Earth's oldest preserved K-bentonites in the ca . 2.1 Ga Francevillian Basin, Gabon

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ID: 305790
2018
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Abstract
Bentonites are the alteration product of volcanic tephra typically preserved in low-energy, sedimentary environments below baseline. Although volcanic tuffs occur throughout the Earth9s history, bentonites older than *ca*. 1.5 Ga have not been described. We present the mineralogy, geochemistry, and age data for K-bentonite beds within the FB Formation in the unmetamorphosed Paleoproterozoic Francevillian Basin, Gabon. The clay mineralogy of the K-bentonites consists predominantly of illite with substantial amounts of kaolinite and trace amounts of long-ordered illite/smectite (R3) mixed layer. The kaolinite content and co-existing 1M and 2M~1~ illites are indicative of diagenetic smectite illitization over a prolonged period of time with minimal burial temperature. Their chemical characteristics suggest derivation from calc-alkaline intermediate to felsic magma, related to continental volcanic arc magmatism in a subduction setting. The zircon grains are relatively small, rounded to sub-rounded, and yield ^207^Pb/^206^Pb dates that have a narrow range with a weighted mean of 2971 ± 13 Ma, consistent with the age of the underlying crystalline calc-alkaline Archean basement granitoids. This age indicates incorporation of zircons from the Archean basement granitoids into the magma during magmatic activity. Considering that the FB Formation bentonites were derived from a volcanic arc developed along the margin of the West Gabonian block and are preserved in the lower part of the Francevillian Basin, we infer that this basin reflects high-rate, but short-lived sedimentation in a pro-foreland basin setting. Paleogeographically, these K-bentonites could thus serve as a potential correlation marker for the Paleoproterozoic Gabonian and adjacent cratons at *ca*. 2.1 Ga. Based on the current records, these are the world9s oldest bentonite beds so far reported.
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Authors Olabode M. Bankole, Abderrazak El Albani, Alain Meunier, Florent Pambo, Jean‐Louis Paquette, Andrey Bekker
Journal american journal of science
Year 2018
DOI
10.2475/04.2018.02
URL
https://doi.org/10.2475/04.2018.02
Keywords Keywords not found

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