Comparison of Thermochronometers in a Slowly Cooled Granulite Terrain: Nagssugtoqidian Orogen, West Greenland

Abstract

Uranium–Pb sphene and apatite, and 40Ar/39Ar hornblende, muscovite and K-feldspar ages from the core of the Proterozoic Nagssugtoqidian orogen, West Greenland, are used to constrain the timing of granulite-facies metamorphism and the subsequent cooling history. Metamorphic monazite growth occurred at 1858 ± 2, 1830 ± 1 and 1807 ± 2 Ma and defines the peak of metamorphism. The uncertainty in the cooling rates has to include the error in the decay constants of the systems used. This source of uncertainty is, however, negligible if a single decay scheme is used or when the age difference between the chronometers is large (>100 m.y.). Over the last two decades increasingly higher closure temperatures have been proposed. This trend reflects the difficulty of determining ‘absolute’ closure temperatures and in using a limited number of closure temperature estimates to infer closure temperatures of other geochronometers. Cooling rates at Ussuit were 2·9 ± 1·7°C/m.y. from 1762 Ma (∼670°C) to 1705 Ma (∼500°C), 1·5 ± 1·1°C/m.y. from 1705 Ma to 1640 Ma (∼410°C), and 0·9 ± 0·4°C/m.y. between 1640 and 1416 Ma (∼200°C). Between 1720 and 1645 Ma cooling rates in Lersletten, ∼60 km north of Ussuit, are indistinguishable from those at Ussuit. After 1645 Ma, however, the area cooled to ∼200°C at a slightly faster rate of 2·6 ± 1·2°C/m.y.