Penrhyn Bodeilas

[SH 318 422]

T.P. Young and W. Gibbons

Introduction

The Penrhyn Bodeilas GCR site (Figure 6.61) preserves one of the best-exposed and most accessible of several intermediate to acid Ordovician intrusions that crop out east of Nefyn in northern Llŷn. The intrusion is particularly interesting in being rich in co-magmatic enclaves, and is considered to represent a sub-volcanic intrusion.

Description

The Penrhyn Bodeilas Granodiorite Intrusion is exceptionally well exposed in coastal outcrops around the headland of the same name. The intrusion is a coarse-grained, greyish coloured granodiorite. It contains crystals of plagioclase 3–4 mm in length and of intermediate composition (An₃₂). Similarly sized clots of manic minerals (hornblende, some clinopyroxene, chlorite and magnetite) occur together within a fine-grained (1 mm) groundmass, mostly of plagioclase together with quartz-feldspar intergrowths. Enclaves include examples of both basic and intermediate composition (' dolerite' and 'andesite' of Tremlett, 1962) (Figure 6.62). A late-stage, more evolved magmatic component is represented by thin aplitic veins, most of which are steep and strike NNE–SSW. Some of these late-stage aplites show chilled margins against the main body of the intrusion (Tremlett, 1962).

Interpretation

The Penrhyn Bodeilas Granodiorite Intrusion is interpreted as one of a suite of Caradoc age sub-volcanic intrusions. It remains unclear whether this intrusion was directly related to the magmatism associated with either of the two Caradoc age magmatic centres in Llŷn (Young et al., in press). The geographical position of the intrusion is marginal to the area of distribution of the Llanbedrog Volcanic Group (Woolstonian), but it lies closer to the more northerly centre. Volcanic rocks from this northern centre include the Upper Lodge and Allt Fawr Rhyolitic Tuff formations both of which are interpreted as ?Soudleyan–Longvillian in age (Young et al., in press). Analyses presented by Croudace (1982) plot in the trachyandesite field on the Zr/TiO₂ vs Nb/Y diagram (Winchester and Floyd, 1977).

Conclusions

The Penrhyn Bodeilas Granodiorite Intrusion is a well-exposed example of a high-level subvolcanic slightly alkaline intrusion belonging to one of the magmatic centres that developed on Llŷn in Caradoc times. It is of particular interest in containing a suite of abundant and compositionally variable co-magmatic enclaves, and a well-developed late aplitic facies preserved as a swarm of steeply inclined dykes.

References