The metamorphic petrology and structure of the district north west of Clifden, CO Galway

Abstract

In the course of the present study the area has been mapped in detail on the six inch scale and a stratigraphical succession has been suggested. The regional metamorphism has been investigated and found to be of amphibolite facies. Metamorphic zones demarcated in pelitic rocks by staurolite, staurolite + sillimanite, and sillimanite + muscovite, have been recognised and the zonal isograds have been mapped. Two post-kinematic granites were intruded into the metasediments, they are adamellitic in composition and are, together with veral other small granite bodies, related to the Galway Granite. The metasediments have been thermally metamorphosed and zones of thermal metamorphism have been mapped, an inner zone of andalusite hornfels, (hornblende hornfels facies), and an outer zone of spotted schist, (epidote actinolite hornfels facies).Fourteen chemical analyses of the metasedimentary rocks were made with the purpose of investigating the question of metasomatism in the regional and thermal metamorphism, and for comparing the metasediments with unmetamorphosed sedimentary rocks. It has been concluded that metasomatism did not occur during either the regional or thermal metamorphism. Chemically the metasediments compare closely with sediments carried into a shelf sea from an adjacent land mass of moderate or low relief and it is concluded that the metasediments were deposited under similar conditions. Three periods of folding have been recognised: 1. East - West, syn-metamorphic 2. East - West, post-metamorphic 3. North - South, post-metamorphic. The first east-west folding resulted in the formation of recumbent, isoclinal nappes. These were subsequently cross folded about an east-west axis to give the present - Connemara anticline which was then deformed by gentle folding about a north-south axis. A swarm of north-south trending dykes was intruded in association with the granite emplacement, and at the same time the area was fractured by a system of faults striking north west - south east, and north east - south west. The area was finally subjected to sub-aerial erosion and glaciated in Pleistocene times.