Abstract
Pb isotopic compositions were determined in rocks from two Proterozoic alkaline complexes in the Gardar Province of South Greenland. Both complexes, Kûngnât Fjeld and Tugtutôq Younger Giant Dyke, embrace a comparable range of compositions (alkali gabbro-ferrosyenogabbro-syenite-granite) but the two lie in contrasting crustal environments. Kûngnât lies within the southern marginal part of the Archaean craton; Tugtutôq lies off the craton in early Proterozoic (Ketilidian) granites. There are significant initial isotopic Pb differences between the two as well as marked Pb isotopic variations within each complex. These latter are attributed to increasing degrees of crustal contamination accompanying crystal fractionation processes whereas the overall differences between the two complexes reflect the differing isotopic signatures of the country rocks, viz. the relatively unradiogenic Archaean host rocks for Kûngnât and the more radiogenic Proterozoic granitic hosts for Tugtutôq.
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