中亚造山带东段晚古生代伸展构造环境的证据:内蒙古双井地区哲斯组沉积学及年代学研究

本研究从沉积学与年代学角度切入,聚焦于中亚造山带东段晚古生代末期的演化历史。以内蒙古双井地区中二叠统哲斯组陆源粗碎屑沉积岩为研究对象,利用沉积剖面实测、岩相古地理分析及锆石年代学测试等方法,厘定其沉积时限与沉积构造环境,以探讨晚古生代末期中亚造山带东段的构造演化历史。来自哲斯组的2个砂岩样品的最年轻谐和锆石组的谐和年龄分别258. 1±1. 9Ma与261. 6±1. 7Ma,在误差范围内相等,限定了研究区内哲斯组地层的最大沉积年龄约为260Ma,处于中二叠世末的Capitanian期;其主要物质来源包括华北板块太古代至早元古代的结晶基底、华北板块北缘早古生代陆缘弧及中-晚泥盆世西别河组。碎屑锆石年代学与沉积学研究表明,研究区内哲斯组所处沉积盆地类型为伸展构造环境下的裂谷盆地,早期具有短时间内快速充填巨厚层粗碎屑沉积的补偿盆地特征,并伴有活跃的同沉积火山与构造活动,发育近岸水下扇相沉积;晚期逐渐转变为过补偿盆地,发育陆相干旱型冲积扇相沉积;表现出沉积环境由海相变为陆相、碎屑物供应充足且近源低成熟度碎屑增多的盆地充填过程。根据沉积相组合及展布、沉积年龄与碎屑锆石主要结晶年龄间隔等特征,结合研究区存在双峰式火山岩及碱性岩带等证据,哲斯组应代表晚古生代末期的裂谷盆地沉积。上述研究提供了中亚造山带东段二叠纪发生陆内伸展过程的证据。

Evidences of the Late Paleozoic extensional setting of the eastern Central Asian Orogenic Belt: Sedimentological and chronological studies of the Zhesi Formation in Shuangjing area, Inner Mongolia

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 clastic 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 2.75~2.49Ga and 1.91~1.61Ga 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 clastics 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.