New petrological and geochronological constraints on the Koralpe–Saualpe–Pohorje Complex (Eastern Alps)

The Koralpe–Saualpe–Pohorje Complex (KSPC) in the Eastern Alps represents a key segment of the Austroalpine basement and hosts the type locality of eclogite. Despite extensive study, its tectono-metamorphic evolution remains debated, particularly regarding a proposed NW–SE increase in pressure–temperature conditions and possible ultra-high-pressure metamorphism in the Pohorje unit. Structural analysis of the KSPC reveals four deformation phases recording a polyphase tectonic evolution: D₁ pre-Alpine deformation with E–W-trending stretching lineation and isoclinal folding, D₂ Alpine deformation with NW–SE-trending stretching lineation and isoclinal folding, D₃ development of Plattengneis fabric with N–S foliation, and D₄ late E–W open folding.

We present new constraints from quartz-in-garnet elastic barometry, Zr-in-rutile thermometry, garnet diffusion modelling and in-situ U–Pb dating of garnet and rutile along a NW–SE transect from Koralpe to Pohorje. This study provides the first application of quartz-in-garnet barometry within the KSPC.

Eclogite samples yield consistent maximum entrapment pressures of ~1.85 GPa across the entire complex, with no systematic spatial variation. Metasedimentary rocks record lower pressures of up to ~1.4 GPa. Zr-in-rutile thermometry indicates uniform peak temperatures of 640 ± 30 °C, likewise showing no thermal gradient. In addition, garnet diffusion modelling suggests short residence times at peak P–T conditions, consistent with rapid burial and exhumation.

Garnet U–Pb ages cluster in the Early Cretaceous (~95–105 Ma), with eclogitic garnet from Koralpe yielding ~112 Ma. A Saualpe metasedimentary sample preserves Triassic garnet cores (~224 Ma) overgrown by Early Cretaceous rims (~115 Ma), indicating polymetamorphic overprinting. Rutile U–Pb ages range from ~98–80 Ma and are interpreted as cooling ages.

Overall, the data suggest that the KSPC is best explained as consisting of several subnappes rather than a single coherent nappe. The absence of a systematic P–T gradient, together with uniform peak conditions, argues against the previously proposed NW–SE metamorphic gradient. Apparent pressure differences between lithologies are most plausibly attributed to fluid-assisted modification of quartz-in-garnet systems rather than primary metamorphic variations. Together with the identified four deformation phases, our results support a homogeneous peak metamorphic overprint within a polyphase tectonic framework.