Posttectonic Cooling of the Nagssugtoqidian Orogen and a Comparison of Contrasting Cooling Histories in Precambrian and Phanerozoic Orogens

40Ar/39Ar and U‐Pb mineral ages provide important constraints for the cooling history of the Paleoproterozoic Nagssugtoqidian Orogen (NO), southwest Greenland. In the central segment of the orogen, hornblende yields late Nagssugtoqidian40Ar/39Ar cooling ages of ca. 1700–1740 Ma. North and south of the orogen, hornblende ages predate Nagssugtoqidian orogenesis. This relationship indicates that metamorphic temperatures exceeding ∼580°C were restricted to a region broadly coinciding with the Nagssugtoqidian Orogen as defined on structural elements. Following peak metamorphism, the southern and central segments of the Nagssugtoqidian Orogen cooled contemporaneously, initially with a rate of 3°C/m.yr. (1762 Ma) that later decreased to 1°–3°C/m.yr. (1400 Ma). The northern segment of the orogen cooled originally slightly faster, with a rate of 5°–7°C/m.yr. at 1740 Ma and subsequently at 1°–3°C/m.yr. Such slow and regionally homogeneous cooling, as observed in the Nagssugtoqidian Orogen, is common to many deeply eroded high‐grade Precambrian orogens but contrasts with the generally much faster and heterogeneous cooling reported for Phanerozoic orogens. The observed differences can be explained without invoking a fundamental change of orogenic processes with time. They mainly reflect a combined effect of sample bias and the increasing chance of a geochronometer to become reset with increasing crustal depth. The difference in cooling histories will generally reflect the different stage of the orogenic cycle during which mineral closure occurred. Mineral ages from Phanerozoic orogens are generally closely associated with a variety of tectonic processes occurring during ongoing orogenesis. In contrast, mineral closure in many deeply eroded Precambrian orogens occurred after termination of orogenesis and reflects homogeneous postorogenic exhumation.