干旱区生态环境知识资源中心

The closure timing of the Paleo-Asian Ocean is a critical issue among subjects of the evolution history of the Central Asian Orogenic Belt (CAOB) since long ago. There is a debate, among different perspectives from many researchers, about if the Paleo-Asian Ocean did exist during Late Paleozoic or not. A comprehensive research on the Late Paleozoic sedimentary history of the southeastern part of the CAOB, where the eastern part of the Paleo-Asian Ocean finally closed, could be helpful on solving this key issue. This research focuses on the Late Paleozoic evolution of eastern part of the CAOB from perspective of sedimentology and chronology. Terrestrial coarse elastic rocks from the Middle Permian Zhesi Formation in Shuangjing area, Inner Mongolia were taken as instance, through a survey of stratigraphic section, analysis of sedimentary facies and paleogeographic environment and zircon chronology to constrain its formation time and sedimentary tectonic setting, and furthermore, to discuss the Late Paleozoic evolution history of eastern the CAOB. The youngest concordant age groups of two sandstone samples from the Zhesi Formation are 258. 1 +/- 1. 9Ma and 261. 6 +/- 1. 7Ma, respectively, which can be considered equal within errors, and thus provide a well constrain on the maximum depositional age of the Zhesi Formation at about 260Ma, i. e., the Capitanian Stage at the end of Middle Permian. The comparison of detrital zircon spectrum between samples and potential provenances indicates that the North China Craton ( NCC) was a major provider of detrital material. Zircons with age between 2. 75 similar to 2. 49Ga and 1. 91 similar to 1. 61Ga may come from the Archean to Late Proterozoic crystalline basement of the NCC. Besides, the Zhesi Formation also shows affinity with the Middle-Late Devonian Xibiehe Formation, which contributed material as well. The Early Paleozoic continental arc along northern edge of the NCC contributed zircons with ages of Early Paleozoic. It can be deduced that the Paleo-Asian Ocean did not exist during Late Paleozoic in consideration that the detrital material from the NCC have reached the southern edge of the Songliao-Hunshandake Block. The research on detrital zircon chronology and sedimentology shows that the Zhesi Formation was deposited in a rift basin under an extensional setting. Rapid filling with tremendous thick coarse elastics in short time as the characteristic of a filled basin and syn-depositional volcanic and structural activity appeared in early stage of the basin, which developed a nearshore subaqueous fan facies. The basin transformed to an overfilled basin gradually in late stage and developed a terrestrial arid fluvial fan facies. The identification of rift basin under extensional setting of the Zhesi Formation denies the possibility of Late Permian to Early Triassic closure of the Paleo-Asian Ocean. Together with the existence of rift basin system, bimodal volcanic rocks and giant alkaline magmatic belt, the extensional setting in Late Paleozoic, after the closure of the Paleo-Asian Ocean in Late Devonian, of the CAOB has been further confirmed.