Sr-isotope variations in a composite sill: crystal-liquid processes and the origin of the Skye granites
Sr-isotope data from the Rubh’an Eireannaich sill, one of a suite of composite (mafic-silicic) minor intrusions associated with the 59 Ma Skye Intrusive Centre, NW Scotland, indicate that both crystal-liquid fractionation and magma-mixing processes have played important roles in its genesis. The data indicate that two distinct magmas were involved in the formation of the sill: a ferrobasaltic andesite magma, generated by contamination and fractional crystallization of regionally available basaltic magma, and a rhyolite magma which has a significant crustal component. Hybrid (intermediate) compositions at the gradational boundaries between the mafic and silicic components of the sill formed by mixing prior to or during emplacement, whilst fractional crystallization of the unmixed silicic magma occurred almost exclusively after mixing. The identification of a significant crustal component within the silicic portion of the sill suggests that the compositionally-related granites of the Skye Intrusive Centre have a similar origin.