New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges,Weng'an and Chengjiang areas,South China |
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| Institution: | 1. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan;2. Department of Earth Science and Astronomy Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan;3. Division of Earth and Planetary Sciences, Graduate School of Science, The University of Kyoto, Yoshidahoncho, Sakyo-ku, Kyoto 606-8501, Japan;4. Division of Ocean-Earth System Science, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan;5. Department of Geology and Key Laboratory for Continental Dynamics, Northwest University, Xi''an 710069, China;6. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan;1. Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK;2. State Key Laboratory of Paleontology and Stratigraphy, NIGPAS, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China;3. Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity, Invalidenstraße 43, 10115 Berlin, Germany;4. State Key Laboratory for Mineral Deposits Research, Nanjing University, 163 Xianlin Road, Nanjing 210023, China;5. EAWAG, Swiss Federal Institute of Aquatic Science and Technology, CH-6047 Kastanienbaum, Switzerland;1. Department of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan;2. Early Life Institute, Northwest University, Xi''an 710069, China;1. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan;2. Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan;3. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan;4. Department of Geology and Key Laboratory for Continental Dynamics, Northwest University, Xi''an 710069, China;5. Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan |
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| Abstract: | The Neoproterozoic and Cambrian were two of the most dramatic periods in the history of the Earth, because large multi-cellular animals first appeared then in the so-called “Cambrian Explosion”. To better understand this event, many paleontological and geochemical studies now focus on rocks in South China, because of the fossiliferous succession and good exposure. Since the recognition of the Yangtze Gorges and Chengjiang area as type localities of the Sinian (Ediacaran) and Meishucunian (Early Cambrian) Systems, both sections have been intensively investigated. In order to decipher the relationships between the evolution of life and surface environmental changes, it is necessary to understand their paleontological, geochemical and geo-chronological constraints.This study presents new chronological constraints for the Cryogenian to Cambrian rocks in the Three Gorges, Weng'an and Chengjiang areas, South China. We discovered two tuff layers, one at the base of the Shuijingtuo Fm at Three Gorges and the other at the bottom of the Dahai Member in Chengjiang. In addition, we collected sandstones from Neoproterozoic strata in the Three Gorges, Chengjiang and Weng'an areas for provenance analysis. Zircons, separated from the tuff layers, provide new Nano-SIMS U–Pb ages of 526.4 ± 5.4 Ma at the base of the Shuijingtuo Fm, and 523.9 ± 6.7 Ma at the bottom of the Dahai Member. The tuffaceous beds occur at an unconformity, and nodules are common in the Three Gorges, Meishucun and Taoying sections, indicating that major and relatively wide-scale volcanic and sedimentological events occurred at ca. 525 Ma. Moreover, carbonate carbon isotope chemostratigraphies at Morocco, Siberia, Three Gorges and Meishucun display different characteristics during the Tommotian. One possibility is that the South China Ocean was separated from an outer ocean at that time. Detrital zircons in sandstones have age populations at ca. 2.7, 1.8, 1.6, 1.0 and from 0.9 to 0.75 Ga. indicating that the paleo-hinterland of the Nanhua and Kangdian rift basins was geologically complex. Despite the lack of ca. 1.6 Ga rocks in the Yangtze and Cathaysia Blocks, these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the Yangtze craton. |
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