The geology of Anglesey

© Craven & Pendle Geological Society

Coedana Granite (SH 381 765)

Caer Claw Quarry, Gwalchmai is a working quarry in the Coedana Granite. Within the Coedana terrane the granite occupies a central position. It runs NE-SW with hornfels to the SE and Central Anglesey Gneisses to the NW. To the SW the granite is exposed on the beach at Porth Nobla and near Rhosneigr.

Coedana Granite Quarry

Greenly (1919) did the first detailed study of the granite. He noted two distinct granite textures - old foliation and mylonitization. Other key works by Shackleton (1954, 1956), Baker (1969) and Gibbons (1983) refined the understanding of the Coedana Granite. However, it was Gibbons’ (1990) who grouped the Coedana Granite and its hornfels with the Central Anglesey Gneisses to form the Coedana Complex. For more detailed information on the Coedana granite I refer you to Horák, J.M., 1993. The Late Precambrian Coedana & Sarn Complexes. PhD thesis, University of Wales, Cardiff.

The Coedana granite can be found in a range of fabrics. Firstly it can be a greenish-pink medium-grained porphyritic granite of is medium-grained, less commonly coarse-grained, with a greenish pink colour.

Non-porphyritic variety is the most abundant in Gwalchmai Quarry. It is usually a found either rich in mafic minerals and biotite to a more paler version variety. granite is the most abundant facies within the Coedana Granite. Its mineralogy varies from that of a relatively mafic, biotite-rich rock, which more closely resembles the porphyritic facies, to a more leucocratic variety. The granite varieties frequently show evidence of ductile deformation. Usually it weathers white to pale green and frequently shows very little evidence of its main igneous texture.

Coedana Granite

Aplite is the least common variety of Coedana Granite. It frequently occupies narrow up to 1.5 m wide cutting the other granite varieties. It is pale pink to cream in colour and contains small muscovite laths.

Summary

The Coedana Granite represents the most extensive outcrop of plutonic igneous rock within the Monian Composite Terrane. It appears to be a typical calc-alkaline volcanic arc granite generated in the late Precambrian Avalonian subduction system.