Abstract

The Ordovician Snowdon Volcanic Centre (SVC) of North Wales comprises a bimodal basalt–subalkaline/peralkaline association emplaced around a caldera within a shallow marine environment. The tectonic setting was associated with closure of the Lower Palaeozoic Iapetus Ocean and cessation of ocean plate subduction. The SVC volcanic products include basaltic lavas and pyroclastic rocks, rhyolitic pyroclastic flow deposits, high-level intrusions, domes, and flows, together with reworked equivalents. A programme of detailed field mapping, sampling, and chemical analysis has been used to evaluate the structure and magmatic evolution of the SVC volcanic system. SVC basalts show a range in chemical characteristics between volcanic arc type and within-plate, ocean island basalt (OIB) type. Subalkaline, silica-oversaturated intermediate intrusions (icelandites) and five chemically distinct groups of extrusive and intrusive subalkaline/peralkaline rhyolites (termed A1, A2, B1, B2, and B3) were emplaced during the evolution of the SVC. This evolution was driven by material and thermal input from basaltic magma. The SVC basaltic lavas were derived as partial melts from a heterogeneous volcanic arc to OIB-type spinel lherzolite mantle and experienced up to 60% olivine gabbro fractionation during storage in sill networks in the sub-crust or lower crust. Some magma batches experienced further fractional crystallization (˜70%) and minor crustal contamination (˜10%) to yield the icelandites. Trace element and Nd isotope data do not favour an origin for the rhyolites by partial or total fusion of likely crustal material, and the five rhyolite groups are regarded as distinct homogeneous batches of magma derived from varied basaltic magmas. The icelandites and peralkaline rhyolites (group B3) result, respectively, from ˜50% and ˜80–90% zircon-free fractional crystallization of SVC basalts. The subalkaline rhyolites (groups A1 and B1) result from ˜80–90% fractional crystallization of subduction-related basalts similar to those of Ordovician basalts which pre-date the Lower Rhyolitic Tuff Formation, and groups A2 and B2 were formed by mixing and homogenization of A1, B1, and B3 magma batches. These data and interpretations provide the basis of a model for the complex evolution of a silicic magma system below the SVC caldera around the time of cessation of Caledonian subduction in North Wales. Rhyolite magma chambers were short lived and discontinuous; the largest was probably disc shaped and was almost entirely evacuated during a >60-km3 ash-flow eruption.

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