Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces within the southern British Caledonides.

At Parys Mountain, Snowdonia and Avoca, bimodal associations of Upper Ordovician basalt and rhyolite with a submarine origin occur within the Caledonian orogenic belt. The rocks have undergone hydrothermal alteration and low-grade metamorphism. The basalts are transitional between volcanic arc and within-plate types; one group of rhyolites (low Zr) is originally of high-K, subalkaline type, the other (high Zr) originally peralkaline. The evidence suggests magma production in a complex tectonic setting, transitional between an active continental margin/island arc and an extensional environment. Associated sulphide mineralization is volcanogenic and probably synsedimentary, the convecting fluids probably driven by high-level rhyolitic magma chambers.-R.E.S.