Age and crustal evolution of crystalline basement in western Ireland and Rockall

Great diversity exists in the age, composition and tectonic history of the crystalline basement in Ireland, northwestern Britain and adjacent Atlantic plateaux. Because the crystalline basement provides the fundamental framework for basin development, an understanding of the temporal and spatial variation in basement geology is an important goal of current research efforts. Economic basement in western Ireland and southwestern Scotland chiefly comprises Proterozoic and Lower Palaeozoic supracrustals. However, small areas of Proterozoic orthogneiss have recently been identified, including exposures of the Rhinns Complex on Inishtrahull and Islay (Muir et al. 1994), and the Annagh Gneiss Complex in north Mayo (Fig. 1; Menuge & Daly 1994). Offshore, Proterozoic gneisses have also been recognized on the Rockall Bank (Morton & Taylor 1991). Until the Proterozoic age of these rocks was recognized, it was generally assumed that the crystalline basement was similar to the Archaean Lewisian Complex, which outcrops in northwestern Scotland and in the Outer Hebrides and forms the basement to the West Shetland Basin. Until recently, our understanding of these basement occurrences has, of necessity, often been based on poor quality geochronology, Nd model ages or geological inferences alone. Recent developments in U-Pb geochronology (air abrasion and reduction in laboratory blanks) allow very small sample sizes to be used and can achieve unprecedented accuracy and precision.