Isotope Geochemistry of Tertiary Igneous Rocks from the Isle of Skye, N.W. Scotland

New Pb isotope analyses are reported for forty-seven basic, intermediate and acid Tertiary igneous rocks from the Isle of Skye and adjacent regions of northwest Scotland. New Sr isotope analyses and two Rb/Sr isochron dates are also reported for the acid igneous rocks. These results are combined with published Sr and Nd isotope data for Tertiary igneous rocks from Skye and adjacent regions in order to examine the genesis and evolution of Tertiary magmas in the Isle of Skye. Pb isotope analyses form a linear array on a Pb/Pb isochron diagram, with a slope yielding an apparent age of 2920 ± 70m.y. (2σ), interpreted as a mixing line between ca. 2900 m.y.-old crustal Pb and 60 m.y.-old mantle-derived Pb. Combined isotopic data support a model of selective crustal Pb, Sr and Nd contamination of basic and intermediate mantle-derived magmas, without significant modification of major-element composition. This contamination process is probably due to the breakdown of LIL-element-enriched accessory mineral phases, and does not necessarily produce linear mixing lines on isotope ratio plots of different elements. Skye Redhills epigranites contain a predominant fraction of acid magma which differentiated from mantle-derived basic precursors, and a subsidiary fraction of upper crustal melt. The proportion of this melt is estimated to rise from less than 10 per cent in the older granites to around 30 per cent in the younger. Isotopic and geochemical data are used to draw conclusions about the differentiation histories of the four principal magma types of Skye: the Fe-rich and Fe-poor members of the Skye Main Lava Series, Preshal Mhor Basalts and Redhills epigranites.