ISOTOPIC EVIDENCE FOR THE EXTENT OF EARLY PROTEROZOIC BASEMENT IN SCOTLAND AND NORTHWEST IRELAND

Tightly clustered Sm-Nd model ages, with an average of 1.96 +/- 0.02 Ga, for the gneiss complex of Inishtrahull indicate coeval development with the early Proterozoic gneiss terrane of Islay. The extent of this terrane, largely beneath the Dalradian Supergroup, is argued to be 100 x 600 km, from northeast Scotland to western Ireland. This is based on the distribution of dated basement in conjunction with Pb, Sr and Nd isotope systematics and inherited zircons in Caledonian granites of the region.