The age of the Garabal Hill-Glen Fyne igneous complex has been determined as 392+ or -4 m.y., by the Rb-Sr method. The components of the complex had an average initial ( 87 Sr/ 86 Sr) 0 isotope ratio of 0.705 + or - 0.003. Geological contamination of marginal components is reflected in the ( 87 Sr/ 86 Sr) 0 values of these rocks which were found to be ca. 0.710, significantly higher than values from uncontaminated samples. The results are in accord with the view that the individual members of the complex had a common source, comparable with regard to the Rb/Sr ratio, to the source of basalts. It is inferred that complete melting of such source material followed by magmatic differentiation is an appropriate mechanism of origin for the complex.