the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jan 2026
A vast Caledonian fan and an Ediacaran arc: The contrasting provenance of Devonian clastics of Brunia (Bohemian Massif)
Abstract. Brunia is a distinctive crustal block within the European Variscides, composed of a late Neoproterozoic arc complex overlain by Ediacaran–early Cambrian cover sequences. Sparse preservation of early Paleozoic strata obscures its pre-Variscan paleogeography. Proposed models suggest Brunia either shared a crustal domain with adjacent parts of the Bohemian Massif, represented a far-eastern extension of Avalonia accreted to Baltica in the early Paleozoic, or maintained long-term connections to Baltica since the late Ediacaran.
To address these uncertainties, we present the first systematic study of detrital zircons (both U–Pb and Lu–Hf isotopic data) from Devonian strata overlying Brunia’s Neoproterozoic basement. Two distinct age-spectral patterns are identified. Type-1, widespread across Brunia, exhibit a near-unimodal late Neoproterozoic peak corresponding to locally preserved arc magmatism. Type-2, display a multimodal spectrum with significant Late Ordovician–Silurian and Paleoproterozoic–early Neoproterozoic age peaks, and only minor late Neoproterozoic input.
The Type-1 pattern reflects predominant recycling of local Brunia sources. Nearly-uniformly positive εHf(t) values in Neoproterozoic zircons contrast with the wide isotopic range typical of other Variscan terranes in Central and Western Europe, but are comparable with values from Avalonian strata in Newfoundland, supporting a Neoproterozoic link between West Avalonia and Brunia.
The Type-2 pattern broadly matches Devonian detrital zircon signatures from the British Isles, the Rhenish and Harz Mountains, Dobrogea, and NW Turkey delineating the northern margin of the Rheic Ocean. Strong similarity to Ordovician–Silurian Scandinavian datasets suggests original derivation from the Caledonides and confirms an Early Devonian connection between Brunia and Baltica.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-86', Martin Jan Timmerman, 12 Feb 2026
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AC1: 'Reply on RC1', Stephen Collett, 04 Apr 2026
We thank the reviewer for his positive assessment and useful suggestions to improve the manuscript. We also apologise for taking time to reply, we were intending to wait for feedback from a second reviewer before responding but address some of the comments raised by the reviewer below:
Comment: “Brunia”: any specific reason for reviving this old name?
Reply: We follow the recommendation put forward in Hanžl et al. (2025) “In this text, we employ the term Brunovistulicum sensu Dudek (1980), who defined the unit as a crystalline basement block (with remnants of Ediacaran to lower Palaeozoic sediments), below the Devonian to Carboniferous sedimentary formations. The term Brunia should be used, if at all, rather to describe the Brunovistulian basement and its pre-Permian cover, all together consolidated during the Variscan Orogeny.”
We also note that far from being an old name that needed reviving, Brunia is widely used as evidenced by the following articles without overlapping author teams published in the past 12 months:
Hamdy, M. M., Zoheir, B. A., Meisel, T. C., Gamaleldien, H., Lasheen, E. S. R., & Abu-Alam, T. S. (2026). Geochemical, OH-Sr Isotopic, and Thermodynamic Insights into the Metasomatic and Metamorphic Evolution of the Saxothuringian Crust: The Case of the Kowadło Ultramafic Suite (SW Poland). Lithosphere, 2026(1), lithosphere_2024_102.
Nowak, M., Tajčmanová, L., Dąbrowski, M., Buisman, I., & Szczepański, J. (2025). Coesite Discovery in Eclogites Confirms UHP Metamorphism in the Orlica‐Śnieżnik Dome (SW Poland). Journal of Metamorphic Geology.
Bezák, V., Klanica, R., Smirnov, M., Vozár, J., & Kováčiková, S. (2026). Major post-collisional shear zones in the magnetotelluric models across the Slovakian Alpides, Bohemian Hercynides and Scandinavian Caledonides. Geological Society, London, Special Publications, 557(1), SP557-2024.
Mazur, S., Palomeras, I., Schiffer, C., Vanderhaeghe, O., & Puziewicz, J. (2026). Lithospheric structure and tectonic memory of the European Variscan belt. Geological Society, London, Special Publications, 557(1), gslspecpub2025-7.
Schulmann, K., Catalán, J. R. M., & Schaltegger, U. (2025). Variscan orogeny: a three oceans problem. Elements, 21(6), 387-393.
Comment: Using the term “strata” for sedimentary rocks is of course OK, but using it also for deformed and metamorphosed amphibolite-facies metasediments is a bit confusing. In the latter case, I suggest you just use “metasediment(s)”. For example, “Ediacaran strata” for me means non-metamorphosed Ediacaran clastic rocks forming a cover to the magmatic and metamorphic basement.
Reply: We accept this point, especially as a means to distinguish between the amphibolite-facies metasediments and the suspected Ediacaran – early Cambrian cover. This change will be made to the revised manuscript.
Comment: Line 132: Ordovician sediments were only found in borehole Bibiela PIG 1. The best reference to the Cambrian – Ordovician cover is the overview paper by Buła et al 2015 (10.7306/gq.1203) and references therein.
Reply: The wording mistakenly used plural, we will also add the reference to Buła et al at this point.
Comment: Lines 194 – 200 and 221 – 230: A few local names and facies names are used here that are not shown in figures. Please have mercy on the reader not acquainted with the local stratigraphy.
Reply: Point accepted, we will make sure in revision that these local names are reduced and where possible incorporated into the figures.
Comment: Lines 376 – 380: why compare a metasediment from the basement (Jabl1) with a sediment from its cover (Česká1)? I could not quite follow the argument.
Reply: Perhaps this was clumsily worded, it was not meant to compare these two samples directly but to highlight that of all the samples with near unimodal Late Neoproterozoic detrital zircon distribution these two were the most significant outliers and give explanation for why they might be outliers.
Comment: Lines 427 – 428: “”single occurrence in the Rhenish Massif” – please more specific: age of deposition and sample number / locality.
Reply: This refers to the Wartenstein Gneiss with data in Dörr and Stein, 2019 and Linnemann et al., 2024. Will be specified in the revision.
Comment: Line 443: “(in Yılmazer et al., 2025)”. These data were debated by Şen (2025; 10.1016/j.precamres.2025.107939) with a reply by Yilmazer (2026; 10.1016/j.precamres.2025.107963).
Reply: Acknowledged, but the debate does not directly impact the point being made.
Comment: Line 511: “Upper Devonian strata of Saxo-Thuringia” – please add here labels A – L of figure 11b.
Reply: Will be done in revision.
Comment: Fig. 1 - why show Brunia in mousy grey? You may want to give it a more conspicuous colour. or frame Brunia and relate to figure 1b.
Reply: An extra frame will be added to highlight the position of 1b
Comment: Fig. 2 – Tišnov, Brana facies etc.: please make sure in Fig. 1 which areas or units are meant.
Reply: Tišnov facies is depicted beneath the symbol for UD52, Branná was not directly depicted as its definition and extent is not well defined, lithologically it is very similar to the Vrbno Facie and is traditionally it is included as part of the Vrbno Facies. The distinction between the two is made purely based on the detrital zircon data presented in this study
Comment: Fig. 9, caption: please lists sample numbers in section S3.
Reply: We accept this is an oversight and will add this information to the revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2026-86-AC1
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AC1: 'Reply on RC1', Stephen Collett, 04 Apr 2026
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CC1: 'Comment on egusphere-2026-86', Armin Zeh, 18 Apr 2026
Attached please find my detailed comments on the ms, which for my opinion needs moderate revisions.
Kind reagrads
Armin Zeh
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AC2: 'Reply on CC1', Stephen Collett, 23 Apr 2026
We thank Prof. Zeh for the insightful and constructive comments on our manuscript. Most of the minor clarifications suggested by the reviewer will be incorporated into the revised version.
Here, we address two substantive points and one technical issue raised in the review.Technical point: “Concordant data are defined as those with a concordia distance of <5%. … this is not clear to me. What does <5% mean (95–105% concordance?), and what are ‘iterative single-grain concordia ages’? Please provide a reference and explain this in the methods. Normally, all ages <1000 Ma are reported as 206Pb/238U ages and >1000 Ma as 207Pb/206Pb ages—is this convention followed here?”
Reply: The reviewer is correct that the conventional approach applies a threshold at ~1000 Ma, whereby ages <1000 Ma are reported as 206Pb/238U and those >1000 Ma as 207Pb/206Pb. However, this “changeover” is precisely what the iterative single-grain concordia age approach seeks to avoid. There is no inherent statistical or geological justification for imposing a fixed transition at 1000 Ma, and doing so can artificially distort age distributions. Particularly in datasets with significant populations around this boundary, as is the case here.
Iterative single-grain concordia ages instead combine the 206Pb/238U and 207Pb/206Pb systems into a single estimate, effectively producing a weighted mean that is independent of arbitrary age thresholds. This approach is implemented in the IsoplotR software and is described in detail by Vermeesch (2021) [cited in our text]. The method itself is not new; it was already incorporated in earlier Isoplot versions (Ludwig, 1998) and has been recommended for detrital zircon studies (e.g., Nemchin and Cawood, 2005; Zimmermann et al., 2018), although it is still not universally adopted.
Similarly, the concordia distance filter (<5%) follows the implementation described in Vermeesch (2021). This method applies a logarithmic transformation to define discordance in a way that is not biased by age or by switching between isotopic systems. Because we recognize that this approach is not yet standard practice, we also provide traditional discordance filters (based on relative age differences) in the data tables, allowing readers to apply alternative filtering criteria if preferred.Point 1: The MGCR is a composite terrane containing (meta)sedimentary rocks from both the Rhenohercynian/Avalonia Domain and the Saxothuringian Domain.
Reply: We fully agree with this point. The composite nature of the Mid-German Crystalline Rise is reflected in several datasets discussed in the manuscript. For example, the Rögis Quartzite of the Ruhla Complex (datasets of Zeh and co-workers) and the Spessart Complex (datasets of Kirchner and Albert) are interpreted as part of the Rhenohercynian Domain and linked to the so-called Caledonian Fan. In contrast, parts of the Odenwald and Ruhla complexes show affinities with Saxothuringian units to the south.
Thus, the MGCR clearly represents a composite zone with mixing of Laurussian- and Gondwanan-derived material. We acknowledge that this complexity was oversimplified in the current text, particularly in the sentence highlighted by the reviewer (line 517). This will be corrected in the revised manuscript. In addition, as outlined below (Point 2), parts of the broader discussion of the MGCR will be streamlined or removed.Point 2: The final section of the discussion could be deleted, as it does not contribute to the main story, which is not about closure of the Rheic Ocean. Discussion of unpublished data (e.g., Linnemann et al., 2025) is also unnecessary.
Reply: We accept the reviewers point that Figure 14 and parts of the associated discussion extend beyond the scope of the new data and take on a review-style character. Accordingly, Figure 14 will be removed, along with speculative elements such as the subdivision into six provenance signals and the discussion of Laurentian versus Baltican contributions to Laurussian strata.
Regarding the final section of the discussion, we partly agree and partly disagree. We accept that commentary on the MGCR is beyond the scope of this study and will remove this accordingly. However, a central aim of the manuscript, as stated in the introduction, is to address the position of the Rheic Suture within the Bohemian Massif. We therefore consider it necessary to retain elements of this last section that returns to this question.
This section (and parts of the previous section) will be revised to avoid speculative or review-style content, while still (i) linking our results to the broader tectonic framework and (ii) outlining future research directions. In particular, we highlight two avenues for further work: (1) the use of zircon Hf isotopic data to test the origin of Stenian zircon populations in relation to Gondwanan sources, and (2) the current lack of robust provenance datasets from Moldanubian units.Citation: https://doi.org/10.5194/egusphere-2026-86-AC2
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AC2: 'Reply on CC1', Stephen Collett, 23 Apr 2026
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RC2: 'Comment on egusphere-2026-86', Brendan Murphy, 20 Apr 2026
Dear Authors:
I thoroughly enjoyed reading your manuscript and learned a lot from it. I attach an annotated pdf with some suggested edits. Some are pedantic, others reflect my compulsion to edit! So feel free to implement the ones that help and ignore the rest. My only scientific comments (which you should feel free to ignore for the purposes of this manuscript) are:
- The similarities between West Avalonia and Brunia indicate that their earliest histories were likely both within the peri-Rodinian (Mirovoi) oceanic realm. But these juvenile isotopic values would be similar no matter where they were originally located in that ocean. So their similarity is important in terms of process (i.e. may reflect a similar intra-oceanic arc origin), but may not imply contiguity at that time (I would expect data from all juvenile terranes in Mirovoi Ocean e.g. Arabian Shield, Tocantins to be similar).
- The potential influence of orographic barriers could be considered. For example, the ancestral Amazon flowed westwards into the Pacific until about 10 million years ago when its course was changed by a pulse of rapid uplift of the northern Andes. Such barriers, as well as changes to those barriers would profoundly affect detrital zircon distribution.
- Regards, Brendan Murphy
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Dear authors,
I have read your manuscript with great interest. It is well researched, contains high quality data for well-chosen samples and the results are discussed in depth. When we analysed the lowermost Basal Clastics east of Brno (Timmerman et al. 2023), we were also surprised to not find convincing evidence for a Devonian age of deposition, although the stratigraphy indicates it. Perhaps the upper part of the Basal Clastics interbedded with the limestones were reworked in the Devonian (hence, more mature?) "versions" of the latest Ediacaran of the lowermost part? I see that for this specific locality, Hf-in-Zrn isotope data do also not offer a clearcut solution.
Having said that, your findings of two types of zircon age spectra for units that are, geologically speaking, not that far apart is very interesting and will trigger further research.
I have only a few general and specific comments (listed below). Other comments can be found as pop-ups in the annotated PDF.
I wish you the best of luck with your future research.
Kind regards,
Martin Timmerman
University of Potsdam, Germany
General
“Brunia”: any specific reason for reviving this old name?
Using the term “strata” for sedimentary rocks is of course OK, but using it also for deformed and metamorphosed amphibolite-facies metasediments is a bit confusing. In the latter case, I suggest you just use “metasediment(s)”. For example, “Ediacaran strata” for me means non-metamorphosed Ediacaran clastic rocks forming a cover to the magmatic and metamorphic basement.
Specific
Line 132: Ordovician sediments were only found in borehole Bibiela PIG 1. The best reference to the Cambrian – Ordovician cover is the overview paper by Buła et al 2015 (10.7306/gq.1203) and references therein.
Lines 194 – 200 and 221 – 230: A few local names and facies names are used here that are not shown in figures. Please have mercy on the reader not acquainted with the local stratigraphy.
Lines 376 – 380: why compare a metasediment from the basement (Jabl1) with a sediment from its cover (Česká1)? I could not quite follow the argument.
Lines 427 – 428: “”single occurrence in the Rhenish Massif” – please more specific: age of deposition and sample number / locality.
Line 443: “(in Yılmazer et al., 2025)”. These data were debated by Şen (2025; 10.1016/j.precamres.2025.107939) with a reply by Yilmazer (2026; 10.1016/j.precamres.2025.107963).
Line 511: “Upper Devonian strata of Saxo-Thuringia” – please add here labels A – L of figure 11b.
Fig. 1 - why show Brunia in mousy grey? You may want to give it a more conspicuous colour. or frame Brunia and relate to figure 1b.
Fig. 2 – Tišnov, Brana facies etc.: please make sure in Fig. 1 which areas or units are meant.
Fig. 9, caption: please lists sample numbers in section S3.
END