CONSTRAINING EPISODES OF LATE ORDOVICIAN VOLCANISM THROUGH U-PB DATING OF K-BENTONITES

The end of the Ordovician marks one of the greatest of the Earth’s mass extinctions. One hypothesis explains this mass extinction as the result of a short-lived, major glaciation preceded by episodes of increased volcanism brought on by the Taconic orogeny. K-bentonites from many localities around the world provide evidence for increased volcanism. However, there is a lack of U-Pb geochronologic data on these ash deposits and some confusion in the biostratigraphy. The aim of this study is to get more precise U-Pb zircon ages from biostratigraphically constrained bentonites which will lead to better correlation of the Upper Ordovician. Zircon grains were extracted from the samples by heavy mineral separation and dated using the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS) at UT-Austin. We report here the first U-Pb zircon ages obtained from the Ontario Trenton Group. One K-bentonite bed from Carden Quarry Ontario, Canada, which is at the top of the Kirkfield Formation of mid-Chatfieldian, Upper Mohawkian age (early Katian), gave a youngest zircon age of 451.2±8 ma, while the weighted mean of 466.0±3.4 ma corresponds with the age of the Red Indian Lake continental magmatic arc in the Appalachians to the east. The youngest zircon fits the position of the ash in the early Katian stage with current dating of its base of 455±1.6ma and of its top of 445±1.5ma. The weighted means suggest that the eruption incorporated zircons from throughout the Red Indian Lake magmatic arc pile. Work in progress will date more of the Trenton bentonites in both Canada and the USA, as well as their possible equivalents in western Europe.