Isotopic evidence for the age and origin of pitchstones and felsites, Isle of Eigg, NW Scotland

An 8 point whole-rock and mineral Rb-Sr isochron for the Sgurr of Eigg yields an age of 52.1 ± 1.0 Ma (2σ). This is the youngest accurate date yet reported for igneous activity in the British Tertiary Province, and indicates that sub-acid rocks of the Small Isles represent one of the latest episodes of Hebridean magmatism. Pb-isotope compositions of these rocks yield an apparent age of 2660 ± 75 Ma (2σ), interpreted as a mixing line between Lewisian, Torridonian and mantle-derived Pb, and indicating the presence of Lewisian basement under Eigg. Pb, Sr and Nd isotopic evidence points to contamination of basaltic differentiates and granulite-facies Lewisian melts by Torridonian sandstone, but precludes significant involvement of amphibolite-facies Lewisian, which may have been largely removed by erosion before deposition of the Torridonian sediments in the Small Isles.