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Gneiss domes preserved throughout the Paleoproterozoic (1870-1820 Ma) Penokean orogenic belt are commonly viewed as collisional features. However, 1700 Ma mica 40Ar/39Ar ages from Archean rocks of the McGrath gneiss dome, Minnesota, are in marked contrast with 1750-1760 Ma mica ages obtained everywhere outside the dome, suggesting the dome may have formed well after compression had ceased. We hypothesize that the bimodal age pattern is a result of crustal excision between Archean basement and Paleoproterozoic cover during extensional collapse of the over-thickened orogen. Removal of structural section and progressive unroofing may have led to the development of a domed basement-cover shear zone. In this respect, the McGrath gneiss dome would represent one of the oldest analogues of a metamorphic core complex. Postorogenic collapse of the Penokean orogen was apparently long-lived, with significant pulses occurring at [approximately]1755 Ma and at [approximately]1700 Ma The sudden and widespread crustal fusion at 1770-1760 Ma that preceded the proposed collapse suggests that rapid sub-crustal lithospheric thinning may have triggered collapse as has been proposed recently for several Phanerozoic collapsed orogens.