Construction and Destruction of Bontău Composite Volcano in the Extensional Setting of Zărand Basin during Miocene (Apuseni Mts., Romania)

Round 1

Reviewer 1 Report

The paper presents results of paleovolcanic reconstruction of the Bontău composite volcano in the Zărand basin of Apuseni Mts in Romania. A special attention is paid to the role of debris avalanches in the evolution of the volcano and in its destruction. In addition the paper treats petrography and petrology of the volcano rocks.

The paper is well structured and written. Data and analyses are presented appropriately. Figures and graphs complement the text.

It is a good paper that merits publication. However, it has some weak points and can be improved.

Weak points of the paper come out of natural circumstances – generally forested field with insufficient number of outcrops prevents a compilation of geological map that would support some of the interpretations and conclusions. Extent of relevant field observations is also limited. In such the situation it might be helpful to construct composite lithological columns for individual parts of the volcano as an alternative way to support major ideas and conclusions. Especially it concerns the following conclusions/aspects:

1) Dominant role of DADs in the late stage of the volcano evolution. This conclusion is not supported well by presented observations. Taking into account description of DADs there is not a clear distinction between DADs and coarse to blocky debris flow deposits. I am afraid that the role of DADs in the late stage volcano evolution is overestimated and that erosion and related debris flows played a more important role. What is presented as DADs accumulations at the western, eastern, southern and northern sides of the volcano might be parts of volcanic apron surrounding the volcano and formed of debris flow and smaller scale DADs as it is the case of the Ruapehu and many other andesite stratovolcanoes.

2) A large scale northern debris avalanche has created depression in the central part of the volcano. I miss some unquestionable arguments. Also the northern DAD does not include a significant proportion of altered rocks that one would expect in the case that this debris avalanche has created a horseshoe shaped depression. Horseshoe shaped depressions are characteristic for andesite stratovolcanoes in the advanced stage of erosion with no history of major debris avalanches. I would assume a much greater role of erosion in creation of depression in the central part of the volcano.

Further comments relted to specific lines:

Lines 2-4: Title of the paper does not correspond well enough with the content. I suggest “Geology and petrology of Bontău composite volcano, Apuseni Mts., Romania” or alternatively “Paleolcanic reconstruction and petrology of Bontău composite volcano, Apuseni Mts., Romania”. Petrology represents 30 % of the paper.

Line 19: instead “is ~ contemporaneous” should be “was ~ contemporaneous”

Lines 25-28 in abstract: should be changed according to suggested changes of the paper text

Line 40 in abstract: instead “MORB-like mantle source“ it should be „MORB-like mantle source magma“.

Line 50: instead „from the southern“ it should be „in the southern“

Lines 59-61: should be changed eventually according to suggested changes of the paper text and conclusions

Lines 85-89: In the light of absence of a geological map supporting interpretations and conclusions I miss composite lithological columns for different parts of the volcano as a way to support interpretations and conclusions

Line 94: instead “Sr-Nd-isotope ratios“ it should be „Sr and Nd isotope ratios“

Lines 104, 110 vs. 110 and elsewhere: Békés-Zărand basin or Zărand basin?

Line 118 figure legend: „Debris avalanche crater of Bontău volcano” is questionable – see my further comments bellow; “High Sr/Y calc-alkaline intrusions” are not visible in the sketch

Line 123: I miss information on essential geology/lithology of basement units

Lines 164-165: There is no argument to interpret pumice-bearing pyroclastic flow deposits as initial explosive events (preceding the dome growth). Rather they represent products of the vulcanian type explosions associated with the dome growth and preceding dome failures producing described block and ash flow deposits.

Lines 169: provide arguments why “tuff and clast supported breccias suggest a phreatomagmatic origin”. I think that rather we are dealing with vulcanian type explosions associated with the dome growth (compare Heiken, G. and Wohletz, K. 1987: Tephra deposits associated with silicic domes and lava flows. GSA Special Pap. 212, 55-76).

Lines 170-172: I find it strange that this kind of pyroclastic rocks (strombolian type breccia) that require rather low viscosity lava would associate with the viscous lava dome growth. Look for an alternative explanation.

Lines 174-179: Not an observed reality – it is interpretation and should be moved into discussion; also this interpretation should be supported by observations that point to the conclusion about the initial explosive activity preceding the dome growth (superposition?). Superposition with block and ash flow deposits is not a valid argument – see the comment to the lines 164-165.

Line 221: why ”secondary associated debris flow deposits“

Lines 222-224: It is not proved well enough, that the central depression is “north-oriented horseshoe shaped-like debris avalanche crater”. An erosion caldera resulting from a combination of debris avalanche events and erosion of altered rocks in the central part of the volcano (as it is in the case of many volcanoes without observed debris avalanche events) seams to me a more probable explanation. Note that altered rocks are not mentioned as a component of debris avalanche deposits.

Lines 250-252: speculation not supported by observation – due to extensive post-volcanic erosion and tectonic displacement the relative elevation of ridges can not be used as an argument.

Lines 271-301: I miss a clear distinction among debris avalanche deposits and debris flow deposits. It seams to me that a part of coarse blocky debris flow deposits is included among debris avalanche deposits. The role of debris avalanche deposits seams to me as overestimated. What is their proportion in the complex of deposits that look like common aprons known at the foot of many stratovolcanoes. It would be helpful if lithology of deposits could be presented also in the form of a composite lithologic column.

Lines 304-306: description of debris avalanche deposits in this case is not convincing

Lines 306-310: speculation – bulldozing would require a uniform thick debris avalanche deposit unit – that is not supported by description. Also here a representative composite lithologic column would be helpful.

Lines 316-324: interpretation – should be moved into discussion

Line 326: diluted debris flow = hyperconcentrated flow

Lines 336-341: How can You claim that the unit is entirely dominated by various types of DADs if the unit is poor in outcrops and those existent are of small-size. Monolithic breccias (line 340) are not characteristic of debris avalanche deposits. Rare outcrops of stratified thin planar and cross-bedded sandstone and siltstone could be interstratified with debris flow and debris avalanche deposits.

Lines 343-346: A debris avalanche corresponding to a major sector collapse of the volcano would show a much different lithology. Such the event is not supported by presented data.

Lines 352-353: the evident horseshoe shape like depression could be alternative interpreted as erosion caldera (due to an intense erosion of altered rocks) as it is the case at many eroded andesite stratovolcanoes. That would not exclude debris avalanche events as processes associated with initial stages of the erosion.

Line 396 table 1: Note that volume of interpreted debris avalanche deposits is 15 times that of remnants of the volcano! That does not fit the presented scenario. Rather we are looking at syngenetic aprons of debris avalanche, debris flow and hyperconcentrated flow deposits as we can see it in the case of the Ruapehu volcano (Hackett and Houghton, 1989).

Line 415 figure: scales of microphotographs is too small, difficult to recognize

Line 415 figure c: it looks like overgrowth by pigeonite (low Ca cpx) that is characteristic for rather dry hot lavas

Line 418: my suggestion replace “aggregate of amphibole with inclusion“ by “amphibole megacryst with inclusions“

Line 424: instead “with two-pyroxene amphibole“ should be „with amphibole–two-pyroxene andesite “ (Note that rock qualifying minerals should start with the less abundant one. Please, check rock names elsewhere).

Lines 435-436: Apparently orthopyroxenes do not show reaction rims of cpx (Fig. 8c) but rather overgrowth by pigeonite (low-Ca cpx) that is characteristic for rather dry hot lavas. Reaction rim should be granular and it would surround all sides of the orthopyroxene grain.

Line 443: the fig. 8f shows rather an amphibole megacrysts with inclusions

Lines 456-458: I find it strange that basaltic andesites are a part of the dome unit. Perhaps it is related to the assignment of the strombolian type pyroclastic rocks to the dome unit (lines 170-172).

Line486: instead “(Supplementary Tables S1.1 and S1.2)“ should be „(Supplementary Table S1.2)“

Line 520: instead “no matter” should be „no matter whether they)“

Line 537: due to their mobility block and ash flows usually accumulate bellow the upper slopes of the volcanic cone

Line 538: how generation of block and ash flows created conditions for DADs generation?

Line 545: instead “extension started ~ 16 Ma up to 12.3-12.1 Ma“ should be „extension that started ~ 16 Ma and lasted up to 12.3-12.1 Ma“

Line 552: include at least one more reference dealing with the problem of DADs generation

Line 557-558: I do not understand how preceding Plinian eruptions and generation of block and ash flows created conditions for DADs generation as it was instability and other aspects of the dome, that were responsible for DADs generation

Lines 569-572: Rather speculative – it should be presented as possibility, not as a proven reality

Line 579: formation of small basins among hummocks of DADs where lacustrine and fluviatile sedimentation occurred is assumption that is not supported by presented observations – it should be presented as assumption

Line 592: instead “with effusive“ should be „continued with extrusive“

Lines 601-602: Hummocky topography formed small basins where lacustrine and fluviatile sedimentation occurred – this should be presented as possibility only.

Line 607-608: instead “as most evolved“ should be „ as the most evolved “

Conclusions should be modified, if authors will accept my suggestions. Essentially that concerns a lesser role of DADs, a greater role of erosion and related debris flows and a substantial role of erosion in creation of depression in the central part of the volcano.

Author Response

Please find attached our answer in red.

 

Author Response File: Author Response.docx

Reviewer 2 Report

General：

This study presents new field survey, petrology and geochemistry data about the Bontău volcanic complex in Apuseni Mts. The volcanic activity lasted for several million years, erupted different volcanic products and construct, and destruct the volcanic edifice many times. The discussion about the relationship of these processes and tectonic evolution is very interesting and greatly put forward the study about the regional volcanology and geological evolution. This paper matches the scope of the special issue of Mineralogy of Remote Sensing. However, this manuscript needs moderate revision before accepting for publication.

Specific:

• The abstract part is too long. It should be re-written and make it concise and readable. I don’t think it need too many details.
• Introduction part: too short, it should let readers know how important of this study, what are problems remained of this topic, and very briefly introduce what this study has done.
• Fig. 1 The legends listed in the figure is im-completed, many structure legends missed. Also, the name the figure is not appropriate, it can be simply named as geological map.
• Methodology part. This can be listed as part of Introduction. I let the editor to judge.
• Local geology. It’s better to introduce regional geological background, such as what caused the extension of the studied region.
• Fig. 4. Similar problems as indicated for Fig. 1, please double check the figure and its legends.
• Line 161, check the sentence.
• Fig. 5, most of the pictures are too small to tell the details. I suggest the authors to put some big ones in Supplementary material.
• Geochemistry, should indicate where these sampled were collected from (volcanic unit).
• When discuss magma evolution, need more diagrams to distinguish the FC/AFC processes, such as Mg# vs. SiO2, etc.
• Figure 11, should add notes to Y and X axis.
• Usually graben/rifting system produce more basalts/rhyolite than andesite, the later is rich in arc system along subduction zone. Is it possible to discuss what kind of mechanism produced the andesite?
• Need more discussion about the magma source(s) and subsequent AFC/FC processes for the samples.

Author Response

Please find attached our answer in red.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I am not completely convinced by Your replies. Unfortunately, nature does not provide enough unquestionable evidence. 

Author Response

We are sorry if some of our answers, however not specified by the reviewer, were not convincing enough. Our interpretations have been essentially based on our field observations and additionally by petrography, geochemistry and Sr and Nd isotope composition. Since the reviewer is a specialist and he is wishing to check his opinions, we would be pleased to have him as a guest in the study area where it will be possible to provide him all the unquestionable evidence provided by the nature.   

Reviewer 2 Report

The revised version has grealty improved. But I am not satified with the discussion about the geochemistry and petrogensis. As a part of tectono-magmatism unit of the NW graben system, the geochemical characteristics of Bontau composite volcano should be compared with that of main Zărand-Brad-Zlatna (Group 1-Z) graben-type basin and several sub-basins Caraci (Group 1.1-C), Săcăramb (Group 1.2-S), Roşia Montană – Baia de Arieş area (Group 2-N) and Deva (Group 3-S) studied in Seghedi et al., 2021. I roughly checked the data, the range of major element contents are different, this paper should describe these differences and discuss why. Simply show the geochemical data of the Bontau volcano is not sufficient for readers to understand the regional geological evolution history.

Author Response

We already point out that the geochemical data have been useful for helping understanding the volcanological problematic and not intending to discuss the regional geological evolution history of the Apuseni Mts. that could be eventually a target for another article. We can confirm that the geochemical data are not different from main Zărand-Brad-Zlatna (Group 1-Z) where the Bontău volcano has developed. To convince the reviewer and the readers we have added the most confident previous geochemical data of the Group 1-Z to the figures we have previously done and made additional few comments. Furthermore we show also the Sr and Nd data for the Group 1.2-S, Group 2-N and Group 3-S, where it is evident that Apuseni magmas are isotopically the most primitive in the Carpathian-Pannonian Region, but as well confirm the range of new data of the Bontău volcano as perfectly fitting in the trend of the few previous data, confirming AFC processes in a rather short range of isotopic Sr.