Ultrahigh-Pressure Metamorphism and P-T-t Paths for the Eclogites from the Central Areas of Sulu Orogen, Eastern China
Round 1
Reviewer 1 Report
The authors describe petrography of eclogite collected from the Guanshan and Yangkou areas in the Sulu orogen. Based on pseudosection modeling, they anticipate the quick subduction and fast exhumation of the metamorphic rocks during subduction and collision between the Yangtz Plate and the North China Plate between Middle Triassic and Early Jurassic. The data are solid and the inference is reasonable. My concerns are listed as follows:
Mineral chemistry in the eclogite. The garnet is almost homogeneous in chemistry but the omphacite is chemically heterogeneous, but the limited data (Fig. 5) are not enough for comparation. Therefore, I suggest that the authors depict all the chemical profiles of the garnet and omphacite samples studied in this manuscript.
About pseudosection modeling. It is noted that strict closed chemical behavior of the eclogite cannot be fulfilled in the metamorphism. Therefore, the authors may ague that they used the “right” chemical compositions of the eclogite to model its metamorphic evolution. Furthermore, the authors may apply geothermobarometers applicable to eclogite, and compare the different results derived from these two different methods.
Specific comments:
Lines 21-22. “and the P-T conditions are constrained at 22–23 kbar/ 600–610 °C and 25–26 kbar/ 605–615 °C in Guanshan”. Such expression is confused to readers. Similar different description of retrograde P-T conditions can be found in Lines 26-28. Please revise accordingly.
Lines 23-24. “The peak P–T conditions (Pmax=33 kbar; T= 685 °C) of Guanshan eclogite” is in contradictive to those in Lines 21-22 (22–23 kbar/ 600–610 °C and 25–26 kbar/ 605–615 °C).
Line 47. The garnet exsolved not only Na-rich Cpx, but also apatite and rutile, as reported by Ye et al. (2000).
Fig. 6. What do the red rectangles, dots and tringles represent? Lines 294-296. Syntax error. Similar phenomena exist in Fig. 7.
Fig. 9. Why not cite the peak P-T conditions (D-II) determined by Ye et al. (2000) for the Yangkou eclogite?
Author Response
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Reviewer 2 Report
The manuscript by Yuan et al., presents PT estimate and PT path of UHP rocks from the Sulu orogen. They studied two eclogite localities from Guanshan and Yankou. Samples from both localities contain relicts of eclogite facies phases but with almost homogenised garnet. PT conditions for the eclogite are estimated using intersections of isopleths of grossular or pyrope with Si in phengite in the pseudosection. The Intersections yield PT conditions of ca 2.3 GPa/600 oC for sample G-10 and three points at 2.5 Gpa 600 oC/35 kbar 700 oC and 820 oC/12 kbar for sample G-11 in the first locality. The eclogite in Yankou locality gives 25 kbar/600 oC and 12 kbar and up to 800 oC for sample Y-32.
The manuscript, in the present form, is very weak and the interpretations are not supported by the data. I understand that the Sulu and Dabie UHP rocks belong among the most studied and well known localities in the world and it is a hard task to bring new information. This can be especially true if the rocks are retrogressed, or their mineral compositions are modified. The analysed garnet from eclogite in both localities shows flat zoning profiles and its core or mantle compositions can be hardly used for prograde or peak pressure conditions. This problem is widely known and discussed in the literature, e.g., see JP, 63, 1-19, 2022.
It is not clear why some PT points are calculated by the intersections of Si in phengite with Xgrs and other with XPy. There is no intersection among different isopleths of garnet. This suggest that the garnet composition cannot be used for any prograde or peak PT conditions.
Other problem is that pseudosections are calculated above 500 oC, where almost all garnet was already created. This can be seen in Fig. 10 with ca 40 mol %) at such PT conditions. This is another argument, which does not support the results of these calculations.
The assumption about the presence of melt during prograde stage at 600 oC is not supported by the data and bring nothing to the problem. The estimated PT conditions during subduction are in the field of hydrous fluids, which is below wet granite melting curve. This means that all plagioclase and quartz belong to replacement phases that could formed either during retrogression or during heating at 800 oC/12 kba, if such PT conditions are supported.
It is not clear, why the PT path from peak pressure conditions goes to high-temperature and further it crosses the point at lower pressure of 12 kbar 800oC. This is in contrast with the results of other research works from the Sulu UHP rocks (e.g., Xu et al., 2009, Li et al., 2018), that assume a cool exhumation PT path. The UHP rocks could exhumed at low temperatures and subsequent could heated at lower pressures (see Faryad and Cuthbert, 2020, ESR).
What was the 800 oC 12 kbar heating source according to the model in Fig. 12 proposed by the authors? Again, answer can be found in the above-mentioned references.
In addition, similar data on strongly retrogressed and modified UHP rocks from the nearby area were recently published by the authors in Minerals (Yuan, 2022). The manuscript in the present form will be only a repetition of previous published paper.
My suggestion, how to improve the manuscript is: to combine the data by the authors with that from the literature, mainly from Xu et al., 2009, Li et al., 2018. Based on these papers a cool subduction PT path was responsible for the Sulu eclogite, which is similar to that obtained by the authors and can be better constrained based on Si in phengite, maybe also by isopleths of omphacite. The authors should rather focus on the granulite facies overprint at ca 12 kbar and bring more PT data. Maybe application of exchange thermobarometers help better to estimate the granulite facies PT conditions. The heating process can be then explained by slab breakoff model during exhumation (see Faryad and Cuthbert, 2020), which is becoming more often used also for the Dabie and Solu UHP rocks.
Author Response
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Reviewer 3 Report
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Comments for author File: Comments.pdf
Author Response
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Round 2
Reviewer 2 Report
The comments addressed have not been considered in the revised version of the manuscript. Probably the main problem is that the authors have problem to understand validities and limitations of thermodynamic modelling in high-grade rocks. They believe that their garnet zoning, which they used for PT estimate was not modified. Considering their PT path and reaching temperature about 800 oC, this cannot be true.
The authors believe that what they calculated using Thermocalc are correct. It could be correct, if the temperature was below 500-550 oC. I referred the authors to the recent paper by Faryad et al., (2022), Journal of Petrology about limitation of thermodynamic modelling, but the author did not even look at this article.
Considering the authors work, I wanted to help them and suggested possible solution, how to use their results, but this also did not happen. Additional explanations to the individual author responses to my previous comments are added to the author response file (minerals-2213348-coverletter.docx).
Comments for author File: Comments.pdf
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