U-PB LA-ICP-MS ZIRCON DATES FROM K-BENTONITES IN THE UPPER ORDOVICIAN OF EASTERN NORTH AMERICA AND BRITAIN

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 (weathered tephras), provide evidence for this volcanism. However, there is a lack of modern precise U-Pb dating of these ashes and some confusion in the biostratigraphy. Key boundary ages have not been accurately determined in the latest International Chronostratigraphic Chart (ICC). Instead, they were estimated on the basis of a few radiogenic isotope dates of low precision from U-Pb dating of multiple zircon grains. The aim of this study is to obtain more precise U-Pb zircon ages from biostratigraphically constrained k-bentonites which will lead to better correlation of the Upper Ordovician relative time scales, as well as timing of eruption pulses. Zircon grains were extracted from the samples by heavy mineral separation and U-Pb dated using the Laser Ablation-Inductively Coupled Plasma- Mass Spectrometer (LA-ICP-MS) at the University of Texas-Austin. Herein, we report U-Pb zircon ages from the Trenton Group, Ontario, Canada, the Utica Shale, New York State and Dob's Linn, Scotland. The youngest age from the top of the Kirkfield Formation in Ontario is 452.3±6.6 Ma, which fits with existing late Ordovician stratigraphic ages. The Utica Shale ages range from 452.3±5.8 to 449.6±7.3 Ma. The youngest dates from the top of the Mohawkian, are also compatible with existing Ordovician ages. At Dob's Linn, Scotland, however, the site of the Ordovician/Silurian Global Boundary Stratigraphic Section and Point (GSSP), the weighted mean age of DL7, a bentonite 5 meters below the GSSP, is 435.6±4 Ma, and the age for DL24L, a bentonite 8 meters above the GSSP is 425.4±5.9 Ma. These ages indicate that the Ordovician/Silurian boundary is somewhat younger than currently accepted (443.4±1.5 Ma) but comparable with the younger age for the mid-Hirnantian in China of 442.99±0.17 Ma (Ling et al., 2019). We are confident that our techniques rule out contamination; even more precise TIMS dating is planned for the most concordant currently dated SIMS zircons, with the aim of more precisely calibrating series and stages with standard international time scales.