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Abstract

22 hornblende K-Ar ages and 10 39Ar-40Ar spectra were obtained for hornblende garbenschists from the Western Tauern Window. The post-kinematic amphiboles were produced during the late Alpine prograde metamorphism (6-10 kb and 500-570° C). Two nearly potassiumfree cummingtonites rimming hornblende yield K-Ar ages of 120 Ma, while the 20 tschermakitic hornblendes scatter between 17 and 37 Ma. The reason for this scatter is excess Ar, possibly incorporated into amphiboles during healing of fractures, now traceable by trails of fluid inclusions. Excess Ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with K-Ar isochrons. It can be quantified in 4 out of 10 hornblendes by 39Ar-40Ar stepwise heating experiments. Ages of 18-20 Ma result for corrected hornblendes. The retentivity of 40Ar, after correction for excess, shows no correlation with chemistry within the narrow compositional range observed; rather, it shows intriguing correlations with irregularities in Ca/K spectra, pointing to a microstructurally controlled mechanism for Ar loss. This observation leads to a critical evaluation of the closure temperature "constant", which apparently depends on an incompletely known number of mineralogical and environmental parameters. In particular those 39Ar-40Ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure. This gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media.