Small volumes of garnet peridotite occur within large dunite bodies in the Western Gneiss Region of the Norwegian Caledonides. Previous Sm-Nd analyses of garnet peridotite yield Proterozoic ages close to the crystallization ages of the host gneisses. In situ (laser-ablation multicollector–inductively coupled plasma–mass spectrometry) Re-Os analysis of sulfides in the garnet peridotites gives a range of Proterozoic and Archean model ages. An Archean (2.7–3.1 Ga) protolith age is supported by whole-rock Re-Os data for sulfide-free dunites from several bodies. The Archean ages imply a melt-extraction event in the lithospheric mantle that predates the growth of the known Proterozoic upper crust in the region.