Pre-Alpine Metamorphic Evolution of the Seckau Complex: Insights from Alpine low-grade metapelitic and magmatic basement units of the Eastern Alps

Pre-Alpine basement units, that originated along the northern Gondwana margin, were integrated into the Austroalpine Nappe System during Alpine nappe stacking. While many experienced significant Alpine overprinting, some units underwent only low-grade metamorphism, preserving valuable records of their pre-Alpine history. The Seckau Complex, part of the Silvretta-Seckau Nappe System, was subject to greenschist facies metamorphism during Eo-Alpine times and retains mineralogical assemblages related to Variscan or even pre-Variscan processes. To deepen the understanding of the pre-Alpine metamorphic evolution of the Eastern Alps, we apply petrological, geochronological, geochemical and geothermobarometric techniques to analyze in particular the metapelitic sections of this basement unit and reconstruct its tectonic and metamorphic history.

The Seckau Complex features diverse metagranitoids, including the Late Cambrian to Early Ordovician Hochreichart Plutonic Suite and the Late Devonian to Early Carboniferous Hintertal Plutonic Suite. These intrusions are hosted by the Glaneck Metamorphic Suite, which primarily comprises garnet-bearing paragneiss and mica-schist, along with amphibolite and tschermakite-bearing gneiss, the latter of potentially magmatic origin. U-Pb zircon dating of paragneisses reveals a detrital provenance with age clusters spanning in the Neoarchean, Paleoproterozoic and Ediacaran (2.7 Ga to 559 Ma). A migmatized paragneiss provided an age of 505 Ma, suggesting that migmatization was likely induced by the intrusion of the Hochreichart Plutonic Suite and indicating a pre-Variscan metamorphism between 559 Ma and 505 Ma (Mandl et al., 2018). Many metapelite samples exhibit a two-stage garnet growth with significant grossular enrichment towards the rims, pointing to a younger metamorphic event likely related to the Variscan orogeny. Results from geothermobarometry and thermodynamic modeling indicate an initial garnet growth at ~550°C and ~0.4–0.5 GPa and rim formation at ~570°C-620°C and ~1.1–1.2 GPa. Zr-in-rutile thermometry yields consistent temperatures of approx. 600°C. Results from monazite dating by EPMA of garnet-bearing mica-schists from the area of Eisenpass provide a weighted average U-Th-total Pb age of 64 ± 3 Ma, suggesting growth of metamorphic monazite during Alpine metamorphism. Garnet-bearing amphibolites show homogeneous garnet composition with subtle spessartine enrichment towards the core, indicating garnet growth along a pro-grade metamorphic path. Geothermobarometry and thermodynamic modeling indicate peak metamorphic conditions of ~620°C and ~0.7–0.8 GPa, again with consistent Zr-in-rutile temperatures of 600-620°C. Geochemical analyses of the metabasites reveal a tholeiitic differentiation trend derived from basaltic to andesitic protoliths. Trace element compositions suggest affinities to MORB and Within-Plate-Lava signatures.