Discussion on low grade metamorphism of the Welsh Basin Lower Palaeozoic succession: an example of diastathermal metamorphism?

Abstract

B. Roberts, J. A. Evans, R. J. Meniman & M. Smith write : In their recent, stimulating paper on low grade metamorphism of the Lower Palaeozoic succession of the Welsh Basin, Bevins & Robinson (1988) claim that, following an initial early period of high heat flow, temperatures remained high in the sedimentary/volcanic rock pile until the end-Silurian to Middle Devonian deformation. The claim rests on three principal lines of evidence: (1) the recognition in North Wales by Roberts and Merriman (1985) of a pre-Arenig burial metamorphic event; (2) the observation that the patterns of distribution of metamorphic grade in the central and southern parts of the basin show a close relation to the basin form, lowest grades being found along the basin margins with grade increasing towards the centre where the succession is thicker and older; (3) on certain ‘critical accounts’ of pre-cleavage, low greenschist facies assemblages developed in the Wenlock of central Wales and the Lower Ordovician of Snowdonia. It is agreed that the Lower Palaeozoic succession now consists of assemblages belonging to the zeolite, prehnite-actinolite, prehnite-pumpellyite and greenschist facies and that the rocks comprise a low pressure facies series, as demonstrated by Robinson & Bevins (1986). What is disputed is the claim for a pre-cleavage ‘diastathermal’ metamorphism (Robinson 1987) with temperatures up to low greenschist facies conditions and, further, that temperatures remained at or about this level in the sedimentary/volcanic rock pile across the basin throughout the Ordovician and Silurian. Relevance of the pre-Arenig metamorphic event in North wales.