Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales

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Abstract
A diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo‐silicate mineralogy, illite crystallinity and bo parameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low‐pressure facies series type, with a geothermal gradient of ∼ 40°Ckm‐1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth‐related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn‐tectonic metamorphic event is apparent.Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote‐actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite‐pumpellyite assemblages in metabasites and it is suggested that the latter represent non‐buffered, and therefore non‐diagnostic, assemblages.