The Age of the Connemara Schists and of their Metamorphism

Extract

In Connemara and South Mayo, three main rock groups have been distinguished. The first group, largely developed in South Mayo (see sketch-map), consists of slates, phyllites, grits, conglomerates and thin limestones, and, although these rocks are much cleaved, sufficient fossils have been found to prove that they vary in age from Arenig to Ludlow. The second group of rocks, occurring principally in northern Connemara, consists essentially of quartzites, limestones and schists, and in this group fossils have not been found. In the pelitic schists of this group, biotite, garnet, staurolite and fibrous sillimanite are of widespread occurrence, proving that the group as a whole is in a state of medium to high grade regional metamorphism. We propose that the term Connemara Schists, which has previously only been used vaguely, should be restricted to this group. The third group, occurring in Southern Connemara, is a gneissic series which was tentatively correlated with the Lewisian Gneiss by early workers, and which is indeed remarkably similar to it in general appearance. Later work by Callaway (1887) and McHenry (1903) has, however, shown that these gneisses are not overlain unconformably by the Connemara schists, but are orthogneisses intrusive into them. Their age, therefore, relative to the Connemara Schists, is not in doubt.