the Yangtze Block during the middle-late Permian.The Chengkou-Western Hubei aulacogen was formed in the northern margin of the Yangtze area under the action of the tensile stress field near the north-south direction. |
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