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

摘要

New biostratigraphical evidence and a high-precision isotope dilution thermal ionization mass spectrometry 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 is stratigraphically below graptolitic mudstones of Slieve Gallion. The U–Pb isotopic and biostratigraphical age constraints closely match an interpolated age for the base of the Middle Ordovician and indicate a Whitlandian age for the upper Tyrone Volcanic Group, which supports the regional correlation with the Ballantrae Complex, Midland Valley Terrane, Scotland.