Abstract

In Snowdonia and Llŷn three prograde stages in metapelites (Gwna Group to Wenlock) are recognized: Stage I, with white mica crystallinity values >0.43Δ°2θ; Stage II, 0.43–0.26Δ°2θ; Stage III, <0.26 Δ°2θ. Isocrysts at 0.03Δ°2θ intervals are drawn separately for pelites above and below the sub-Arenig unconformity. Approximately concordant isocryst patterns characterize the unconformity in the northwestern and southeastern limbs of the Snowdonia Syncline, whereas discordant patterns are developed northwest of the Aber–Dinlle Fault and at the closure of the Ynyscynhaiarn Anticline. The contrasting patterns are the result of a pre-Arenig burial metamorphism. In post-Tremadoc pelites belts of steep metamorphic gradient coincide with belts of high strain, and a broad area of higher grade Caradoc pelites coincides with the core of the Snowdonia Syncline. The later metamorphism is related to Caledonian deformation rather than to burial. Pelite alteration adjacent to small, intermediate to acid plutons suggests emplacement before the Caledonian regional metamorphism. A close correlation exists between metabasite zones and metapelite stages.

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