Polyphase metamorphism of Austroalpine basement units in the Eastern Alps: hints to Early Devonian subduction

The Austroalpine nappe stack includes basement units with a partly polyphase pre-Alpine history that were part of the northern Gondwana margin before the Variscan orogeny. Although Alpine metamorphism strongly overprinted many of these units, parts of the Silvretta-Seckau Nappe System preserve a complex record of earlier metamorphic events. This study reconstructs the pressure-temperature-time (P-T-t) evolution of the Seckau and Speik complexes, two key elements of the Silvretta-Seckau Nappe System, to refine models for the tectonic evolution of the Eastern Alps.

Metapelitic rocks of the Seckau Complex (Glaneck Metamorphic Suite) document a polyphase metamorphic evolution. Garnet textures and compositions record two distinct growth stages. Early garnet nucleation occurred at approximately 550°C and 0.4-0.5 GPa, followed by rim growth at higher pressures (1.1-1.4 GPa) and temperatures of 570-620°C, the latter being characteristic for conditions of Eo-Alpine metamorphism. Zr-in-rutile thermometry consistently yields temperatures around 600°C, corroborating these estimates. Locally, EPMA monazite ages of metapelites in the range of ~68-64 Ma indicate Late Cretaceous metamorphic overprinting, suggesting a tectonic affinity of parts of the Seckau Complex with the adjacent Koralpe-Wölz Nappe System. The Glaneck Metamorphic Suite is associated with plutonic suites, that are related to magmatic episodes from the late Cambrian-Early Ordovician (Mandl et al., 2018) through early Carboniferous to the late Permian, as constrained by U-Pb zircon ages from calc-alkaline and predominantly peraluminous metagranitoids with I- to S-type characteristics.

In contrast, the Speik Complex preserves evidence of high-pressure metamorphism related to Early Devonian oceanic subduction. This ophiolitic unit comprises serpentinized ultramafics, (garnet-) amphibolites, rare eclogites, and subordinate gneisses and marbles. Eclogites contain garnet, omphacite/clinopyroxene, amphibole and zoisite, typical of high-pressure metamorphism. Garnet textures show homogeneous compositions with spessartine-rich cores, while others display two-stage growth with rims having higher grossular and pyrope contents. Geothermobarometry and thermodynamic modelling indicate peak conditions of 600-650°C at 1.3-2.0 GPa. Whole-rock geochemistry shows a tholeiitic trend, with dominantly MORB, but also arc-related affinities, confirming an oceanic protolith. U-Pb zircon ages from metabasaltic dikes within serpentinite (403-395 Ma), together with Sm-Nd garnet-whole rock isochrons on amphibolite (413-406 Ma) and ⁴⁰Ar/³⁹Ar amphibole cooling ages of ~397 Ma (Faryad et al., 2002) constrain high-pressure metamorphism in the Early Devonian, preceding Variscan continental collision. Metagranitoids of the Speik Complex yield late Cambrian (503-493 Ma) ages, consistent with recently published ages from Guan et al. (2025). However, a metagranitoid sample from the same area yields a middle Permian age of 272.3 ± 3.2 Ma.

Together, these results indicate that the Seckau Complex preserves a polyphase metamorphic history from pre-Variscan to Alpine times, whereas the Speik Complex represents remnants of oceanic lithosphere as part of an Early Devonian suture zone, related to subduction of an oceanic basin that formed along the northern Gondwana margin (Neubauer et al., 2022; Finger & Riegler, 2023). Their combined P-T-t paths highlight a complex mosaic of continental and oceanic domains later assembled during Alpine orogeny.