Clay Mineral Evidence for Low-Grade Caledonian and Variscan Metamorphism in South-Western Dyfed, South Wales

D. Robinson, R. A. Nicholls and L. J. Thomas
Department of Geology, The University, Bristol, UK
Department of Geology, The University, St. Andrews, Fife, Scotland

Summary: Illite crystallinity determinations on Palaeozoic pelitic rocks, whose stratigraphic range runs from Lower Cambrian to Westphalian, indicate that anchimetamorphism has affected both the Lower and Upper Palaeozoic sequences. Two metamorphic episodes are in evidence, with the earlier, Caledonian, being of slightly higher grade. The higher anchizone crystallinity values are recorded from the Fishguard area in which mineral assemblages of the prehnite-pumpellyite facies have recently been recognized in basic igneous rocks. The later metamorphic episode has affected rocks to the south of the Variscan front. Here crystallinity values are low anchizone but some straddle the boundary with the diagenetic state. The Pembroke coalfield lies in this southern area and has coals largely of anthracite rank with volatile matter contents of between 10.1 and 5% Grade of metamorphism as indicated by crystallinity and by coal rank data from the Pembroke coalfield shows anomalous results to that described from the main South Wales coalfield. A neo-formed 2M illite from the Variscan spaced cleavage is described with analytical and X-ray diffraction data.

Mineralogical Magazine; September 1980 v. 43; no. 331; p. 857-863; DOI: 10.1180/minmag.1980.043.331.06
© 1980, The Mineralogical Society
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