The “Permian event” in the Eastern European Alps: Sm–Nd and P–T data recorded by multi-stage garnet from the Plankogel unit

Abstract

Kyanite–staurolite–garnet micaschists from the Plankogel unit, Saualpe–Koralpe region, Eastern European Alps, document a poly-metamorphic evolution. Textures indicate two stages of garnet growth, interpreted as (i) relics of an early, pre-Alpine metamorphic event and (ii) newly-grown garnet of Alpine age. The age of the first crystallisation event during prograde Permian metamorphim is constrained by Sm–Nd dating of the almandine-rich garnet core domains. Mineral–wr ages of nine high-Sm/Nd garnet single crystal core fractions from three distant localities (Laaken, SE Koralpe; Plankogel, NW Saualpe; Friesach, E Gurktal Alps) range between 275.0 ± 2.7 and 264.1 ± 3.6 Ma. The pooled mean regression age of just the garnet fractions is 269.6 ± 6.2 Ma (MSWD = 4.3; n = 9), with an initial Nd isotope composition of εNd(270) = − 8.6. Leaching experiments and whole rock analyses revealed slight Nd isotope disequilibrium between garnet porphyroblasts, REE-rich inclusions, and the matrix of the polyphase mineral assemblage. Kyanite inclusions in the inner and sillimanite in the outer parts of the garnet cores, decreasing grossular and increasing pyrope contents indicate variable pressure/temperature conditions during Permian garnet growth. Modelling of mineral assemblages in pressure–temperature pseudosections suggests that the first-generation, Permian garnet crystallized at about 0.46 GPa and 540 °C, whereas the eo-Alpine garnet documents distinctly higher pressures of 1.0 GPa and 600 °C. The data document regional prograde metamorphism in metapelites, at low or moderate pressure conditions, during the Middle Permian. Crustal-scale extensional processes and heat input from the mantle, induced by ongoing Paleotethys subduction and Neotethys opening in the course of the separation of Europe and Africa-related Adria were responsible for this low-P tectonothermal event.