Petrology and geochemistry of the Fishguard Volcanic Complex, Wales

The Fishguard Volcanic Complex represents an important volcanic episode which occurred within the Welsh sector of the British Caledonide region during early Ordovkian times. A variety of extrusive and contemporaneous intrusive rocks are present, ranging in composition from basic, through intermediate to acidic. Mineral and whole‐rock chemical analyses have been determined on a representative suite of rocks from the complex and, despite secondary alteration effects, the original tholeiitic characteristics of the suite can be discerned. The variation observed is considered to result largely from low‐pressure crystal fractionation, although the origin of the acidic magma remains speculative in view of subsequent extensive recrystallization. The rocks of the Fishguard Volcanic Complex formed in the Welsh Basin, which has been likened by earlier workers to a marginal basin. Comparison of the geochemical characteristics of the Fishguard Volcanic Complex with that of documented marginal basin rocks appears to favour this contention.