Pressure-temperature-time evolution of Austroalpine metamorphic rocks from the southeastern Pohorje Mountains

We have studied eclogite, garnet clinopyroxenite, and garnet-bearing micaschist and gneiss from the southeastern flank of the Pohorje Mountains (Mts.) in order to better understand the pressure-temperature (P-T)-time evolution of these rocks. Geochronology was performed by in-situ analyses of monazite in different textural positions with an electron microprobe and a laser-ablation inductively coupled plasma mass-spectrometer. P-T trajectories were obtained by thermodynamic modelling considering strongly the chemical zoning of garnet and mica and the mineral inclusions in these phases. In addition, we calculated the influence on intracrystalline cation diffusion on garnet zoning also to gain time constraints.

Two high-pressure (HP) events were proved for metamorphic rocks of the Pohorje Mts. These events occurred at temperatures between 570-650 °C for micaschist and 670-740 °C for eclogite + garnet clinopyroxenite in Late Cretaceous and Eocene times. In addition, we found that a micaschist sample taken close to the Pohorje pluton was partially overprinted in the Miocene (18.9±0.2 Ma) by this intrusion at depths of 30-32 km. Thus, the subsequent uplift of the Pohorje pluton and its surrounding occurred at a mean rate of 1.6-1.7 mm/a. The studied metamorphic rocks were also significantly exhumed probably soon after the Eo-Alpine event that had led to peak pressures up to about 2.3 GPa. This exhumation was accompanied by cooling. Another burial process followed during which Eo-Alpine rocks were significantly overprinted at peak pressures up to 2.4 GPa in the Eocene. For example, two generations of potassic white mica (phengite) formed in micaschist. The Eo-Alpine one was relatively coarse grained, whereas the Eocene generation replaced this coarse-grained phengite by newly grown small flakes. No indications for ultrahigh-pressure metamorphism were found.

We interpret our findings, including previous results on rocks of our study area in the Pohorje Mts., in a geodynamic context as follows: A first collision of continental (micro)plates occurred in the Late Cretaceous after a branch of the Neotethys Ocean was closed. The subduction of the corresponding oceanic plate including sediments on top led to eclogite (+ HP garnet clinopyroxenite) and HP micaschist which were exhumed during the continent-continent collision in an exhumation channel. About 45 Ma after this Eo-Alpine collisional event, another part of the Neotethys Ocean was closed followed by a second collision of continental (micro)plates. This process led to clearly overthickened crust and deep burial of rocks residing in the Eo-Alpine exhumation channel. Exhumation of the studied metamorphic rock units, probably mainly caused by surface erosion, followed this Eocene collisional event. A particular event in the Miocene is characterized by intrusions of large volumes of acidic magma. These intrusions formed the Pohorje pluton, which produced discernable contact metamorphism, for instance in micaschist, close to its margin.