Lacking provincial fauna, establishing autochthoneity or allochthoneity of domains in Precambrian orogens typically requires comparison of precollisional attributes across suspect terrane boundaries. We have measured the Pb isotopic compositions of K-feldspar grains in epoxy mounts and rock slabs by a rapid laser ablation–inductively coupled plasma mass spectrometry method with sufficient precision and accuracy to define two large Pb isotopic domains in the Paleoproterozoic Nagssugtoqidian-Rinkian orogen of West Greenland. These domains are interpreted to correlate with and help delineate the Rae and North Atlantic cratons. Their boundary coincides with a recently proposed location of the main collisional suture of this orogen, thus supporting this interpretation. This study validates a rapid analytical Pb isotopic method that will be useful in addressing a range of problems that requires knowledge of crustal affinity in Precambrian terrains.