Petrogenesis of Ordovician igneous rocks in the southern part of the Welsh Basin

During early Ordovician times volcanic and high-level intrusive activity occurred at numerous centres across North Pembrokeshire. Previous work suggested a geochemical transition in this activity from calcalkaline through to tholeiitic with time. However, new data indicate more extensive calc-alkaline magmatism and that both magma types were coeval. The origin of the calcalkaline magmas remains equivocal, but the tholeiitic magmas appear to have been derived from a source similar to N-type MORB, variably modified by supra-subduction zone fluids, combined with some fractionation during ascent. These data are consistent with emplacement in a supra-subduction zone marginal basin.