A wealth of P–T–t information from metasediments in the HP–UHP terrane of the Pohorje Mountains, Slovenia, elucidates the evolution of the Eastern Alps

Abstract

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.

Chromium in minerals as tracer of the polycyclic evolution of eclogite and related metabasite from the Pohorje Mountains, Slovenian Eastern Alps

“…Such high pressures were reached twice during different metamorphic events discernible by the Cr zonation in garnet. The derivation of the polycyclic HP metamorphism in this study supports the finding by Li, Massonne, Koller, and Zhang (2021), Li et al (2023) that both a Late Cretaceous and a Palaeogene orogenic event affected the Pohorje Mountains. Thus, our study has major implications for the geotectonic evolution of the south‐easternmost Alps.…”

“…They relate the second event to Palaeogene HP metamorphism that is known for regions of the Eastern Alps especially in and around the Tauern window (e.g., Thöni, 2006). The similarities from these terranes support a similar Late Cretaceous and Palaeogene HP evolution in the Pohorje Mountains (Li et al, 2023).…”

“…demonstrated that the choice of the geothermobarometric methods significantly influenced the calculated conditions. Studies of metasedimentary country rocks suggested a similar but lowerrange in Late Cretaceous peak-pressure conditions between 13 and ≥35 kbar at 600-850 C (Herg & Stüwe, 2018;Hurai et al, 2010;Jan ak et al, 2009;Jan ak, Froitzheim, et al, 2015;Krenn et al, 2009;Li et al, 2023;Li, Massonne, Koller, & Zhang, 2021). Li, Massonne, Koller, and Zhang (2021) reported an additional P-T loop during the Cenozoic [between 26.2 ± 2.8 (2σ) and 47.9 ± 10.8 Ma] leading to peak pressures of 18.5-23 kbar at temperatures somewhat below 600 C. This HP event was related by these authors to the one that is more discernible in the Tauern window (Figure 1b; 17-24 kbar, ≤635 C) at 31-<45 Ma (Dachs & Proyer, 2002;Holland, 1979;Hoschek, 2004;Hoschek et al, 2010;Spear & Franz, 1986;Stöckhert et al, 1997: Thöni, 2006).…”

“…Miller and Konzett (2005) demonstrated that the choice of the geothermobarometric methods significantly influenced the calculated conditions. Studies of metasedimentary country rocks suggested a similar but lowerrange in Late Cretaceous peak‐pressure conditions between 13 and ≥35 kbar at 600–850°C (Herg & Stüwe, 2018; Hurai et al, 2010; Janák et al, 2009; Janák, Froitzheim, et al, 2015; Krenn et al, 2009; Li et al, 2023; Li, Massonne, Koller, & Zhang, 2021). Li, Massonne, Koller, and Zhang (2021) reported an additional P–T loop during the Cenozoic [between 26.2 ± 2.8 (2σ) and 47.9 ± 10.8 Ma] leading to peak pressures of 18.5–23 kbar at temperatures somewhat below 600°C.…”

“…Metamorphism in the Pohorje nappe occurred during the Late Cretaceous: (1) U–Pb ages of zircon in eclogite, metapelite and orthogneiss (Miller, Mundil, et al, 2005; Janák et al, 2009; Chang et al, 2020: 89–93 Ma) (2) Sm–Nd garnet ages of eclogite (Miller, Mundil, et al, 2005: 87–95 Ma), gneiss and micaschist (Thöni, 2003: 84–95 Ma), (3) a Lu–Hf garnet age of eclogite [Thöni et al, 2008: 93.3 ± 2.8 (2σ) Ma], (4) in situ U–Th–Pb ages of monazite in metapelite determined with the electron microprobe (EMP; Krenn et al, 2009; Li, Massonne, Koller, & Zhang, 2021: 90–106 Ma) and (5) in situ U–Pb ages of monazite in metapelite obtained by laser ablation inductively coupled plasma mass spectrometry [LA‐ICP‐MS; Li et al, 2023: 90.62 ± 2.78 (2σ) Ma].…”