Nd and Sr isotopic data are reported for metallic iron-bearing volcanics from Disko Island, Greenland. These rocks are the products of contamination of mantle-derived picritic to tholeiitic magmas by Cretaceous to early Tertiary sediments through which they erupted. They offer a unique opportunity to study contamination processes because representatives of both uncontaminated parent magmas and the assimilated sediments can be sampled. The sediments include shales, mudstones and sandstones. They have uniform Nd isotopic composition, but are heterogeneous in Nd and Sr concentrations and Sr isotopic composition, and appear to be mixtures of two Sr components. A metallic iron-bearing flow at Asuk, which is one of the earliest contaminated sequences in the West Greenland basalt province, can be modelled as a simple mixture of uncontaminated olivine tholeiite and an "average" sediment composition. None of the analyzed sediments or sediment-derived xenoliths alone can represent the bulk contaminant material. A high degree of contamination (> 30%) is indicated by Nd, Sr and Mg mixing calculations. A metallic iron-bearing subvolcanic intrusive complex, the Hammers Dal Complex, can be modelled by combined assimilation and fractional crystallization (AFC), although unique solutions cannot be obtained because the exact constitution of the fractionating phases and the composition of the bulk contaminant are not known. High levels of contamination (as much as 30%) and fractionation are indicated by AFC model calculations. Important inferences from this study are: (1) even in apparently simple cases where both parents and contaminants can be directly sampled, contamination processes appear to be complex; (2) chemically and isotopically heterogeneous contaminants may be mixed and homogenized during assimilation so that the "bulk" contaminant cannot be recognized among exposed rocks; (3) in such cases it may appear that AFC is required to explain observed chemical and isotopic trends, when in fact only simple mixing occurred; (4) because processes and endmembers are different to resolve even in this apparently simple case, contamination must always be difficult to prove or disprove if it occurred much deeper than the site of final magma solidification.
