Garnet Sm–Nd data from the Saualpe and the Koralpe (Eastern Alps, Austria): chronological and P–T constraints on the thermal and tectonic history

Abstract

Garnets from recrystallized, staurolite‐ and kyanite‐bearing mica schists from the central Saualpe basement, representing the host rocks of the type‐locality eclogites, give concordant Sm–Nd garnet–whole‐rock isochron ages between 88.5±1.7 and 90.9±0.7 Ma. The millimetre‐sized, mostly inclusion‐free grains show fairly homogeneous element profiles with pyrope contents of 25–27%. Narrow rims with an increase in Fe and Mn and a decrease in Mg document minor local re‐equilibration during cooling. According to phengite geothermobarometry, peak metamorphic conditions at 90 Ma were close to 20 kbar and 680 °C and similar to those recorded by the eclogites. The garnet rims record about 575 °C/7 kbar for the final stages of metamorphism. A phengitic garnet–mica schist, sampled at the immediate contact with the Gertrusk eclogite, gave a garnet–whole‐rock Sm–Nd age of 94.0±2.7 Ma.Garnet porphyroclasts separated from a pegmatite–mylonite of the Koralpe plattengneiss near Stainz are unzoned and show spessartine contents of 15%. Composition and Sm–Nd ages of close to 260 Ma point to a magmatic origin for these garnets.The garnet data from the Saualpe document an intense Alpine metamorphism for this part of the Austroalpine basement. The mica schists recrystallized during decompression and rapid exhumation, at the final stages of and immediately following a high‐P event. The Koralpe data show that high Alpine temperatures did not reopen the Sm–Nd isotope system, implying a closure temperature in excess of c. 600 °C for this isotopic system in garnet.