Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

Cisneros, Miguel; Barnes, Jaime D.; Behr, Whitney M.; Kotowski, Alissa J.; Stockli, Daniel F.; Soukis, Konstantinos

doi:https://doi.org/10.5194/se-12-1335-2021

Articles | Volume 12, issue 6

https://doi.org/10.5194/se-12-1335-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-12-1335-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 6

Research article

|

15 Jun 2021

Research article |

| 15 Jun 2021

Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

Miguel Cisneros, Jaime D. Barnes, Whitney M. Behr, Alissa J. Kotowski, Daniel F. Stockli, and Konstantinos Soukis

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Final revised paper (published on 15 Jun 2021)
Supplement to the final revised paper
Preprint (discussion started on 14 Sep 2020)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

SC1: 'Electron probe measurements', Ioannis Baziotis, 20 Sep 2020
- AC1: 'Reply to I. Baziotis comments', Miguel Cisneros, 22 Sep 2020
RC1: 'Comment', John Schumacher, 06 Nov 2020
- AC2: 'Reply to J. Schumacher comments', Miguel Cisneros, 17 Dec 2020
RC2: 'Review of MS No.: se-2020-154 - Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece', Valentin Laurent, 18 Nov 2020
- AC3: 'Reply to V. Laurent comments', Miguel Cisneros, 17 Dec 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Miguel Cisneros on behalf of the Authors (17 Dec 2020) Author's response

ED: Referee Nomination & Report Request started (04 Jan 2021) by Federico Rossetti

RR by Valentin Laurent (20 Mar 2021)

Suggestions for revision or reasons for rejection

Reviewer: Valentin Laurent

The manuscript "Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece " by Miguel Cisneros, Jaime D. Barnes, Whitney M. Behr, Alissa J. Kotowski, Daniel F. Stockli, and Konstantinos Soukis submitted for publication in Solid Earth (2nd submission) combines elastic thermobarometry (quartz-in-garnet and quartz-in-epidote barometry) with oxygen isotope thermometry to quantify the pressure-temperature (P-T) evolution of peak to retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU). As I said in my 1st review of this paper, the work presents interesting P-T data, especially the ones obtained from epidote and Qtz/Calcite boudins as they allow to add robust constraints on the retrograde evolution of the CBU. However, and even if some of my previous comments on this paper has been considered (e.g. literature is better referenced now) or at least discussed by the authors, I still have major concerns with somebof the main conclusions of this paper:
1) Maximum P-T conditions of the CBU.
2) Results of this study show cooling during exhumation and do not support a phase of reheating at 10-12 kbar.
3) Implications of the results on the tectonometamorphic history and exhumation model of the CBU.

In my opinion the work has to be profoundly reconsidered before publication. This is what I already recommended in my 1st review and clearly the paper has not been profoundly reconsidered in this 2nd version. For me, this paper can’t be published in its actual state. It is a shame as there are interesting data that could complement our actual understanding of the retrograde evolution of the CBU.

Major concerns:

1) Maximum P-T conditions of the CBU

I did insist in my 1st review that pressure results obtained by the authors on garnet crystallization could be interpreted as not representing maximum P conditions as these results match almost perfectly the P conditions reported by Laurent et al. (2018) for their 1st event of garnet crystallization on Syros (16-18 kbar). In their response to my comment, the authors argued that they have several lines of evidence to say that these are the maximum P-T conditions reached by the CBU (note that only a pressure was determined, the maximum temperature is just inferred from previous studies). In my opinion this is a major issue of the present work… In their paper, the authors are presenting and discussing the different thermobarometric estimations previously obtained in the CBU of both Syros and Sifnos islands, opposing the studies yielding ~1.5 – 1.6 GPa to the studies yielding 1.9 Gpa. This is great but it is not enough. The CBU (and more precisely the Upper Cycladic Nappe as described by Grasemann et al. 2018) is observed in other Cycladic islands such as Tinos, Andros, Ios, Sikinos, Milos and even Naxos. And on practically every of these islands, the most recent published articles that have estimated the peak P-T conditions of the CBU yields consistent results 1.9 GPa (e.g. Lamont et al. 2020 on Tinos; Huet et al. 2015 on Andros; Augier et al. 2014 on Sikinos; Grasemann et al. 2018 on Milos; Peillod et al. 2021 on Naxos; peak P-T conditions are poorly constrained on Ios, see Huet, 2010). Additionally, we can add another new published study on Syros, Gyomlai et al. (2021) (not discussed in the reviewed study as published after re-submission) who found maximum P-T conditions on Syros to be ~2 GPa (I will come back on this study in point 2 as results show a reheating phase at 1.0 – 1.2 GPa that contradicts another conclusion of this work).
There is also a sentence in the response to my comments that worries me… The authors says ‘However, we consider it more likely that different techniques are recording different pressures’. What does that mean? Different thermobarometric techniques yield to different maximum P-T conditions for the same rocks? But in this case, it also means that some of these techniques can’t be used to retrieve maximum P-T conditions in these rocks. And in this case, should we better trust ‘conventional thermobarometry’ as stated by the authors, a technique that have proved to be reliable in every HP-LT belt observed all over the world, or the relatively new technique used by the authors in this study? Perhaps, the question that should be treated in this paper is more: why is the elastic thermobarometry technique yielding to lower P-T conditions than more conventional thermobarometry? Note that Peillod et al. (2021) used the Qtz-in-Grt technique in rocks of the CBU in Naxos and find maximum pressures of ~ 2 GPa.
Lines 86-87 the authors write ‘The range of previous P-T conditions reflects the lack of comprehensive studies that combine structural geology, petrology, and thermobarometry across the CBU’. I already told the authors that they can’t say a statement like that. The CBU is one of the best studied HP-LT belts worldwide for 40-50 years now. In the literature, there are a multitude of research groups and studies that have combined structural geology, petrology and thermobarometry in the CBU at a level of integration that is far more advanced than in this study (e.g. Ring’s group; Grasemann’s group; Jolivet’s group; Lister’s group; and so many others...). I personally dedicated most of my PhD thesis on determining the tectonometamorphic history of the CBU by coupling and integrating structural geology (Laurent et al., 2016; Roche et al., 2016), thermobarometry (Laurent et al., 2018) and geochronology (Laurent et al. 2017) with more than 8 months spent at studying in the field the CBU on Syros, Sifnos, Tinos and Ios. So no, the range of previous P-T conditions doesn’t reflect the lack of comprehensive studies that combine structural geology, petrology and thermobarometry across the CBU. Such P-T range is observed in the literature of most HP-LT belts across the world and is just the result of the numerous different research groups that have studied these rocks since decades. It is important to note that the implications of the results of this study on the tectonometamorphic history and exhumation model of the CBU is not really clear (see my points 2 and 3 below).

2) Results of this study show cooling during exhumation and do not support a phase of reheating at 10-12 kbar.

Another main conclusion of this work is that the P-T path obtained for the CBU on Syros show a constant cooling during retrogression, from peak to greenschist-facies P-T conditions. While I quite agree with that, I am not really sure that the data obtained in this study are sufficient to make such conclusion. The authors make it clear in their paper that the entrapment temperature (Ttrap) of quartz inclusions in garnet (garnet growth temperature) is estimated at 500-550°C based on good agreement between previous studies on the maximum temperature reached by CBU rocks from Syros. Ttrap for the ep2 population is deduced from oxygen isotope thermometry of quartz-calcite boudin-neck precipitates (411 ± 23°C). However, Ttrap for the ep1 population is not constrained (in this study or any previously published studies) and estimated as being intermediate between garnet and ep2 growth (~400-500 °C). This last hypothesis means that the P-T path can’t show anything else than constant cooling during exhumation. Trotet et al. (or anyone else) could legitimately argue that if you consider a Ttrap of 500-550 ˚C for Ep1 (as this is not constrained in the study), your P-T path would show a 1st event of isothermal exhumation from peak P-T conditions to blueschist facies conditions and then a 2nd phase of cooling from blueschist to greenschist facies conditions (something really similar to what Trotet et al., 2001 proposed). In summary, what I want to highlight here is that the conclusion that the P-T path proposed in this study shows constant cooling during exhumation is only based on the unconstrained assumption of Ttrap for Ep1 that can’t yield to another result.
My second comment is that this study claims their results don’t support the reheating phase at 10-12 kbar and from ~500 to 550˚C proposed by Laurent et al. (2018). I did insist in my 1st review that for me, the results of this study don’t contradict the existence of this reheating phase but rather add more constrains on the greenschist P-T conditions during exhumation. The authors of this study completely have the right to disagree with what Laurent et al. (2018) proposed (even more considering that this is shows by only 1 garnet). However, I would like to highlight that a newly published study (published after the resubmission of this study – Gyomlai et al. 2021 in Lithos) has determined exactly the same reheating phase after looking to fluid-rock interactions and metasomatism in the northern block-in-matrix structures on Syros. A difference is that they were able to constrain this reheating phase from rocks located at the top of the CBU on Syros, implying that all subunits of the CBU has undergone this reheating phase. As mentioned in my previous review, this reheating phase has already been described in the CBU of Tinos and Andros and has also recently been shown in the CBU of Naxos (Peillod et al., 2021). So here again, I think that if we consider the previously published studies in different Cycladic islands where the CBU is observed (and again some very recently published studies that the authors can’t have considered as published after re-submission), it seems that there are various pieces of evidence suggesting the existence of this reheating phase, contradicting one of the main conclusion of this study.

3) Implications of the results on the tectonometamorphic history and exhumation model of the CBU.

Last thing I would like to highlight is about one of the promises of this study, which was to provide a ‘more robust’ P-T-D path ‘than what is commonly possible with conventional thermobarometry’ (as this is important to determine the tectonometamorphic history of the CBU and exhumation mechanisms). In my opinion the present study doesn’t propose a more robust P-T-D path for reasons exposed above and hereafter. The P-T-D path presented in Figure 7 is mostly incomplete, with only the Dt2 deformation event being shown. What about the other fabrics described in this study? And more importantly, what about the numerous previously published structural studies on the CBU of Syros, Sifnos and the other Cycladic units? There is no discussion of the previously published P-T-D path in the CBU. As actually written, it is practically impossible to understand the view of the authors about the full tectonometamorphic evolution of the CBU. The short section (Section 6.4) on the implications for exhumation mechanisms where an exhumation model of the CBU is proposed is not enough detailed in my opinion. Be more specific on what is exactly the input of your study on the tectonometamorphic history and the exhumation model of the CBU? When do you think exactly there is a rotation from N-S to E-W stretching lineations (after peak metam or later during exhumation?) and make it clear how your data support this model (actually I don't see any measurements of N-S stretching lineations in your Fig. 1). Should we understand that, in your opinion, there is no record of deformation acquired during back-arc setting in Syros? You presented different exhumation models (e.g. subduction channel vs. extrusion wedge models) in the geological setting and it would be great to come back on this here. Why do you prefer the subduction channel model compare to other models? Perhaps illustrating your exhumation model would help. I think that this section should be entirely redrawn and further developed to clearly expose the implications of the results on the tectonometamorphic history of the CBU.

I have written some other comments on a PDF annotated version of the manuscript.

Referee Report: PDF

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ED: Reconsider after major revisions (20 Mar 2021) by Federico Rossetti

AR by Miguel Cisneros on behalf of the Authors (22 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (29 Apr 2021) by Federico Rossetti

AR by Miguel Cisneros on behalf of the Authors (03 May 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (04 May 2021) by Federico Rossetti

ED: Publish as is (04 May 2021) by Federico Rossetti (Executive editor)

AR by Miguel Cisneros on behalf of the Authors (05 May 2021) Manuscript

Short summary

Constraining the conditions at which rocks form is crucial for understanding geologic processes. For years, the conditions under which rocks from Syros, Greece, formed have remained enigmatic; yet these rocks are fundamental for understanding processes occurring at the interface between colliding tectonic plates (subduction zones). Here, we constrain conditions under which these rocks formed and show they were transported to the surface adjacent to the down-going (subducting) tectonic plate.