Garnet Sm-Nd and U-Pb systems: A case study of a granulite from the European Variscan belt |
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Authors: | Fukun Chen |
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Affiliation: | Institut für Mineralogie, Petrologie und Geochemie, Universität Tübingen, 72074 Tübingen, GermanyInstitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029 ,China |
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Abstract: | This study presents zircon and garnet ages of a mafic granulite from the high-grade Variscan basement of the Black Forest, Germany and discuss isotope closure temperature of garnet Sm-Nd and U-Pb systems. Zircon grains yield 207Pb/206Pb ages between ∼340 and ∼414 Ma by the U-Pb and evaporation methods. In contract, garnet dating gives Sm-Nd and Pb-Pb isochron ages of (398±3) Ma and (411±14) Ma, respectively, which are older than most of zircon ages. These data imply that most of zircons lost radiogenic Pb, probably due to metamictization or recrystallisation during the granulite-facies metamorphism (−800°C) at −340 Ma. Garnet Sm-Nd and U-Pb systems preserve chronological information of pro-grade metamorphism, probably profiting from a fluid-absence metamorphic environment. These results demonstrate that garnet mineral can be a better candidate than zircon mineral to date high-grade metamorphism by the U-Pb and Sm-Nd methods in some cases. |
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Keywords: | Variscan granulite garnet Sm-Nd U-Pb |
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