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

Abstract

Bentonites are the alteration product of volcanic tephra typically preserved in low-energy, sedimentary environments below baseline. Although volcanic tuffs occur throughout the Earth's 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 2M₁ 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 ²⁰⁷Pb/²⁰⁶Pb 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 world's oldest bentonite beds so far reported.