Provenance of the early Palaeozoic volcano-sedimentary successions from eastern part of the Central Sudetes: implications for the tectonic evolution of the NE Bohemian Massif

Abstract

The Kamieniec Metamorphic Belt (KMB) and the Doboszowice Metamorphic Complex (DMC) expose a fragment of the pre-Variscan volcano-sedimentary cover preserved in the Fore-Sudetic Block in the NE part of the Bohemian Massif. We present the age of detrital and magmatic zircon grains and the bulk rock chemical composition of rock samples from the KMB and the DMC to better understand the evolution of the early Palaeozoic Gondwana margin. The zircon age spectra were acquired by U–Pb LA–ICP–MS dating and represent two groups that differ by maximum depositional age (MDA). The paragneiss from the DMC displays the MDA at 456 Ma, whereas the mica shist from the KMB displays the MDA at 529 Ma. Older age peaks in both groups of samples are represented by the Neoproterozoic and less frequent the Paleoproterozoic and Archean. The data presented indicate that the rock successions were sourced from the Cadomian orogen and deposited in the basins that developed on the Gondwana margin. Our results support the suggestion that the crystalline basement in the eastern part of the Fore-Sudetic Block has an affinity to the Trans-Saharan Belt or West African Craton and was part of a Gondwana shelf. The final stage of evolution of the studied successions was related to the Variscan thermal overprint. Based on presented data, we support the idea that the suture separating the Brunovistulian domain from the rest of the Gondwana-derived terranes is not related to the closure of the Rheic Ocean and represents a local feature.

1. Introduction

Several crustal fragments of the Bohemian Massif comprise Neoproterozoic-Early Ordovician volcano-sedimentary successions formed at the northern Gondwana margin. In the Neoproterozoic and early Cambrian, northern part of Gondwana was a long volcano-magmatic arc formed in response to subduction of oceanic domains identified in the literature as the Iapetus and Tornquist Oceans (e.g. Linnemann et al. 2007; Nance et al. 2012; Domeier, 2016; Boris, Domeier & Jakob, 2021; Oriolo et al. 2021) or the Paleo-Pacific Ocean (Merdith et al. 2021). During the late Cambrian and Early Ordovician, the arc was split into two parts called Avalonia and Cadomia, and separated by the newly opened Rheic Ocean (e.g. Linnemann et al. 2004; Linnemann et al. 2008; Nance & Linnemann, 2008; Oriolo et al. 2021). This resulted in the late Cambrian-Early Ordovician times in transformation of the Gondwana active margin into a passive margin, although the mechanism responsible for this transformation is still debated (e.g. Winchester et al. 2006; Linnemann et al. 2008; Sláma et al. 2008b; Nance et al. 2010; Díez Fernández et al. 2012; Hajná et al. 2018). Subsequently, during the closure of the Rheic Ocean, remnants of the Neoproterozoic basement were trapped between the colliding continents of Gondwana and Laurussia and recycled in the Variscan belt of Europe. Subsequently, remnants of the Avalonian and Cadomian basement were dispersed throughout the Variscan belt of Europe and juxtaposed along the Rheic suture (Fig. 1). The suture is relatively well documented in the western and central sectors of the Variscan belt of Europe (e.g. Linnemann et al. 2007; Pérez-Cáceres et al. 2015; Kirchner & Albert, 2020). However, in its eastern part, that is, in the Sudetes, the location of the suture is still poorly recognized. Some authors discussed the presence of the Rheic suture between the Central and East Sudetes (e.g. Mazur et al. 2012; Jastrzębski et al. 2013; Jastrzębski et al. 2015; Oberc-Dziedzic et al. 2018; Jastrzębski et al. 2020; Śliwiński et al. 2022), while others questioned its existence in this fragment of the Variscan belt of Europe (Soejono et al. 2010; Collett et al. 2021, 2022). Unfortunately, this fragment of the Variscan orogen is partly located on the Fore-Sudetic Block, where the metamorphic basement is mostly covered by Cenozoic sediments. Therefore, unravelling the location of the suture in this portion of the Sudetes and the nature of the exposed basement is disputable, and every new piece of evidence becomes important.

Figure 1. Peri-Gondwanan terranes of Southern and Central Europe (modified from Franke, 1989; Linnemann et al. 2007; Nance et al. 2008). AM – Armorican Massif, IM – Iberian Massif, FMC – French Massif Central, BM – Bohemian Massif, RM – Rhenish Massif, MC – Midland Craton, B – Brunovistulicum, Ga- Ganderia, SPZ – South Portuguese Zone, OMZ – Ossa-Morena Zone, CIZ- Central Iberian Zone, SxZ – Saxothuringian Zone, EFZ – Elbe Fault Zone, ISF – Intra Sudetic Fault, DFZ – Dolsk Fault Zone, OFZ – Odra Fault Zone, IS – Iapetus Suture, RS – Rheic Suture STS – Saxothuringian Suture, TS – Thor Suture.

In this paper, we present new LA–ICP–MS U–Pb data on zircons and bulk chemical composition of the volcano-sedimentary successions from the Kamieniec Metamorphic Belt (KMB) and the Doboszowice Metamorphic Complex (DMC) exposed in the eastern part of the Fore-Sudetic Block (Fig. 2). The data are used to describe the provenance of the detrital material, the time of deposition and the tectonic setting of the formation of the volcano-sedimentary successions in the eastern part of the Fore-Sudetic Block. This allows us to propose a new model for the development of the Cambro-Ordovician rocks cropping out in the eastern part of the Fore-Sudetic Block from the early Palaeozoic to the Carboniferous. The model is generally consistent with those proposed for other Late Neoproterozoic and Cambro-Ordovician sequences exposed in the Bohemian Massif (Jastrzębski et al. 2017; Hajná et al. 2018; Collett et al. 2020; Soejono et al. 2020; Collett et al. 2022; Soejono et al. 2022). Our data combined with already published results shed new light on the development of the passive Gondwana margin and the nature of the tectonic suture separating the Central and East Sudetes. Furthermore, the presented data provide important time limitations on the Variscan tectonothermal event responsible for recycling of the Cadomian crust.

Figure 2. Geological sketch map of the Sudetes after Mazur et al. (2006). Abbreviations: BU – Bardo Sedimentary unit; OSD – Orlica-Śnieżnik Dome, ISF – Intra-Sudetic fault; KM – Kłodzko massif; KMB – Kamieniec Metamorphic Belt; DMC – Doboszowice Metamorphic Complex; LM – Lusatian massif; NKG – Nysa Kłodzka Graben, NAM – Niedźwiedź amphibolite massif; LH – Lipowe Hills Massif; SBF – Sudetic boundary fault; SMB – Staré Město Belt; TB/STS – Teplá-Barrandian/Saxothuringian suture; Nth – Nyznerov thrust. Abbreviations inset: EFZ – Elbe Fault Zone, MGH – Mid-German Crystalline High; MO – Moldanubian zone; MS – Moravo-Silesian zone; NP – Northern Phyllite zone; OG – Odra granitoids, OFZ – Odra Fault Zone, RH – Rhenohercynian zone; SX – Saxothuringian zone. Age assignments: Pt – Proterozoic; Pz – Palaeozoic; Cm – Cambrian; Or – Ordovician; D – Devonian; C – Carboniferous; 1 – Early; 2 – Middle.

2. Geological setting

The KMB and the DMC are fragments of the crystalline basement, emerging from below the younger Cenozoic cover of the Fore-Sudetic Block (Fig. 2). The KMB forms c. 25 km long and 5 km wide longitudinal belt between the Góry Sowie Massif and the Niemcza Shear Zone in the west and the Strzelin Crystalline Massif (including the Lipowe Hills) in the east (Fig. 2). A volcano-sedimentary succession exposed in the KMB is dominated by mica schists intercalated with scarce paragneisses, marbles, quartz-graphite schists, eclogites, and felsic volcanics (Fig. 3). The latter are interpreted as tuffs or lava flows (Dziedzicowa, 1966). On the contrary, the DMC is a c. 6 km long exposure of crystalline basement located south-east of the KMB and directly west of the Niedźwiedź Amphibolite Massif (NAM, Fig. 2). The DMC may be divided into a western and an eastern parts. The western part exposes the Doboszowice orthogneiss with protolith age dated on zircons using U–Pb SHRIMP at 488 ± 6 Ma (Mazur et al. 2010) or at 494 ± 5 Ma using U–Pb ICP–MS (Jastrzębski et al. 2023). The eastern part is mainly composed of the migmatic Chałupki paragneiss interleaved with scarce mica schists and metabasalts of unknown age (Fig. 3). The protolith age of the volcano-sedimentary successions exposed in the KMB and DMC is poorly known. Available data suggest that the maximum depositional age (MDA) of the KMB and the adjacent Lipowe Hills mica schists is in the range of c. 560–570 Ma (Oberc-Dziedzic et al. 2018; Jastrzębski et al. 2020), while the Chałupki paragneiss from the DMC has a late Cambrian MDA (Jastrzębski et al. 2023).

Figure 3. Geological sketch map of the Kamieniec Metamorphic Belt and the Doboszowice Metamorphic Complex.

The volcano-sedimentary successions comprised in the KMB and DMC suffered a Variscan metamorphism. According to several authors, it was related to continental collision between two Gondwana-derived crustal domains represented by the Saxothuringian and Brunovistulian (e.g. Schulmann et al. 2009; Chopin et al. 2012; Jastrzębski et al. 2020; Szczepański et al. 2022; Szczepański & Goleń, 2022). However, the metamorphic records preserved in the KMB and DMC are strikingly different. The mica schists of the KMB underwent HP-LT metamorphism at c. 490^oC and 18 kbar which was overprinted by LP-HT episode at c. 530–580^oC and 3–7 kbar (Szczepański et al. 2022; Szczepański & Goleń, 2022). On the other hand, the migmatized Chałupki paragneiss bears record of HT metamorphism at max. 670^oC and 8 kbar (Puziewicz et al. 1999; Szczepański & Marciniak, 2018). The age of the tectonothermal event recorded in the KMB has recently been estimated at c. 330 Ma (Jastrzębski et al. 2020) and in the DMC at c. 346–341 Ma based on ICP–MS U–Th–Pb dating of monazite (Jastrzębski et al. 2023). This is in agreement with Lu-–Hf and Sm–Nd garnet dating of the Chałupki paragneiss, which shows that these rocks were metamorphosed between 347 ± 3.6 Ma and 337.3 ± 6.6 Ma (Szczepanski et al. 2022).

3. Analytical methods

Bulk rock chemical analyses of 53 mica schists, 10 paragneisses and 17 quartz-feldspathic rocks (interpreted as metarhyolites, see below) were performed at Acme Analytical Laboratories Ltd. (Vancouver, Canada) and are summarized in Supplementary Table 1. The location of all the samples used in this study is shown in Fig. 3. The location and a brief petrological description of the geochronological samples from the studied units are given in Table 1. Major and trace element abundances were determined using ICP-MS following lithium metaborate fusion and nitric acid digestion of 0.2 g of representative whole-rock powder. Loss on ignition (LOI) was measured by weight difference after ignition at 1000^oC. The detection limits are within 0.01% for major elements, between 0.05 and 0.5 ppm for most trace elements. The detection limits for all analysed elements are given in Supplementary Table 1. Geochemical diagrams were designed using the R software environment (R Core Team, 2021).

Table 1. Location and brief petrological description of geochronological samples from the studied area. The abbreviations of mineral names are after Whitney and Evans (2010)

Mineral separation was carried out at the Kraków Research Centre, Institute of Geological Sciences, Polish Academy of Sciences following conventional techniques involving crushing, sieving, heavy liquids and Frantz magnetic separator. Zircon grains were hand-picked under a binocular so that representative population was achieved. Cathodoluminescent images of zircons were obtained using a JEOL electron microscope (15 kV and 1 nA) at the Institute of Geological Sciences, University of Wroclaw, Poland.

In situ U-Pb detrital zircon dating of five samples by laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) was conducted at the Kraków Research Centre, Institute of Geological Sciences, Polish Academy of Sciences. The analyses were carried out using an excimer laser (ArF) RESOlution by Resonetics (now Applied Spectra) equipped with S155 dual-volume, large format sample cell. Laser ablation was coupled with ICP-MS XseriesII by ThermoFisher Scientific. Ablation took place in He which was mixed downstream with Ar and small amount of nitrogen to enhance sensitivity of ICP-MS. Before reaching the plasma, the carrier gas passes through a signal smoothing manifold. We applied a 32 µm diameter laser spot of about 3 J/cm² energy fired with 5 Hz frequency (analytical parameters are summarized in Supplementary Table 2). The main ablation is preceded by two surface cleaning shots and gas blank measurement. Zircon Z91500 (Wiedenbeck et al. 1995) was used as a primary standard, and zircons GJ-1 (Jackson et al. 2004) and/or Plešovice (Sláma et al. 2008a) were frequently measured for quality control. Typically, two primary standards were measured every six or seven unknowns (Supplementary Table 3). For each analytical session, the secondary standards yielded Concordia ages accurate within ≤ 1% precision (two relative standard deviations).

The data were reduced using Iolite 3 software (Paton et al. 2011). The data treatment along with the error propagation method are presented in Paton et al. (2010). No common Pb correction was applied. All analyses are presented on the Concordia plots using Isoplot/Ex 4.15 (Ludwig, 2008), whereas the KDE diagrams contain only zircons with ≤ 10% discordance (% discordance =  [1 – (²⁰⁶Pb/²³⁸U age)/(²⁰⁷Pb/²³⁵U age)] × 100%). For KDE, we used ²⁰⁶Pb/²³⁸U ages except for zircons older than 1 Ga, for which we used ²⁰⁷Pb/²⁰⁶Pb ages. The diagrams were constructed using the R software environment (R Core Team, 2021). The MDA of the sedimentary protolith was defined according to Dickinson and Gehrels (2009) using the youngest graphical detrital zircon age peak on an age probability plot and the mean age of the youngest two or more grains that overlap in age at 1σ. In our interpretation of MDA, we ignored single-grain age peaks. All mineral abbreviations are after Whitney and Evans (2010).

3.a. Petrography of geochronological samples

Two mica schist samples (PK007and PK023) and quartz-feldspathic schist (metarhyolite PK026) from the KMB, and two samples of migmatitic paragneiss from the DMC (MD01-32 and MD04-01) were subjected to LA ICP-MS U-Pb zircon dating (Fig. 4, Tables 2–6). Petrography, mineral assemblages and the P-T history of mica schists from the KMB were described in detail by Szczepański et al. (2022) and Szczepański and Goleń (2022). In this study, we used the same set of samples, and therefore here only a brief description of the mica schist from the KMB used for the geochronological study is given. The mica schist PK007 (Fig. 4a) is a fine-grained rock that contains two mineral assemblages. The M1 assemblage consists of the first generation of garnet – Grt1 forming the cores of the garnet grains and Cld + Ph + Pg + Chl + Rt + Qz forming inclusions predominantly in Grt1. Quartz, phengitic white mica, chlorite and rutile are also present in the rock matrix of the examined sample. The M2 assemblage is represented by the second garnet generation – Grt2, which defines narrow rims growing on the first garnet generation (Grt1) and Ms + Bt + Pl + Chl + Ilm + Qz. The mineral assemblage M2 is observed as inclusions in the Grt2 as well as in the rock matrix. Sample PK023 (Fig. 4b) is a medium- to coarse-grained mica schist that contains two mineral assemblages. The M1 assemblage represented by Cld + Ph + Pg + Ep + Chl + Rt + Qz is mostly observed as inclusions in garnet porpyroblasts. The M2 assemblage represented by Ms + Bt + Mrg + Pl + St + Ilm + And + Qz is partly observed as inclusions in garnet grains and also occurs in the rock matrix.

Figure 4. Macro- and microphotographs of representative lithologies from the Kamieniec Metamorphic Belt and the Doboszowice Metamorphic Complex: (a) fine-grained mica schist sPK007, (b) coarse-grained mica schist PK023, (c) fine-grained metarhyolite PK026, (c) medium-grained migmatic Chałupki paragneiss MD01 and (d) medium-grained migmatic Chałupki paragneiss MD04.

Table 2. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample PK007

Table 3. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample PK023

Table 4. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample PK026

Table 5. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample MD01-32

Table 6. Laser ablation ICP-MS U–Pb isotopic data of detrital zircons from sample MD04-01

Quartz-feldspathic (metarhyolite) PK026 sample (Fig. 4c) is a fine-grained rock containing a mineral assemblage composed of Qz + Kfs + Ms + Bt + Opq. The sample contains K-feldspar porphyroblasts reaching up to 2 mm in diameter that are dispersed in a very fine-grained matrix consisting mainly of a mixture of recrystallized K-feldspar, quartz and rare white mica flakes. Due to this petrographic and mineral evidence coupled with geochemical features (see section 3.c.), we identify sample PK026 (together with the whole group of quartz-feldspathic rocks) as a metarhyolite.

A detailed description of the paragneisses from the DMC has already been published (Puziewicz et al. 1999), and therefore only a brief petrographic description of the analysed samples is presented here. Sample MD01-32 (Fig. 4d) is a medium-grained garnet-bearing paragneiss containing two mineral assemblages. The M1 assemblage consists of rare high Si white mica grains (Ms1) together with rutile needles, and the M2 assemblage consists of Grt and low Si white mica (Ms2) accompanied by Pl + Bt + Ilm. Sample MD04-01 (Fig. 4e) is a medium-grained paragneiss that contains one mineral assemblage comprising low Si white mica (Ms2) and Pl + Bt + Rt + Ilm. Both paragneiss samples show elongated quartz-feldspathic patches of variable size (up to 2–3 cm in length) that are aligned parallel to the penetrative foliation (Fig. 4d, e).

3.b. Whole-rock geochemistry of metasedimentary rocks

Fifty-three mica schist samples and ten paragneiss samples from the KMB and the DMC were analysed for major and trace element composition (Fig. 3, Supplementary Table 1). The two rock suites differ in the chemical composition of the major elements. This is expressed by a higher average SiO₂ value (72.5 wt%) in the analysed paragneisses compared to mica schists (64.4 wt%). On the contrary, the mica schists have a higher Al₂O₃ content (17.15 wt%) compared to the paragneisses (14.25 wt%). Similarly, the average sum of MgO and FeO (7.8 in the mica schists and 4.0 in the paragneisses) and the average sum of Na₂O, K₂O and CaO (4.3 in the mica schists and 1.7 in the paragneisses) are different.

Major element data can be used to distinguish carbonates and soils from more clastic sediments and to classify greywacke along with arkose and quartz-rich sandstones using binary plots. A ternary plot utilizing SiO₂, Al₂O₃+Fe₂O₃ and CaO+MgO (Hasterok et al. 2018) shows that all the samples fall into the pelite field close to the psammite field (Fig. 5a). On the classification diagram of Wimmenauer (1984), the studied samples show a rather limited range of SiO₂/Al₂O₃ ratio and are scattered between the greywacke and pelite fields, while a minority of samples show a composition typical of pelitic greywacke and arkoses (Fig. 5b). The diagram proposed by Herron (1988) shows that in terms of chemical composition the analysed rocks resemble mostly shales and wackes, while minority of samples are chemically similar to arkoses (Fig. 5c). On the other hand, the diagram proposed by Pettijohn et al. (1987) shows that inspected rocks are scattered in the fields of greywacke, litharenite and arkose (Fig. 5d). In terms of trace element abundances when normalized to the composition of the upper continental crust, the studied mica schist and paragneiss samples show quite consistent patterns (Fig. 5e, f). All samples studied display strong Sr-negative anomaly and, in the case of the mica schists, variously developed U-negative and Nb-positive anomalies. In addition, the paragneiss samples show variously developed Zr and Hf negative anomalies and variously developed Sc negative anomalies. The DMC paragneisses also exhibit variable Cs content.

Figure 5. Geochemical characteristics of the studied metasedimentary rocks from the Kamieniec Metamorphic Belt and the Doboszowice Metamorphic Complex. (a–d) Classification diagrams after (a) Hasterok et al. (2018), (b) Wimmenauer (1984), (c) Herron (1988) and (d) Pettijohn et al. (1987). (c, d) Upper continental crust (UCC)-normalized major element pattern for investigated samples from the Kamieniec Metamorphic Belt and the Doboszowice Metamorphic Complex. Normalization factors after Taylor and McLennan (1995). MS – mica schists, PG – paragneisses.

3.c. Whole-rock geochemistry of metarhyolites from the KMB

Metarhyolites were analysed for major and trace element composition (Fig. 3, Supplementary Table 1). They have 73.3–81.8 wt% of SiO₂, 9.8–14.9 wt% of Al₂O₃, 0.39–2.73 wt% of Fe₂O₃ ^tot, 0.08–1.53 wt% of MgO and 5.11–9.89 wt% Na₂O+K₂O.

The total alkali versus SiO₂ plot (TAS diagram, not shown) reveals that these rocks are rhyolites of subalkaline affinity, which is consistent with petrographic evidence (Fig. 4c). Diagrams based on immobile trace element ratios confirm that these rocks have the composition corresponding to that of rhyolites (Fig. 6b), while on the Co versus Th plot (Hastie et al. 2007) they fall in the field of high-K calc-alkaline and shoshonitic series but near the borderline with calc-alkaline series (Fig. 6c). The A/CNK (molar Al₂O₃/(CaO + Na₂O + K₂O)) and A/NK (molar Al₂O₃/(Na₂O + K₂O)) values (both 1.1–2.3) indicate that they are peraluminous (Fig. 6a), whilst in terms of CIPW norms are corundum-normative (1.1–7.6%).

Figure 6. Geochemical data for the meta-rhyolites of the Kamieniec Metamorphic Belt. (a) Aluminousity diagram of Shand (1943) with divisions after Maniar and Piccoli (1989). (b) Zr/Ti–Nb/Y classification diagram of Winchester and Floyd (1977). (c) Th–Co classification diagram of Hastie et al. (2007). (d) Nb–Y discrimination diagram of Pearce et al. (1984). ORG – orogenic granites, syn-COLG – syn-collisional granites, VAG – volcanic arc granites, WPG – within-plate granites. (e) Meta-rhyolites plotted against fields for granitic liquids experimentally derived from pelites, greywackes and amphibolites (data from Patiño Douce (1999) compiled by Jung et al. 2009). (f) Th/Yb–Ta/Yb geotectonic classification diagram of Gorton and Schandl (2000). OA – oceanic arcs, ACM – active continental margins, WPVZ – within-plate volcanic zones, WPB – within-plate basalts, MORB – mid-ocean ridge basalts. See text.

The investigated rocks show negative correlations of SiO₂: well defined with Al₂O₃ and Fe₂O₃ ^tot, and less distinct with TiO₂, MgO and CaO (Fig. S01), which may indicate a fractionation and removal of plagioclase, biotite, apatite, and ilmenite. Given the peraluminous composition, it could be conjectured that the negative correlations of Al₂O₃, FeO^tot and MgO against SiO₂ imply cordierite-crystallizing magmas.

The concentrations of compatible trace elements are low, for example, Co (0.3–2.2 ppm), V (8–20 ppm) Sc (2–11 ppm) or below detection limit (Cr, Ni). The rocks have variable concentrations of incompatible trace elements (e.g. Zr: 57–160 ppm, Ba: 353–1707 ppm, Y: 24–50 ppm, Th: 9.6–20 ppm and ΣREE: 99–212 ppm), and among them only a few show but weak trend with increasing SiO₂ (Zr and Nb negative, Th positive; Fig. S02). In the case of large ion lithophile elements (LILE), this lack of correlation most probably indicates element mobility under metamorphic conditions. The rare earth elements (REE) chondrite-normalized (CN, Sun & McDonough, 1989) profiles show enrichment of light rare earth elements (LREE) over mid rare earth elements (MREE) and heavy rare earth elements (HREE) ([La/Sm]_CN = 2.81–5.47, [La/Yb]_CN = 3.80–13.6). The resulting REE patterns have a negative slope for LREE and are flat for MREE–HREE (Fig. 7a). The fractionation of MREE from HREE is weak as [Gd/Yb]_CN ranges from 1.0 to 1.5. The rocks show a pronounced Eu anomaly (Eu/Eu* = 0.11–0.36, Fig. 7a), and assuming a divalent oxidation state of Eu, it may indicate either fractional crystallization of plagioclase or that during partial melting this mineral was present in the source. On upper continental crust-normalized variation diagrams (Fig. 7b), meta-rhyolites show variable enrichment in LILE and Th but strong depletion in high-field-strength elements (HFSE, i.e. Nb, Ta, Ti, P, Zr, Hf) and in Sr; the latter negative anomaly is probably related to the Eu anomaly. These significant and negative anomalies support the fractionation of the Pl + Bt + Ap + Ilm mineral assemblage, presumably joined by some zircon and perhaps monazite, as shown by negative trends of Zr versus SiO₂ (Fig. S02) and [La/Yb]_CN versus Ce_CN (not shown). However, these anomalies may also characterize the melt source, as upper and/or middle crust have developed depletion in high-field-strength elements (Taylor & McLennan, 1985).

Figure 7. Trace element diagrams for the studied samples of meta-rhyolite and metasediments from the Kamieniec Metamorphic Belt: (a) REE patterns normalized to chondrite, (b) immobile incompatible elements normalized to primitive mantle. Normalization factor after Sun and McDonough (1989). Dark shaded area is for mica schists form the KMB, while light shaded area is for the Doboszowice paragneiss for the DMC, (c) chondrite-normalized REE patterns for investigated samples from (c) the Kamieniec Metamorphic Belt and (d) the Doboszowice Metamorphic Complex. Normalization factors after Sun and McDonough (1989). Symbols for metasedimentary rocks as in Fig. 5.

3.d. Internal structure of analysed zircon grains

3.d.1. Sample PK007

The zircon population of sample PK007 ranges from 40 to 152 μm with a mean length of 80 μm. It is dominated by grains with an elongation of less than 2 (84%), while zircons with an elongation in the range of 2 to 3 are much less frequent (16%). The zircon grains examined are often ovoid (32%) to well-rounded (38%). However, euhedral crystals are an important group (30%). Most of the grains in the examined sample are homogeneous (60%), while 40% of them show inherited cores. The very small group of grains (4%) have dark, thin rims.

3.d.2. Sample PK023

Sample PK023 contains grains ranging in length from 51 to 149 μm with a mean length of 83 μm. Most of the zircons analysed have an elongation of less than 2 (64%), while grains with an elongation greater than 2 form a much smaller group (36%). A large group of zircons are rounded to euhedral crystals (66%), while ovoid to well-rounded grains are less common (34%). Most grains are homogeneous (74%), and only a small group of zircons (18%) show inherited cores. Examined zircons often show dark, thin rims (44%) or represent dark, homogeneous grains (8%).

3.d.3. Sample MD01-31

The zircons in sample MD01-32 range in length from 56 to 121 μm with a mean length of 85 μm. Half of the grains analysed have an elongation of less than 2. The sample is dominated by euhedral crystals (55% of the population). However, well-rounded grains are also common (45%). Most zircon crystals are homogeneous (75%), while grains with inherited cores are much less frequent (25%). Zircons with dark, thin rims make up 40% of the population.

3.d.4. MD04-01

Zircon grains in sample MD04-01 range in length from 57 to 166 μm with a mean length of 92 μm and an elongation mostly below 2 (73%). Most grains are euhedral (78%), while rounded and well-rounded zircons are common (22% of the population). A relatively large group of grains have dark and thin rims (57%).

3.d.5. Sample PK026

The zircon population of sample PK026 ranges from 54 to 164 μm with a mean length of 121 μm. The population of grains with elongation greater than 2 is relatively abundant (40%). The zircon grains examined are predominantly euhedral (95%) and homogenous (83%) crystals. A small group of zircons (17%) show inherited cores.

In summary, cathodoluminescence images of the investigated zircon population show that within the metasedimentary samples most of the investigated grains show similar characteristics in terms of brightness, internal texture and elongation. Most of the grains are euhedral with locally corroded faces and show moderate cathodoluminescence and fine oscillatory zoning of clearly magmatic origin (e.g. PK007-2-36, Fig. 8) of which some are well rounded (e.g. PK023-40, Fig. 8). Some zircons have textures indicative of metasomatic alterations (e.g. MD04-1-12, Fig. 8). Some zircons from both groups contain xenocrystic cores (e.g. PK026-26, Fig. 8). Interestingly, especially in samples PK023, MD01-32 and MD04-01, a number of grains show thin and dark rims (e.g. PK023-21, MD04-1-91, MD04-1-116, Fig. 8 and S03) or represent dark, homogeneous grains with patchy or no internal zoning pattern (PK023-18. Fig. 8) probably related to metamorphic growth. It is worth noting that the population of zircon grains documented in the sample PK026 differs slightly from the remaining samples. The majority of grains in this sample are characterized by long euhedral grains with high elongation of greater than 2 and with oscillatory zoning of clearly magmatic origin supporting our interpretation that this sample represents metarhyolite (Fig. 8).

Figure 8. Examples of cathodo luminescence images of zircon grains from: (a) mica schists PK007 and PK023, (b) paragneiss MD01 and metarhyolite PK026 and (c) paragneiss MD04. Grain numbers and ages (Ma) refer to Tables 2–6; Uncertainties are 1σ.

3.e. U-Pb zircon dating

Laser ablation ICP–MS U–Pb dating identified more than 80 % of zircons as younger than 1 Ga (Figs. 9 and 10, Tables 2–6). Although the samples show significant differences in their KDE age spectra, they all share the presence of the prominent 590–600 Ma peak (Fig. 10). In the case of sample PK007, this peak dominates the entire spectrum but is additionally accompanied by a significant small group of zircons of about 1.0 Ga (Fig. 10a). Sample PK023 contains, in addition to the common Ediacaran peak mentioned above, a prominent younger peak of 400 Ma tailing to about 320 Ma representing the youngest age component in the latter sample (Fig. 10b). The paragneisses contain a very pronounced peak of 480–510 Ma, which is dominant in sample MD01-32 (Fig. 10c, d). The two paragneisses also contain several smaller peaks ranging from about 770 to 1000 Ma (Fig. 10c, d). All samples contain several minor groups of grains of Proterozoic and Archean age (Fig. 10).

Figure 9. U–Pb Concordia diagrams for samples from the DMC and KMB. Concordia plots were done using the Isoplot/Ex 4.15 Excel add-in (Ludwig, 2008). (a) micaschist PK007, (b) micaschist PK023, (c, d) metarhyolite PK026, (e) paragneiss MD04-01 and (f) paragneiss MD01-32.

Figure 10. Binned frequency-density plots of detrital zircons from: (a–d) analysed samples (N = number of analyses). For zircons older than, 1 Ga ²⁰⁷</sup>Pb/²⁰⁶</sup>Pb ages were taken for interpretation and the ²⁰⁶Pb/²³⁸U ages for younger grains. Frequency-density plots were designed via the R software environment (R Core Team, 2021). KDE bandwidth is 30 Ma.

Except for the four metamorphosed sedimentary rocks presented above, we additionally analysed the quartz-feldspathic schist PK026, which we interpreted on account of its mineral composition, zircon morphology and bulk geochemical features as a metarhyolite (see sections 3a, 3c and 3d). The vast majority of analyses plot between 450 and 550 Ma (²⁰⁶Pb/²³⁸U ages). The most common group of analyses defines a clear cluster around 510 Ma, which we interpret as the time of rhyolite emplacement (Fig. 9c, d). Some younger ages we interpret as the result of Pb loss due to a Variscan event. A group of three concordant analyses reflects inherited domains of 630–645 Ma, and ²⁰⁷Pb/²⁰⁶Pb ages define the minimum zircon crystallization ages of the two oldest domains defined by single analyses as 2593 ± 21 and 1536 ± 76 Ma (Fig. 9c, d).

4. Discussion

4.a. Potential influence of metamorphism on whole-rock geochemistry

Both metasediments and metaryolites have undergone a medium- to high-temperature metamorphism, which could have mobilized the large-ion lithophile elements (e.g. Na, K, Ba, Rb and Cs). However, any large-scale remobilization of REEs, Th, Zr, Sc, Cr and Co seems unlikely as the patterns observed on the spider plots are quite uniform in the examined samples suite (Figs. 5 and 7). Furthermore, these elements are thought to be largely transferred to clastic sediments during weathering and transport, thus reflecting the signature of the parent material (Bhatia & Crook, 1986; McLennan, 1989).

4.b. Provenance signature based on chemical composition of metasediments

The La/Sc and Th/Co ratios are commonly considered useful provenance indicators (Cullers, 2002). The samples used in this study show La/Sc (1.02 to 10.57) and Th/Co (0.48 to 30.7) ratios typical of felsic rocks (Fig. 11a). Similarly, TiO₂ (0.04 to 1.08) and Ni (2 to 82.8) concentrations, as well as Hf/Yb and La/Th ratios, are mostly characteristic of the felsic source in the examined samples (Floyd et al. 1990; Fig. 11b, c). These conclusions are confirmed by the Th/Sc versus Zr/Sc diagram (Fig. 11d). Since these elements are transferred quantitatively from source to sink sediment, their ratio can be used to infer the composition of the source rock and the sedimentary processes it suffered (McLennan et al. 1993). Zirconium is mainly retained in zircon, and thus the Zr/Sc ratio is a useful index of zircon enrichment in sedimentary rocks. However, the Th/Sc ratio is a good index of igneous differentiation processes because Th is typically an incompatible element, whereas Sc is compatible in igneous rocks. Therefore, the Zr/Sc ratio is considered a proxy for the degree of recycling, whereas the Th/Sc ratio reflects the average composition of the source rocks (McLennan et al. 1993). Consequently, igneous differentiation results in a positive linear correlation of the Th/Sc versus Zr/Sc ratios. Interestingly, the analysed samples show Zr/Sc (7.8 to 53.4) and Th/Sc (0.6 to 7.0) ratios intermediate between rhyolite and granite source rocks (Fig. 11d). Furthermore, relatively low values of the Zr/Sc ratio and low Zr concentration (0.47 to 0.78) suggest that sediment recycling and reworking did not influence the final composition of the studied metasediments (Floyd et al. 1990; McLennan et al. 1993).

Figure 11. Diagrams illustrating lithology of the source area after (a) Cullers (2002), (b) Floyd et al. (1989), (c) Hladil et al. (2003) and (d) the influence of sediment recycling and zircon enrichment on chemical composition of the investigated quartzites after McLennan et al. (1993). (e, f) Discrimination diagrams showing tectonic setting of deposition of the protolith to the mica schists and paragneisses of the Kamieniec Metamorphic Belt and the Doboszowice Metamorphic Complex. (after Bhatia & Crook, 1986). Abbreviations: PM – passive margin, CIA – continental island arc, ACM – Active continental margin, OIA – oceanic island arc. d) Discriminant-function multi-dimensional diagram after Verma and Armstrong-Altrin (2013). Samples fall in the field typical of arc-related sediments. Symbols as in Fig. 5.

The REE pattern of the source rocks is preserved in clastic sediments and can therefore be used to distinguish between a felsic and a mafic dominated source of clastic sediments (Taylor & McLennan, 1985). Felsic rocks are characterized by higher LREE/HREE ratios and negative Eu anomalies, whereas mafic rocks generally have lower LREE/HREE ratios accompanied by little or no Eu anomalies (Cullers, 1994). The samples examined generally show high (LREE/HREE)_CN ratios (0.8 to 14.5 with the mean value of 8.7) and moderate to strong negative Eu_CN anomalies (1.34 to 18.94 with the mean value of 2.46, Fig. 7c, d).

In summary, the ratios of La/Sc, Th/Sc, Th/Co, Zr concentration and the REE distribution patterns of the studied sedimentary successions from the KMB and DMC indicate that they were generally derived from a felsic source and, as shown by the Eu anomaly, the Zr/Sc ratio and Zr concentration, they were not significantly affected by alteration during transport (Cullers, 1994; Cullers, 2000).

4.c. Tectonic setting of deposition

Several diagrams allow for the identification of the tectonic setting of a source region (e.g. Bhatia, 1983; Roser &Korsch, 1986). However, unravelling the tectonic setting of the deposition of metamorphosed sedimentary sequences should rely on immobile trace elements. Therefore, we first applied the ternary plots Sc-Th-Zr, Th-Co-Zr and Th-La-Sc of Bhatia and Crook (1986). These plots show that the samples analysed have a typical trace element composition of sediments derived from erosion of the continental arc or active continental margin (Fig. 11e–g). However, as shown by Verma and Armstrong-Altrin (2013), major elements can also provide reliable information on the tectonic setting of the sedimentary basin. They proposed two new discriminant functions based on major elements for the tectonic discrimination of siliciclastic sediments and suggested that the functions are insensitive to element mobility. Indeed, the studied metasedimentary rocks have the chemical composition of the major element typical of arc-related sandstones demonstrating the usefulness of this technique even for metamorphosed rock suites (Fig. 11h).

In summary, the chemical composition of the mica schists and paragneisses from the KMB and DMC, in terms of main and trace elements, is typical of sediments derived from erosion of suprasubduction complexes.

4.d. Age of deposition of the KMB and DMC volcano-sedimentary succession

The youngest prominent density peaks defined by detrital zircon grains in the mica schists of the KMB (samples PK007 and PK023) range from 610 to 600 Ma (Fig. 12a, b). However, both samples reveal less prominent density peaks at 540 (sample PK007) and 520 Ma (sample PK023) (Fig. 12a, b). On the other hand, paragneiss samples from the DMC (samples MD01-32 and MD04-01) have the youngest prominent density peaks at 480–470 Ma (Fig. 12c, d). Consequently, these peaks represent the youngest graphical age peaks controlled by more than one grain and could therefore be interpreted as MDAs for these samples (Dickinson & Gehrels, 2009) (Fig. 12a–d). However, we additionally calculated MDA using the mean age of the youngest two or more grains that overlap in age at 1σ (Dickinson & Gehrels, 2009). Using this approach, the MDA for sample PK007 is 528.8 ± 8.8 Ma (MSWD = 1.02), 529 ± 19 Ma (MSWD = 1.4) for sample PK023, 468 ± 27 (MSWD 2.3) for sample MD04 and 455.7 ± 7.4 (MSWD = 0.27) for sample MD01. The only prominent peak documented in the metarhyolite sample from the KMB has an age of 510 Ma and is interpreted as a magmatic age (Fig. 12e). Considering the differences between the MDAs calculated by the two methods, we finally decided to use the ages calculated using the mean age of the youngest two or more grains that overlap in age at 1σ. Consequently, the results presented suggest that the studied volcano-sedimentary successions exposed in the eastern part of the Fore-Sudetic Block may represent two different rock sequences in terms of MDA. The mica schists exposed in the KMB show an early Cambrian MDA at c. 529 Ma, whereas the paragneiss of the DMC represent an Upper Ordovician sequence with a MDA of c. 456 Ma. In addition, an early Cambrian sequence of the KMB was injected by several rhyolitic sills at c. 510 Ma. Our interpretation of the studied metarhyolites as sills is supported by their very consistent chemical composition and consistent textural features, with no evidence of, for example, degassing. This allows us to reject the suggestion that the metarhyolites represent tuffs or lava flows as suggested by Dziedzicowa (1966). Moreover, it should be emphasized that younger age peaks documented for samples PK023 and MD04 are mostly defined by dark grains or thin dark rims formed due to metamorphism (Fig. 12b, c, see section 4.h.).

Figure 12. (a–e) Histograms with kernel density estimates of the Phanerozoic and Neoproterozoic zircon age populations from the dated mica schists and paragneisses of the Kamieniec Metamorphic Belt and the Doboszowice Metamorphic Complex. Bin width for histogram is 20 Ma, and bandwidth for KDE is 15 Ma. Orange rectangles and black lines are for MDA calculated using the mean age of the youngest two or more grains that overlap in age at 1σ (MDA_G). Yellow dots are for MDA calculated using the youngest graphical age peaks controlled by more than one grain (MDA_YG). Vertical axis is relative age probability. (f) Diagram showing Th/U zircon ratios versus the age showing the chemical variation of magmatic and metamorphic zircon domains for the studied samples. The colour code refers to the investigated samples, while the different dot shapes refer to the different types of zircons.

Our estimate of MDA of the mica schists from the KMB and Chałupki paragneiss form the DMC is c. 30 Ma younger compared to previously published maximum depositional ages (Oberc-Dziedzic et al. 2018; Jastrzębski et al. 2020; Jastrzębski et al. 2023). Consequently, in terms of a MDA, we interpret the volcano-sedimentary successions exposed in the KMB equivalent to the early Cambrian sedimentary sequences exposed in other parts of the Variscan belt of Europe. The early Cambrian succession exposed in the Central Sudetes is represented by metamorphosed pelites with supra-subduction geochemical signature (e.g. Szczepański & Ilnicki, 2014). On the other hand, the Upper Ordovician sequence exposed in the DMC has no equivalent in the Sudetes, but it was documented in the Teplá-Barrandian Unit (Drost et al. 2011). The only Ordovician sequence known from the Sudetes is of Early Ordovician in age and is often represented by metamorphosed quartzarenites showing geochemical signature typical of sediments deposited on a passive continental margin (e.g. Žáčková et al. 2010; Mazur et al. 2012; Szczepański & Ilnicki, 2014; Szczepański et al. 2020). However, similar to described from the DMC Ordovician paragneiss and mica schist lithologies with geochemical signature indicating erosion of supra-subduction rock complexes have recently been documented from the Teplá-Barrandian of the Bohemian Massif (Hajná et al. 2017; Soejono et al. 2020).

In conclusion, we suggest that the volcano-sedimentary successions of the KMB and DMC represent fragment of the early Palaeozoic siliciclastic cover deposited on the Gondwana margin.

4.e. Provenance constraints

Figure 13 compares the age spectra from the KMB and DMC with those from various cratonic areas of Gondwana.

Figure 13. (a) Tentative simplified reconstruction of Western Gondwana extended passive margin during the Early Ordovician, modified after Domeier (2016) and Torsvik (2017). Abbreviations: CA – Carolina, GA – Ganderia, WA – West Avalonia, EA – East Avalonia, IB – Iberia, AM – Armorica, BV – Brunovistulia, BH – Saxothuringia, Tepla-Barrandian and – Moldanubia, KMB – Kamieniec Metamorphic Belt, DMC – Doboszowice Metamorphic Complex. Red arrows indicate the main directon of sedimentary transport. Types of lithospheric boundaries: black-white – spreading ridge, grey – subduction, blue – transform. (b) Kernel density plots for detrital zircons from: Kamieniec Metamorphic Belt (this study), Doboszowice Metamorphic Complex (this study), Amazonian Craton (Gaucher et al. 2008; Geraldes et al. 2014; Pankhurst et al. 2016), West African Craton (Abati et al. 2010), Trans-Saharan Belt – Tuareg Shield (Henry et al. 2009), Trans-Saharan Belt (Peucat et al. 2003; Abdallah et al. 2007; Bendaoud et al. 2008; Bosch et al. 2016), Saharan Metacraton (Meinhold et al. 2011) and Baltica (Valverde-Vaquero et al. 2000; Kristoffersen et al. 2014; Kuznetsov et al. 2014). Kernel density plots were designed via the R software environment (R Core Team, 2021). Abreviations: Ph – Phanerozoic, Pt1 – Neoproterozoic, Pt2 – Mesoproterozoic, Pt2 – Paleoproterozoic, A – Archean.

The youngest age spectra defined by detrital grains recognized in the investigated rocks are typical of the Cambro-Ordovician sedimentary cover of the Cadomian crust exposed in the Sudetes and other parts of the Variscan belt of Europe (Figs. 12–14; e.g. Fernández-Suárez et al. 2002; Linnemann, 2007; Linnemann et al. 2008; Žáčková et al. 2010; Drost et al. 2011; Mazur et al. 2012; Košler et al. 2014; Strachan et al. 2014; Soejono et al. 2020). The youngest population of zircon grains recognized in the metasedimentary succession analysed from the DMC was probably derived from the erosion of Cambro-Ordovician granites widely exposed in the Saxothuringian (Zieger et al. 2018 and references therein), Teplá-Barradian (e.g. Žák et al. 2013) and Moldanubian zones (Žák et al. 2023). An example of such intrusion is the Doboszowice orthogneiss dated at 488 ± 6 Ma, which is exposed in the DMC (Mazur et al. 2010). Furthermore, zircon grains ranging in age from ca. 490 to 520 Ma were most likely supplied by Cambrian granites known from the Sudetes exemplified by, for example, Śnieżnik or Izera orthogneiss (e.g. Turniak et al. 2000; Lange et al. 2005; Mazur et al. 2010; Zieger et al. 2018). Neoproterozoic grains could have been derived from granitoid intrusions dated at c. 540 Ma that were reported from several fragments of Cadomian basement exposed in the Bohemian Massif (Linnemann & Heuse, 2001; Tichomirowa et al. 2001; Mingram et al. 2004; Żelaźniewicz et al. 2004; Jastrzębski et al. 2021; Tabaud et al. 2021; Śliwiński et al.et al. 2022; Soejono et al. 2022). However, several detrital zircons were most probably supplied by volcano-sedimentary successions represented by, for example, the Stronie and Młynowiec formations in the Orlica-Śnieżnik dome (Jastrzębski et al. 2010; Mazur et al. 2012; Mazur et al. 2015), the Zwethau and the Rothstein formations in the Saxohuringian zone (e.g. Elicki, 1997; Linnemann, 2007) or from glaciomarine sedimentary rocks documented in the Elbe zone and the North Saxon antiform in the southeastern part of the Saxothuringian zone (Linnemann et al. 2018). Consequently, the anhedral Neoproterozoic zircons documented in the studied rocks may represent recycled material supplied by local sedimentary successions. This may explain the low abundance of the oldest, Paleoproterozoic or Archean zircons, in the studied samples.

Figure 14. Comparison of zircon age spectra obtained for various crustal domains from the Sudetes shown on the geological sketch map of the Sudetes after Mazur et al. (2006). Kernel density plots for detrital zircons from: 1. Karkonosze (Žáčková et al. 2010), 2. Orlica-Śnieżnik Dome (Szczepański et al. 2020); 3. Góry Sowie Massif (Tabaud et al. 2021); 4a. Kamieniec Metamorphic Belt (this work), 4b. Doboszowice Crystalline Massif (this work and Jastrzębski et al. 2023), 4c – Lipowe Hills (Oberc-Dziedzic et al. 2018); 4d. Kamieniec Metamorphic Belt (Jastrzębski et al. 2020); 5. Brunovistulia (Friedl et al. 2004; Mazur et al. 2010), 6. Staré Město Belt (Śliwiński et al. 2022). Abbreviations: STS – Saxothuringian suture, OSD – Orlica-Śnieżnik Dome, DMC – Doboszowice Crystalline Massif, KMB – Kamieniec Metamorphic Belt, NZ – Niemcza Zone, LH – Lipowe Hills, NAM – Niedźwiedź amphibolite Massif, Pth – Paczków thrust, Nth – Nyznerov thrust, SMB – Staré Město Belt. Abbreviations in inset: C-Or – Carboniferous-Ordovician, Cm – Cambrian, Pt1 – Neoproterozoic, Pt2 – Mesoproterozoic, Pt2 – Paleoproterozoic, A – Archean.

The detrital zircon age spectra clearly show the paucity of Mesoproterozic zircons pointing to a period of magmatic and volcanic inactivity in the source area, as there are only a few zircon grains of this age documented in the investigated rocks (Figs. 10 and 13). This Mesoproterozoic age gap in zircon age spectra is commonly reported from the West African Craton and from the Trans-Saharan Belt (e.g. Tuareg Shield) (Fig. 13). Interestingly, the Saharan Metacraton, Amazonia and Baltica can be excluded as a potential source of detritus, since these cratonic areas are characterized by the presence of Mesoproterozoic zircons (Fig. 13).

In summary, we suggest that the data presented here advocate for the West African provenance for the studied volcano-sedimentary succession. Available detrital zircon ages from Cambro-Ordovician sedimentary successions covering the Saxothuringian, Teplá-Barrandian and Moldanubian zones as well as the Armorican Massif clearly indicate that all these Neoproterozoic crustal fragments were derived from the same cratonic areas and Cadomian arc-related basement located at the northern periphery of Gondwana (Figs. 13 and 14; corresponding references are given in Fig. 14). Consequently, we support suggestions that during the Cambro-Ordovician time, Cadomian crustal fragments were dispersed along the northern periphery of Gondwana, most probably forming an extended shelf that developed on a passive continental margin (e.g. Drost et al. 2011; Žák & Sláma 2018; Soejono et al. 2020; Collett et al. 2021; Tabaud et al. 2021; Tabaud et al. 2021; Collett et al. 2022).

4.f. Petrogenetic interpretation of the KMB volcanism

The zircons from metarhyolite sample are dominated by an age peak of 510 Ma that we correlate with the time of magmatic emplacement (Figs. 9c, d and 12e). Rare Neoproterozoic and Archaean inheritance points to the involvement of an older crustal component in magma genesis (Fig. 9c, d).

The derivation of metarhyolites protolith from continental crust material is implied by the peraluminous and corundum-normative composition, depletion in HFSE (Figs. 6 and 7) and by low Zr/Nb (8.2–23) and Nb/Th (0.23–0.69) (Supplementary Table 1). Depletion in HFSE together with enrichment in LREE, a flat MREE-HREE profile, elevated Th content and high Th/Nb are also consistent with a subduction-related environment of magma origin (e.g. Pearce et al. 1984; Pearce & Peate, 1995, Schandl & Gorton, 2002).

Jung et al. (2009) through compilation of experimentally derived liquids devised a diagram for discrimination of granitoid sources (Fig. 6e). The plot shows that the KMB metarhyolites are related to a greywacke precursor (with no input from mantle-derived sources). This inference is consistent with their peraluminous affinity, whilst the plagioclase-bearing residue in the source possibly explains the negative Eu and Sr anomalies. Furthermore, immature, quartz-feldspathic-rich sources (e.g. psammites, paragneisses, etc.) that are able to supply peraluminous, cordierite-crystallizing liquids, require higher melting temperatures (>850–875°C; Sylvester, 1998, Moyen et al. 2017). Sylvester (1998) showed that low to moderate values of Al₂O₃/TiO₂ in strongly peraluminous granitic rocks may serve as a proxy of hot, melt-forming conditions. Moderate values of Al₂O₃/TiO₂ (52–228, av. 108 ± 48) of the studied metarhyolites seem to confirm these premises.

Although some of the discrimination diagrams support an active continental margin as a possible tectonic environment (Fig. 6f), we interpret this as a signature of inherited geochemical features pertinent to the tectonic setting of sedimentary precursor to the KMB meta-rhyolites. Additionally, collisional or post-collisional affinity is suggested (Fig. 6d and S04a). Furthermore, the discrimination diagram of Verma et al. (2013) with probability of 65% points to the collisional setting of the KMB meta-rhyolites. However, within-plate environment is also apparent (probability of 34%; Fig. S04b). Given no affinity to alkaline, A-type granitoid magmas, the latter indication possibly echoes the progression of tectonic setting from post-collisional towards anorogenic, presumably extensional regime (Fig. S04c and d).

The Cambrian age (ca. 510 Ma) and geochemical features of the KMB meta-rhyolites (S-type, peraluminous magmas from post-collisional, transitional setting) emphasize their similarities to post-orogenic (meta-)granites from the Saxothuringian domain in the West Sudetes, notably, ca. 504 Ma Rumburk granites and ca. 500–511 Ma Izera-Kowary gneisses (Oberc-Dziedzic et al. 2005, Pin et al. 2007, Oberc-Dziedzic et al. 2009) and those from the Moldanubian and Tepla-Barandian domains (e.g. Žák, Kraft & Hajná, 2013; Žák et al. 2023), but not the Brunovistulian domain. Thus, we consider the origin of the meta-rhyolites as another facet of magmatic pulses concurrent with the gradual change of tectonic environment marking the end of the Cadomian orogeny (arc-Gondwana continent collision) and the onset of Cambro-Ordovician rifting. These genetic links are highlighted by sparse inherited zircon ages of meta-rhyolites (ca. 560, 630 and 2590 Ma, Fig. 9c, d). They are close to the age peaks reported from the Rumburk granites and Izera-Kowary gneisses, and resemble those of Lusatian greywackes (1.1–2.6 Ma and 540–730 Ma; Kröner et al. 1994, Linnemann et al. 2004) considered as their sedimentary precursors formed in the Cadomian arc.

The lack of HP conditions during Cadomian arc-continent collision and limited crustal thickening (e.g. Dörr et al. 2002) are consistent with the inferred hot conditions of the KMB rhyolitic magma generation and correlate with intrusions into transitional crust at the peripheries of Gondwana that were postulated by tectonic model of Linnemann (2007) for the late Cambrian in the Saxothuringia (e.g. Zieger et al. 2018).

4.g. Is the Rheic suture exposed on the Fore-Sudetic Block?

The presented zircon age spectra demonstrate that the volcano-sedimentary successions of the KMB and DMC are typical of the Gondwana-derived terranes now exposed in the Bohemian Massif and exemplified by the Saxothuringian, Teplá-Barrandian and Moldanubian zones (see Žák & Sláma, 2018). It is worth noting that some volcano-sedimentary successions exposed in the Brunovistulian zone also show similar detrital zircon signature (Soejono et al. 2022). However, this domain is characterized by a paucity of magmatic and volcanic rocks of c. 500 Ma that are commonly exposed in the KMB and DMC. Furthermore, the existence of extensive Ordovician cover seems to be rather typical for such Gondwana-derived terranes like Saxothuringia, Moldanubia or Teplá-Barrandia. The only Ordovician strata documented in the Brunovistulian domain includes light green clay-rich siliceous rocks, interbedded with fine-grained quartz sandstones that has only been encountered in a restricted area in the northern part of Upper Silesia (see Kalvoda et al. 2008). It is worth noting that an affinity to the Saxothuringian domain was previously suggested by Jastrzębski et al. (2020, 2023) and Oberc-Dziedzic et al. (2018) for rocks of the DMC, Lipowe Hills and KMB (sites 4b, 4c and 4d in Fig. 14, respectively). Furthermore, similar age spectra were also reported by Tabaud et al. (2021) for the Góry Sowie Massif (site 3 in Fig. 14). On the other hand, Tabaud et al. (2021) suggested that the rocks exposed to the east of the Góry Sowie Massif and to the west of the Strzelin Massif represent the Moldanubian zone. However, it is worth noting that Chopin et al. (2012) defined the Moldanubian zone as a recycled passive margin of the Saxothuringian zone. Therefore, the results of our study combined with existing data allow us to identify the volcano-sedimentary successions now exposed in the longitudinal belt between the Góry Sowie massif in the west and the Lipowe Hills in the east as part of the Gondwana-derived terranes now exposed in the Bohemian Massif including Saxothuringia, its recycled part described as Moldanubia or Teplá-Barrandia (Fig. 14). Furthermore, similar detrital zircon age spectra of the late Neoproterozoic (Ediacaran) and Cambro-Ordovician volcano-sedimentary sequences were recently documented from the Stare Město Belt and the Bunovistulian terrane (sites 6 and 5 on Fig. 14, respectively, Collett et al. 2021; Śliwiński et al. 2022; Soejono et al. 2022). This implies that mentioned Gondwana-derived terranes and Brunovistulia were located within the extended Gondwana shelf at that time and were sourced from the same cratonic area (e.g. Collett et al. 2021; Śliwiński et al. 2022; Soejono et al. 2022). Furthermore, early Cambrian–Early Ordovician metabasalts of the Staré Město Belt and the Letovice ophiolite as well as the NAM (considered as the northern continuation of the Staré Město Belt on the Fore-Sudetic Block at the western tectonic boundary of the Brunovistulia; Fig. 14) all show intra-continental geochemical signatures (Awdankiewicz, 2008; Soejono et al. 2010; Collett et al. 2021). Therefore, by combining the geochemical signal of the aforementioned metaigneous complexes with the provenance signature of the associated metasedimentary successions, we support the previous assumptions that the Saxoturingia–Brunovistulia boundary is a remnant of the initial intracontinental rift that developed on the extended Gondwana shelf that was later closed during the Variscan orogeny (see, e.g. Collett et al. 2021; Soejono et al. 2022). Consequently, we are inclined to support the hypothesis that the Rheic suture is not exposed in the eastern part of the Bohemian Massif (e.g. Mazur et al. 2020; Collett et al. 2021; Soejono et al. 2022).

4.h. Variscan thermal event recorded by the volcano-sedimentary succession

The youngest zircons documented in the studied metasedimentary samples (PK023 and MD04-01, Fig. 12b, c) reach ages of c. 330–320 Ma. We interpret all ages in the range between 450 and 320 Ma as reflecting lead loss due to the Variscan metamorphism. Several such young grains have been documented in samples PK023, MD04-01 and MD01-32 and are characterized by the appearance of thin dark rims or patchy zonation (Fig. 8). These features are characteristic of crystals that have experienced fluid interaction, for example, as a result of metamorphism (e.g. Spandler et al. 2004). Furthermore, dark zircon rims or dark homogeneous grains are mostly characterized by a low Th/U ratio, which is typical for metamorphic zircons (Fig. 12f). We would like to emphasize that many such dark and thin zircon rims were documented in the samples examined but could not be dated due to their small dimensions. (Fig. S03). We suggest that the youngest zircons in our collection, with ages ranging between 330 and 320 Ma, most likely represent the age of the Variscan thermal overprint that caused Pb loss in some of the examined zircon grains.

4.i. From the Gondwana active margin to the Variscan collision of Cadomian terranes

Several early-middle Cambrian to Lower Ordovician sedimentary successions have been documented in the Variscan basement of the Bohemian Massif including the Münchberg Massif as well as the Teplá-Barrandian, the Saxothurignian and the Moldanubian domains (e.g. Linnemann et al. 2000; Drost et al. 2011; Košler et al. 2014; Hajná et al. 2018; Koglin et al. 2018; Tabaud et al. 2021). These volcano-sedimentary successions are mainly characterized by immature graywacke protoliths with chemical composition indicating deposition in a supra-subduction environment (Drost et al. 2011; Koglin et al. 2018; Tabaud et al. 2021).

The data presented here indicate that the early-middle Cambrian and Ordovician sediments exposed in the KMB and DMC were deposited in a sedimentary basin sourced by a Cadomian orogen and their geochemical signal point to detritus derivation from a magmatic arc system. Therefore, in terms of provenance signature, it was the same or very similar detritus that fed the volcano-sedimentary successions exposed in the Saxoturingian domain including the neighbouring Orlica-Śnieżnik dome, the south-eastern part of the Karkonosze-Izera Block, the Góry Sowie massif, the Staré Město Belt and also the Brunovistulia in the East Sudetes (Fig. 14, and references therein). The most interesting is that in some of these areas the Early Ordovician rock successions represent light quartzites interpreted as mature quartzitic sandstones deposited on a passive continental margin (e.g. the Goszów Quartzites from the Orlica-Śnieżnik dome, Mazur et al. 2012; Szczepański et al. 2020). This suggests that the sedimentary basin that developed along the northern margin of Gondwana was active during the whole Ordovician and was fed by detritus characterized by varying proportions of mature cratonic material and immature arc detritus. Therefore, we support the idea that the Cambro-Ordovician basin at the northern margin of Gondwana developed above a retreating subducting oceanic slab represented by the Iapetus Ocean (e.g. Arenas et al. 2007; Díez Fernández et al. 2012; Hajná et al. 2018; Soejono et al. 2020). The retreat of a subducting oceanic slab was responsible for the continental extension and the formation of a passive Gondwana margin. This process led to lithospheric thinning and astenospheric mantle upwelling responsible for crustal anatexis and the formation of granitoid intrusions at c. 520–480 Ma as proposed earlier by, for example, Soejono et al. (2020). Postulated lithospheric thinning and extension was also responsible for the separation of Cadomian crustal fragments that formed the extended Gondwana shelf and the development of local intracontinental rifts, remnants of which are now exemplified by the NAM metabasalts on the Fore-Sudetic Block (Fig. 15a).

Figure 15. Simplified tectonic model for early Palaeozoic–Carboniferous evolution of the KMB and DMC. See text for explanation.

The Cambro-Ordovician extension is well documented in the post-Cadomian successions within the Variscan belt of Europe due to the occurrence of 517–480 Ma granitoid intrusions exposed, for example, throughout the Saxothuringian zone (e.g. the Izera-Karkonosze Massif), but also widely exposed in the Teplá-Barrandian and Moldanubian zones. Although metagranitoids from the Izera-Karkonosze Massif have a chemical composition typical of volcanic arc granites (e.g. Turniak et al. 2000; Zieger et al. 2018), Oberc-Dziedzic et al. (2005) and Pin et al. (2007) interpreted them on the basis of Sm-Nd isotopic data and bulk rock geochemistry as either typical S-type post-collisional or anorogenic and related to continental break-up. The latter idea and the relationship of the metagranitoids to continental break-up was recently supported by Jastrzębski et al. (2017).

The final stage in the evolution of the rock successions exposed in the KMB and DMC was the closure of the intracontinental rift, remnants of which are found in the NAM on the Fore-Sudetic Block, followed by subduction of fragments of Gondwana continental shelf below Brunovistulia during the Variscan Orogeny (Fig. 15b). The results of classical geothermobarometry, thermodynamic calculations and quartz in garnet elastobarometry point to burial of the KMB volcano-sedimentary succession to depths of ∼65–70 km, which was followed by its imbrication, exhumation and folding (Szczepański et al. 2022; Szczepański & Goleń, 2022; Fig. 15b, c). The age of maximum burial of the rock complexes within the postulated subduction zone is unknown and requires further investigation. The data presented in this study and previously published isotopic dating of monazite grains suggest that the last stages of exhumation occurred c. 320–330 Ma (Jastrzębski et al. 2020). However, more recently published U-Th-Pb monazite data indicate that DMC experienced metamorphic episode at ca. 346–340 Ma (Jastrzębski et al. 2023). This is confirmed by Lu-Hf and Sm-Nd garnet dating of the Chałupki paragneiss, which shows that these rocks were metamorphosed between 347 ± 3.6 Ma and 337.3 ± 6.6 Ma (Szczepański et al. 2022). A scenario proposed here involving Cambro-Ordovician rifting followed by Variscan collision of terranes is consistent with previously proposed models for the development of the Cadomian and Cambro-Ordovician successions exposed in the Bohemian Massif (Martínez Catalán et al. 2020; Collett et al. 2021; Martínez Catalán et al. 2021; Tabaud et al. 2021; Soejono et al. 2022).

5. Conclusions

The volcano-sedimentary successions from the eastern part of the Fore-Sudetic Block (the KMB and the DMC) provide new input to understand the pre-Variscan and Varciscan history of the Cambro-Ordovician rocks outcropping in the eastern part of the Central Sudetes. Bulk rock chemistry and detrital zircon age spectra obtained using LA–ICP–MS U–Pb show that the mica schists of the KMB represent a succession equivalent to the Stronie Formation of the Orlica-Śnieżnik dome, while there is no exact time equivalent in the Sudetes for the Chałupki paragneisses now exposed in the DMC. The only Ordovician rock succession exposed in the Sudetes is represented by the Goszów quartzites of the Orlica-Śnieżnik dome. The data presented indicate that the volcano-sedimentary successions exposed in the eastern part of the Fore-Sudetic Block were sourced from the West African Craton or, more probably, from part of the Trans-Saharan Belt of Gondwana. Bulk rock geochemistry suggests that the studied volcano-sedimentary successions were deposited on the Gondwana margin during their transformation from an active to a passive setting within the back-arc environment. The youngest single zircon grains documented in the metasediment samples show ages of c. 330–320 Ma that we interpret as reflecting lead loss owing to the Variscan thermal overprint. Our data, combined with published results, suggest that the suture separating the Brunovistulian and Saxothuringian terranes is an intracontinental rift closed during the Variscan Orogeny, rather than a continuation of the Rheic suture.