The geology of the Nefyn‐Llanaelhaiarn area of north Wales

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This area of Wales is a westerly continuation of the northern limb of the Snowdon syncline and stretches from the Pre-Cambrian volcanic series in the north to just above the abbreviated Snowdon Volcanic Series in the south. Large parts of the intervening area consist of Caledonian intrusive rocks, but the middle part of the sedimentary succession has yielded scattered graptolites, most and probably all, of Llanvirn age. The exposed volcanic rocks are largely of ignimbrite character while the Caledonian intrusions are of four types (for which chemical analyses are given). The sequence of these four rocktypes and of the structural events of the district is considered.

The geology of the Clynnog‐fawr district and Gurn Ddu hills of Northeast Lleyn

Tremlett

1964

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The geology of the Chwilog area of Southeastern Lleyn (Caernarvonshire)

Tremlett

1965

Due partly to folding but mainly to faulting, the Caradocian volcanic rocks outcrop in three districts within this area, and this repetition of outcrop reveals the variation shown by this dominantly rhyolitic volcanic series. A thick lower group of submarine bedded tuffs, thinning and fading out northwestwards, is followed unconformably by subaerial andesite and rhyolitic rocks in the north, but these volcanic rocks are absent or represented by bedded tuffs in the southern districts. The overlying sediments are generally poorly exposed but have yielded Upper Longvillian fossils from a number of localities. Caledonian intrusions are neither so numerous nor so varied as in northern Lleyn, but include granodioriteporphyries and felsites. Explosion breccias are present at the margins of some of the felsites. Nine chemical analyses indicate the variety of composition in the volcanic series while two analyses of Caledonian felsites are also supplied.

Geochemistry of an ordovician basalt‐trachybasalt‐subalkaline/peralkaline rhyolite association from the Lleyn Peninsula, North Wales, U.K

Leat

Thorpe

1986

The Caradoc volcanic rocks of the Lleyn Peninsula represent a major episode of rhyolite-dominated eruption (total volume up to 560 km3) within the Southern British Caledonides. New geochemical data reveal the original chemical characteristics of the basic, intermediate, and acid lavas. The basalts are interpreted as having predominantly within-plate character and are associated with intermediate lavas (trachybasalts) and high-Zr, originally peralkaline, trachytes, and rhyolites. A low-Zr, originally high-K, subalkalic group of rhyolites, and a basalt with transitional island arc tholeiite-MORB characteristics are unrelated to these lavas. The volcanic rocks are interpreted to have been erupted onto a complex, rapidly evolving, active continental margin dominated by crustal tension.