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

Abstract

Mineral assemblages within metabasites from the paratectonic Caledonides of Wales show that the grade of metamorphism was predominantly within the prehnite-pumpellyite facics, although greenschist facies assemblages occur in the more central parts of the region. General agreement with this pattern of metamorphism is Seen in recent studies of illite crystallinity and conodont colour. Comparison with other regions suggests that all of the paratectonic Caledonides of the British Isles has suffered a low-grade of regional metamorphism during late Caledonian times.

A re-appraisal of the petrogenesis and tectonic setting of the Ordovician Fishguard Volcanic Group, SW Wales

“…The rocks analysed in this study have all undergone low-grade sub-greenschist facies metamorphism (Bevins & Rowbotham, 1983; Robinson & Bevins, 1986) and this is reflected in the abundance of chlorite observed in the thin-sections along with less common pumpellyite, prehnite and epidote. Varying degrees of albitization have affected the rocks and so the concentrations of Al, Ca and Na in particular have been modified (Bevins, 1982).…”

“…(zeolite to low-grade greenschist facies) metamorphism, primarily due to burial (Bevins & Rowbotham, 1983;Bevins & Robinson, 1988;Robinson et al 1999).…”

“…The overall consensus is that the Ordovician rocks of Wales were emplaced in a supra-subduction back-arc basin, in a mostly submarine environment on immature continental crust on the southeast side of the Iapetus Ocean (Bevins, 1982; Bevins, Kokelaar & Dunkley, 1984; Kokelaar et al 1984 b ; Leat et al 1986). All of the Lower Palaeozoic rocks in Wales and the Welsh Borderland were affected by low-grade (zeolite to low-grade greenschist facies) metamorphism, primarily due to burial (Bevins & Rowbotham, 1983; Bevins & Robinson, 1988; Robinson et al 1999).…”

Alteration fabrics and mineralogy as provenance indicators; the Stonehenge bluestone dolerites and their enigmatic “spots”

“…Thorpe et al (1991), in their comprehensive (but essentially geochemical) review of the geological sources and mode of transport of the Stonehenge bluestones, provided a very brief petrographic description of the dolerites and noted the anhedral form of the Preseli dolerite spots, their irregular/unsystematic distribution (from sometimes forming up to around 15% of a rock surface to only one spot being visible on an area of ca. 1 m 2 ) and their metamorphic mineralogy, and concluded that they represent porphyroblasts arising from the low-grade metamorphism described by Bevins and Rowbotham (1983) and . Thorpe et al (1991) did, however, describe for the first time the chemistry of the spots compared to the host dolerite, recording that the spots have markedly higher concentrations of Al 2 O 3 , Sr and CaO and greatly depleted concentrations of the incompatible elements Nb, Y and Zr when compared to the host dolerite.…”

The ordovician (caradoc) volcanic rocks of montgomery, Powys, N. Wales

“…With the exception of early compaction (1 above) and the late illitization and pumpellyite replacement (7 above) which are probably attributable either to deep burial diagenesis or to a low grade regional metamorphic event thought to have affected this area (Bevins and Rowbotham 1983), the diagenetic sequence 2-6 is typical of marine volcanic sandstones (Almon ef af. 1979;Surdam and Boles 1979;Lee and De Vreis Klein 1986).…”