Abstract
Located between two large Late Palaeozoic rhyolitic Caldera systems (the Tharandter Wald Caldera (TWC) and the Altenberg–Teplice Caldera (ATC)) in the eastern Erzgebirge region, the extended NE–SW-trending rhyolitic Sayda–Berggießhübel dyke swarm (SBDS) was emplaced into Variscan gneisses. The dykes expose massive coherent rhyolite (CR), and four pyroclastic bodies reveal welded rhyolitic tuffs (WRT).
Overlying the CR dykes, the WRT form subcircular topographic elevations of up to 1 km in diameter. These have been interpreted by previous authors as erosional remnants of the ATC ignimbrite sheet. Another hill located on a dyke is built up of a columnar jointed rhyolitic lava dome or laccolith. An abandoned quarry near Mulda exposes sills propagating from a dyke into weathered gneiss. The WRT display a fluidal moderately to mostly vertically dipping welded texture with fiamme-like fragments up to 90 cm long. Strike orientation of the fiamme varies greatly. Gneiss lithic fragments make up to 2 vol.% of the analysed area of rock samples. The WRT are columnar jointed. The columns have a predominantly horizontal–subhorizontal orientation.
In outcrop, rock slab and thin section maximum particle size and the relative proportions of fiamme, phenocrysts in fiamme, lithic fragments and of crystal clasts (QZ, KF, PL, BT) in tuff have been measured. The correlation of the data with the regional geology shows that the welded rhyolitic tuff bodies resemble individual vent systems. Compared to the fiamme, the tuff matrix has a higher crystal clast content, indicating that pyroclastic fractionation has taken place.
Comparison of granulometric and geochemical SBDS data with ignimbrite samples from TWC and ATC confirms a close relationship between the SBDS and ATC. The SBDS is a possible vent area for the ATC. The textural data, the orientation of both fiamme and cooling columns, and other field relations suggest that the WRT of the SBDS represent welded fall-back tuffs. They formed in vents positioned on an active magmatic dyke system.
Fissure ignimbrites, pyroclastic dykes, ash-flow fissure vents and intra-vent welded tuffs are frequently used terms in volcanological literature when dealing with dyke or funnel-shaped pyroclastic bodies (e.g. Cook 1968; Koronovsky 1971; Ekren & Byers 1976; Awdankiewicz 1999; Aguirre-Díaz & Labarthe-Hernández 2003). Formation scenarios for the pyroclastic domains include ‘frozen-in’ upwards-directed movement of erupting fragmented magma, and intra-vent fall-back tuffs (e.g. Almond 1971; Reedman et al. 1987; Branney & Kokelaar 1994; Kano et al. 1997; Sparks et al. 1999).
A number of pyroclastic bodies are exposed in circular and elongated massive rhyolitic dyke swarms related to the Tharandter Wald Caldera (TWC) and the Altenberg–Teplice Caldera (ATC) in the eastern Erzgebirge region in Saxony/Germany (Fig. 1). One of the dyke systems, the rhyolitic Sayda–Berggießhübel dyke swarm (SBDS), contains pyroclastic rocks that have been considered as erosional remnants of the ATC ignimbrites in previous studies (Schust 1980; Wetzel 1984).
Geological sketch map depicting the northeastern part of the Erzgebirge region in Saxony; the map features the Late Palaeozoic magmatic rocks with the prominent NE–SW-trending Sayda–Berggießhübel dyke swarm (SBDS), the Altenberg–Teplice trapdoor caldera system (ATC) and the circular Tharandter Wald Caldera (TWC). Inset map: position of map in Germany; the box shows the location of Fig. 2 (modified after Sächsisches Landesamt für Umwelt und Geologie, 1995).
Three-dimensional measurements of fiamme and of cooling columns, as well as petrography and granulometry, have been used for a quantitative characterization of the SBDS pyroclastic (WRT) and massive coherent rhyolitic rocks (CR). Furthermore, we compared the granulometry and geochemistry of the SBDS pyroclastic facies (WRT) with the TWC and ATC ignimbrites. As will be shown in the next section, the SBDS pyroclastic rocks (WRT) have been interpreted as intra-vent welded fall-back tuffs.
Geological setting
The Erzgebirge region, situated in the south of Saxony in eastern Germany, is part of the NE–SW-trending Fichtelgebirge–Erzgebirge Anticline (Mingram & Rötzler 1999). The Erzgebirge region comprises a crustal pile of metamorphic rocks that formed during the Variscan orogeny which affected large parts of Europe in the Late Palaeozoic (Wilson et al. 2004). In the course of the Cenozoic, uplift and north-vergent block tilting led to the present-day morphology. In a late phase of the Variscan orogeny (Late Carboniferous–Early Permian) mainly acidic magmatism affected the entire Erzgebirge region. This led to the emplacement of voluminous granites (Förster et al. 1999) and dyke swarms like the SBDS, and to the evolution of extended volcanic systems like the ATC and TWC (Tischendorf 1989).
In the eastern part of the Erzgebirge region, more than 350 dykes constitute the SBDS cross-cutting the Variscan basement (Fig. 1). They reach lengths from a few metres up to several tens of kilometres, and widths from decimetres to several hundred metres. Three generations of rhyolitic dykes have been distinguished macroscopically, as well as by age and geochemistry (Thiergärtner 1962; Pälchen 1968; Wetzel 1984). In this paper only the first generation of rhyolitic dykes with welded rhyolitic tuffs will be considered. Synchronously with, or shortly after, the emplacement of the first SBDS generation the formation of the Altenberg–Teplice Caldera (ATC) took place (Wetzel 1984; Benek 1991). The ATC ignimbrites have been subdivided into a lower and an upper emplacement unit (Schönfeld and Altenberg–Teplice Ignimbrite, respectively). In a late phase, the ATC was intruded by voluminous coarsely porphyritic magmas, the so-called ‘Granitporphyr’ (the porphyritic granite dyke in Fig. 2).
Topographical and geological map of the investigated area with stereograms of columnar jointing and different facies types in rhyolite dykes of the Sayda–Berggießhübel dyke swarm (SBDS).
Presumably, at the same time, the rhyolitic Tharandter Wald Caldera (TWC) formed (Benek 1980; Stanek & Renno 2001) (Fig. 1). Only the caldera-fill facies is preserved and it consists of older quartz-poor and younger quartz-rich ignimbrites (Benek 1980; Stanek & Renno 2001). Pyroclastic rhyolitic rocks similar to the WRT presented here have been reported from the circular ring-dyke system of the TWC (Benek 1980).
In first half of the nineteenth century Beust (1835) investigated the porphyritic rhyolitic rocks in the Freiberg region. In the course of the first complete geological mapping campaign of Saxony, Sauer (1886), Schalch (1887) and, later, Reinisch (1920, 1927) provided detailed geological maps and petrographic descriptions. Considering mainly the tectonic framework, Moesta (1928) postulated a formation of the SBDS under a strike-slip regime. Schreiter (1933) described xenolitic gneiss inclusions in the rhyolitic dyke of the Turmberg area. Further petrographical and geochemical studies on the SBDS have been carried out by Thiergärtner (1962, 1966), Pälchen (1968) and Lai (1978). Schust (1980) was the first to reveal the pyroclastic nature of the Burgberg rocks and interpreted them as erosional remnants of the ATC ignimbrite sheets (see also Wetzel 1984).
Both the coherent rhyolitic (CR) and the pyroclastic facies (WRT) of the SBDS form morphological elevations. This is the consequence of a strong resistance of the finely crystalline felsic rocks, compared to the coarse metamorphic host rocks, to weathering that affected the region during Cretaceous–Neogene times. Field information for the present contribution has been gathered from hand samples, boulders, scattered natural outcrops, as well as abandoned and active quarries.
Methods
Apart from mapping and sampling in the field area (Fig. 2), the orientations of fiamme cutting lineaments on random rock surfaces or, where possible, planes of fiamme have been measured. These and the orientation of cooling columns were plotted in stereograms (Figs 2 and 7). As shown in Figure 3, in the coherent facies (CR) we distinguish between matrix, phenocrysts and xenoliths. In the pyroclastic facies (WRT) the fiamme contain matrix and phenocrysts. Around the fiamme there is tuff matrix with crystal clasts and lithic fragments.
Schematic sketch depicting the nomenclature used in this contribution: (a) of the coherent rhyolitic facies (CR); and (b) of the welded rhyolitic tuff (WRT).
Petrographic, granulometric and geochemical investigations have been carried out on SBDS rocks, and, for comparison, on selected samples from the nearby Tharandter Wald (TWC) and Altenberg–Teplice caldera systems (ATC). The quantitative granulometric analysis included point counting and maximum particle size measurements in thin sections, as well as digital image analysis on scanned rock slabs of decimetre size (cf. Mock et al. 2003) (Table 1). By these means, we determined the amount of phenocryst and crystal clasts in the matrix, the crystal/matrix ratio, as well as the maximum particle size measurements of crystal clasts, phenocrysts, xenoliths and lithic fragments in the different lithologies (Tables 1, 2, 3). With the welded rhyolitic tuff domains, we estimated the portion of fiamme in tuff matrix, and the phenocryst/matrix ratio inside the fiamme (Table 1).
Results of measurements on rock slabs from pyroclastic domains; area portions of fiamme and lithic fragments and of quartz and feldspar phenocrysts in fiamme
Areal amount of phenocrysts/crystal clasts measured on rock slabs in matrix of coherent rhyolite (CR) and in matrix of welded rhyolite tuff samples (see Figure 4 for nomenclature)
Results from thin-section measurements: maximum particle size (mps) of phenocrysts/crystal clasts; values are averages of the five largest phenocrysts/crystal clasts; point counting (each thin section has a minimum of 1000 points)
All area measurements of crystal clasts/phenocrysts and matrix have been carried out on rock slabs as the accuracy is higher compared to point counting of thin sections (Mock et al. 2003) (Table 3). A number of 200–300 crystal clasts or phenocrysts, respectively, were measured on each rock slab in order to gain statistic significance (Mock & Jerram 2005). Only maximum crystal clast/phenocryst size, volume % (vol.%) of biotite and the volume % (vol.%) ratio of feldspars (K-feldspar/plagioclase) were determined in thin sections (Table 3). Length and height determination of fiamme, maximum particle size measurements of xenoliths in the coherent facies (CR), and lithic fragments in the pyroclastic facies (WRT) have been carried out in the field, as xenoliths and lithic fragments are scarce and fiamme length reach up to 90 cm.
One of our major goals was a quantification of the textures using granulometric methods. For the interpretation of the resulting data, we were aware of restrictions regarding the statistic significance and of two and three dimensions (Higgins 2002; see the discussion in Mock et al. 2003). With this in mind, only very clear trends and distinct differences in the data set were used for interpretation.
Facies and texture of the Sayda–Berggießhübel dyke swarm (SBDS) near Burkersdorf
Field observations and textural analysis of rock slabs and thin sections allow for a distinction between two main SBDS rock types: (i) a coherent rhyolitic facies (CR) that crops out in a system of dykes, sills and domes; and (ii) a welded rhyolitic tuff facies (WRT) forming roundish monadnocks up to 1 km in diameter (Fig. 2). Both facies contain scarce xenoliths or lithic fragments, respectively, consisting mostly of angular gneiss fragments, typically 2 cm in diameter (rarely up to 8 cm). In addition, at Burgberg, coherent rhyolitic fragments, typically 2 cm in diameter (rarely up to 14 cm), originating from dykes have been observed in the WRT.
The SBDS near Burkersdorf formed in three stages (Fig. 2):
A NE–SW- and NW–SE-striking network of rhyolitic dykes of relatively constant width (decimetres up to 60 m) presumably formed co-genetic to NE–SW-striking rhyolitic dykes that have a curved course and a strong variation in width (up to 600 m) over a short distance (Fig. 2). At Burgberg and Turmberg, these widened dyke sections host welded rhyolitic tuff (WRT) domains. The Röthenbacher Berg consists of an isolated coherent rhyolitic domain (CR), 1 km in diameter, which has blobs of vesiculated intermediate rocks suggestive of magma mingling (Fig. 6f). Distinction between an extrusive (lava dome) and an intrusive formation (laccolith) was not possible because top contacts are not exposed. Four locations depicted in Figure 2 revealed dominantly obliquely to horizontally dipping cooling columns, oriented perpendicular to the dyke margins or, in the case of the lava dome at Röthenbacher Berg, in a radial arrangement. Centimetre- to metre-thick rhyolitic sills were observed to have intruded the welded rhyolitic tuff of SBDS (Fig. 4) and into weathered gneiss (Fig. 5). The first-stage dykes expose domains with fluidal texture and with brecciation (Fig. 2);
The first-stage dykes were cross-cut by 200–700 m-thick coarsely porphyritic rhyolitic dykes (‘Granitporphyr’) (Fig. 2), which belong to the late intrusive phase of the ATC; and
NW–SE-striking intermediate–basic dykes cross-cut dykes of stages (i) and (ii) (Fig. 2).
(a) Outcrop on the SE slope of Burgberg; complex intrusion of coherent rhyolite (CR) into the welded rhyolitic tuff (WRT); (b) intrusive contact (scale: hammer 26 cm); (c) small coherent rhyolitic (CR) dyke in welded rhyolitic tuff (WRT) (folding rule: 2 m); and (d) small coherent rhyolitic sill in welded rhyolitic tuff (WRT) (folding rule: 1.20 m).
(a) Part of an abandoned quarry near Mulda with apophysis of coherent rhyolite intruding into weathered Variscan gneiss; and (b) photograph of the coherent rhyolite (CR) apophysis intruding the Variscan gneiss.
The coherent rhyolitic facies (CR)
For the CR the thin section and the chemical composition are evidence for a rhyolitic composition (Tables 1, 2, 3, 4). The matrix of this massive rock type consists of a fine-grained homogenous mosaic of feldspar and quartz. In places, spherulitic texture formed. The coherent rhyolitic rocks have a phenocryst content of between 4 and 20% (Table 2; Figs 6g & h and 9b) with a crystal size up to 4 mm (Table 3). Quartz/feldspar ratios vary from 0.5 to 1.5, and K-feldspar is more common than plagioclase (ratio 3–11; Fig. 9b and Table 2). Traces of biotite (up to 1 mm, Table 3) and opaque minerals also occur. Micrographic intergrowth of K-feldspar and quartz has been observed.
Photographs and microphotographs of welded rhyolitic tuff (a–e) and coherent rhyolitic rocks (f–h) from samples of the Sayda–Berggießhübel dyke swarm (SBDS): (a) photograph of pyroclastic facies (WRT) at Burgberg: dark tuff matrix with lighter fiamme and numerous crystal clasts; (b) sample of pyroclastic facies (WRT) at Turmberg with fiamme and gneiss lithic fragment (GL); (c–e) welded rhyolite tuff from hill SSW Turmberg and Burgberg: cryptocrystalline matrix with coarser crystallized fiamme and quartz (QZ) and feldspar phenocrysts (KF; PL) and lithic fragments, crossed nicols; (f) magma mingling of coherent rhyolite (CR) with more mafic, vesiculated material (VM) at Röthenbacher Berg; (g) a typical sample of coherent rhyolite (from Turmberg): reddish matrix with quartz (QZ) and feldspar phenocrysts (FS); and (h) coherent rhyolitic facies: quartz (QZ) and plagioclase (PL) crystal clasts in microcrystalline matrix, crossed nicols.
Geochemical data of samples of the Sayda–Berggießhübel dyke swarm (SBDS) (analysed by ACMELAB Vancouver Canada)
The welded rhyolitic tuff domains (WRT)
Welded rhyolitic tuff facies has been recognized at Burgberg and at a nearby small hill, at Turmberg and a knoll SSW of it, and at Kahle Höhe in the east of the mapping area (Fig. 2). Unfortunately, the transition from a coherent rhyolitic feeder dyke below the fragmentation level to a welded rhyolitic tuff domain above it is not exposed. Outcrops, rock slabs and thin sections show a crystal-rich matrix with fiamme-like elongated porphyritic fragments with a strong flow foliation (Fig. 6a and b). Intentionally, the term ignimbrite is avoided here because there is no physical evidence for the fiamme-like elongated porphyritic fragments representing compacted pumice. No vesicles have been observed inside the fiamme. Maximum crystal clast size in the matrix and phenocryst size in the fiamme matrix is 2.7 mm (Table 3) (see Fig. 3 for nomenclature).
The WRT matrix contains varying amounts of crystal clasts (14–44%; Fig. 9a and Table 2), the quartz–feldspar ratio ranges between 0.8 and 2.6 (Fig. 9b and Table 2). Apart from idiomorphic shapes, fragments of crystal clasts have been observed that presumably formed during explosive eruption (Fig. 6c and d) (Best & Christiansen 1997). The welded rhyolitic tuff domain SSW of Turmberg contains a significant amount of xenocrystic white mica in the matrix, which originated from the fragmentation of metamorphic country rock during the explosive eruption.
Fiamme makes up to 24 vol.% of the WRT domains (Table 1). They have ragged margins, varying aspect ratios (0.04–0.3; Fig. 9d) and lengths of up to 90 cm. The fiamme show pectinate crystallization texture near the margins, and fine crystalline–mosaic-like, occasionally spherulitic, texture in the centre. This, and the ‘splayed, brush-like ends’ (Branney & Sparks 1990) of the fiamme, infer that the shape formed from welding-compaction and shearing. At Turmberg, folding of the foliation is present. The 3D orientation of the fiamme has been measured in three domains at 20 locations (presented in stereograms in Fig. 7). Orientation of foliation is predominantly vertical–subvertical with a strong variation in strike.
Topographical map with the outcrops of volcanic rocks of (a) Burgberg and (b) Turmberg with stereograms of orientation of plane normals, and cutting lineation of fiamme and different facies types. Blocky material= blocks of welded rhyolitic tuff.
Quartz/feldspar phenocryst ratios in the fiamme vary between 0.9 and 2.1 (Table 1). Crystal contents in the fiamme range from 10 to 25% (Fig. 9a), consistently lower compared to that of the matrix of the host tuff (Fig. 9a). This is interpreted to be the result of removal of fine ash during explosive eruption (pyroclastic fractionation: Sparks & Walker 1977).
Granulometric measurements revealed a number of significant differences between the WRT bodies of Burgberg and Turmberg, such as crystal content of the tuff matrix (Fig. 9a) and of the fiamme matrix (Fig. 9c), the quartz phenocryst content (Fig. 9c) and size (Fig. 9e) in the fiamme, and the length and aspect ratio of the fiamme. These observations clearly indicate that the Burgberg and Turmberg welded rhyolitic tuff domains formed independently of each other.
Three WRT domains have columnar jointing. The columns have diameters ranging between 10 and 30 cm (Fig. 8a). At Burgberg, orientation is vertical–subhorizontal; at the hill SSW of Turmberg, subhorizontal oriented columns have been measured (Fig. 2); and Turmberg revealed a subhorizontal radial complex arrangement (Fig. 8b).
(a) Columnar jointing in welded rhyolitic tuff at Turmberg (folding rule: 0.40 m) (from Kuhnt & Ritzmann 2002); and (b) orientation of cooling columns, measured as lineation, in different outcrops at Turmberg (see the legend on Fig. 2); note: some measurements belong to the surrounding coherent rhyolitic facies.
In two welded rhyolitic tuff bodies (Turmberg, and a hill SSW of Turmberg), centimetre-thick sandstone-filled dykes with clear-cut margins have been observed (‘S’ in Fig. 2). These clastic dykes contain abundant biotite and, subordinately, rhyolitic rock fragments and white mica. These dykes might have formed during a late phreatic activity. However, a later sedimentary infill of open cracks cannot be ruled out.
Granulometric comparison of the SBDS welded rhyolitic tuff and coherent rhyolitic domains
Phenocryst and crystal clast content and the quartz/feldspar ratio apparently suggest that the WRT at Burgberg and Turmberg and the CR comprise individual units (Fig. 9a–d). However, as mentioned previously, intra-fiamme granulometric data are more reliable for comparison of magma batches. With this in mind, Figure 9c indicates a similarity of the Turmberg welded rhyolitic tuff domain with the SBDS coherent rhyolitic rocks.
Results of quantitative texture analysis (see Fig. 3 for nomenclature): (a) comparison of phenocryst content in the fiamme and crystal clasts in the tuff matrix of pyroclastic rocks (WRT) of the Sayda–Berggießhübel dyke swarm (SBDS), data see Tables 1 and 2; (b) area ratio of quartz and feldspar phenocrysts (or crystal clasts, repectively) v. matrix; data partially in Table 3; (c) comparison of quartz v. total phenocryst content in fiamme of pyroclastic rocks and in matrix of coherent rhyolitic rocks; data see Tables 1 and 2; (d) aspect ratio v. maximum particle size (mps) of fiamme (note: this diagram shows field measurements not presented in tables, one fiamme reached a length of 90 cm – not displayed here); and (e) mps of alkali feldspar v. mps of plagioclase phenocrysts/crystal clasts (legend for all diagrams is given in Fig. 9d).
Comparison of SBDS welded rhyolitic tuff domains with the TWC and ATC ignimbrites
Crystal clast/matrix ratios of the SBDS WRT domains and the ATC and TWC ignimbrites are similar, only the Burgberg WRT domain contains slightly more crystal clasts (Fig. 9b). Relative amounts of quartz and feldspar crystal clasts of SBDS equals that of ATC ignimbrites, TWC flow deposits clearly display less quartz (Fig. 9b). As pyroclastic fractionation can change the original magmatic granulometry, phenocryst content inside the fiamme is more representative of the erupted magma. These data (Fig. 9c) also show a similarity between the SBDS welded rhyolitic tuff domains and the ATC ignimbrites, and a marked lower quartz content for the TWC deposits.
Maximum crystal clast size of the SBDS, ATC and TWC rocks is displayed in Table 3 and, for feldspar, in Figure 9e. TWC deposits are characterized by relatively large plagioclase crystal clasts compared to those present in ATC and SBDS rocks. Compared to the rest, TWC ignimbrites have relatively small quartz crystal clasts (Table 3). Crystal clast size in SBDS WRT and in ATC deposits is similar. In conclusion, the granulometry shows close similarities between SBDS and ATC rocks, and allows for a relatively clear separation of TWC ignimbrites.
In order to complete the database of the granulometric findings, 14 whole-rock samples of the SBDS have been analysed geochemically (Table 4). The data were compared with published analytical data from the TWC ignimbrite (15 analyses: Stanek & Renno 2001) and from the ATC ignimbrite (15 analyses: Lai 1978; Seltmann & Schilka 1995; Breiter et al. 2001).
All rocks considered show a rhyolitic composition (Fig. 10a). An immobile element plot of Zr v. TiO2 allows for a distinction between TWC ignimbrites, on the one hand, and the SBDS and ATC rocks, on the other. Only the coherent rhyolitic facies (CR) at Hühnerberg has high TiO2 values, similar to the TWC rocks (Fig. 10b). TWC samples have relatively high Zr/Hf ratios compared to those of the SBDS and the ATC (Fig. 10c). Only a few samples of the TWC show low Zr/Hf ratios, these rocks are more highly evolved and have been generated through diffusion processes in a stratified magma chamber (Renno pers. commun. 2006). Samples that plot in the CHARAC field (box in Fig. 10c) represent trace-element behaviour in a geochemical system controlled by CHArge and RAdius (Bau 1996). Many SBDS samples apparently display a non-CHARAC trace-element behaviour; typical for evolved rocks (Bau 1996). Altogether, chemical compositions of the ATC and the SBDS rocks have much in common (see also Wetzel 1984), whereas the TWC ignimbrites are apparently quite different.
Position of the CR and WRT of the SBDS and of the ATC and TWC ignimbrites: (a) in the Nb/Y–Zr/TiO2 diagram after Winchester & Floyd (1977); (b) in the TiO2–Zr diagram; and (c) in the Zr/Hf–Y/Ho diagram; box in (c) CHARAC field (explanation see text); geochemical data for Altenberg–Teplice Caldera ignimbrite (ATC) from Lai (1978), Seltmann & Schilka (1995) and Breiter et al. (2001); for Tharandter Wald Caldera (TWC) ignimbrites from Stanek & Renno (2001), legend for all diagrams is given in (a); see also Table 4.
Discussion
Models of pyroclastic dyke formation
Pyroclastic dykes are a fascinating interface between the subvolcanic intrusive and the extrusive explosive processes in intermediate–silica-rich volcanic systems. They belong to a large range of geological forms summarized under the term ‘clastic dykes’. Clastic dykes comprise, on the one hand, products of sedimentary processes, such as neptunian dykes (open fissures filled with sediments from above: Smart et al. 1988) and sedimentary intrusive dykes often related to seismic activity. They can also form during volcanic eruptions. Here, the spectrum ranges from non-welded tuffs associated with (phreato-) magmatic eruption centres to welded ignimbrite-like domains often found in dykes and vents of large caldera systems, such as described here from the eastern Erzgebirge region.
Essentially two scenarios were discussed in literature for the formation of welded pyroclastic dykes and vents: (1) product of ‘frozen-in’ upwards-directed movement of a mixture of hot molten magma fragments and gas; and (2) intra-vent fall-back from a collapsing eruption column. Both scenarios are quite similar with respect to the resulting textures. Only the second scenario implies that the fragmented material temporarily formed part of the eruption column, and that pyroclastic fractionation (winnowing of fine ash) was possible prior to deposition and welding compaction.
Scenario (1) was the interpretation of studies in large ignimbrite provinces, the formation of which often was related to huge fissure eruptions (Bonnichsen 1982). Extended fissures up to 460 m wide and with horizontal extension of several kilometres filled with welded pyroclastic material were described from the Pliocene Verkhnechegemsk volcanic Complex in northern Caucasus (Koronovsky 1971), from the Basin-and-Range-related Mid-Cenozoic volcanic provinces in Sierra Madre Occidental in Mexico (Aguirre-Díaz & Labarthe-Hernández 2003) and in Nevada in western USA (Ekren & Byers 1976). Small Cenozoic trachytic dykes emplaced in basaltic country rocks display evidence of agglutination of pyroclastic material on vent walls and around large host rock blocks (Wolff et al. 1989).
The model of a formation from an upwardly moving dispersion has also been favoured for pyroclastic dykes in classical caldera systems, e.g. present in the Cenozoic magmatic province in northwestern UK and Ireland (Mull Volcanic Centre in Scotland: Sparks et al. 1999; Slieve Gullion ring-dyke in Ireland: McDonnell et al. 2004). In addition, syneruption block movement and inflation–deflation during caldera evolution have been accounted for by compression, ductile shearing and closing of pyroclastic dykes (Jurassic Sabaloka Caldera in northern Sudan: Almond 1971; Ordovician Bad Step Tuff in the British Lake District: Branney & Kokelaar 1994).
Similar to the SBDS pyroclastic bodies (WRT) in dimension and shape, a 700×400 m plug of welded tuff has been studied in Weolseong, southern Korea by Reedman et al. (1987). These authors concluded that the funnel-shaped pyroclastic domain had been formed according to Scenario (1) by prograding agglutination of pyroclastic material at the walls of the vent. Kano et al. (1997) disagreed, comparing the Weolseong plug with the Shiotani conduit fill in SW Japan. These authors favour Scenario (2) for the formation of welded pyroclastic domains in funnel-shaped vents. During eruption column collapse, large portions of pyroclastic material fall back into the vent. The lower part of the accumulating deposit undergoes welding compaction caused by overburden and by heating from underlying magma. Compaction causes shearing and reomorphic folding along the vent margin leading to a foliation dipping towards the vent centre by 60°–90°.
Evolution of the SBDS
The SBDS comprise seven lithofacies types of coherent rhyolite and of welded rhyolitic tuff. Among the coherent rhyolitic facies (CR) field relations, granulometry and geochemical composition allows distinctions to be made between pre- to syn- and post-WRT generations. The pyroclastic rocks at Burgberg contain some fragments of coherent rhyolitic material (CR) accounted for by a pre-existing coherent rhyolitic dyke below. Certain SBDS coherent rhyolitic rocks (CR) may resemble subfragmentation-level equivalents of the welded rhyolitic tuff domains (e.g. the Turmberg domain; Fig. 9b and c), however, compelling evidence for a co-genetic source–product relationship is lacking.
The SBDS welded rhyolitic tuff domains (WRT) have a subcircular form. Most of the fiamme are (sub-) vertically oriented, but with a strong variation in strike direction; the cooling joints, at least of the Turmberg outcrops, display a complex radial arrangement. All these observations suggest a funnel-shaped (v. fissure-shaped) vent as described by Reedman et al. (1987) and Kano et al. (1997).
Pyroclastic flow deposits are markedly fines-depleted, as movement of pyroclastic flows is facilitated by fluidization that leads to a winnowing of fine ash (Sparks & Walker 1977). Intra-vent or -fissure processes described in Scenario (1) presumably do not allow for a significant pyroclastic fractionation. However, the SBDS data clearly indicate a concentration of crystal clasts in the tuff matrix between the fiamme. This is one important argument which means that the Reedman et al. (1987) model is not applicable for the SBDS pyroclastic bodies (WRT). Also, the Reedman et al. scenario should lead to a strong concentric compositional and textural zonation of the pyroclastic material, and a complete closure of the vent by lateral agglutination from an upwardly moving dispersion seems unrealistic. As a consequence, we favour the fall-back scenario model of Kano et al. (1997) for the formation of the SBDS pyroclastic domains. However, this implies that ballistic fall-back of larger particles and convection collapse in the eruption column leads to pyroclastic fractionation.
The WRT fiamme of SBDS show no traces of vesicles. However, since (sub-) plinian eruptions can be assumed, first boiling must have taken place. This is supported by the presence of phenocryst fragments (Best & Christiansen 1997) and of broken quartz phenocrysts that consist of partially annealed fragments with different optical orientations (Fig. 6e). It is possible that destruction of pore space and vesicles in the fiamme took place during welding compaction by resorption of volatiles back into the hot glass (Sparks et al. 1999).
Based on petrographic and geochemical studies, Schust (1980) and Wetzel (1984) discussed a genetic relationship between the SBDS and the ATC system. Our geochemical data and granulometric results confirm the assumption of a common magmatic source. The interpretation of the SBDS pyroclastic domains (WRT) as erosional remnants of the ATC ignimbrite sheet, however, can be discarded for the following reasons: (i) The SBDS welded rhyolitic tuff bodies reveal horizontally–subhorizontally oriented cooling columns and vertical–subvertical orientation of fiamme; both features are uncommon with ignimbrite sheets; (ii) The neighbouring pyroclastic domains of Burgberg and Turmberg have contrasting fiamme length and aspect ratio, as well as differing crystal clast content, rendering the interpretation as erosional remnants of one ignimbrite sheet unlikely; and (iii) Furthermore, sediments or soils should be exposed in the contact zone between the pyroclastic domain and the country rock (i.e. the metamorphic basement).
Consequently, the welded rhyolitic tuff bodies of SBDS may represent a vent system for the Altenberg–Teplice Caldera system. In this case the SBDS intra-vent fall-back tuff obviously represents the final stage of the ATC eruption.
Conclusions
The NE–SW-trending SBDS acted as a complex feeding system for rhyolitic magma in the eastern Erzgebirge region during the Late Palaeozoic. Apparently, magmas with different volatile content ascended. Low-volatile batches led to the formation of the lava dome or laccolith complex at Röthenbacher Berg. Here, the only evidence of synchronous ascent of intermediate magma and resulting magma mingling is exposed. Volatile-rich phases led to the formation of explosive centres, remnants of which are preserved in the SBDS in the form of the WRT domains. The subcircular welded rhyolitic tuff domains at Burgberg and Turmberg represent individual explosive vents where different magmas experienced subplinian or plinian eruptions (Fig. 11a). Pyroclastic flows might have moved away from the vents, but owing to later erosion deposits are not preserved. Granulometric and geochemical data suggest a genetic relationship of the WRT and CR with the ATC ignimbrites. However, the SBDS WRT domains do not resemble erosional remnants of ATC ignimbrite sheets.
Possible model of formation of vent-like welded rhyolitic tuff occurrence near Burkersdorf/Frauenstein in Saxony (for an explanation see the text).
Towards the end of the eruption at each vent, pyroclastic material fell back into the vent during eruption column collapse. A portion of fine ash did not fall back but was blown away, resulting in enrichment of crystal clasts in the tuff matrix between the fiamme. The fallen back lava fragments were hot enough to undergo welding–compaction with shearing and reomorphic folding near the walls of the vent. Presumably, existing intra-clast pore space and intra-fiamme vesicles were destroyed by volatile resorption into the hot glass (Fig. 11b).
During cooling of the dense intra-vent fall-back tuff (sub-) horizontally orientated cooling columns formed. Phenocryst-poor coherent rhyolitic magmas intruded into the vent-fill under brittle conditions (Fig. 11c).
Mesozoic–Cenozoic weathering and erosion modelled the coherent rhyolitic and pyroclastic domains of the SBDS to the present-day monadnocks, as the felsic rocks are more resistant to weathering compared to the coarse, polycrystalline metamorphic host rocks (Fig. 11d).
Acknowledgments
K. Breiter (Czech Geological Survey, Prague) is thanked for enabling access and sampling of drilling Mikulov 4. K. Stanek and A. Renno (both TU Bergakademie Freiberg) kindly placed TWC geochemical analyses at our disposal. M. Magnus (TU Bergakademie Freiberg) and A. Mock (now with Numerical Rocks, Norway) kindly introduced quantitative image analysis to C. Winter. H. Förster is acknowledged for fieldwork assistance. We highly appreciate a careful review by S. Passey which led to a substantial improvement of this contribution.
- © The Geological Society of London 2008
References
Textural analysis of a Late Palaeozoic coherent-pyroclastic rhyolitic dyke system near Burkersdorf (Erzgebirge, Saxony, Germany)
