New petrological and geochronological results from the Koralpe-Saualpe-Pohorje Complex (Eastern Alps)

Abstract

The Koralpe-Saualpe-Pohorje Complex (KSPC) in the Eastern Alps stretches from SE Austria to NE Slovenia and hosts the type locality for eclogite. Although the KSPC has been studied for decades, some aspects of its tectonometamorphic evolution are still controversial. There is, for example, an ongoing discussion, if the Pohorje unit experienced ultra-high-pressure (UHP) conditions during the Eoalpine orogeny. The KSPC is part of the Austroalpine basement units and was interpreted to represent a coherent (U?)HP nappe consisting mainly of gneisses and metasedimentary rocks, with abundant eclogite lenses embedded. Some of the Austroalpine basement units, including the KSPC, experienced a long-lived Permian-Triassic tectonometamorphic event, where gabbros intruded into a thinned crust, which experienced eclogite facies conditions during the Late Cretaceous. A metamorphic field gradient with an increase in peak pressure-temperature (PT) conditions from NW to SE, and UHP conditions for Pohorje, has previously been proposed based on thermodynamic modelling, geothermobarometry and the discovery of diamond in fluid inclusions in garnet. To unravel the metamorphic evolution of the KSPC, we applied quartz-in-garnet elastic barometry, Zr-in-rutile thermometery and in-situ U-Pb dating of garnet and rutile from eclogite and metasedimentary rock samples along a NW-SE transect. This is the first application of quartz in garnet elastic barometry within the KSPC in order to determine the entrapment pressures of the quartz inclusions. The eclogite samples yielded maximum pressures of 1.9 GPa across the KSPC, indicating no pressure increase from the NW to SE. The metasedimentary rocks show overall lower pressures with a maximum of ca. 1.4 GPa. Zr-in-rutile thermometry yielded uniform temperatures of 640 (±30)°C, indicating no temperature gradient. The novel approach of in-situ garnet U-Pb dating was conducted to decipher potentially different metamorphic events. Garnet from the Koralpe, Saualpe and Pohorje metasedimentary rocks yielded Early Cretaceous dates ranging from ~95 to 105 Ma, similar to eclogitic garnet from the Koralpe with ~112 Ma. Additionally, garnet from a Saualpe micaschist yielded Late Triassic cores (~224 Ma) and Early Cretaceous rims (~115 Ma). Rutile throughout the KSPC yielded Late Cretaceous U-Pb dates of ~98–83 Ma (eclogites) and ~87–80 Ma (metasedimentary rocks).The results of this study suggest that the KSPC represents a coherent nappe. The recorded maximum pressures and temperatures are identical throughout the KSPC. The lower pressure for the metasedimentary rocks is interpreted to be the result of viscous relaxation in garnet due to the presence of fluids during metamorphism. The obtained garnet U-Pb dates from both eclogites and metasedimentary rocks are interpreted as Late Cretaceous bulk crystallization ages that reflect prograde to peak metamorphic garnet growth. The Late Triassic U-Pb dates from the Saualpe garnet cores are in line with the existing literature proposing a polymetamorphic cycle for the KSPC. The rutile dates are interpreted as cooling ages. The lack of a metamorphic field gradient may imply a different tectonical setting for the KSPC than previously proposed, and warrants further investigation.