Petrology and geochemistry of lower to middle Ordovician igneous rocks in Wales: a volcanic arc to marginal basin transition

Abstract

Volcanic activity was widespread in Wales during Ordovician times. There are marked petrological and geochemical differences between late Tremadoc volcanism and that of Arenig to Llandeilo age. The earlier activity, represented by the Rhobell Volcanic Complex and the Trefgarn Volcanic Group, comprises a fractionation continuum which is basalt-andesite-dacite-rhyodacite and is similar to that of modern volcanic arcs. In contrast, the later activity, as seen in the Fishguard Volcanic Complex and the Aran Volcanic Group, is represented by basalt-rhyolite bimodal sequences. The basic rocks are similar to those of ensialic marginal basins and the acidic magmas were most probably generated by crustal fusion. The contemporaneous Builth Volcanic Group has some arc-like characteristics but the variation is here interpreted as due to the influence of subduction zone components in marginal basin magma generation. The change from the Tremadoc to later volcanism reflects an arc to marginal basin transition.