Evolution of the Lower Permian Rochlitz volcanic system, Eastern Germany: reconstruction of an intra-continental supereruption

Extensional tectonics in the Late Paleozoic Central Europe was accompanied by rift magmatism that triggered voluminous intracontinental caldera-forming eruptions. Among these, the Lower Permian Rochlitz Volcanic System (RVS) in the North Saxon Volcanic Complex (Eastern Germany, Saxony) represents a supereruption (VEI 8, estimated volume of 1056 km3) of monotonous rhyolites followed by monotonous intermediates. Mapping, petrography, whole-rock geochemistry along with mineral chemistry and oxygen isotopes in zircon display its complex eruption history and magma evolution. Crystal-rich (> 35 vol%), rhyolitic Rochlitz-α Ignimbrite with strong to moderate welding compaction erupted in the climactic stage after reheating of the magma by basaltic injections. Due to magma mixing, low-volume trachydacitic-to-rhyolitic Rochlitz-β Ignimbrite succeeded, characterized by high Ti and Zr-values and zircon with mantle δ18O. Randomly oriented, sub-horizontally bedded fiamme, and NW–SE striking subvolcanic bodies and faults suggest pyroclastic fountaining along NW–SE-oriented fissures as the dominant eruption style. Intrusion of the Leisnig and the Grimma Laccoliths caused resurgence of the Rochlitz caldera forming several peripheral subbasins. In the post-climactic stage, these were filled with lava complexes, ignimbrites and alluvial to lacustrine sediments. Significant Nb and Ta anomalies and high Nb/Ta ratios (11.8–17.9) display a high degree of crustal contamination for the melts of the RVS. Based on homogenous petrographic and geochemical composition along with a narrow range of δ18O in zircon Rochlitz-α Ignimbrite were classified as monotonous rhyolites. For the Rochlitz-β Ignimbrites, underplating and mixing with basic melts are indicated by Mg-rich annite–siderophyllite and δ18O < 6.0 in zircon. The wide spectrum of δ18O on zircon suggests an incomplete mixing process during the formation of monotonous intermediates in the RVS.


Introduction
Eruptions forming voluminous crystal-rich ignimbrites are often related to reheating of crystal mush in the magma reservoir; underplating/injection of basic melts causes magma mingling, mixing and homogenization (Smith 1979;Bachmann et al. 2002;Bachmann and Bergantz 2004;Christiansen 2005;Folkes et al. 2011). This may result in the formation of monotonous intermediates known from many subduction-related settings (Hildreth 1981;Huber et al. 2012;Bachmann and Huber 2016;Kaiser et al. 2017). In these magmatic systems, melt extraction may result in zoned crystal-poor rhyolitic magma batches (Bachmann and Bergantz 2008a). Watts et al. (2016) reported on numerous supereruptions leading to high-volume, crystal-rich rhyolitic monotonous ignimbrites which display a limited variation in whole-rock geochemistry and open-system magmatic processes with a complex evolution (Cooper and Wilson 2014).
The late to post-orogenic phase of the Variscan Orogen in central Europe features many silica-rich, high volume volcanic complexes with large, crystal-rich ignimbrite sheets, associated with lava complexes and subvolcanic units (Timmerman 2008;Repstock et al. 2018). Among these volcanic complexes, studies have been carried out on the Ora caldera complex in northern Italy (Willcock et al. 2013;Willcock and Cas 2014), the Donnersberg Formation in the Saar-Nahe Basin in south-western Germany (Lorenz and Haneke 2004;Boy et al. 2012) and the Northeast German Large Igneous Province (Benek et al. 1996;Paulick and Breitkreuz 2005;Breitkreuz et al. 2007;Geißler et al. 2008). Further examples are the Halle Volcanic Complex in central Germany ; Fig. 1) and the Intra-Sudetic and North Sudetic Basins in Poland and Czech Republic (Awdankiewicz 1999).
The North Saxon Volcanic Complex (NSVC) is located east of the city of Leipzig (Eastern Germany, Saxony) and extends over 54 × 54 km. It contains at least two large nested caldera systems (Fig. 2), the Wurzen Volcanic System (WVS, Repstock et al. 2018) and the Rochlitz Volcanic System (RVS; Eigenfeld 1978). Like in the San Juan Mountains (Lipman 1976(Lipman , 1997, partial erosion of the NSVC enabled deep insight in climactic and post-climactic pyroclastic deposits and subvolcanic bodies. Partial erosion also removed caldera-outflow facies for both, WVS and RVS.
It has to be noted that the WVS is one of the first examples of monotonous intermediates recorded to have formed in an intra-continental setting.
A spectrum of eruption styles has been reported for the climactic phase of large caldera systems. On the one hand, an initial eruption column accompanied by Plinian fallout may collapse, forming voluminous ignimbrites (e.g., Taupo Volcanic Zone, Wilson et al. 2006; Minoan eruption of the Santorini Volcanic Field, Johnston et al. 2014;Valles Caldera, Cook et al. 2016a, b). On the other hand, pyroclastic fountaining (Branney and Kokelaar 2002) from fissures or ring dykes is the dominant eruption style of many large caldera systems (Long Valley Caldera, Hildreth and Mahood 1986; San Juan Mountains, Bachmann et al. 2000; Western Snake River Plain, Branney et al. 2008; Ora Formation, Willcock et al. 2013). Pyroclastic fountaining along  (Fig. 7). Arrows and numbers depict dip direction and dip angle of long axis of fiamme in Rochlitz Ignimbrites 1 3 NW-SE-striking fissures was the predominant eruption style assumed for the WVS (Repstock et al. 2018).
For the RVS, we combine data from previous studies with new data on physical volcanology and petrography from outcrop and drillings (aspect ratio and orientation of fiamme, granulometry, maximum particle size). New whole-rock geochemistry, biotite chemistry and δ 18 O on zircon are provided. The results obtained will be used to reconstruct the evolution of one of the largest eruptions in central Europe during the Late Paleozoic. This study depicts several eruption stages of the RVS, and the magma evolution of a supervolcano in an intra-continental setting.

Geological setting
The NSVC, situated east of Leipzig (Figs. 1,2), covers an area of c. 1600 km 2 and represents a nested caldera complex hosting the large Rochlitz and the Wurzen Volcanic Systems, with diameters of 54 and 26 km, respectively (Figs. 1, 2;Röllig , 1976Eigenfeld 1978;Repstock et al. 2018). Although northern Saxony is largely covered by Quaternary (peri-) glacial deposits, local outcrops, many quarries and hundreds of exploration drillings allow for good insight into the distribution of NSVC lithotypes. First data on wholerock geochemistry of NSVC units were published by Wetzel et al. (1995), characterizing the volcanic rocks as subalkaline to slightly alkaline with a rhyolitic to slightly trachytic composition. Radiometric ages, ranging from 300 to 287 Ma, were published by Wendt et al. (1995) and Hoffmann et al. (2013).
According to Walter (2006) the volcaniclastic and epiclastic succession of the NSVC can be subdivided into four formations (Fig. 3). The lowermost Kohren Formation overlies metamorphic basement rocks and comprises alluvial to fluvial sand-and siltstones as well as debris flow deposits intercalated with basic to silica-rich pyroclastics and lavas (Walter 2006). Biostratigraphy suggests a deposition age close to the Permian-Carboniferous boundary (Barthel et al. 1976). Hoffmann et al. (2013) obtained a radiometric age of 298.3 ± 9.1 Ma (U/Pb SHRIMP on zircons) for a tuff at the base of the formation. In previous studies (e.g., Hohl and Wilsdorf 1966;Walter 2006Walter , 2012, the Leisnig Porphyry, a complex silica-rich magmatic body of 16 km in E-W-extension and up to 220 m in thickness, has been interpreted as a lava flow belonging to the Kohren Fm. (Figs. 2, 3). Within the Leisnig unit, Hohl and Wilsdorf (1966) distinguished a "normal" facies dominated by feldspar phenocrysts from biotite-and quartz-dominated domains (Fig. 5). In the center of the Leisnig Porphyry, two dyke complexes, partially in pitchstone facies with large lithophysae (Götze et al. 2020), were recognized (Hohl and Wilsdorf 1966).
The Rochlitz Fm. consists of various welded ignimbrites, with a minimum thickness of 400 m (Röllig 1976), overlying both the Kohren Fm. and the Variscan basement (greywacke,schist,phyllite,quartzite and gneiss;Figs. 2,3). Variation in the modal mineral content has been applied to distinguish four depositional units (R I , R II , R III , R N ; Röllig et al. 1970, Eigenfeld 1978. This subdivision was supported by Wetzel et al. (1995) using geochemical data. Based on joint measurements, Eigenfeld (1978) allocated each unit as an individual cooling unit. Unit R I has been dated with SHRIMP U/Pb on zircon at 294.4 ± 1.8 Ma (Hoffmann et al. 2013). These authors emphasized, the analyzed unit yielded no inherited zircon.
The WVS evolution is characterized by magma mingling and mixing due to underplating and reheating by basaltic melts (Repstock et al. 2018). Applying different radiometric methods, Wendt et al. (1995) established an age of 287 ± 3 Ma for the WVS ignimbrites and Hoffmann et al. (2013) determined a SHRIMP U/Pb age on zircon of 289.3 ± 4.1 Ma for a rhyolitic porphyritic dyke cutting through the granite porphyry.
Local hydrothermal activity and widespread Mesozoic and Cenozoic subtropical weathering led to intense alteration in central Europe (Dill et al. 2015). In places, the NSVC volcanics were kaolinized up to 40 m depth (e.g., Kemmlitz Porphyry, Fig. 6; Schwerdtner and Störr 1983;Walter 1991). At the "Rochlitzer Berg" (for location see Online Resource Table 1), alteration formed an outstanding facies of the Rochlitz Ignimbrite called Rochlitz Porphyry Tuff with high porosity, a prominent eutaxitic texture and a bright red colour (Fig. 4A). It is utilized as dimension stone for more than 1000 years.

Methods
Samples were taken from 69 outcrops and quarries, in addition with 16 samples from drill cores (Online Resource Fig. 1). Besides, 49 drilling reports have been considered. For establishing the emplacement mode of the subvolcanic bodies, detailed mapping and evaluation of drilling descriptions were carried out. Petrographic analyses were accomplished on 106 thin sections. Phenocryst mode and maximum particle size (MPS, four measurements per thin section, Online Resource Table 3) on minerals and fiamme were measured by applying ZeissAx-ioVision 4.9 and JMicrovison software. Long-axis strike and dip of fiamme were determined for constraining the eruption style and vent locations ( Fig. 2; Online Resource Table 2). The aspect ratio of fiamme (ratio of height and length) has been calculated and classified according to Quane and Russell (2005). 77 geochemical bulk-rock analyses on whole-rock samples (0.5-2 kg per sample, Online Resource Table 4) from the RVS volcanic rocks were carried out at Bureau Veritas in Vancouver (Canada). Analyses on major element oxides, minor and trace elements were measured with inductively coupled plasma mass spectrometry (ICP-MS), and the elements Mo, Cu, Pb, Zn, Ni, As, Cd, Sb, Bi, Ag, Au, Hg, Tl and Se by inductively coupled plasma atomic emission spectroscopy (ICP-AES).
Composition of biotite (Online Resource Table 6) and feldspar (Online Resource Table 7) was investigated using at a JEOL RX8900d wd/ed combined micro analyzer at the Institute of Material Science, TU Bergakademie Freiberg. The applied spectral lines, standards and counting times are presented in Online Resource Table 5. Additional analyses were performed at the Berlin Museum of Natural History-Leibniz Institute for Evolution and Biodiversity Science, using a JEOL JXA-8500F field emission electron probe micro analyzer. Operating conditions for both were set to 15 keV and 20nA with a beam diameter up to 2 µm. Analyses at the EPMA in Berlin were calibrated on Smithsonian international mineral standards. For spectral lines, standards and peak counting times on each analyzed element see Online Resource Table 5.
Zircon crystals have been mechanically separated from samples using high-voltage pulse SelFrag in the Geological Institute of TU Freiberg. For concentration gravity separation was performed. The zircon was manually picked  Tuff) from "Rochlitzer Berg"; B fine grained phreatic dyke crosscutting Rochlitz-α Ignimbrite at "Rochlitzer Berg" in NW-SEdirection; C Part of a 40 cm thick rhyolitic flow-foliated sill intercalated in the "Lastauer Tuff" (locality 23/5/13/2 in Online Resource Table 1) representing a possible early intrusion phase of the Leisnig Laccolith and mounted. The standard TEMORA II (one standard per three unknowns) was also mounted.
Oxygen isotopes have been quantified on zircon (Online Resource Table 8) at a SHRIMP II multi-collector ion microprobe at Micro-area Analysis Laboratory of PGI-NRI, Warsaw. The configuration of the instrument and applied analytical procedure were based on Ickert et al. (2008). The oxygen isotope data were corrected using the POXI MC software (P. Lanc). Oxygen isotope ratios are reported in standard per mille notation relative to Vienna Standard Mean Ocean Water.

Structural outline of the RVS caldera
In the vicinity of the town of Rochlitz and at the southern margin of the Mügeln Subbasin, the Rochlitz Ignimbrites rest on Variscan basement with an erosional unconformity. The outline of the RVS is marked by andesitoids of the Kohren Fm. occurring at the northeastern and eastern margin as well as in the Borna Subbasin (Gläßer 1983 ; Fig. 2). Outcrops and drillings reveal that the RVS is bordered by subvertical faults in the N, NE and SW, which are assumed to have acted as volcanotectonic normal faults during caldera subsidence (Fig. 2). As depicted in profile 3 (Fig. 2), the southern part of the Rochlitz caldera is characterized by a piecemeal-like, intense block faulting (Branney and Kokelaar 1994).
Porphyritic laccoliths with granitic composition emplaced both in the central RVS and at its northeastern margin (Fig. 2). Emplacement of the laccoliths in the center led to a caldera resurgence forming several subbasin. The central laccoliths of the RVS strike NW-SE, like the subvolcanic bodies of the WVS (Repstock et al. 2018). A small block of the Variscan basement was uplifted in the central RVS forming a horst like structure (Fig. 2).

Lithostratigraphic framework of the RVS
Based on the new data set the classification of the Rochlitz Fm. had to be modified (see Table 1). Granulometric and geochemical data do not allow for a distinction between the subunits R I and R II (Figs. 9, 10). However, the new wholerock immobile trace element data (Ti, Zr, P), and biotite geochemistry are helpful to differentiate R III and R N from R I and R II . Therefore, the units R I and R II are grouped to the new unit Rochlitz-α and R III will be renamed as Rochlitz-β. The subunit R N will be renamed to Rochlitz-N for a coherent labeling. Following the geochemical data presented here, this sub-unit can be divided into the Schöna and Thalheim subtypes. Based on our textural investigation, a new nomenclature for some RVS volcanics and subvolcanics has been established (Table 1).

Rochlitz ignimbrites
In the southern part of the NSVC, a continuous belt of outand subcrops of the welded Rochlitz Ignimbrites occurs. The belt measures c. 54 km from the Borna Subbasin in the west to the Mügeln Subbasin in the east (Fig. 2) covering an area of c. 880 km 2 . In this belt, numerous exploration drillings (≥ 100 m deep) were drilled into the Rochlitz Ignimbrites, such as Borna 1/56 (146 m), Saalhausen 1/22 (224 m), and Ballendorf 1/61 (119 m; Fig. 7). Drilling Frauendorf 1/54 completely perforated the Rochlitz Fm. exposing 398.7 m thickness for the Rochlitz Ignimbrite (Fig. 7). Further drillings, which also document a thickness of the RVS ignimbrites up to 400 m were used to construct profiles depicted in Fig. 2. Besides, isolated outcrops and a few drillings at the northern and northeastern margin of the NSVC exhibit welded ignimbrites considered as Rochlitz Ignimbrites ( Fig. 2; see also Röllig et al. 1970, Walter 2006. Fallout and surge deposits related to the Rochlitz Ignimbrites have not been observed in the NSVC (Table 1).
As depicted in Fig. 2, the lowermost unit Rochlitz-α shows the largest distribution, whereas Rochlitz-β is restricted to an area in the northwest of Colditz. West of Oschatz and northeast of Wurzen (Fig. 2), Rochlitz Ignimbrites occur locally summarized as Rochlitz-N (Eigenfeld 1978). However, field relation and drillings do not expose transitions from Rochlitz-α to Rochlitz-β Ignimbrites.
All Rochlitz Ignimbrite units comprise phenocrysts of plagioclase, alkali feldspar, quartz, biotite and pseudomorphs of chlorite after pyroxene (Online Resource Table 2). Contents of quartz, plagioclase and K-feldspar vary significantly within a single unit, however, the broad spectrum allows no differentiation between the units. As a consequence, a subdivision of RVS ignimbrites based on modal composition, as proposed by Eigenfeld (1978), cannot be maintained. The Rochlitz Ignimbrites have crystal contents between 35 and 47 vol% (Online Resource Table 3). Maximum particle size (MPS) ranges between 2.0 and 4.5 mm for quartz and feldspar (Online Resource Table 3). Scarce lithics (granulites, sandstones and volcanic rocks) occur with a MPS of 1 cm ( Fig. 8A-D). Frequently, small agglomorates of feldspar, quartz and Fe-Ti-minerals and micrographic intergrowth of quartz and biotite have been documented (Fig. 8E). The groundmass consists of a fine mosaic of quartz and K-feldspar. Typical characteristics of the Rochlitz Ignimbrites are a homogenous mineral and structural content, a strong to moderate welding compaction (Fig. 8H) and embayments on plagioclase and quartz (Fig. 8F, Online Resource Fig. 3A, B). Domains of partially devitrified vitrophyric Rochlitz-α with perlitic texture occur near Lastau (Fig. 8C), Ebersbach and Korpitzsch (Online Resource Table 1; Eigenfeld 1978, Jentsch 1979. In some areas of the central RVS, Rochlitz-α shows spherulites formed on the margin of fiamme (Fig. 8I).
The Rochlitz Ignimbrites are rich in fiamme with sizes up to 13 cm (Drilling Meltewitz 1/66, Figs. 7E, 8A-D). The aspect ratio of fiamme displays a eutaxitic texture ( Fig. 8A-C, Online Resource Fig. 3C, D), ranging between II and V according to Quane and Russell (2005 ; Online  Resource Table 2), with no significant vertical trend (Fig. 7E). Measurements on fiamme in natural outcrops and quarries reveal subhorizontal bedding and no preferred orientation of elongation on a regional scale ( Fig. 2; Online Resource Table 2). The fiamme of Rochlitz-N are oxidized and hematized resulting in a reddish color (Fig. 8B), whereas the other units have black to green fiamme. Fiamme of Rochlitz-α display sometimes a porphyritic structure and have a low compaction (rank II-IV) compared to Rochlitz-β fiamme with rank IV-V. The latter contains less phenocrysts.

Post-climactic processes and rock units (Oschatz Fm.)
The rhyolitic Leisnig Laccolith comprises three different lithotypes: a "normal" feldspar-dominated type and quartzor biotite-rich types (Fig. 5;Hohl and Wilsdorf 1966). All lithotypes have 70-90 vol% of groundmass and MPS of quartz, feldspar and biotite phenocrysts is 3.0 mm, 5.0 mm and 2.5 mm, respectively. Some domains are biotite-rich (3-6 vol%), compared to the normal type with 1-3 vol%. Throughout the accessible outcrops near the top margin of the laccolith, coherent facies with flow foliation and trains of vesicles are present. Flow foliation dips sub-horizontally to steeply eastward and southward, sub-parallel to the top margin of the laccolith (Fig. 5). Textures of carapace facies typical for rhyolitic lava flows, such as (ex-) glassy and pumice domains, spherulites and lithophysae, and, most important, brecciated domains (Manley and Fink 1987;Manley 1996) have not been observed.
Drillings close to Seifersdorf (Fig. 5) reveal a maximum thickness of 220 m for the Leisnig Laccolith and an offset of its base of c. 180 m. Furthermore, at the river bank of the Zwickau Mulde, south of Colditz, a 40 cm thick weakly porphyritic, rhyolitic sill intruded in the "Lastauer Tuff" at the base of Rochlitz-α (Pietzsch 1962;Fig. 4C; locality 23/5/13/2 in Online Resource Table 1).
NE-SW-striking phreatic dykes are crosscutting the Rochlitz Ignimbrites at Scheergrund and in an active quarry at "Rochlitzer Berg" (Fig. 4B; Online Resource Table 1). In summary, we interpret the Leisnig Porphyry as a laccolith complex emplaced into the tuffs and sediments of the Kohren Fm. below the Rochlitz Ignimbrites. The small sill near Lastau is, due to the similar geochemical composition and its position is considered as cogenetic to the emplacement of the Leisnig Laccolith. The formation of the phreatic dykes, crosscutting the Rochlitz Ignimbrites, may have been related to vapor explosions caused by the emplacement of the Leisnig Laccolith.
In the central RVS, the microgranitic Grimma Porphyry intruded in the Rochlitz Ignimbrites (Figs. 2, 3). The NW-SE-oriented subvolcanic body extends 8 × 4 km. The Grimma Porphyry is characterized by an aplitic crystalline groundmass with large phenocrysts (up to 1.5 cm) of smoky quartz dominating over altered feldspar and biotite. The phenocrysts make up 13-22 vol% of bulk rock. At the eastern margin several sills (protolobes acc. to Hutton 2009) occur, depicting a laccolith structure for the Grimma Porphyry ( Fig. 2 profile 1) leading to renaming the subvolcanic complex as Grimma Laccolith.
In the Borna Subbasin the post-Rochlitz succession comprises lava, pyroclastic rocks and clastic sediments (clayand sandstones, coal; Figs. 6, 7A, B). The slightly porphyritic Buchheim Volcanics are strongly welded ignimbrite (rank V, Quane and Russell 2005) with a maximum thickness of 120 m interbedded with fallout tuff. Its basal part is vitrophyric, in places. The Buchheim Ignimbrite has a high crystal content of 35-47 vol% (Online Resource Table 3). The Frohburg Porphyry (50 m thickness) is an ignimbrite with eutaxitic texture. Its crystal content between 15 and 19 vol% is low compared to the other volcanics of the RVS. In contrast, the Mügeln Subbasin in the eastern RVS contains predominantly clastic sedimentary rocks (lacustrine to alluvial) with intercalations of tuffs and coals (Figs. 6,7D,E). At the base of the succession, the at least 70 m thick Kemmlitz porphyritic rhyolite lava occurs ( Fig. 6; Götze et al. 2017), which is actively being quarried for kaolinite (Schwerdtner and Störr 1983).
Furthermore, in the Meltewitz Subbasin between Schöna and Oschatz, Rochlitz-α Ignimbrite is overlain by a 110 m thick monomict, matrix-to clast-supported conglomerate (Fig. 7E). The porphyritic rhyolite lithics (∅ ≤ 7 cm) may have been transported by debris flows from a lava dome (not exposed by drilling so far) which presumably formed along the RVS caldera margin. The conglomerate is overlain by a 57 m thick succession of fallout tuffs and reworked, in parts cross-bedded volcanoclastic deposits. The uppermost unit in drilling Meltewitz 1/66 (Fischer 1968) is the 45 m light gray Dornreichenbach Ignimbrite (Figs. 2, 7E)-a fiamme-and plagioclase-rich, moderately to weakly welded ignimbrite (rank III-IV, Quane and Russell 2005). The total crystal content of the ignimbrite varies from the lower part with 43 vol% to the upper part with 33 vol%. It comprises 24 vol% of feldspar, 12 vol% of quartz, and 1.3 vol% of biotite with 5-9 vol% of lithics. Since pyroxene and amphibole are rare, the Dornreichenbach Ignimbrite can clearly be distinguished from the nearby Wurzen ignimbrites .
Whole-rock analyses of the Rochlitz Ignimbrites reveal a wide range of alteration (Fig. 9); however, Rochlitz-β and Rochlitz-N display the least alteration (Online Resource Fig. 2). Ignimbrites of Rochlitz-β seem to have a trachydacitic to rhyolitic composition before alteration started. In contrast, the original composition of Rochlitz-N ignimbrites was rhyolitic. The unaltered composition of Rochlitz-α is difficult to determine because of the different alterations as sericitization and K-feldspar crystallization. Considering the alteration index (Ishikawa et al., 1976) and the Carbonate-Chlorite-Pyrite Index (Gifkins et al., 2005) the predominant alteration of the volcanic rocks of the RVS is a hydrothermal sericitization and K-metasomatism leading to higher values of K (Fig. 9). The kaolinized samples of the "Rochlitzer Berg" experienced depletion of alkali metals (< 1 wt% Na 2 O and 2.5-3.4 wt% K 2 O, Online Resource Table 4). The high silica contents are a consequence of this depletion.
Mantle-normalized trace element pattern shows a common trend with pronounced negative anomalies of Ba, Nb, Ta, Sr, P and Ti for all Rochlitz Ignimbrites (Fig. 11A). A small variation in Zr is evident. Rochlitz-β shows less pronounced negative anomalies. Significant differences are depicted for Pb (Fig. 11A). Both Rochlitz-N domains, Schöna and Thalheim (Fig. 2), display positive Pb anomalies, whereas the main Rochlitz types are characterized by moderate negative anomalies. The low degree of alteration for Rochlitz-β ignimbrites is confirmed by a significantly weak Ba anomaly. The strong variation observed for Pb and Sr in the Rochlitz subunits bears, at least in parts, magma genetic significance (e.g., Dostal et al. 1979;Vogel et al. 2004), in spite of the strong mobility of these elements during weathering and alteration of feldspar.
The chondrite-normalized REE patterns of the Rochlitz volcanics have a similar trend, with Rochlitz-β showing a less pronounced negative Eu anomaly and lower HREE values (Fig. 11B). One sample of R II shows enriched HREE raising the mean values (Fig. 11B).
The subvolcanic Leisnig Laccolith has a chemical composition similar to the Rochlitz Ignimbrites (Fig. 10A, C). Mantle-normalized trace element pattern of the Leisnig Laccolith is similar to the Rochlitz Ignimbrites; however, with a less pronounced negative Eu-anomaly and overall depleted REE (Fig. 11D). The Nb/Ta ratios (~ 12.5) are lower than for the Rochlitz Ignimbrites (~ 14.6) showing a slightly enhanced continental contamination (Green 1995).
Previous studies of Röllig et al. (1970) and Wetzel et al. (1995) depicted a lava surrounding the Grimma laccolith, the Gattersburg Porphyry. Our investigation revealed a pyroclastic eutaxitic texture and mineral content similar to the Rochlitz Ignimbrites (Online Resource Table 3). Similarities are also shown by the geochemical data and mantle-normalized trace element patterns (Figs. 10, 11).
Post-Rochlitz volcanic and subvolcanic units can be distinguished from the Rochlitz Ignimbrites using geochemical analyses. The Dornreichenbach Ignimbrite of the Meltewitz Subbasin shows the highest silica content within a narrow range (74-75 wt%). The concentration of the alkalis (Na 2 O + K 2 O) ranges between 6.3 and 9.0 wt%, which presumably displays secondary depletion or enrichment ( Fig. 10A; Online Resource Fig. 2). In contrast, the Frohburg Ignimbrite has the highest alkali concentration (up to 9.8 wt%; Fig. 10A). It can be further distinguished from the Rochlitz Ignimbrites by relatively low Nb/Y values (Online Resource Table 4). The significantly low La/Nb and Nb/ Ta ratios (Online Resource

Mineral chemistry and δ 18 O in zircon
K-feldspar from Rochlitz-α and -β Ignimbrites has sanidine signature, whereas Rochlitz-N contains ternary feldspar (Fig. 12, Online Resource Table 7). The composition of biotite in all Rochlitz Ignimbrite units ranges from siderophyllite to annite. Biotite of the Rochlitz-β Ignimbrite covers the largest element distribution at the octahedral site (Mg# from 24.4 to 52.7, Fig. 13A, Online Resource Table 6). Biotite in Rochlitz-α and Rochlitz-N Ignimbrites reveals a narrow Fe-rich compositional range, with FeO-concentrations around 28.6 and 25.9 apfu for Rochlitz-α (Fig. 13A). All biotite of the RVS plots in the field of primary magmatic biotite (Fig. 13D). Along with iron, the manganese concentration is high in Rochlitz-α (R II ) biotite compared to those of other units (Fig. 13B). This may indicate crystal growth in more evolved melts, whereas the Mg-rich annite-siderophyllite from Rochlitz-β crystallized in less evolved parts of the Rochlitz magma reservoir. The highest Ti-concentrations can be reported for the Mg-rich annite-siderophyllite of Rochlitz-β (up to 10.4 wt%; drilling Ballendorf 1/61; Fig. 13C, Online Resource Table 6).
Crystallization conditions in the Rochlitz magma reservoir can be explored by applying the Ti-in-biotite thermometry of Henry et al. (2005) and barometry of Wu and Chen (2015). This thermometer requires conditions of XMg (Mg/∑ Mg+Fe ) in a range of 0.275 to 1.000 and Ti between 0.04 to 0.60 apfu (Henry et al. 2005). For Rochlitz-α, a crystallization temperature of 732 ± 30 °C and 768 ± 30 °C for Rochlitz-β have been obtained. The pressure can be estimated with 4 ± 2 kbar. Assuming an average density of 2.8 g/ cm 3 for the Late Palaeozoic crust in this region, we estimate the crystallization depth at around 15 ± 7 km.

Eruption style forming the RVS climactic ignimbrites
Classic models of caldera evolution comprise thick mostly welded caldera fill, thin but widely distributed caldera outflow and ring dyke feeding systems (Lipman 1984;Cole et al. 2005). Thick ignimbrite packages may also form outside a caldera, as documented for the Bishop Tuff of the Long Valley Caldera (Hildreth 2004;Chamberlain et al. 2014), requiring a pre-existent deep topography. In pre-climactic stage of the RVS evolution (Fig. 16, stage 1; Table 2), andesitoid lava complexes of the Kohren Fm. formed close to the future RVS ring fault system (Figs. 2, 16;Gläßer 1983). On the other hand, Repstock et al. (2018) interpreted NW-SE-trending central subvolcanic bodies and dykes to represent major feeding fissures of the WVS (see also Branney et al. 2008 for the western Snake River system). Similarly, the Grimma Laccolith emplaced as a NW-SE-trending subvolcanic body in the central RVS. In the southeastern extension of it, the presence of the multi-vent Leisnig Laccolith complex gives further evidence for a fissure-shaped feeding system of RVS (Fig. 2). The Buchheim Volcanics overlying the Rochlitz-β Ignimbrite in the west of the RVS represent a possible reactivation on the NW-SE-trending vent assumed for Rochlitz-β. In contrast to classic caldera with uniform piston subsidence, the Rochlitz caldera floor fragmented forming a piecemeal caldera with numerous vents according to Roche et al. (2000). Randomly oriented fiamme as in the Rochlitz Ignimbrites are typical for these eruptions (Scafell Caldera, Branney and Kokelaar 1994). Intra-caldera faults in the west and in the Mügeln and Meltewitz Subbasin (Fig. 2) support this interpretation.
During the climactic stage of the Rochlitz caldera (stage 2 phase 1), high volumes of crystal-rich, rhyolitic and strongly to moderately welded ignimbrites formed (Rochlitz-α;Figs. 8D, 15;rank II-IV, Quane and Russell 2005). The high and homogenous welding compaction is a result of a fast deposition without significant breaks. No fallout or surge deposits, at the base or as intercalations in the Rochlitz Ignimbrites have been documented making Plinian eruption phases unlikely. Instead, we infer pyroclastic fountaining from NW-SE-trending fissures to be one of the dominant eruption styles.
Towards the termination of the climactic stage (stage 2 phase 2), the eruption of low-volume, crystal-rich, rhyolitic to trachydacitic magmas took place close to the western (Rochlitz-β) and northeastern margins (Rochlitz-N) of the Rochlitz caldera (Fig. 16). Scarcity of lithics in the Rochlitz system can be explained by rapid sealing of conduits by ascending magmas. It is also corroborated by the lack of inherited zircon in Rochlitz-α (Hoffmann et al. 2013).
The stratigraphic relation between Rochlitz-α and -β is not well established. A clear contact between these subunits has not been documented in outcrops or drillings so far. Drilling Ballendorf 1/61 (Fig. 7C) displays c. 110 m of Rochlitz-β Ignimbrite without reaching the base. The ignimbrite is followed directly by the Buchheim Volcanics of the Oschatz Formation. Other drilling reports in this area supply information about tuffs and metamorphic basement underlying Rochlitz-β, lacking Rochlitz-α Ignimbrite. These observations can be explained by an assumed paleo-topographic high in the Ballendorf area present during the deposition of Rochlitz-α. Much of Rochlitz-β Ignimbrite may have been eroded, leaving an erosional relict in the area northwest of Colditz (Fig. 2).

Magnitude of RVS climactic eruption
For magma volume calculation of the climactic Rochlitz-α Ignimbrite, a constant minimum thickness is estimated at 400 m (Fig. 7). The extension under the Wurzen Caldera is unknown, because no drilling reached the Rochlitz Ignimbrites underlying the Wurzen Ignimbrites. Assuming that the RVS ignimbrites completely underlie WVS (1300 km 2 ), a volume of c. 533 km 3 of preserved caldera-fill results. Confining the calculation to the continuous out-and subcrop belt in the southern NSVC (880 km 2 ), a volume of 352 km 3 can be assessed for the preserved caldera-fill ignimbrites. Sub recent calderas produced similar volumes of caldera fill, caldera outflow and co-ignimbritic ash deposits (Mason et al. 2004). Thus, using the conservative distribution, the total erupted volume of the RVS can be estimated as 1056 km 3 . According to Cook et al. (2016a, b), these calculations should be treated cautiously. Even this conservative volume estimation would result in a VEI of 8 (Newhall and Self 1982) rendering the Rochlitz event a supereruption (Rampino and Self 1992), comparable to the Permian Ora Fm. in northern Italy (1290 km 3 , Willcock et al. 2013). RVS would thus make one of the largest caldera systems in central Europe. Furthermore, the RVS is significantly larger than the WVS (c. 199 km 3 , Repstock et al. 2018) that formed a few million years later in the NSVC.

Post-climactic evolution
Presumably early in the post-climactic phase (stage 3 phase 1) the Leisnig Laccolith emplaced at the south-eastern margin of the RVS resulting in resurgence and formation of intra-caldera subbasins (Fig. 16;Fridrich et al. 1991;Kennedy et al. 2012). The magma intruded into volcaniclastic deposits of the Kohren Fm. underlying the Rochlitz Ignimbrites (among others the Wendishain ignimbrite; Fig. 5). The magma intruded into the still unconsolidated sediments. They can be considered as a suitable emplacement level between high strength metamorphic basement below and the thick welded Rochlitz Ignimbrites above (Awdankiewicz et al. 2004;Rocchi and Breitkreuz 2018). The dyke/ pitchstone domains located southwest of Leisnig and to the north of Wallbach represent feeding systems for the Leisnig Laccolith (Fig. 5). The two isolated outcrops of the quartzrich rhyolite in the basement south of Erlbach (Fig. 5) might be considered as additional feeding systems. This implies Fig. 9 Alteration after Gifkins et al. (2005). Chlorite-Carbonate-Pyrite Index (CCPI) vs. alteration index after Ishikawa et al. (1976)  The presence of NE-SW-striking phreatic dykes crosscutting Rochlitz Ignimbrites at Scheergrund and in a quarry at Rochlitzer Berg ( Fig. 4B; Online Resource Table 1) support the thesis of the Leisnig Laccolith. The dykes may have originated from phreatic explosions that occurred during the emplacement of the Leisnig Laccolith. The small sill at Lastau may represent an early intrusion phase of the Leisnig subvolcanic complex like the elongated, sub-horizontal bodies in the Henry Mtns. in Utah (Horsman et al. 2009). In drillings, tuff intercalations occur between Leisnig Laccolith protolobes which we consider as rafts of host rocks engulfed by rhyolitic melt. The new interpretation of the Leisnig Porphyry as a laccolith implies emplacement after the climactic eruption of the Rochlitz Ignimbrites, and consequently it does not belong to the Kohren Fm. Apparently, after the climactic phase, magma ascended from the reservoir, leading to the resurgence of the caldera like in the Caetano Caldera and Valles Caldera (stage 3 phase 1; John et al. 2009;Kennedy et al. 2012).
The coarse crystalline Grimma Laccolith, which intruded the Rochlitz Ignimbrites, reveals textural and geochemical differences to the weakly crystalline Leisnig Laccolith suggesting an origin from different magma batches (Fig. 16). Due to the geometry of the Grimma Porphyry with protolobes (Hutton 2009), documented in various outcrops and drillings (Online Resource Fig. 2 profile 1), it can be also  Pearce et al. (1996). For data see Online Resource Table 4. D TiO 2 vs. Zr diagram (for symbols see Fig. 9) 1 3 defined as a laccolith. The intrusion of subvolcanics at the end of the climactic phase of caldera eruptions is a typical feature of long-lasting magmatic systems (e.g., Chiaradia et al. 2009).
The monomict volcaniclastic conglomerate in the Meltewitz drilling presumably derived from a collapsed lava dome in the Meltewitz Subbasin indicates that post-climactic magmatism was also active at or near the RVS ring fault system    (Figs. 6, 16). Thus, we interpret that the emplacement of the Leisnig Laccolith (max. thickness of 220 m) preceded the deposition in these subbasins (Fig. 16).

Magmatic evolution
The overall magmatic trend of the Rochlitz caldera-forming eruption is one from a homogenous, rhyolitic crystalrich melt (monotonous rhyolite) for Rochlitz-α to a typical monotonous intermediate composition for Rochlitz-β (Figs. 14, 15;Hildreth 1981). Repstock et al. (2021) applied a range of geothermo-and barometers to constrain the depth of the WVS magma reservoirs between 15 and 40 km. The biotite geothermo-and barometry presented here yielded a depth of 15 km for the magma chamber of the RVS. With an error of ± 7 km the uppermost magma chamber can be postulated with ~ 8 km, comparable to the estimates for the monotonous rhyolite Ongatiti ignimbrite (Cooper and Wilson 2014).
Reheating by underplating enables the eruption of crystal-rich magma is a widely accepted concept (Bachmann et al. 2002;Bachmann and Bergantz 2004;Huber et al. 2009Huber et al. , 2012. For the eruption of the voluminous rhyolitic and crystal-rich Rochlitz-α magma intense reheating had to occur (Fig. 16). In contrast to similar eruptions of crystalrich magmas (Ongatiti Ignimbrite, Cooper and Wilson 2014;WVS, Repstock et al. 2018) apparently magma mingling was not important for the RVS as suggested by lack of fiamme and minerals with basic composition. Instead, magma mixing is indicated by resorption textures in plagioclase and quartz in Rochlitz-β Ignimbrite (Fig. 8F, Online Resource Fig. 3A, B) showing disequilibrium between magma and crystals (Patiño Douce and Johnston 1991;Matthews et al. 2012). Wetzel et al. (1995) already discussed magma mixing as the dominant process for RVS. Convection in the upper part of the magma chamber resulted in homogenization forming voluminous rhyolitic magma batches as it has been stated for the Nevada Volcanic Field (Henry and John 2013), the Ongatiti Ignimbrite (Cooper and Wilson 2014) and the Caetano Tuff (Watts et al. 2016). Due to the homogenous and distinct biotite composition (Fig. 13) and the narrow range in δ 18 O of zircon of Rochlitz-α (Fig. 14), these ignimbrites constitute large-volume monotonous rhyolites (Fig. 15).
The slightly more basic composition of the Rochlitz-β Ignimbrite together with Nb/Ta ratios up to 17 suggests magma mixing with underplating basic magma that resulted in a monotonous intermediate composition (Fig. 16;Green 1995;Bachmann and Bergantz 2008b;Huber et al. 2012). The significant negative Ti anomaly (Fig. 11) indicates fractionation of Fe-Ti-minerals like titanomagnetite and ilmenite. These minerals can be found in glomerocrysts, which are cumulates of a less evolved magma (e.g., Ellis et al. 2013). The pseudomorphs of pyroxene together with the absence of amphibole in RVS, indicate injection of H 2 O-poor, basic magma. This is supported by low δ 18 O in some zircon rims. Due to the large heterogeneity of δ 18 O in the Rochlitz-β zircon, it can be assumed, that the homogenization process, typical for monotonous intermediates, remained incomplete in RVS.
The high Nb/Ta ratios  show a lower degree of differentiation (Green 1995;Ballouard et al. 2015) in contrast to the values of 9.1-10.3 and 9.8-9.9 of the post-climactic Dornreichenbach and Frohburg Ignimbrites, respectively (Online Resource Table 4). The lower Nb/Ta ratio and a more pronounced negative Eu anomaly suggest a higher degree of plagioclase fractionation with a higher oxidation state for these post-climactic ignimbrites of the Oschatz Formation (Online Resource Fig. 4, Green 1995). The higher degrees of crustal contamination, indicated by a La/Nb ratio of 2.0-2.6 for the Dornreichenbach Ignimbrite and of 2.0-2.3 for the Frohburg Ignimbrite, significant negative Nb and Ta anomalies and low Ti values support this interpretation (Thompson et al. 1984;Cook et al. 2016a, b).
In the Borna Subbasin, the Buchheim Volcanics display REE pattern different from the neighboring Frohburg Ignimbrite suggesting origin from two separate magma batches. A less pronounced negative Eu anomaly of the Buchheim Volcanics implies a less differentiated magma. Furthermore, based on Nb/Ta ratio of 14.7-15.8 this magma also experienced a lower degree of crustal contamination.
The trachydacitic composition of Rochlitz-β suggests origin from a less evolved magma compared to Rochlitz-α, which is also supported by the presence of Mg-rich biotite. The Mg-poor and Fe-rich biotite of Rochlitz-α reflects a more differentiated magma. The large range of XMg of Rochlitz-β biotite suggests presence of different magma zones in the reservoir, resulted from basic intrusions in the crustal magma chamber, tapped by several fissure eruptions. The Mg-rich biotite may represent early cumulates that were extracted after homogenization (Lubbers et al. 2020). In granitic melts, Mn and Fe contents increase in biotite with  (Nachit et al. 2005). Data from Rochlitz-N are from subtype Thalheim. Biotite of Rochlitz-β has higher Ti and Mg contents than other Rochlitz Ignimbrites higher differentiation (Miller and Stoddard 1981). Furthermore, the high values of Mn (0.03-0.06 apfu, 0.2-0.4 wt%) in Rochlitz-α biotite (compared to Rochlitz-β, 0.01-0.02 apfu, 0.0-0.1 wt%; Fig. 13B), the more pronounced negative Eu-anomaly (Fig. 11) and the high silica content (Fig. 10) of Rochlitz-α Ignimbrite indicate a higher differentiation.
The RVS magma evolution, displaying a trend from voluminous rhyolitic ignimbrites to monotonous intermediates, can be compared with other magmatic centers like the Halle Volcanic Complex Słodczyk et al. 2015Słodczyk et al. , 2016, the Ongatiti Ignimbrite in New Zealand  (Cooper and Wilson 2014) and the Caetano Caldera (Watts et al. 2016) in the Nevada Great Basin.
The Halle Volcanic Complex (HVC), located northwest of the RVS, is dominated by large laccolith units (Fig. 1). According to Słodczyk et al. (2015) the chemical composition is similar to RVS. The range of δ 18 O in zircon is very limited compared to the variation in RVS (Fig. 14). Thus, the HVC can be considered as a "non-erupted monotonous rhyolite". The synchronous development (300-290 Ma, Breitkreuz et al. 2009;Hoffmann et al. 2013) in an intracontinental extensional setting (Romer et al. 2001), and similar geochemical composition suggest a cogenetic evolution of the RVS and the HVC.
The Ongatiti Ignimbrite and the Caetano Caldera display similarities in the magmatic evolution, whereas the geological setting differs from the tectonic environment of RVS. Both are classified as monotonous rhyolites (Cooper and Wilson 2014;Watts et al. 2016). Watts et al. (2016) postulated an evolution of monotonous rhyolites to monotonous intermediates due to basic injections leading to an eruptible mush for the rhyolitic, crystal-rich magma. Biotite in the Caetano Tuffs shows similar trends in Mg and Ti enrichment in the upper units like Rochlitz-β. The initial phase of both systems is characterized by extrusion of andesite melts. Injection of low-SiO 2 magma into a SiO 2 -rich magma chamber induced magma mixing and the formation of a homogenous high-SiO 2 cap and a heterogeneous less evolved lower part. After the eruption of the high-volume, crystal-rich Caetano Tuff, a less siliceous magma erupted. The Lower Caetano Tuff of the Caetano Caldera displays a narrow δ 18 O range in zircon (7.59-9.72‰, Watts et al. 2016;Fig. 14) like Rochlitz-α leading to interpretation as monotonous rhyolite. Resorption structures in plagioclase of the Upper Caetano Tuff evidence for injections of basic magma and mixing processes forming monotonous intermediates like Rochlitz-β.
With a calculated melt temperature of 755 °C, a crystal content of approx. 30% and a volume of more than 1000 km 3 , the evolution of the RVS is comparable to the eruption conditions of the Ongatiti Ignimbrite of the Taupo Volcanic Zone, New Zealand (more than 500 km 3 , temperature between 770 and 820 °C, 20-30 vol% crystal content, Cooper and Wilson 2014). These authors postulated a depth of 4-6 km for the Ongatiti magma reservoir, which is comparable to the calculated minimum depth of 8 km and a mean depth of 15 km estimated for RVS.

Conclusions
Petrographic features, whole-rock analyses, mineral chemistry of biotite and oxygen isotopes in zircon allow to characterize the different ignimbrites and other (sub)volcanic rocks of the RVS. The Rochlitz-α Ignimbrite can be classified as a high-voluminous monotonous rhyolite. In contrast, the following low-volume Rochlitz-β Ignimbrite has a trachydacitic composition. High TiO 2 and Zr content in whole-rock and lower δ 18 O in zircon reflect origin from a monotonous intermediate magma which experienced magma mixing by basic injections into a crustal magma chamber.
The evolution of the RVS can be summarized as follows: 1. The minimum volume of RVS can be estimated at 1056 km 3 suggesting a magnitude of VEI 8. The caldera extension (880 km 2 ) is comparable to other large calderas with VEI 8, thus, the RVS represents one of the biggest eruptions in central Europe. 2. The RVS comprises large-volume monotonous rhyolites as well as monotonous intermediates erupted in phase 1 of the climactic stage. Reheating by underplating and injections of basic magma led to the eruption of the crystal-rich melts. 3. During the second phase of the climactic stage, basic injections into a crustal magma chamber caused magma mixing forming a monotonous intermediate composition prior to the eruption of Rochlitz-β Ignimbrite. 4. Magma mixing was incomplete as reflected by the wide range of oxygen isotope ratios in zircon of Rochlitz-β. 5. We consider central fissure eruptions along NW-SEtrending faults as the main eruption style of the RVS resembling characteristics of a piecemeal caldera. Pres-  Fig. 16 Evolution model of the RVS with stages S1-S3. Stage 1) Pre-caldera eruptions during Kohren Fm.; Stage 2 phase1) Climactic phase of the Rochlitz caldera with deposition of voluminous, welded rhyolitic Rochlitz-α ignimbrites; Stage 2 phase 2) Termination of the climactic phase with deposition of Ti-and Zr-rich, rhyolitic to trachydacitic welded ignimbrites of Rochlitz-β and -N; Stage 3 phase 1) Early post-climactic resurgence phase; emplacement of the Leisnig Laccolith caused resurgence of the Rochlitz caldera and formation of the intracaldera subbasins; emplacement of the Grimma Laccolith in the apex of the resurgent dome; extrusion of the rhyolitic, porphyritic Kemmlitz Lava in the Mügeln Subbasin; Stage 3 phase 2) Late post-climactic phase: eruption of low-volume, strongly welded ignimbrites (Buchheim Volcanics) in the Borna Subbasin; deposition of sediments (e.g., Saalhausen beds, Fig. 6) and low-volume pyroclastic deposits (e.g., Dornreichenbach Ignimbrite); display of the caldera complex is vertically exaggerated ence of additional vents is displayed by dyke and laccolith complexes at the caldera margin. 6. The rhyolitic subvolcanic units in the caldera center can be confirmed as laccolith complexes, emplaced below and into Rochlitz Ignimbrites, respectively, during the post-climactic stage. Their intrusion led to a caldera resurgence forming three subbasins in the periphery. 7. The subdivision of the Rochlitz Ignimbrites into four subunits (R I-III and R N ), as proposed in previous studies, cannot be maintained. Instead, based on geochemical composition, a new subdivision is postulated. Rochlitz-α and -β Ignimbrites dominate. Besides, Rochlitz-N Ignimbrites locally occur can be further classified as Schöna type in the North and Thalheim type in the East. 8. An increasing differentiation in the magma chamber after the eruption of the monotonous intermediates is shown by the numerous low-volume ignimbrites erupted in the second phase of the post-climactic stage. 9. A correlation of the post-climactic succession (Oschatz Formation) in the three subbasins is not possible due to their different lithostratigraphy; however, a general cogenetic evolution in post-caldera depressions can be assumed.
Acknowledgements Frank Eigenfeld generously allocated data and 208 additional thin sections related to his Ph.D. project. The research was supported by the Saxon State Office for Environment, Agriculture and Geology, Department of Geology and by the Geology Department of the Technical University of Freiberg. Thanks go to the Saxon State Office for Environment, Agriculture and Geology and to the Wismut GmbH for allowing access to drilling documentation and cores. Results from master theses of Axel Rommel and Jan Rehda (2017, 2018, respectively, both TU Bergakademie Freiberg) have been incorporated in this study. We thank Lutz Hecht and Peter Czaja for EPMA analyses as well as Kirsten Born, all Leibnitz Museum für Naturkunde Berlin, for SEM measurements. We are very grateful for the careful reviews by Sergio Rocchi and Karsten Obst.
Funding Open Access funding enabled and organized by Projekt DEAL. The research was supported by the Saxon State Office for Environment, Agriculture and Geology, Department of Geology and by the Geology Department of the Technical University of Freiberg.
Availability of data and materials Data used in the manuscript are available by sending an email to the corresponding author and in the electronic supplement.
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