‘New’ geology in the western Mourne Mountains – the Trainor’s Rocks gritstone-conglomerate

A hitherto unrecorded pebbly gritstone–microconglomerate unit outcrops on the hilltop southwest of the Leitrim River valley, south of Hilltown in the Mourne Mountains. The ~400m by 1 km outcrop adjoins the Mournes G4 Granite which forms a roof contact underlying the pebbly gritstone. The occurrence is regionally significant because this is one of the youngest (~430 Ma) coarse clastic units within the Ordovician-Silurian Southern Uplands – Down-Longford Terrane. The unit 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 extracted from samples of the (highly indurated) conglomerate 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. It seems likely that the gritstone-conglomerate was deposited as a submarine channel fan deposit during the final stages of closing of the Iapetus Ocean, while the northwestern hinterland underwent episodic tectonic unroofing in response to strike-slip movements. More speculatively, the gritstone-conglomerate might be the source rock for alluvial gold occurring in modern sediments of the Leitrim River.