Provenance of the Trainor's Rocks microconglomerate, Northern Ireland: a mid-Silurian (Hawick Group) submarine channel fan deposit in the closing Iapetus Ocean

Abstract

A pebbly gritstone–microconglomerate outcrop at Trainor's Rocks in the Mourne Mountains is one of the youngest (∼430 Ma) coarse clastic units within the Ordovician-Silurian Southern Uplands – Down-Longford Terrane. The ∼400m 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 SHRIMP, yield predominantly Ordovician U-Pb ages of 450-490 Ma, peaking at ∼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 Neoarchaean 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.