Myrmekite and strain weakening in granitoid mylonites

Ceccato, Alberto; Menegon, Luca; Pennacchioni, Giorgio; Morales, Luiz Fernando Grafulha

doi:https://doi.org/10.5194/se-9-1399-2018

Articles | Volume 9, issue 6

https://doi.org/10.5194/se-9-1399-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/se-9-1399-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 9, issue 6

Research article

|

10 Dec 2018

Research article |

| 10 Dec 2018

Myrmekite and strain weakening in granitoid mylonites

Alberto Ceccato, Luca Menegon, Giorgio Pennacchioni, and Luiz Fernando Grafulha Morales

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Final revised paper (published on 10 Dec 2018)
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Preprint (discussion started on 01 Aug 2018)
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Interactive discussion

Status: closed

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

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RC1: 'Referee comment', Anonymous Referee #1, 02 Aug 2018
- AC2: 'Response to Referee #1 comment', Alberto Ceccato, 14 Oct 2018
SC1: 'metasomatism during myrmekite replacement', Bernardo Cesare, 02 Aug 2018
- AC1: 'Response to Short comment SC1 on "Myrmekite and strain weakening in granitoid mylonites"', Alberto Ceccato, 03 Aug 2018
RC2: 'reviewer comment', Elena A. Miranda, 17 Sep 2018
- AC3: 'Response to Referee #2 comment - Elena A. Miranda', Alberto Ceccato, 14 Oct 2018
AC4: 'Rebuttal Letter', Alberto Ceccato, 14 Oct 2018

Peer-review completion

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

AR by Alberto Ceccato on behalf of the Authors (15 Oct 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Oct 2018) by Florian Fusseis

RR by Anonymous Referee #1 (24 Oct 2018)

Suggestions for revision or reasons for rejection

Review comments on "Myrmekite and strain weakening in granitoid mylonites" by Ceccato et al.

I reviewed this paper previously, and recommended it to be major revision. In the revised version of the manuscript, main text and figures are modified essentially along with my comments. However, some points in the manuscript as described below should be reconsidered or revised. After sufficient modification, I recommend the revised manuscript will be accepted.

(1) P2, L5–: According to Kretz (1983), abbreviation of plagioclase is Pl, not Plg.
(2) P4, L1: Please delete "EM".
(3) P5, L5: Where are "encircled clusters" in Fig. 3d.
(4) P8, L10: There is no "section 6.1.2.". It may be "section 7.2.1".
(5) P11, L28: The unit of the gas constant should be "J K–1 mol–1", "J/K/mol" or "J/(K·mol)".
(6) P12, L12–14: At least for me, I cannot find any "details" in supplementary material.
(7) P13, L26–27: Which do you prefer grain size reduction of Qtz resulted from higher strain or higher stress at the margins of the monomineralic layers? If you prefer the former, grain size of Qtz in the layers is not the steady-state one. Is this OK?
(8) P14, L7–8: Please describe the stress condition for "3–4 orders of magnitude increase of strain rate".
(9) P14, L9: Please delete "for a reaction progress factor of  = 0.25, i.e.", because the sentences related to the reaction progress have been omitted in the revised main text.
(10) P14, L30–31: Where do the values of strain rate and differential stress come from?
(11) Fig. 9b: For the model myrmekite, both Pl and Qtz deformed by dislocation creep and by diffusion creep in the dislocation- and the diffusion-creep fields, respectively? If so, grain sizes of Pl and Qtz for the deformation mechanism switch are similar to each other. Perhaps, the mechanism switch of the modeled myrmekite is mainly controlled by that of plagioclase. By the way, the sentence of P11, L19–20 maybe not correct; grain size is not set to be 7 µm in Fig. 9b, but a variable.
(12) Fig. 9c: For the model myrmekite, grain sizes of Qtz and Pl are set to be 7 µm, and they would not change during deformation. In this case, 7 µm Pl deformed mainly by diffusion creep in the investigated conditions, whereas 7 µm Qtz deformed by diffusion creep at lower differential stresses and by dislocation creep at higher stresses. If so, please describe it in the main text.

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RR by James Gilgannon (21 Nov 2018)

RR by Anonymous Referee #4 (21 Nov 2018)

Suggestions for revision or reasons for rejection

Comments on the manuscript: «Myrmekite and strain weakening in…” by Ceccato, Menegon, Pennacchioni and Morales

This is the second round of reviews after two reviewers have already commented on the manuscript. The comments of the previous reviews have been taken into account during the preparation of this revised version, and the manuscript seems to be more or less ready for publication. Every publication can be improved in one way or another, and the number of different opinions about such changes will depend on the number of reviewers chosen. In order not to extend the review process ad infinitum, I will restrict my comments to some general thoughts and to some aspects, which are incorrect in the present manuscript.

The manuscript documents a strain gradient with parallel reaction progress in a granitoid. The data appears to be of very good quality, and the conclusions are mostly sound and all based on the data presented. I have enjoyed reading the manuscript and I think the material presented is of very good quality.

General aspects:
(1) The P,T-estimates for the myrmekite reaction seem to be very low with 4kb and 420-460C. I am sure that the pseudosection modeling was carried out at the state of the art, and I did not check this in the PhD thesis or other papers. I do not know why the temperatures are so low, but they seem to be too low given some experimental data by Goldsmith, Matthews, and Spear. The problem is the following: The CaAl-silicate (zoisite, epidote) forming reaction is the reaction which separates the stability of albite + CaAl silicate from the stability field of an intermediate plagioclase. For the formation of myrmekite, you need to be on the high temperature side of this reaction, because the Al-conserving feldspar replacement reaction is the reason for the quartz precipitates in the symplectite microstructure. The CaAl-silicate reaction delineates the boundary between greenschist and amphibolite facies, at least in mafic rocks. The only way to stabilize an intermediate plagioclase at low temperatures is at very low pressures, e.g. 3 kb and lower. The 4kb given here seem to be too high for that, so that the lower temperature end of the 420-460C interval certainly is too low, because that definitely is in the greenschist facies. Furthermore, epidote-quartz veins are described in the text, so that greenschist facies conditions clearly are reached.
I think that part of the problem is that the shear zones described here have a fairly extended temperature history. In figure 2, there is a well-documented strain and reaction evolution in the protomylonite and mylonite with increasing amounts of myrmekite. The ultramylonite is described as a mixture of plagioclase, epidote, biotite, etc. So, for me this looks like a first stage of the deformation history, in which the protomylonite and mylonite have formed (in the amphibolite facies, and with the myrmekite reaction), and, subsequently, the ultramylonite with a greenschist facies assemblage has formed. If biotite continues to change its composition in the amphibolite facies part of the shear zone during later lower P,T conditions, the temperature estimates may be rather low as a consequence of that, but I can only speculate on the reasons for the low temperature estimate.
The temperature estimate is critical for the rheology calculations, of course. It would be a good idea to make it clear that the temperature estimates chosen for the rheological modeling are at the minimum limit for these shear zones.

(2) The rheological model seems to follow an approach by Dimanov and Dresen (2005) and Platt (2015). As Dimanov and Dresen express it so well in their paper: “At this point it seems impossible to suggest a suitable continuum model that captures the material behavior … offering more than a purely phenomenological or qualitative description.” And they have tried to model only two phases for which they have had good experimental data. The attempt made here is a very complex model with lots of different parameters, deformation mechanisms, combinations of materials, etc. I personally think that by making models more complicated we do not learn more or come closer to explaining what happens in nature. But that is my personal opinion, and as the model in this manuscript follows some approach already presented in the literature, it is up to the authors what they want to publish. For example, the den Brok model for quartz is making rather special assumptions about island and channel structures and he makes it clear that his model produces much faster strain rates than conventional diffusion creep models. But Platt (2015) chooses this model as something that is available, without giving good reasons for using it. It would perhaps have been more instructive to present a simpler model here with only end-member rheologies.
Another aspect of the rheological discussion may be that it should be more clearly stated that the model more or less assumes something close to a Reuss lower bound, because it is assumed that the weak layers are connected. It is not clear that this assumption is also made for the granitoid case, for example.
Detailed comments:

p.2, line 6-9: Myrmekite formation is ALWAYS the result of a chemical metastability of K-feldspar, not of a stress concentration. The location, where the reaction takes place, may be controlled by the stress field (among other factors).

Figure 6: The surface fraction of quartz boundaries is a wrong label on the x-axis, or the data is plotted incorrectly. The quartz surface area fraction should not be 55-60%, if quartz makes up only 18 vol% of the mixture, as described in the text. The points should be plotted mirror-symmetrically on the other side of the vertical 50% line. It then also is more consistent in that the feldspar grain boundaries plot close to their expected frequency.

p.8, line 28: Why does the quartz coarsening imply grain size reduction in plagioclase? They could both grow.

p.10, line 21: please omit “strong”. There is a CPO, but not a strong one.

Figure 9: the largest quartz grain size in Figure 9a is plotted as 30 microns, but the measured grain size distribution gives modes of 20 for the small and 70 microns for the large fraction (Figure 7d). I did not understand why, but I might have missed something here.

p.14, line 16-20: this inference cannot really be made, because the myrmekite reaction is a chemical reaction, and its progress is not dependent on rheology. It is correct that the strain and the reaction progress are connected here, but it is not possible to argue for more progress of the reaction based on a weakening of the rock. In other words, the reaction correlates with strain, but it does not necessarily depend on strain rate.

p.15, line 20: The phase mixing does not impede dynamic recrystallization – the grain size sensitive processes are more efficient in terms of rheology, that is all.

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ED: Publish subject to minor revisions (review by editor) (21 Nov 2018) by Florian Fusseis

AR by Alberto Ceccato on behalf of the Authors (28 Nov 2018) Author's response Manuscript

ED: Publish as is (29 Nov 2018) by Florian Fusseis

ED: Publish as is (29 Nov 2018) by Federico Rossetti (Executive editor)

AR by Alberto Ceccato on behalf of the Authors (30 Nov 2018) Author's response Manuscript

Short summary

Metamorphic fine-grained reaction products make continental crust rocks weaker. Microstructural processes related to the transformation of strong K-feldspar into weak aggregates of plagioclase and quartz during crustal deformation have been investigated through electron microscopy. Rheological calculations show that the occurrence of even small amounts of weak aggregates, whose deformation is mainly diffusion-assisted, would lead to a decrease in rock viscosity of several orders of magnitude.