A major doleritic and gabbroic sill complex, with minor microtonalitic components, cuts Ordovician strata to the east of Fishguard. Their age has long been equivocal. New geochemical data, combined with petrographic analyses, indicate that the intrusions represent an eastward extension of the Ordovician (Llanvirn) Fishguard Volcanic Complex lavas and high-level intrusions of the Strumble Head region. In addition similar doleritic intrusions crop out to the west of Strumble Head, as far as Traeth Llyfn.
The data presented here confirm earlier conclusions that the FVC has tholeiitic affinities and that compositional variations largely result from low-pressure fractional crystallization. The petrogenetic relationships of the microtonalites are, however, as yet equivocal. Incompatible element discriminant diagrams indicate that the parental basic magmas were derived by melting of a slightly depleted mantle source similar to N-type MORB, to which had been added a minor supra-subduction zone LIL element component. The data are consistent with emplacement of the magmas in a marginal basin environment.