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标题：Chronological constraints on the Paleoproterozoic Francevillian Group in Gabon
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作者：Yusuke Sawaki ; Yusuke Sawaki ; Mathieu Moussavou 等
期刊名称：Geoscience Frontiers
印刷版ISSN：1674-9871
出版年度：2017
卷号：8
期号：2
页码：397-407
DOI：10.1016/j.gsf.2016.10.001
语种：English
出版社：Elsevier
摘要：Abstract The Francevillian Group in Gabonese Republic was recently established as a typical sedimentary sequence for the Paleoproterozoic. However, its age is rather poorly constrained, mainly based on Rb-Sr and Nd-Sm datings. This study reports new zircon data obtained from Chaillu massif and N'goutou complex, which constrain the protolith age of the basement orthogneisses and the igneous age of an intrusive granite, respectively. Most zircons from the orthogneisses are blue and exhibit oscillatory zoning in cathode-luminescence images. Zircons with lower common lead abundances tend to be distributed close to the concordia curve. Two age clusters around 2860 Ma and 2910 Ma are found in zircons plotted on the concordia curve. Based on the Th/U ratios of zircons, these ages correspond to the protolith ages of the orthogneisses, and the zircons are not metamorphic in origin. Syenites and granites were collected from the N'goutou complex that intrudes into the FA and FB units of the Francevillian Group. The granitoids exhibit chemical composition of A-type granite affinity. Half of zircons separated from the granite are non-luminous, and the remaining half exhibit obscure internal textures under cathode-luminescence observation. All zircon grains contain significant amounts of common lead; the lead isotopic variability is probably attributed to the mixing of two components in the zircons. The zircon radiogenic 207Pb/206Pb ratio is 0.13707 ± 0.0010, corresponding to a 207Pb/206Pb age of 2191 ± 13 Ma. This constrains the minimum depositional age of the FA and FB units. Furthermore, the FB unit consists of manganese-rich carbonate rocks and organic carbon-rich black shales with macroscopic fossils. Based on our age constraints, these organisms appeared in the study area just after the last Paleoproterozoic Snowball Earth event, in concert with global scale oxidation event encompassing the Snowball Earth. Graphical abstract Display Omitted Highlights • The protoligh age of basement orthogneiss near Franceville ranged from 2860 to 2910 Ma. • The syenite in N'goutou complex intruded into FA and FB units at 2191 ± 13 Ma. • Macroscopic organisms in FB unit appeared shortly after the Snowball Earth events.
关键词：KeywordsenGabonFrancevillian GroupPaleoproterozoicN'goutou complexZirconMC-ICP-MS