Comparison of Thermochronometers in a Slowly Cooled Granulite Terrain: Nagssugtoqidian Orogen, West Greenland |
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| Authors: | WILLIGERS B J A; KROGSTAD E J; WIJBRANS J R |
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| Institution: | 1DANISH LITHOSPHERE CENTRE, ØSTER VOLDGADE 10, DK-1350 COPENHAGEN K, DENMARK
2VRIJE UNIVERSITEIT, FACULTY OF GEOLOGY, DE BOELELAAN 1085, 1081 HV AMSTERDAM, THE NETHERLANDS |
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| Abstract: | Uranium–Pb sphene and apatite, and 40Ar/39Ar hornblende,muscovite and K-feldspar ages from the core of the ProterozoicNagssugtoqidian orogen, West Greenland, are used to constrainthe timing of granulite-facies metamorphism and the subsequentcooling history. Metamorphic monazite growth occurred at 1858± 2, 1830 ± 1 and 1807 ± 2 Ma and definesthe peak of metamorphism. The uncertainty in the cooling rateshas to include the error in the decay constants of the systemsused. This source of uncertainty is, however, negligible ifa single decay scheme is used or when the age difference betweenthe chronometers is large (>100 m.y.). Over the last twodecades increasingly higher closure temperatures have been proposed.This trend reflects the difficulty of determining ‘absolute’closure temperatures and in using a limited number of closuretemperature estimates to infer closure temperatures of othergeochronometers. 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 1640Ma (410°C), and 0·9 ± 0·4°C/m.y.between 1640 and 1416 Ma (200°C). Between 1720 and 1645Ma cooling rates in Lersletten, 60 km north of Ussuit, are indistinguishablefrom those at Ussuit. After 1645 Ma, however, the area cooledto 200°C at a slightly faster rate of 2·6 ±1·2°C/m.y. KEY WORDS: 40Ar/39Ar and U–Pb geochronometers; granulite metamorphism; slow cooling; T–t path |
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