AbstractThe Caradoc volcanic rocks of the Lleyn Peninsula represent a major episode of rhyolite‐dominated eruption (total volume up to 560 km3) within the Southern British Caledonides. New geochemical data reveal the original chemical characteristics of the basic, intermediate, and acid lavas. The basalts are interpreted as having predominantly within‐plate character and are associated with intermediate lavas (trachybasalts) and high‐Zr, originally peralkaline, trachytes, and rhyolites. A low‐Zr, originally high‐K, subalkalic group of rhyolites, and a basalt with transitional island arc tholeiite‐MORB characteristics are unrelated to these lavas. The volcanic rocks are interpreted to have been erupted onto a complex, rapidly evolving, active continental margin dominated by crus