New age constraints for the Ordovician Tyrone Volcanic Group, Northern Ireland

New biostratigraphical evidence and a high precision ID-TIMS U-Pb zircon age provide refined age constraint for the Ordovician Tyrone Volcanic Group of the Tyrone Igneous Complex. In a graptolite fauna from Slieve Gallion, the presence of Isograptus victoriae lunatus, the index fossil of the victoriae lunatus graptolite zone, indicates a correlation with the Australasian Castlemainian (Ca1) Stage. The U-Pb zircon age of 473 ± 0.8 Ma dates a volcanic arc-related rhyolite body that sits stratigraphically below graptolitic mudstones of Slieve Gallion. The U-Pb isotopic and biostratigraphical age constraints match closely with an interpolated age for the base of the Middle Ordovician (Sadler & Cooper 2004), and indicate a Whitlandian age for the upper Tyrone Volcanic Group, which supports the regional correlation with the Ballantrae Complex, Midland Valley Terrane, Scotland