A Comparison of Fission-Track Ages of Apatite and Zircon to the K/Ar Ages of Illite-Smectite (I/S) from Ordovician K-Bentonites, Southern Appalachian Basin

Abstract

Fission-track ages of apatite and zircon from the middle Ordovician Deicke K-bentonites, collected over a wide area of the southern Appalachian basin, bracketed the previously determined K/Ar ages (270-300 Ma) of illite/smectite (I/S) separated from these K-bentonties. The zircon fission-track ages ranged from $306 \pm 32$ to $410 \pm 50 Ma$, while the apatite fission-track ages ranged from $165 \pm 14$ to $223 \pm19 Ma$. Basin wide, the zircon fission-track ages were partially reset, while the apatite fission-track ages were completely reset. The partially reset zircon fission-track ages at the distal margin suggest that the K-bentonites were affected by a pulse of heated solutions, 175°-200°C, for a duration of at least 1 m.y. as opposed to being heated by deep burial. The fission-track data are consistent with previous data suggesting that I/S formed by reaction with heated saline solutions expelled during Alleghanian orogeny. These fission-track ages are also consistent with apatite fission-track ages measured from the Middle Devonian Tioga ash bed and Late Devonian Hampshire Formation in the Appalachian basin of Virginia, West Virgina, and Maryland. The synchronous nature of the reset apatite fission-track ages throughout the southern Appalachian basin indicates that the section heated by Alleghanian fluids remained buried to some extent, then underwent a widespread unroofing/cooling during the Triassic-Jurassic. The discordance of the apatite fission-track ages with the measured K/Ar ages of I/S indicates that the blocking temperature of argon in illite, or I/S, is higher than the commonly accepted closure temperature of $100^{\circ} \pm 20^{\circ}C$ for fission track retention in apatite.