A U-Pb zircon age constraint for the Ordovician Tyrone Plutonic Group ophiolite, Tyrone Igneous Complex, Northern Ireland : implications for plate models and Grampian orogenic amalgamation [abstract only]

Geological history and age determination of the Tyrone Igneous Complex and Tyrone Central Inlier are key to the understanding of Grampian orogenic amalgamation of the Caledonides. The Tyrone rocks are recognized as belonging to the Midland Valley Terrane (Bluck et al. 1992), and a strong regional link has been made between the Tyrone Igneous Complex and the Ballantrae Complex of Girvan (Bluck 1985) which together compose the Tyrone-Girvan sub-terrane. In a broader context, the Tyrone Igneous Complex appears to fit most closely with the Notre Dame Subzone of the Dunnage Zone of Newfoundland, as described in Van Staal et al. (1998). The Ordovician Tyrone Igneous Complex is comprised of the ophiolitic Tyrone Plutonic Group and overlying arc-related Tyrone Volcanic Group. Together they structurally overly sillimanite-grade paragneisses of the Tyrone Central Inlier which, based on detrital zircon age profiling, appear to be of Upper Dalradian Laurentian affinity (Chew et al. in press). The Tyrone Igneous Complex and Tyrone Central Inlier are pinned together by a suit of arc-related tonalitic-granitic intrusives. Age constraints for the Tyrone Volcanic Group come from U-Pb dating of zircons from a high level, syn-depositional rhyolite at 473 ± 0.8 Ma, and in addition from a biostratigraphical correlation established by the presence of Isograptus victoriae lunatus, the index fossil of the victoriae lunatus graptolite zone, from Slieve Gallion, which indicates a correlation with the Australasian Castlemainian (Ca1) Stage of the middle Arenig (Cooper et al. 2008). Until now the age of Tyrone Plutonic Group has been based on a magma mixing relationship between gabbro and tonalite at Craigballyharky. The tonalite has been dated at 472 +2/- 4 Ma, and was taken as evidence for the age of the ophiolite and for the timing of obduction (Hutton et al. 1985). Although their mixing with gabbro is clear, the tonalites have a volcanic arc geochemical signature. The geological scenario would therefore require obduction of contemporaneous ophiolite and an active volcanic arc. This paper presents a new U-Pb zircon age of 480 ± 1 Ma for layered gabbros (normaltype MORB) of the Tyrone Plutonic Group at Scalp. This direct age for the ophiolite is older than that suggested by the Craigballyharky mixing relationships and dating of tonalite. When viewed against the 473-464 Ma U-Pb zircon age range of the arc-related intrusive suite (Noble et al, 2004), it makes more geological sense that the ophiolite is significantly older. This age difference allows time for establishment of northwards directed subduction after obduction (~ 480 and 475 Ma), followed by development of a volcanic arc (~ 475 and 465 Ma) as depicted by Chew et al. (opt cit).