Petrology and geochemistry of late Precambrian volcanic rocks of the St. David's area, Pembrokeshire, South Wales (U.K.)

Abstract

Late Precambrian volcanic rocks near St. David's, Pembrokeshire (U.K.) constitute a bimodal basaltrhyolite assemblage. A new structural interpretation changes the presently accepted stratigraphical succession, the main effect of which is to place the Rhosson basalts at the base rather than at the top of the sequence. The olivine bearing basalts are subaerial and are normative olivine tholeiites. They have immobile element distributions characteristic of non arc environments. Unusual types of basaltic agglomerate and lapilli tuffs are described some of which may be tuff-lavas or hot debris flows. The rhyolitic rocks comprise lavas, breccias and ash flows, the latter being reported for the first time from the Precambrian of this area. They were also subaerially erupted and the bimodal basalt-rhyolite association may be a post calcalkali phase of eruption on continental crust; possibly in an ensialic back arc environment.