Abstract
A pebbly gritstone–microconglomerate outcrop at Trainor's Rocks in the Mourne Mountains is one of the youngest ( c . 430 Ma) coarse clastic units within the Ordovician–Silurian Southern Uplands–Down–Longford Terrane. The c . 400 m by 1 km outcrop lies within fine-grained Hawick Group (Wenlock) strata, and is rich in extraformational clasts including granite, rhyolite, andesite, basalt, vein quartz and metamorphic rocks, plus intraformational rip-up clasts of mudstone. Detrital zircons, analysed by sensitive high-resolution ion microprobe (SHRIMP), yield predominantly Ordovician U–Pb ages of 450–490 Ma, peaking at c . 470 Ma, coincident with arc-related magmatism in the Midland Valley Terrane. Whole-rock geochemical data are consistent with derivation from a calc-alkaline continental arc, with clast provenance matching 473–464 Ma arc volcanic and intrusive rocks from the Tyrone Igneous Complex. A small proportion of analysed zircons have Proterozoic and Neoarchean ages typical of sediments derived from the Dalradian Supergroup (Grampian Terrane). The youngest zircon analysed, 435 ± 8 Ma, may indicate that magmatism continued during closure of Iapetus in the Llandovery. In these samples there is no evidence for Gondwana-derived sediment. The influx of detritus from the volcanic arc–Laurentian hinterland suggests episodic tectonic unroofing in response to syndepositional strike-slip movements.
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