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Abstract

Samples from the dacitic Kroksfjordur volcanic center, NW Iceland, have calc-alkaline compositions contrasting the alkaline to tholeiitic composition commonly observed for felsic rocks from ocean islands or plateaus (JÓNASSON, 2006). New major, trace element, and isotope data show that these samples display a distinct depletion of Nb (La/NbPM = 2.7 ± 0.4; 1σ) and enrichment of Pb (Ce/Pb PM = 0.7 ± 0.1) in primitive mantle normalized trace element plots, and high Na/K ratios (3.6 ± 0.3). The Sr-Nd-Hf-Pb isotopic compositions preclude the involvement of old continental lithosphere in the petrogenesis of the dacitic melts. The limited variation in major element data and the small though systematic variation of La/Nb and Ce/Pb ratios with TiO2 and Ce concentrations, respectively, suggest that the dacites were derived by partial melting of a mafic protolith comprising plagioclase + clinopyroxene + amphibole + Fe-Ti oxides. Inverse modelling shows that the calculated trace element composition of the source is similar to that of mafic crustal xenoliths hosted by one of the dacitic bodies. This observation suggests that partial melting of Icelandic mafic lower crust, most likely during magmatic underplating, has produced the calc-alkaline dacites at Kroksfjordur.