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In addition, garnet dates can now be used to place quantitative constraints on initial cooling from high- and ultrahigh temperature—a temperature segment that is inaccessible to other thermochronometers. Nouvelles Données Géochronologiques sur le Volcanisme Paléogène des Alpes Occidentales: Existence d'un Evènement Magmatique Bref Généralisé (Séance Spécialisée, Soc. Cores with a very low iron-magnesium ratio are surrounded by a second garnet generation which is very similar to the Alpine generation in the other two samples.The two coarser-grained samples yield scattered ages between 26.9 ± 9.8 Ma and 62.7 ± 1.8 Ma for individual garnet-whole-rock pairs as the analysed garnet fractions display very different Hf ratios.However, the age and duration of the high-pressure stage remains the subject of ongoing debate.32 Ma Sr–Rb ages interpreted to date eclogite-facies metamorphism (Glodny et al., 2005) appear too young in traditional tectonic reconstructions of plate collision in the Alps.In one of the other two, coarser-grained samples' garnet chemistry is identical.The third sample, however, shows complex zoning in large garnet crystals.
A six-point isochron of this sample yields 32.8 ± 0.5 Ma (MSWD = 1.06), interpreted as the age of Alpine eclogite-facies metamorphism.Instead, the isotope dates are interpreted to reflect partial loss of radiogenic Nd, and are used to constrain the systematics of the garnet chronometers at high temperature.The data constrain the grain sizes and thermal histories for which garnet dates can be validly linked to prograde or peak conditions.In contrast, Sm–Nd dates are equal to or younger than the 2641–2637 Ma age of peak metamorphism.The roles of concurrently digested inclusions and core–rim Lu/Sm fractionation are quantitatively evaluated and excluded as causes for the observed age heterogeneity.