Lu–Hf dating, petrography, and tectonic implications of the youngest Alpine eclogites (Tauern Window, Austria)

Abstract

Isotopic dating of metamorphic minerals places fundamental constraints on the rates and mechanisms of burial and exhumation in collisional orogens. The Eclogite Zone in the Tauern Window has been the focus of many studies on subduction-related high-pressure metamorphism. However, the age and duration of the high-pressure stage remains the subject of ongoing debate. 32Ma 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. These ages have either been interpreted to indicate extremely rapid exhumation from more than 60kilometre depth to mid-crustal levels within 1Mayears or to date retrogression subsequent to high-pressure metamorphism.We present element distribution maps and lutetium–hafnium (Lu–Hf) garnet ages of three samples from the Eclogite Zone. All samples display almost unaltered eclogite-facies assemblages and garnets preserve growth zoning. Lu–Hf ages are thus considered as formation ages recording metamorphism towards peak-pressure conditions. In the sample with the smallest grain size, garnet shows regular bell-shaped element distributions with respect to manganese and the iron–magnesium ratio. A six-point isochron of this sample yields 32.8±0.5Ma (MSWD=1.06), interpreted as the age of Alpine eclogite-facies metamorphism. In one of the other two, coarser-grained samples' garnet chemistry is identical. The third sample, however, shows complex zoning in large garnet crystals. 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±9.8Ma and 62.7±1.8Ma for individual garnet-whole-rock pairs as the analysed garnet fractions display very different 176Hf/177Hf vs. 176Lu/177Hf ratios. This scatter reflects varying degrees of mixing between Alpine and pre-Alpine garnet fractions as represented by the cores of the third sample. The results confirm the Rb–Sr-whole rock ages of Glodny et al. (2005). Despite the problems this result causes for conventional tectonic reconstructions, the eclogites from the Eclogite Zone in the Tauern Window have to be considered as Lower Oligocene in age and are thus the youngest eclogites of the Alps identified so far.