EARTH’S OLDEST PRESERVED K-BENTONITES IN THE CA. 2.1 Ga FRANCEVILLIAN BASIN, GABON
Résumé
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 Kbentonite
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 calcalkaline
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.