Sr isotope chemistry of contaminated Tertiary volcanic rocks from Disko, central West Greenland

The Sr isotope chemistry of 26 samples of Tertiary volcanic rocks from the Vaigat and Maligat Formations on Disko are presented together with 5 samples of potential sedimentary contaminants from Disko and Nugssuaq. The volcanic rocks include one primitive picrite; 11 basalts, 8 andesites, 4 dacites and 2 rhyolites. Except for two basalts, all the basaltic to rhyolitic rocks are distinctly enriched in radiogenic Sr and this together with petrographical observations is taken as evidence for reaction with crustal rocks. The widespread xenoliths and xenocrysts point to Mesozoic to early Tertiary sediments as the major contami­nants and a shale and a sandstone composition have accordingly been chosen as model contaminants. Assimilation and Fractional Crystallization (AFC) calculations indicate that mafic silicates were the predominant fractionating phases in the contaminated members of the Vaigat Formation, whereas plagioclase crystallization and equilibration played a dominant role in the contaminated members of the Maligat Formation. The Kuganguaq Member in the Vaigat Formation cannot be modelled with a shale contaminant, but easily with a sandstone contaminant, and one dacite sample in the Nordfjord Member of the Maligat Formation is best modelled with sandstone contaminant. For all the other rocks, there is evidence of a dominating shale contamination. The most strongly contaminated rock on Disko analysed for Sr isotopes is a rhyolite from the Nordfjord Member, whereas another rhyolite showed evidence of less contamination but very extensive feldspar fractionation. None of the silica-enriched rocks on Disko appear to be formed by closed system fraction­ation of a basic uncontaminated magma.