Early Ordovician volcanism in Dyfed, SW Wales

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Abstract
A review of early Ordovician volcanism in SW Dyfed, Wales is presented in the light of recent work. There were a number of major and minor volcanic episodes within the area during Arenig and Llanvirn times. For the most part, submarine volcanism occurred within a restricted basin, in which black muds were deposited and to which coarse terrigenous material had very limited access. Basic and intermediate magmas were generally erupted as flows. Acidic magmas were also erupted quietly, to produce thick, frequently autobrecciated flows and domes. Volcaniclastic rocks resulted by deposition from sediment gravity flows originating on unstable flanks of lava domes. Evidence for occasional episodes of explosive activity is provided by sediment gravity flows carrying a high proportion of pyroclastic debris. Subaerial or shallow water conditions are suggested only for the oldest volcanic rocks on Ramsey Island. Numerous basic and less frequent intermediate and acid high-level intrusive sheets have a close temporal, spatial and genetic relationship with the volcanic rocks.