INITIATION OF IAPETUS SUBDUCTION UNDER IRISH AVALONIA

The Dowery Hill Member of metamorphosed basalt, dolerite and siltstone is here recognized as the oldest exposed volcanic unit of the Lower O rdovician Ribband Group, with a minimum age of early Arenig. Peperites and resedimented hydroclastic breccia demonstrate a volcanic origin f or the basalts. The igneous rocks are tholeiitic, with a trace element geochemistry indicative of a subduction-modified fertile mantle sourc e, which we interpret as recording an early stage of volcanic are evol ution. The member is therefore the oldest known component of the volca nic are generated by subduction of Iapetus oceanic lithosphere under s outheastern Ireland Subduction started earlier than predicted by curre nt plate tectonic models, and these should be re-evaluated.