MINERALOGICAL MAGAZINE, JUNE 1985, VOL. 49, PP. 451-456 Pumpellyite-dominated metadomain alteration at Builth Wells, Wales evidence for a fossil submarine hydrothermal system? R. E. BEVINS Department of Geology, National Museum of Wales, Cardiff, CF1 3NP, Wales ABSTRACT. Two metamorphic alteration styles have been observed in basic lavas and volcaniclastic rocks of the Buitth Volcanic Series of Ordovician age exposed at Llanelwedd Quarry, in the Builth lnlier, Wales. The first alteration style is characterized by the development of a mineral assemblage comprising albite+chlorite+ sphene+ pumpellyite _ prehnite + calcite + white mica, although original textures are largely preserved. This alteration pattern corresponds with that developed in volcanic and volcaniclasticrocks elsewherein Wales, and which is attributed to the effects of burial. The second alteration style is characterized by the development of metadomains dominated by pumpellyite or, more rarely, prehnite. In the metadomains no original textures are seen in hand specimen.This alteration pattern has, so far, not been observed elsewhere in Wales, and it is here suggested that it might result from a submarine hydro- thermal circulation system. KEVWOROS: pumpellyite, metadomains, metamorph- ism, Builth Wells, Wales. THE Builth Inlier, Powys, Wales, exposes a sequence of sedimentary, volcanic and volcaniclas- tic rocks which accumulated at the margin of the Welsh Basin during Ordovician times. The exten- sive quarry at Llanelwedd (National Grid Ref. SO 051 521), 1 km northeast of Builth Wells, provides an excellent section through westward-dipping basic lavas and associated volcaniclastic rocks of Llanvirn age, which form a part of the Builth Volcanic Series (Jones and Pugh, 1949). Elsewhere within the series, but not exposed at Llanelwedd, are andesitic to dacitic lavas and intrusions, and rhyolitic tufts. Minor intrusions of basic composi- tion occur in the inlier, but these invade strata of Caradoc age, and are not related to volcanism associated with the development of the Builth Volcanic Series. Basic lavas of the Builth Volcanic Series are typically feldspar-phyric, strongly vesicular and generally massive, although pillowed flows and hyaloclastites are locally developed. This, coupled ~) Copyright the Mineralogical Society with the nature of the associated volcaniclastic sediments both above and below the lavas, suggests that, for much of the time, lava eruption and emplacement occurred in a submarine environ- ment. Jones and Pugh (op. cit.), however, described the presence of a shore-line environment during accumulation of the sequence, and it is likely that 'volcanic islands' existed periodically during development of the pile. The volcaniclastic sand- stones which overlie the lava pile (the Newmead Sands) appear to have been deposited in a shallow marine environment and provide clear evidence of contemporaneous erosion. Chemically the basalt-andesite-dacite-rhyolite volcanic rocks of the Builth Volcanic Series show calc-alkaline affinities (Furnes, 1978), which con- trasts with the predominantly tholeiitic basalt- rhyolite character of lavas from other Ordovician sequences in the Welsh Basin (Kokelaar et al., 1984; Bevins et al., 1984). The Lower Palaeozoic rocks of the Welsh Basin and adjacent areas have suffered low-grade regional metamorphism, generally within the prehnite- pumpellyite facies (Bevins and Rowbotham, 1983), although there is probably a complete range from zeolite facies through to greenschist facies (Bevins et al., in press). Metamorphic alteration has been attributed to the effects of burial (Bevins and Row- botham, op. cit.), although it has been suggested that in Snowdonia and Ll~n the grade may have been influenced, at least in part, by the intensity of deformation (Merriman and Roberts, 1985). Metamorphic alteration in the Builth lnlier. Two metamorphic alteration styles can be observed within the volcanic and volcaniclastic rocks of the Builth Volcanic Series at Llanelwedd. First, and most widely developed, is a mineral assemblage in basic lavas and volcaniclastics comprising albite + chlorite + sphene + pumpellyite • prehnite + cal- cite + white mica, sometimes with relict clinopyro- xene. These secondary minerals occur in varying