Central European Variscan Basement in the Outer Carpathians: A Case Study from the Magura Nappe, Outer Western Carpathians, Poland

Round 1

Reviewer 1 Report

This is a detailed descriptive paper on the geological setting and composition (diverse methods used) of a large granitic boulder contained within Eocene deposits of a nappe in the outer Carpathian Mountains of SW Poland.  Because the lithology is intact as opposed to just being detrital mineral fragments, and because of the detailed study of the petrography, major, minor, trace elements, REE, U-Pb, Sr and Nd data obtained, the authors have very competently characterised this unusual boulder and discussed in a very intelligent and informed way the origin and likely setting of this boulder.

It is very well written and although one might say that the analytical methods need not be described in quite so much detail (instead perhaps referencing other publications from the same labs), I enjoyed reading all of the methods descriptions which I found very competent and thorough.  The U-Pb methods are clear and the authors present data on standards that show that the methods are reliable and as precise as can be expected with the methods used.  I found the high Ba-Sr, low Sr87/86, and positive epsilon Nd quite interesting for a sample of granite with nearly 70% silica – which has been interpreted sensibly as having some mantle component during its formation in late Variscan time around 316 Ma.  All in all there is very little to criticise in the manuscript because the writing is clear, they adequately describe the geological setting and the unusual boulders, they  present very good methods and results sections on their data, and have a sensible and interesting interpretation of the setting of both the origin of the granite but also its deposition in the Eocene deposits.  The only thing I might have said could be added to the study is thermochronology on apatite and zircon (ie fission track and/or U-Th-He dates) that could give some insight into when the sample first cooled below about 200C and how much it was buried/heated during Eocene sedimentation as well as later nappe stacking/burial, and then followed by Alpine type exhumation.  This might have allowed more direct and complete description of the full geological history.  Nevertheless the study is really very competent and easy to read and grasp.

I commend the authors who did the work.

I would not suggest any major or even minor amendments other than a few typos I have discovered through detailed reading as follows:

Line 345 has ‘varieties’ twice and ‘are’ twice

Captions to tables 5&6 do not related to the U-Pb/Sr-Nd tables but seem copied from elemental composition table titles- easy to fix.

 21 January 2021

Author Response

Response to the referee comments.

I would not suggest any major or even minor amendments other than a few typos I have discovered through detailed reading as follows:

Line 345 has ‘varieties’ twice and ‘are’ twice

Captions to tables 5&6 do not related to the U-Pb/Sr-Nd tables but seem copied from elemental composition table titles- easy to fix.

We thank Reviewer 1 for the careful review. The small mistakes mentioned in the comments are corrected  (e.g. former line 345 – current line 358) and the descriptions of Tables 5 & 6 were improved -  thank you for highlighting!

The only thing I might have said could be added to the study is thermochronology on apatite and zircon (ie fission track and/or U-Th-He dates) that could give some insight into when the sample first cooled below about 200C and how much it was buried/heated during Eocene sedimentation as well as later nappe stacking/burial, and then followed by Alpine type exhumation.  This might have allowed more direct and complete description of the full geological history.  Nevertheless the study is really very competent and easy to read and grasp.

In case of the Eocene from the Magura Nappe, isotopic dating of uplift would not add much as the paleontological data provide a very precise constraint, so we even have not planned that. However, other parts of the Carpathians it could be very helpful.

Reviewer 2 Report

See the attached file 

Comments for author File: Comments.pdf

Author Response

Response to the referee comments.

We thank Reviewer 2 for the detailed and very careful review of our paper.

Despite the tiny size of the sample rock, this study is consistent owing to the supposed variscan basement.

As part of our response, we would like to emphasize that in the Outer Carpathians, these olistoliths are very big, and have important implications for Carpathian geology. The size of the crystalline exotic blocks is also not tiny – it is spectacularly big, when compare not only to other localities in the Magura Nappe, but also in the other Carpathian nappes and in the Alps. The only exceptions are two localities in the Subsilesian Nappe, described by the team in 2019, but without any connection to the Variscan basement.

So, I suggest to add a scheme of the alpine mecanism for the nappe emplacement and a scheme of the european variscan belts with location of the present studied area.

As the Alpine mechanism of nappe formation is widely known we feel it is best not to describe again this story. Moreover, citation [1] is the chapter from the newest version of the Encyclopedia of Geology (2021) devoted to the Carpathians and describes the main processes and models.

In this paper we deal with only one (albeit the largest) nappe – the Magura Nappe - in the Outer Carpathians. While we plan to write a paper describing findings from the other Outer Carpathian nappes and provide comparison with the Alpine nappes, it is beyond the scope of the current paper. We have a new figure (figure 7) showing the Paleogeography of the peri-Gondwanan domains on the Laurussia and European margins which could be interesting for the wider audience. In the revision we have also added important remarks about the Late Paleozoic paleogeography of the Protocarpathian region (see below).

Keywords : add ; calk-alkaline granites, Carpathian, Variscan, Poland,

keywords: we added Carpathians, Magura Nappe and granitoids (our granitoids are not calc-alkaline).

A North-South cross section crossing the outer Carpathian zone is necessary to understand the structure of this arc.

The suggested cross-section is added (new Figure 1 B), as well as the general map of Europe (new Figure 1 A) showing the location of the Carpathians (Referee 3 suggested this). Citations to these parts of this figure were consequently changed throughout the manuscript.

Could we have a photo of the Boulder (may be in figure 2)

The photograph of the exotic block is already included (Figure 2A) This same photo is also used to show the macroscopic structure of the rock. It was impossible to make a photo of the entire olistostrome due to the nature of the exposure in the field (it is a stream exposure with lots of vegetation around that we cannot remove).

-Line 129 : what means LOI

Line 129 (current line 130) – LOI (loss on ignition) is now explained.

What means the chemical results. Variscan Island arc ? Do you have sone indications on the

Variscan belt structure (from the adjacent areas) ? What is know in the adjacent areas on the

Cadomian, Devonian and Variscan orogens which should be applied to the Silesian zone ?

Results – The development of the Variscan arcs has been widely studied and discussed in the literature, and our was not to discuss global Variscan tectonics. However, we have included portions of text related to both the petrological interpretation (section 5.1, lines 307-310, which also answers the question raised by the reviewer on line 324 – see below) and on the Moldanubicum development (section 5.2, lines 362-375).  We have also added a new Figure 7 showing the paleotectonic setting of the Variscan orogen in Europe. We also have included a new citation (Moyen et al., 2017) explaining how our T_DM ages and inherited zircon cores link in with a mixed Cadomian/Caledonian and Paleoproterozoic source for the post-tectonic granites in the Variscan belt of Europe (line 308). We would also like to emphasize that in this paper we concentrate on the Magura Nappe, not the Silesian zone which has a completely different basement ( section 5.2. current lines 344-346 and cited paper [7] – Gawęda et al. 2019).

Line 309 -this assumption require a simplified scheme.

Discussion – former line 309 – we removed this sentence.

Line 321-What is the name of this Neoproterozoic ocean ?

Former Line 321 - Unfortunately we have too few data to speculate about the name of the Neoproterozoic Ocean - in case of the exotic blocks it is impossible to make the required paleomagnetic measurements. In the future, we plan to examine in detail the petrology and geochemistry of these gabbro-diorite exotics.

Line 324- Did you have an idea of the Neoproterozoic and Cadomian zircons provenance ?

Northern Gondwana margin is not very constrained.

Former Line 324 – see above where the new lines 307-310 include an interpretation after Moyen et al. (citation [37]).

Line 345 : delete the second « are »

Line 345 – done

Figure 7 : A cross section with the present structure is needed.

Former Figure 7 - cross-section through the present structure is included in Figure 1 (now Figure 1 B).

Conclusion.

Too short. Please, give some informations on the Moldanubian unit. The Jan Golonka

paleogeographic reconstructions are mondialy known. Could you provide an appropriate

one in the variscan times, with location of the area?

Conclusions – we added remarks on the Moldanubian unit as suggested but in the Discussion chapter, as the conclusions should be concise. A paleogeographic map for the Late Devonian made by Jan Golonka (co-author) that illustrates the position of the Moldanubian has been included.

References .

1. References on the adjacent areas are well cited, but a non familiar geologist with these

country, will be disheartened.

References – we added some new important references relevant to our paper, while avoiding too many references dealing with the basic geology of Europe.

Reviewer 3 Report

Dear Editor and Authors,

After reading the manuscript, the main issues that came up are:

1. Interpretations and conclusions based on the available data are not justified. Authors should compare their results from a wider range of studies published in international peer-reviewed journals.

In the manuscript, only one granitoidic rock has been studied with two samples. The authors draw interpretations and conclusions based on the geochemistry of only these two samples, without making a comparison to published data from other locations (this should done both in Figures and in text). In addition, the authors state that the rapakivi structure (or antirapakivi structure) is one reason why the magma is originated from the mantle. To my knowledge the scientific view of formation processes behind rapakivi structure do not relate to this thing. This part of the discussion is also without any reference to peer-reviewed articles.

Based on the manuscript the age data seems valid, but presenting the whole-rock geochemical data and the discussion related to it needs more work. The limitations of the geochemical data are not discussed at all, which is very important thing to do.

2. Language and use of terminology of the manuscript are not adequate and need improvement.

Some of the terms are used a bit loosely (e.g. block, exotic block, clast, exotic clast, exotic crystalline clast), and terms used in the article should be defined more clearly and same term should be then applied throughout the article, if referring to same thing. There are some terms, which are not written in English or they contain typos (e.g. celsjan). It should be stated clearer, whether the authors refer to one granitoid or two samples of that granitoid (now, in places it is written “granitoids”, which gives an impression that several different granitoid samples where studied). This comment applies also to table texts. There are also some parts that are unclear, what authors want to say, for example: “rapakivi mantling of K-feldspar”.

3. Illustrations need to be reconsidered. Here are some examples of things that I suggest to check carefully:

Illustrations are not consistent. Figure 1 would benefit of an inset map showing the location of the Carpathian mountains in Central Europe more larger scale than it is evident now. Used symbols etc. varies. The figures would be more clear, if references to subsection of figures would be done in some other way than lines crossing the figures/legends. In places, font size is too small, and in Fig. 2c and 2d the scale bars are too small. In Fig 3 it is unclear which symbols refer to sample 1 and which sample 2. In Fig 3d, for example the legend contains two samples with number 1. Use of capital letter to refer to figures varies throughout of the manuscript.

Author Response

Response to the referee comments.

Interpretations and conclusions based on the available data are not justified. Authors should compare their results from a wider range of studies published in international peer-reviewed journals.

In the manuscript, only one granitoidic rock has been studied with two samples. The authors draw interpretations and conclusions based on the geochemistry of only these two samples, without making a comparison to published data from other locations (this should done both in Figures and in text). In addition, the authors state that the rapakivi structure (or antirapakivi structure) is one reason why the magma is originated from the mantle. To my knowledge the scientific view of formation processes behind rapakivi structure do not relate to this thing. This part of the discussion is also without any reference to peer-reviewed articles

Thank you very much for all the comments, improving our manuscript.

Introduction: We have added more information about the two samples. We wish to emphasize, and it is stated clearly in the paper, that there are no other crystalline exotic blocks bigger then centimeter-size pebbles in Magura Nappe. Hence we compare the geochemistry with Moldanubian granitoids (see section 5.1. and 5.2).

Regarding the reviewer’s remarks on the rapakivi structure we note only the antirapakivi mantling as a fact (defined already in the text – in the same sentence – see lines 233-234), later we made mistake writing “rapakivi” in place of “antirapakivi” (changed now – current line 297– thank you for mentioning our mistake), but it was interpreted together with Zr saturation temperature and geochemistry as having “at least a component of mantle derivation” (current line 299). Final justification was based on isotopic and geochemical data (see Table 6 and the Discussion chapter).

Language and use of terminology of the manuscript are not adequate and need improvement.

Some of the terms are used a bit loosely (e.g. block, exotic block, clast, exotic clast, exotic crystalline clast), and terms used in the article should be defined more clearly and same term should be then applied throughout the article, if referring to same thing. There are some terms, which are not written in English or they contain typos (e.g. celsjan). It should be stated clearer, whether the authors refer to one granitoid or two samples of that granitoid (now, in places it is written “granitoids”, which gives an impression that several different granitoid samples where studied). This comment applies also to table texts. There are also some parts that are unclear, what authors want to say, for example: “rapakivi mantling of K-feldspa

Regarding the use of the English language, one of the authors is a native speaker (Prof. David Chew, Trinity College Dublin) – see the Author Contributions notes. The paper has been edited thoroughly for the English during the revisions stage.

It should be noted that we used synonyms for “block” e.g. clasts etc. to avoid excessive repetition of the phrase “exotic block”, following the suggestion of the reviewer “exotic block” has now been consistently used throughout the manuscript, and we have changed “granitoids” to “granitoid”.

 Illustrations need to be reconsidered. Here are some examples of things that I suggest to check carefully:

Illustrations are not consistent. Figure 1 would benefit of an inset map showing the location of the Carpathian mountains in Central Europe more larger scale than it is evident now. Used symbols etc. varies. The figures would be more clear, if references to subsection of figures would be done in some other way than lines crossing the figures/legends. In places, font size is too small, and in Fig. 2c and 2d the scale bars are too small. In Fig 3 it is unclear which symbols refer to sample 1 and which sample 2. In Fig 3d, for example the legend contains two samples with number 1. Use of capital letter to refer to figures varies throughout of the manuscript.

The scale is now better shown on Figure 2, as well as description of samples on Figure 3. The term “celsjan” was replaced by “celsian”. In Figure 1 we now include the position of the Carpathian chain in Europe (current Figure 1A). Capital letters are already used to refer to the figures in the manuscript text, according to Minerals rules.

Round 2

Reviewer 3 Report

Dear Authors and Editor,

please see my comments from the attached file.

Comments for author File: Comments.pdf

Author Response

Dear reviewer, the figures 3 & 6 were improved according to your suggestions (grey field of published data was included). We also include the informtion about the data source. Thank you very much for you effords to improve the manuscript.

Best regards

Aleksandra Gawęda