The geology of the Strumble Head - Fishguard region, Dyfed, Wales

Abstract

An investigation was undertaken on the volcanic and volcaniclastic rocks forming the Fishguard Volcanic Group of Lower Ordovician age, which crop out in the Strumble Head - Fishguard area of Southwest Dyfed (North Pembrokeshire).
The nature of the lavas and volcaniclastic rocks suggests that this episode of vulcanicity was entirely subaqueous in aspect.
It is shown that a variety of magma types were available for penecontemporaneous extrusion and intrusion at a high level. The form assumed by the extrusions depends primarily upon magma composition, which also largely determines magma viscosity. In addition these factors, plus the eruption depth, have also governed the development of volcaniclastic and pyroclastic debris. Basic magma was erupted quietly, and resulted in a thick lava pile with only a limited production of volcaniclastic and pyroclastic material. Similarily acidic magma also appears to have been erupted quietly in this area, although as a result of its viscosity it produced thick flows and domes, and abundant related autobreccias and collapse breccias.
From an examination of whole-rock major and trace element analyses of a representative suite of rocks it is demonstrated that the intrusions and extrusions, which have tholeiitic characteristics, are comagmatic and that the majority of the igneous rocks examined are related to each other by a process of high-level crystal fractionation.
The rocks of the area suffered low-grade regional metamorphism during the Caledonian Orogeny, indicated by the presence of pumpellyite and prehnite within the meta basites.
In spite of this alteration, clinopyroxene remains as a metastable, relict phase within the meta-basites. From a microprobe study of the composition of these clinopyroxenes, it is clearly demonstrated that the rocks have tholeiitic affinities in addition to the fact that the composition of clinopyroxene within igneous systems is dependant upon cooling history.