Chapter 6 Wales, south and east of the Menai Strait

Introduction

J.M. Horák and W. Gibbons

The two sites covered by this chapter lie to the south-east of the Menai Strait Fault System (Figure 6.1), which is a major Precambrian crustal boundary (Figure 1.1). In contrast to sites dealt with in Chapter 7, which include outboard components of the Avalonian subduction system, such as blueschists, this chapter deals with rocks formed within a magmatic arc located inboard of the former subduction zone (Figure 1.4). The sites cover volcanic and volcaniclastic sequences belonging to the Pebidian Supergroup (St David's Peninsula GCR sites) and the Arfon Group (the Llyn Padarn GCR site), which are the principal outcropping representatives of the Cymru Terrane (see Chapter 1).

Geochemical studies reveal that the Arfon Group and Pebidian Supergroup rocks both possess similar calc-alkaline signatures (Bevins et al., 1995), supporting their joint placement within a single terrane. That the magmatism of the two sequences was broadly contemporaneous is further suggested by U-Pb zircon ages, of 614 ± 2 Ma for the Arfon Group (Tucker and Pharaoh, 1991) and 587 t Ma for the St David's Granophyre, which is the youngest Pebidian component (Patchett and Jocelyn, 1979). Such ages place these igneous rocks within the main phase of Avalonian arc magmatism, as described by Gibbons and Horák (1996), identifiable in southern Britain and more extensively in the Avalonian of Maritime Canada. The two sites, however, show several contrasting features, the most notable of which is the fact that Arfon Group magmatism is dominated by acidic ash-flow tuffs, whereas the Pebidian Supergroup is predominantly composed of basic volcanic rocks.

There are important geochemical differences between the Pebidian Supergroup, the Coomb Volcanic Formation of the Llangynog site and the Johnston Diorite–Benton Volcanic Group, as summarized by Bevins et al. (1995) and discussed further in the introduction to the St David's site. Whereas the former is calc-alkaline, the latter two form part of a bimodal magmatic suite with a more pronounced within-plate component. Such geochemical differences are the basis for suggesting that the Coomb Volcanic Formation and the Pebidian Supergroup may belong to different terranes within the Avalon Composite Terrane (Chapter 1). The putative terrane boundary is taken as the south-western extension of the Welsh Borderlands Fault System (Figure 6.1).

It should be noted that there are two possible occurrences of Arfon-type pyroclastic rocks on Anglesey, these being the welded, acidic ash-flow tuffs of the Bwlch Gwyn Tuff and Baron Hill Formation (Reedman et al., 1984). According to Gibbons and Horák (1996) these rocks, which occupy isolated exposures within the Berw Shear Zone (Figure 7.1), represent fragments of the Cymru Terrane that were tectonically interleaved with the Monian Composite Terrane following cessation and dismemberment of the Avalonian arc (Figure 1.4).

The Sarn Complex and Parwyd Gneiss are further components of the Cymru Terrane but they occur in tectonic contact with the Monian Gwna Group in the Llŷn area and are for convenience described in Chapter 7. The Parwyd Gneiss is represented at the Braich y Pwll to Parwyd GCR site by a single exposure of heavily retrogressed garnet amphibolite and felsic gneisses within the Llŷn Shear Zone (the south-western extension of the Menai Strait Fault System traversing Llŷn, (Figure 7.1)). The metamorphic or igneous protolith age of the gneiss is unknown, and although Beckinsale et al. (1984) undertook Rb-Sr analysis on samples from this outcrop, the result that was obtained, of 542 ± 17 Ma, represents the age of low-grade retrogression of the gneiss. Beckinsale et al. (1984) considered the gneisses and the adjacent Sarn Complex to the south-east to be separate entities, whereas Gibbons (1980), Gibbons and Horák (1990, 1996) and Horik (1993) grouped these two units together as the Sarn Complex. The Sarn Complex includes poorly exposed, highly heterogeneous, calc-alkaline plutonic rocks ranging from gabbros to evolved leucogranites, some of these lithologies being seen at the Penrhyn Nefyn GCR site. Horák (1993) has confirmed the magmatic age of this igneous suite as 615 ± 2 Ma, on the basis of U-Pb zircon data, and suggested that it represents the most northwesterly occurrence of the main Avalonian arc (i.e. the Cymru Terrane) in southern Britain.

The main value of the two sites that are featured in this chapter rests upon the record they provide of some of the oldest geological events in southern Britain. Together with the evidence from the Coomb Volcanic Formation of the Llangynog site, they show how the Avalonian Precambrian arc changed with time, from one with a typical calc-alkaline signature to one involving a more complex, intraplate-type chemistry during later rifting of the arc. Such characteristics record the transition from steady-state subduction to extreme oblique subduction leading eventually to arc dissections (Figure 1.4), processes that are observed in modern and recent arc systems.

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