Comparison of thermochronometers in a slowly cooled granulite terrain: Nagssugtoqidian orogen, West Greenland

Uranium-Pb sphene and apatite, and Ar-40/Ar-39 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 metamorphi sm and the subsequent cooling history. Metamorphic monazite growth occurred at 1858 +/- 2, 1830 +/- 1 and 1807 +/- 2 Ma and defines the peak of metamo rphism. The uncertainty in the cooling rates has to include the error in th e decay constants of the systems used. This source of uncertainty is, howev er, 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 i ncreasingly higher closure temperatures have been proposed. This trend refl ects the difficulty of determining 'absolute' closure temperatures and in u sing a limited number of closure temperature estimates to infer closure tem peratures of other geochronometers. Cooling rates at Ussuit were 2.9 +/- 1. 7 degreesC/m.y. from 1762 Ma (similar to 670 degreesC) to 1705 Ma (similar to 500 degreesC), 1.5 +/- 1.1 degreesC/m.y. from 1705 Ma to 1640 Ma (simila r to 410 degreesC), and 0.9 +/- 0.4 degreesC/m.y. between 1640 and 1416 Ma (similar to 200 degreesC). Between 1720 and 1645 Ma cooling rates in Lersle tten, similar to 60 km north of Ussuit, are indistinguishable from those at Ussuit. After 1645 Ma, however, the area cooled to similar to 200 degreesC at a slightly faster rate of 2.6 +/- 1.2 degreesC/m.y.