Tremadoc to Llanvirn volcanism on the southeast side of the Harlech dome (Rhobell Fawr), N Wales

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Abstract
Summary: A volcanic episode, initiated in the late Tremadoc and persisting into the lower Llanvirn, is represented around Rhobell Fawr (SH 7825) by basaltic lavas and their breccias (the Rhobell Volcanic Group), by dolerite, microdiorite and microtonalite intrusions, by a rhyolite tuff and by the basic and intermediate constituents of certain sedimentary rocks. Initially basalts were erupted, at a variable rate, subaerially from fissures. This effusive phase was followed by eruptions of andesite and dacite, which, in turn, were followed by explosive eruption of rhyolite. A dyke regime in the west of the area represents the sub- and intravolcanic parts of a major site of eruption and was the source of the volcanic group. The rocks of the Rhobell episode are genetically related by amphibole-dominated fractional crystallisation of mantle-derived basaltic magma and are similar to those of modern destructive plate margin environments. The Rhobell fractionation series, from silica-undersaturated to quartz-normative, is closely paralleled by that at Grenada, at the southern end of the Lesser Antilles island arc. The tectonovolcanic evolution of the area is consistent with the development of a N-S basement fracture situated beneath the eastern margin of the Harlech Dome.