Mid-Caradoc magmatism in central Llyn, rhyolite petrogenesis, and the evolution of the Snowdonia volcanic corridor in NW Wales

Abstract

Ordovician arc magmatism in the Welsh Basin culminated in the development of a NE-SW corridor of intense volcanism extending for > 65 km in northwest Wales from Conwy through Snowdonia to Llanbedrog in the central Llŷn Peninsula. New data from central Llŷn reveal four eruptive phases: Llŷn Phase 1 produced up to 800 m of subalkaline rhyolites and acid ash-flow tuffs (Pen-ychain Rhyolitic Complex), interpreted as the SW margin of the Llwyd Mawr/Pitts Head caldera. The Complex thins rapidly westwards from the eruptive centre, and lies within clastic sediments and minor basaltic lithic tuffs of the Cwm Eigiau Formation (Soudleyan-Longvillian). Llŷn Phase 2 is recorded by a 120 m thick pyroclastic flow (Allt Fawr Rhyolitic Tuff Formation) which interrupted Longvillian marine sedimentation (Cwm Eigiau Formation below; Dwyfach Formation above). This ash-flow tuff is more alkaline than the products of both Llwyd Mawr and Snowdon, and is interpreted as having flowed from an as yet unidentified source in northern Llŷn. Llŷn Phase 3 involved the growth of a major centre which erupted over dominantly marine siltstones (Dwyfach Formation) locally to produce > 1000 m of lavas and tuffs that define the Llanbedrog Volcanic Group. This Llanbedrog volcanism evolved from intermediate to acid compositions, with early basaltic trachyandesites (Penmaen Formation) followed by up to 275 m of rhyodacites (Foel Ddu Rhyodacite Formation) and climaxing with up to 500 m of rhyolitic ash flow tuffs (Carneddol Rhyolitic Tuff Formation). The Llanbedrog Volcanic Group (and associated intrusions) define a geochemically coherent fractionation trend along the subalkaline/alkaline boundary. Around the Llanbedrog volcanic edifice an apron of volcaniclastic-rich sediment (Yoke House Formation) graded outwards into surrounding deeper marine conditions where Dwyfach silts and muds continued to be deposited. Faunal evidence suggests that eruptions took place across the Longvillian to Woolstonian boundary, and final foundering of the Llanbedrog centre was followed by deposition of graptolitic mud (Nod Glas Formation) in Woolstonian times. Llŷn Phase 4 erupted up to 100 m of arc tholeiitic basaltic lavas and tuffs on to these marine muds. Recognition of these four phases on Llŷn confirms the transience of the Snowdonia volcanic corridor, within which all eruptions took place over a few million years inside the span of three mid-Caradoc stages (Soudleyan, Longvillian, Woolstonian) and two graptolite zones (Dicranograptus clingani and Diplograptus multidens). With the exception of early andesitic eruptions in the northeast (Foel Fras) and southwest (Penmaen) of the corridor, magmatism was bimodal and dominated by rhyolite. At least six great (>500 m) rhyolitic ash-flow tuff sequences were erupted (Conwy, Capel Curig, Pitts Head/Llwyd Mawr/Pen-ychain, Snowdon, Llanbedrog and Crafnant). Basaltic eruptions spatially associated with these centres typically constructed strombolian cones and/or accumulated within submarine depressions. Rhyolite petrogenesis is attributed to some combination of sub-crustal melt ponding and fractionation plus partial remelting of older (Avalonian) arc basement within an actively deforming lithospheric corridor of intense magmatic activity. The transient burst of mid-Caradoc volcanicity in northwest Wales represents the climax of some 38 Ma of arc magmatism across the Welsh basin and the sudden cessation of eruptions presumably records a change from oblique subduction to transcurrent movements along this part of the Iapetus margin.