Contrasting views exist in the literature in regard of the evolution of metamorphic rocks in the southeastern Pohorje Mountains (Mts), located in the southeastern Eastern Alps. Major debated points are whether micaschists have experienced ultrahigh‐pressure metamorphism in the Late Cretaceous (Eo‐Alpine) and whether they were continuously exhumed or experienced a multiple subduction‐exhumation process from that time on. Therefore, we studied micaschist sample 18Slo39 with two generations of garnet and phengitic muscovite from this area. Our detailed study of this rock included petrographic observations, chemical analyses of minerals with the electron microprobe, pseudosection modeling, conventional geothermometry, and monazite in‐situ U‐Th‐Pb dating using laser‐ablation ICP mass spectrometry. The following results were obtained: The studied micaschist was subject to a peak‐pressure of 1.31±0.14 GPa at 603±26 °C in Eo‐Alpine times: 90.62±2.78 (2σ) Ma (Stage I). Contact metamorphism at pressure‐temperature conditions of 0.66±0.10 GPa and 577±23 °C was induced by the intrusion of the Pohorje pluton (Stage III). We determined an early Miocene age of 18.33±0.43 (2σ) Ma for this intrusion. Based on this study and the previously reported data for a micaschist (16Slo12) taken in the vicinity of sa 18Slo39, a geodynamic model is proposed for the region of the Pohorje Mts considering Eo‐Alpine subduction of oceanic crust and European continental crust, of which the micaschist was part of. Another high‐pressure event in the Eocene (Stage II) was the result of intracontinental subduction due to transpression by the Periadriatic fault system which separates the Eastern Alps from the Southern Alps. This type of subduction gave rise to magma generation and ascent to form the Pohorje pluton, which caused contact metamorphism in its vicinity.

<p>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.</p> <p>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.</p> <p>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.</p>

<p>Controversy remains concerning (1) the ultrahigh-pressure (UHP) or high-pressure (HP) nature of metamorphic rocks at the southeastern flank of the Pohorje Mountains (Mts.) and (2) corresponding different geotectonic scenarios such as deep subduction of a coherent continental slab or fragments of the continental crust. For example, eclogites from this area were reported to have experienced peak pressure-temperature (P-T) conditions of 30-37 kbar and 710-940 °C (Janák et al., 2004; Vrabec et al., 2012) or 18-25 kbar and 630-750 °C (Sassi et al., 2004; Miller et al., 2005). Therefore, we studied an eclogite body (actually a garnet pyroxenite) from the southeastern Pohorje Mts. using mainly the electron microprobe for mineral analytics and the computer program PERPLE_X for thermodynamic modelling in order to decipher the P-T evolution of this rock. Millimetre-sized garnet shows a peculiar zonation. The virtually Cr-free and unzoned core (Grt1) contains c. 16 mol% almandine, 18.5 mol% grossular (+andradite), 65 mol% pyrope, and 0.5 mol% spessartine components (Alm16Grs18.5Pyr65Sps0.5). The average composition of the slightly zoned mantle (+rim) (Grt2) is Alm16Grs20Pyr63.5Sps0.5 with 0.22 wt% Cr<sub>2</sub>O<sub>3</sub>. X-ray maps (KαCr-radiation) of garnet show a sharp boundary between Grt1 and Grt2 and demonstrate that the shape of Grt1 is irregular. Cr-free amphibole, (clino)zoisite, kyanite, staurolite, and rutile are enclosed in garnet. Inclusions in Grt2 are also Cr-bearing amphibole and (clino)zoisite. The matrix consists of garnet and Cr-bearing amphibole, clinopyroxene (around 17 mol% jadeite+acmite), and (clino)zoisite as well as some Cr-free kyanite.</p><p>After thermodynamic modelling, we interpret the studied rock as follows: An olivine- and hornblende-bearing gabbro with some chromite experienced a first metamorphism at about 22 kbar and 730 °C. The rock was subsequently exhumed and cooled leading to significant corrosion of garnet. A second metamorphism, which was recognized thanks to the different and mappable Cr contents in garnet, led to considerable growth of Grt2 and other Cr-bearing minerals at the expense of chromite at P-T conditions around 23.5 kbar and 710 °C. These conditions are compatible with those derived by Sassi et al. (2004) and Miller et al. (2005) and support the view that no UHP eclogite exists in the Pohorje Mts. The two metamorphic events could be related to Cretaceous and Palaeogene HP events which were recently reported by Li et al. (2021) from the Pohorje Mts.</p><p>Janák et al., 2004. Tectonics 23, TC5014.</p><p>Li et al., 2021. Journal of Metamorphic Geology 39, 695-726.</p><p>Miller et al., 2005. Contributions to Mineralogy and Petrology 150, 70-84.</p><p>Sassi et al., 2004. Lithos 78, 235-261.</p><p>Vrabec et al., 2012. Lithos 144, 40-55.</p>