The Lough Nafooey arc, in the western Irish Caledonides, collided with Laurentia during the Early Ordovician. This event is recorded in the stratigraphy of the South Mayo trough, the preserved forearc basin of this system. Lavas at the base of the oldest Lough Nafooey Group show intraoceanic arc composition. ϵNd(t) decreases and light rare earth elements (REEs) become more enriched upsection in the Lough Nafooey Group (ca. 495– 481 Ma), reflecting early collision with Laurentia. The subsequent Tourmakeady Group (ca. 481–470 Ma) is rhyolitic, light REE enriched, and has strongly negative ϵNd(t). These rocks were erupted during the Grampian orogeny. The Rosroe and Mweelrea Formations (<470 Ma) show wide scatter of La/Sm and Nb/Zr values, suggesting mixed mantle sources. This correlates with rapid exhumation of the adjacent Connemara metamorphic terrane. The chemical evolution of the arc supports models of collision, orogeny, and tectonic unroofing within ∼15 m.y., and shows that genesis of magmas more enriched than continental crust can occur during arc-continent collision, clarifying the potential role of arc volcanism in continental crust formation.