Basalts of the Bedded Pyroclastic Formation, Snowdon Volcanic Group, North Wales of Longvillian (Caradoc) age, are heterogeneous in trace element composition, forming a mixing trend between ocean island basalt (OIB)-type and magmatic arc basalt. The two end-member compositions were partial melts of two distinct upper mantle sources. The OIB end-member is similar to Icelandic OIB, and is the only known example of basalt of this type in the Ordovician-Silurian Welsh Basin. Since ocean island basalt magma type is thought never to erupt above well-established subduction zones, the eruption of the Longvillian OIB provides a minimum age for the end of subduction of Iapetus oceanic lithosphere below the Welsh Basin. A model for the evolution of the mantle below the Welsh Basin is proposed which accounts for the geochemistry of magmas erupted from the Tremadoc to the early Silurian.