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Clay mineral evidence for low-grade Caledonian and Variscan metamorphism in south-western Dyfed, south Wales

Published online by Cambridge University Press:  05 July 2018

D. Robinson ,
R. A. Nicholls  and
L. J. Thomas
D. Robinson
Affiliation:
Department of Geology, The University, BristolUK
R. A. Nicholls
Affiliation:
Department of Geology, The University, BristolUK
L. J. Thomas
Affiliation:
Department of Geology, The University, St. Andrews, FifeScotland
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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.

Type
Research Article
Information
Mineralogical Magazine , Volume 43 , Issue 331 , September 1980 , pp. 857 - 863
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1980

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