Basaltic, andesitic and rhyolitic volcanism was widespread during Ordovician time in the Welsh, Basin. Minor Silurian volcanic rocks occur in the Welsh Borderland and in Pembrokeshire (Dyfed), where they represent the youngest volcanic activity associated with the Caledonian orogeny in Wales. The Skomer Volcanic Group comprises c. 760 m of lavas and pyroclastic rocks with minor sedimentary rocks which are best exposed on Skomer Island but crop out over 43 km from the Smalls and Grassholm (in the Irish Sea) to mainland Pembrokeshire. New chemical analyses have been used to characterize these rocks and to compare them with earlier Ordovician volcanic rocks. The Skomer Volcanic Group comprises hawaiite/mugearite lavas and rhyolitic flows and pyroclastic rocks. The rhyolitic rocks are subdivided into a high-Zr group, (originally of peralkaline composition) and an unrelated low-Zr group. The hawaiite/mugearite lavas and high-Zr rhyolites are considered to form a single group, related by fractional crystallization, analogous to basalt–hawaiite–mugearite–comendite associations erupted in within-plate oceanic and continental settings. However concentrations of high field strength elements (Ti, Zr, Nb, Ta, Hf) suggest that the parent magmas may have been derived from a within-plate (ocean island basalt) source modified by the effects of earlier or contemporaneous Caledonian subduction.

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