Important features resulting from hypotheses

30 MP-LT Garnet Phyllite Unit

This unit consists of garnet-bearing phyllites, graphitic phyllites and chloritoid- bearing phyllites with garnet and feldspar. Also, the amphibole schist and quartzite bodies occur here (Rötzler et al., 1998; Schumacher et al., 1999). The P-T conditions were estimated by TWEEQ-method (Berman, 1988) to 480°C and 9.5 kbar. These estimations match the garnet-phengite (Green and Hellman, 1982) and garnet-chlorite (Ghent et al., 1987) thermometer and phengite barometer (Massonne, 1991).

LP-LT Phyllite Unit

Low grade, graphite/feldspar bearing and feldspar free phyllites with bodies of amphibole schists belong to that unit. The maximum temperature is estimated to be up to 300°C as the quartz is recrystallized dynamically with low-pressure conditions of about 2 kbar based on low Si apfu content. However, chloritoid phyllites record pressures around c. 13 kbar, 520°C (Jouvent et al., 2021).

Transition zones

Figure 2 shows two transition zones, which separates Gneiss-Eclogite Units from Gneiss-amphibolite Unit and Micaschist-Eclogite Unit. They are referred to as large scale shear zones consisting of strongly deformed and retrograded rocks in the textural appearance of mylonites and ultramylonites. Rötzler et al. (1998) estimated the metamorphic peak condition of metasediments to 470–560°C at 12 kbar and 470°C at 9 kbar.

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The structural position and spatial distribution of those units / nappes in the Erzgebirge shows some differences too. The map of spatial distribution of Konopásek and Schulmann, (2005) is shown in Figure 3. Rötzler and Plessen, (2010) interpretation is illustrated on the map in Figure 5 and in the cross-section in Figure 4. These maps are merged in Figure 2b in the simplified geological map after Kryl et al. (2021) with the legend (Fig. 2c) referring to both hypotheses. It is noticeable that the main difference in spatial distribution of the units is near to Mnišská skála locality, which is pointed out in Figure 2b by the yellow star. Rötzler and Plessen (2010) suggest this area to be Gneis-Eclogite Unit I (HP-HT) and on the other hand Konopásek and Schulmann, (2005) describe it as an Upper Crystalline nappe (UHP-HT). It is important to mention a presence of a small orthogneiss body in the vicinity of the Přísečnice water reservoir, which was reported as UHP-HT by Konopásek and Schulmann (2005). It corresponds to the Mnišská skála locality studied and described in detail in chapter IV.

Another important feature is the presence of other quartzofeldspathic rocks than orthogneiss, such as granulites and granofels. Willner et al. (1997) describes granulites from the Zöblitz complex, which is another reason why Konopásek and Schulmann, (2005) consider the area in the vicinity of Mnišská skála as an Upper Crystalline nappe (UHP-HT).

Another difference of these hypotheses is the inverse interpretation of two allochthonous units as UHP-HT above HP-HT (Upper Crystalline nappe above Lower Crystalline nappe) in the Czech part by Konopásek and Schulmann (2005) and as HP-HT above UHP-HT (Gneiss-Eclogite Unit I above Gneiss-Eclogite Unit II) in Germany by Rötzler and Plessen (2010). The Mnišská skála orthogneiss together with the UHP-HT Eger Crystalline Complex to the south (Kotková et al., 2011; Závada et al., 2018, 2021) occurs in the hanging wall of the studied allochthonous HP-HT GEU I, which led Konopásek and Schulmann (2005) to propose the inverted metamorphic nappe stack of these units. On the other hand, direct contact of the allochthonous UHP-HT GEU II with the lowermost parautochthonous GAU led other authors (Willner et al., 2000; Rötzler and Plessen, 2010) to propose the inverted metamorphism only across this boundary.

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The rest of the overlying units were interpreted to show a normal metamorphic field gradient.

These differences will be cleared out at the discussion and conclusions part of this thesis.

3 Brief summary of most recent and accepted advances

Saxothuringian unit is considered as a lower plate that subducted underneath the Teplá-Barrandian unit during Variscan orogeny (Konopásek et al., 2001; Mingram et al., 2004; Schulmann et al., 2014; Peřestý et al., 2017, 2020). The central part of Erzgebirge (Fig. 2b) consists of several allochthonous and parautochthonous units (Konopásek et al., 2001; Schmädicke and Evans, 1997; Konopásek and Schulmann, 2005; Rötzler and Plessen, 2010; Collett et al., 2020, Jouvent et al., 2021). The area forms an antiformal dome with Kateřina-Reitzenhain granite- gneiss amphibolite unit (GAU) in the core (Fig. 2b,c). GAU is understood as a (para-)autochthonous (Konopásek and Schulmann, 2005; Collett et al., 2020) and is affected by MP-HT metamorphism at ~6.5–13.5 kbar and ~600–820 ◦C (Kröner et al., 1995; Rötzler, 1995, Kryl et al., 2021). The protolith ages of parautochtonous GAU granitoids point to several magmatic pulses from Neoproterozoic to Cambro- Ordovician (~560–500 Ma). On the other hand, the allochthonous orthogneisses overlying GAU have strictly Ordovician protolith (Kröner et al., 1995; Košler et al., 2004; Mingram et al., 2004; Tichomirowa and Sergeev, 2012; Collett et al., 2020).

The metabasites and metasedimentary rocks of allochthonous units (Fig. 2b) locally preserve high- or ultra high- (UHP) pressure conditions as documented by coesite and diamond (Schmädicke et al., 1992; Klápová et al., 1998; Nasdala and Massonne, 2000; Massonne, 2001). The allochthonous units had been divided by Schmädicke et al. (1992, 1995) into three isolated HP units, when HP unit 1 coressponds to GEU II, HP unit 2 to GEU I and HP unit 3 to Micaschist-Eclogite unit. This division is based on their peak P-T conditions. Bottom HP unit 1: 850 ◦C,

>29 kbar, middle HP unit 2: 650–750 ◦C, 24–26 kbar, top HP unit 3: 600–650 ◦C, 20–23 kbar, Fig. 2d. The peak metamorphic conditions of the HP unit 1 were later re-evaluated to record up to ~40–60 kbar and ~1000–1100 ◦C (Massonne, 1999;

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Hwang et al., 2001, 2006). P–T conditions of orthogneiss in the HP unit 2 were constrained by Kryl et al. (2021) to ~10–16 kbar and ~600–740°C. The area in between isolated HP units is occupied by lower-grade gneiss, interpreted as transition shear zones (Sebastian, 1995; Rötzler et al., 1998). Konopásek et al.

(2001) and Konopásek and Schulmann (2005) alternatively divide the area into the parautochthonous lower-grade gneiss unit that is overlain by two crystalline nappes: the HP-MT Lower Crystalline nappe (consisting of the HP unit 2 and partly also 3), and the HP-HT Upper Crystalline nappe (HP unit 1, fig. 1b,c).

The UHP and HP metamorphism is of Variscan age and has been constrained by U–Pb zircon ages to ~340 Ma, just slightly older than Ar–Ar cooling ages (~336–

333 Ma) (Schmädicke et al., 1995; Kröner and Willner, 1998; Werner and Lippolt, 2000; Tichomirowa et al., 2005, Hallas et al., 2021). The Eger Crystalline

Complex (Fig. 2b), included in the Upper Crystalline nappe of Konopásek and Schulmann (2005), yield U–Pb monazite ages between 353 and 338 Ma (Závada et al., 2021).

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CHAPTER II