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

Abstract

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.

Geochemical variation in the Penmaenmawr intrusion (North Wales)

“…Howells et al (1991) accepted these conclusions on the age of the intrusion and followed Ball and Merriman (1989) in assigning it to the ®rst eruptive cycle of Snowdonia (the Llewelyn Volcanic Group). Howells et al agreed with Kokelaar et al (1984), Leat et al (1986), Leat and Thorpe (1986) and Ball and Merriman (1989) in placing the Snowdonian Caradocian magmatism in an ensialic marginal basin environment. Ball and Merriman (1989) recognized two distinct dierentiation suites in the ®rst magmatic cycle, with the majority of the rocks derived from a basic parent, similar to the Foel Grach basalt, but the Penmaenmawr and adjacent small intrusions and some of the extrusive rocks forming a low-Zr group and coming from a dierent basic parent which is not exposed.…”

Snowdon basalts and the cessation of Caledonian subduction by the Longvillian

“…The Tremadoc to Llanvirn volcanic rocks are thought to represent a transition from a Tremadoc magmatic arc, to an early Ordovician extensional, suprasubduction zone marginal basin (Bevins et al 1984;Kokelaar et al 1984;Kokelaar 1986). However, mafic volcanic rocks erupted in Caradoc times have geochemical affinities transitional between those of magmatic arcs and 'within-plate', environments (Croudace 1982;Leat et al 1986;Leat & Thorpe 1986u, 19866) and similar to those of the early Silurian (Llandovery) volcanic rocks of the Skomer Volcanic Group (Thorpe et al 1989) which represents the last major volcanism in the Welsh Basin.…”

“…Ten samples of extrusive basalts from the BPF have been analysed by instrumental neutron activation analysis (INAA) at the Open University. These include 9 new analyses together with sample LY34 from Leat et al (1986). This method allows precise and accurate measurement of Th, Ta, Hf, and a range of REE to relatively low levels (Potts 1987).…”

Volcanic clasts in Silurian conglomerates of the Midland Valley (Hagshaw Hills inlier) Scotland, and their meaning for Caledonian plate tectonics

“…5. In addition, a horizontal dashed line is drawn at 2100 ppm Ti (-0.35 % TiO2) to indicate the field of rhyolites which according to LEAT et al (1986) have usually less than 0.35 % TiO2 and plot below this line.…”