Lu-Hf garnet dating of eclogite from Hochgrößen (Speik Complex, Lower Central Austroalpine): Evidence for Silurian (c. 438 Ma) high-pressure metamorphism

In the eastern European Alps, lenses of eclogite occur within a serpentinite body at Hochgrosen in northern Styria, Austria. These rocks belong to the Speik Complex, a subducted meta-ophiolite in the Lower Central Austroalpine basement, and show geochemical characteristics (relative abundances of Ti, V, Sr, Sc, and Si) of MORB magmatites with a backarc basin affinity. Three samples of partly retrogressed eclogites were studied using Lu-Hf geochronology on whole rock and garnet separates. One sample yields a well-defined four-point Lu-Hf isochron at 437.9 ± 0.8 Ma (Early Silurian). Although partly retrogressed to amphibolite facies, prior equilibration in the eclogite facies is confirmed by the occurrence of omphacite (33.2% jadeite component) inclusions in garnet. Garnet shows a bell-shaped Lu concentration profile, indicating that the Lu-Hf age is related to garnet growth, i.e. eclogite-facies metamorphism. The two other samples were more strongly retrogressed and did not yield isochrons, probably because of Variscan metamorphic overprint. With this new age, the Hochgrosen eclogite is now the oldest eclogite dated so far in the Austroalpine nappes and reflects subduction at the northern margin of Gondwana during the Early Silurian.