Polyphase deformation and metamorphism in the Llyn Ogwen area of Snowdonia, North Wales

Structural mapping of Upper Ordovician (Caradoc) vo;caniclastic rocks aroiu nd Llyn Ogwen, North Wales has revealed a hitherto unrecorded, kilometre-sc ale, Ramsay 'type-3' refold pattern. Each phase of folding is associated wi th an axial planar low grade, metamorphic fabric. The Caradoc aged, sub-vol canic Ogwen microgranite, contains two fabrics which are contiguous with th ose in the metasedimentary rocks. Recognition of two tectono-metamorphic ev ents may simply explain the discrete Late Silurian and Early Devonian isoto pic ages for deformation in North Wales and has implications for models of Acadian tectonics and the use of K/Ar and Ar-40/Ar-39 dating techniques in slate belts.