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

Abstract

SummaryIllite 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.

Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales

“…1980). Nevertheless, there has been an increasing interest in the metamorphic character of the region, with Bevins (1978) describing prehnite-pumpellyite assemblages in metabasites from the Fishguard Volcanic Complex of North Pembrokeshire and Robinson, Nicholls & Thomas (1980) identifying both Caledonian and Variscan metamorphic events in south-west Wales using clay mineral and illite crystallinity data. Subsequent regional surveys have been undertaken which utilize metabasite assemblages (Roberts, 1981;Bevins & Rowbotham, 1983) and illite crystallinity (Cronshaw, 1985;, whilst additional data have also become available which are based on conodont colour index (CAI) determinations (Bergstrom, 1980;Savage & Bassett, 1985;Aldridge, in press) and graptolite reflectance studies (Oliver, in press).…”

Low‐grade metamorphism within the Welsh sector of the paratectonic Caledonides

“…It is well established that illite crystallinity increases with metamorphic grade; as the grade increases, illite becomes more ordered, and accordingly the shape of the illite (001) peak becomes sharper and the peak:background ratio increases. Robinson et al (1980) examined illites from Lower Palaeozoic pelites of southwest Dyfed, and concluded that they represented illites of anchizone metamorphism. Therefore they agree with the indication of prehnite-pumpellyite facies metamorphism in metabasites as reported here (if the correlation of Kisch (1974) is followed), and earlier by Bevins (1978).…”

“…It has commonly been referred to as being unmetamorphosed, or as belonging to 'the non-metamorphic Caledonides' (Kelling 1978;Anderson and Owen 1980;Soper 1980). However, in the past few years, a number of workers (Bevins 1978;Oliver 1978;Robinson et al 1980;Oliver and Leggett 1980;Roberts 1981;Bevins et al 1981) have shown that parts of the paratectonic Caledonides have been metamorphosed within the prehnite-pumpellyite facies and lower part of the greenschist facies.…”

A white mica crystallinity study of the Berwyn Hills, North Wales

“…White mica crystallinity has been widely used to calibrate the low-grade metamorphism of rocks in the Welsh Basin (Robinson et al, 1980;Roberts & Merriman, 1985;Robinson & Bevins, 1986;Mackie, 1987;Pratt, 1990). However, there is disagreement regarding the factors that controlled crystallinity variation in Wales.…”