GEOCHEMICAL CHARACTER OF NEOPROTEROZOIC VOLCANIC-ROCKS IN SOUTHWEST WALES

Volcanic rocks of Neoproterozoic age form several fault-related inlier s in Wales and the Welsh Borderland. Those in North Wales and the Bord erland have been investigated recently, while those in southwest Wales remain little studied. In southwest Wales, basic to intermediate lava s and related volcaniclastic rocks and silicic tuffs in the St David's area comprise the Pebidian Supergroup, which is intruded by minor bas ic intrusions, while the Coomb Volcanic Formation, exposed farther eas t around Llangynog, is composed of basic and silicic lavas and silicic tuffs, intruded by a single dacite sill and numerous dolerite sills. New geochemical data show that both the Pebidian and Coomb basalts hav e subduction-related characteristics. The Coomb basalts are geochemica lly similar to those of the Uriconian Group of the Welsh Borderland, w hile the Pebidian basalts bear a similarity to volcanic rocks recovere d from the Bryn-teg Borehole, in the Harlech Dome. Uriconian-type volc anic rocks may underlie much of the Midlands Microcraton, with the Wel sh area underlain by volcanic rocks of Pebidian type.