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.

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