Metamorphic Age Comparison and Its Implications between the Zuoquan and Zanhuang Complexes in the Central North China Craton, Based on LA-ICP-MS Zircon U–Pb Dating
Round 1
Reviewer 1 Report
My comments are listed below;
1) Figure 1 needs work - increase the size of the legend so that the patterns are identifiable. The Jiao-Ling-Ji belt is mentioned in text but not included in Fig 1a
2) Line 97 - 126: The geological setting requires more detail. What is the age of the basement (oldest rocks) within the region? What are the reported the ages of the TTG gneisses, the supracrustal units and granites. This is important as it would help place your c.2.5 Ga dates into regional context. Are there rocks in the region that could serve as protolith to the metamorphic rocks in the study.
3) line 131- 154: The description of the samples requires more detail. What are the PT estimates for the units sampled, how do they relate to published estimates for the region.
4) line 159-161: Please provide an description of the analytical technique used for the CL analysis
5) line 198-199: you mention that there is no 'rim-core texture' in sample CZ23, but Figure 5a clearly shows rims with a bright luminescence on zircons 21 and 24. You make no mention of these rims and their signficance?
6) line 250-265: You mention observing two groups of zircons. I would consider changing the order of your groups, so that group 1 is the oldest (i.e. the cores) and group 2 the youngest.
7) line 296-326: the discussion on the metamorphic origin of the zircons analyzed in this study is weak. Please reference previous studies that outline the growth of metamorphic zircons in the PT ranges you have taken samples from. Explain why are there no inherited grains in rocks from the Zouquan complex. Provide more references for how one can identify metamorphic zircons, do they match your conclusions?
8) line 323-326: Provide more explanation to support the metamorphic origin of the zircons with high U/Th ratios in the zircons from the Zouquan complex. While it is true that high U/Th ratios have been found in metamorphic zircons, at present such high ratios have typically been associated with extremes of metamorphism. Do the estimates of peak metamorphism in your samples support this?
9) line 328-365: Did the Zouquan complex witness metamorphism at c.1.85 Ga? Your data indicates that there was no metamorphic zircon growth at this time.
10) line 367-396: the section of tectonic implications is weak. You need to provide a more detailed explanation of why the c.1.85 Ga collision is not recorded by the zircons in Zouquan complex. Did the the rocks in the Zouquan not approach anatexis at 1.85 Ga. Provide more detail to support regional metamorphism in the Zouquan at c.1.85 Ga
11) Line 397-406: The conclusions are not supported by the data. In line 398 you state that 'two groups of metamorphic ages c.2.5 Ga and 1.85 Ga, were both recorded in the Zuoquan and Zanhuang metamorphic rocks. Your data does not support this conclusion as your study found no 1.85 Ga ages in the Zouquan complex. You could state previous work has identified the younger event.
Author Response
Dear Professor,
Hope everything goes well with you.
Many thanks for the insightful comments and suggestions on our manuscript. We have made all the necessary revisions according to your advices. Please see the attachment for the details.
Best regards,
LingLing XIAO
Corresponding author, on behalf of all the authors
December 1, 2019
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Metamorphic age comparison and its implications
between the Zuoquan and Zanhuang complexes incentral North China Craton, based on LA-ICP-MS
zircon U-Pb dating
Ling-Ling Xiao 1, 2 and * Min-Hua Chen 3
Reviewer:
A brief summary
This study deals with the Zuoquan and Zanhuang Complexes located within the Trans-North China Orogen, North China Craton. The researchers document U-Pb zircon geochronology of mainly metamorphic zircon combined with REE analayses. Two main groups of metamorphism are dated, at ca. 2.4 Ga and ca. 1.85 Ga and the aim of the paper is to see if both ages occur in both complexes or not.
Broad comments:
Introduction
The introduction is missing a clear statement of the aim of the study.
Some examples:
In orderto place new constraints on the timing of metamorphism and/or protolith age?
We present in this study... or
In this work we investigate...
This paper aims to...
We identify/propose...
a new set of combined U-Pb and REE analyses on zircon...
By applying this combinedmethod...
tocharacterize the metamorphic history...
Stress the importance of combining the metamorphic zircon ages to the REE data. Especially if other studies have not done this before.
Discussion
Give an explanation for the Eu anomalies or at least discuss the feature.
Could there be a decoupling between the REE patterns and the U-Pb age? Can the REE patterns really be linked to the metamorphism?
You did not plot the Concordia age of the samples, because the ages were slightly spread out over the Concordia line. How do you explain this?
Why not discuss the protolith age of sample ZZ17-8. It is quite interesting why this sample has preserved the protolith age and not the other samples. Please explain. Has this to do with the protolith itself or the metamorphic conditions?
Conclusions
If the period between 1.97-1.80 Ga is considered to represent one subduction-collision-uplift cycle then what do these ages represent? Is 1.97 Ga the age of prograde metamorphism? And are the ages recorded at 1.8 Ga due to uplift?
Also, do you only have zircon growth at peak temperatures? If you look in the literature it becomes clear that this is not always the case!
References
It strikes me that the only non Chinese reference is the one from Ludwig (2003) for Isoplot. I understand that you need these references for the background geology, however zircon U-Pb dating and REE of high grade rocks is a well-studied subject in petrochronology. Therefore more general references explaining zircon processes are needed.
The reference list of this paper is really lacking a large of amount of reference about zircon behaviour during metamorphism in general, analytical methods
Specific comments:
Line 17 - correspondence OR relationship
Line 18 - Do you mean origin with 'formation of the basement'?
Line 26 - unclear what is meant with 'probably formed'. Is part of the sentence missing here? Please rewrite.
Lines 26-27 - this entire sentence is too long and messy. Rewrite.
Line 28: Formation at different crustal levels. What do you mean with formation? These rocks were metamorphosed at different crustal levels? The formation was sedimentary or are they intrusive rocks?
Line 38 - model = study
Line 42 - How can the first staged of metamorphism be at 1.95 Ga? What happened at 2.5 Ga?
Line 45 - Why 1.90 Ga? This is then a 4th recognized stage of metamorphism...
Line 46 - coherent?
Lines 49-51 - Rewrite. For example...
These complexes commonly record the metamorphic age of c. 1.85 Ga, and there is an agreement that the complexes where involved in the tectonic cycle of subduction-collision-uplift of the TNCO (REF).
Lines 58-60 - are part of the description of Fig. 1
Line 61 - delete 'progressively'
Lines 61-62: However, another metamorphic event has been identified at c. 1.95 Ga...
Line 62 - 'One common view holds' bad English
Line 69 - space missing between to 1.90 Ga.
Line 81 - to the amalgamation of the Archean...
Line 79 - Why say 'a group of metamorphic ages' Please be more specific if the age spread is large. For example Metamorphic ages ranging between 2.55-2.45 Ga. Also, are all these metamorphic ages based on Zircon U-Pb dating? Again be more specific here.
Line 82 - Better to say studies instead of researchers. For example
Previous studies have shown that...
Lines 83-84 - Also rewrite. Unclear what you mean here. For example: Another interpretation is that the Fuping Arc was already part of the Eastern Block at c. 2.5 Ga... Or using accretion instead of amalgamation.
Line 86: 'Regardless, the data draw attention to a need to discriminate effects' bad English rewrite.
Line 89: major instead of important
Line 90- On zircon?
Line 91: the Precambrian rocks. Why 1.84 Ga?
Lines 95-96: To contribute valuable information on this topic, we focus on the spatial distribution of ???and the corresponding relationship between the two groups of metamorphic ages in this study.
Rewrite this sentence. Bad English and confusing.
I understand that you want to focus on the importance of the spread of metamorphic ages in the different rock types throughout the two complexes. However, this sentence is confusing. You could refer to the two metamorphic events as is shown by the two clustered groups of metamorphic ages.
Line 100 - is = lies
Lies 101-106 - Is there any age data available on these rocks? Be more specific here. For example do not use the term early Precambrian for Archean when you later specify Paleoproterozoic and Mesoproterozoic.
Lines 101 - 110 - Rewrite this section. I suggest that you start directly with the new division. If you want to you can just refer shortly to the old division:
The Zanhuang Group (52) is now subdivided by...
Or previous known as the Zanhuang Group (52)...
Line 104 - in Archean?
Line 106-107 - This is inconsistent with line 115
Line 11: felsic amphibole/biotite-plagioclase gneiss. What kind of rock is that? Is it a paragneiss? Is it amphibole or biotite or amphibole and biotite? Is there no quartz in these gneisses? Why not? Why is it felsic then?
Line 112 - typical metapelites? What do you mean?
Line 112 - delete only
Line 115 – up to the transition region? Also, what do you mean with ‘during the same tectono-thermal event’? The same as???
Line 116 – ‘Three generations of metamorphic mineral assemblages’ Do you mean prograde, peak and retrograde? Be clearer here…
Line 123 – ‘as evidenced’ bad English
Line 123 – metamorphic event is not the same as deformation (consistent schistosity and gneissosity). Also, why not refer to this as foliation? Rephrase
Fig. 2 - Which samples do these photo's represent? Is it ZZ17-8 and ZZ29-2?
Line 138 - 140 - No plagioclase? And what kind or pyroxene?
Line 150 - Why?
Line 153 - See the text for details
Line 157 - If you picked more than 250 grains for each sample, why were only so few analysed?
Line 159 - photographed in reflected and transmitted light. Why? Did you use this for something?
Section 3.2. Analytical methods and data reporting needs to be more specific. Please follow the recommendations on http://www.plasmage.org/recommendations/home.html
Lines 180-181 - Reference?
Line 187 - True but I would say there are at least two different patterns to see within the REE distribution patterns. Is this feature seen by a different method as well? Is there a trend?
Line 199 - Reference?
Line 200 – Did the double analyses give similar results?
Fig. 5 It would be nice to see all the grains that are pointed out in the figs are found in all figures. For example I am missing No 18 in Fig 5a, b, d and e etc...
Line 216 - to have been subjected to
Line 217 - "Zircon domains with relatively homogeneous luminescence were analyzed." Was this in order to only analyserecrystalised zircon? And why was zircon no 24 analysed then. This grain obviously did not have a homogeneous CL luminescence.
Lines 222 – 224 The CL and HREE patterns are most consistent with the grains having formed or been extensively modified during metamorphism and growth with garnet and plagioclase. This is an interpretation, move to the discussion. Also, reference?
Line 237 - Most spots show a flat HREE pattern without negative europium anomalies
Really? In Fig. 7c it clearly looks like most analyses do have a negative Eu anomaly...
Line 253 and 258 – Move interpretations to the discussion. Also, provide more references for your statements. Corfu (2004) Reviews in Mineralogy and Geochemistry (Zircon) is a good reference on Zircon textures.
Also are low Th/U really prove of metamorphism? Please give a reference to this statement. This statement has been the subject of a lot of discussion, and it seems that it does not hold for high T metamorphism… Read the new zircon chapter from Rubatto (2017) Reviews in Mineralogy and Geochemistry, Petrology: Methods and applications
Lines 272- 273 – Representing an igneous event? You mean the age of the protolith?
Again, it would be better if the interpretations of this section were moved to the discussion.
Fig. 8 – It would be interesting to see the REE pattern and the CL picture of analysis no 24 since you put it in Group 1, even though it has an old age similar to group 2. Is this solely based on the low Th/U level?
Fig. 9 It would be interesting to see the REE pattern of the two analyses on the metamorphic rims no 37 and 38, that are slightly younger. Do you show a different pattern?
Also, the yellow pattern, to which grain/analysis does it below and why is it different from the rest?
Lines 297-299 – Again where are the references for these statements? This goes for the whole of the discussion.
Line 303 - Abies-leaf- and fan-shaped zoning. Why use this term? I could not find it in the literature.
Line 305 - Taxitic? Why use this term?
Lines 315 – 316 This result is obtained because the HREE prefer to fractionate into garnet during the metamorphic process. Again, where is the reference? And also why not analyze the garnet?
Line 320 – Is this a common feature, if so where is the reference. If not how do you explain this?
Line 323 -326 – Good that you address this point, but these references that you give are not sufficient enough.
Lines 336-337 How do you know that the garnet was part of the peak mineral assemblage?
Lines 338- 339 – Funny sentence. After the protolith had formed? Of course otherwise it is not the protolith… Also, what was the protolith?
Line 347 – Prepeak? Or Prograde? Also, what were the PT-conditions?
Lines 363 - 365 - How is this possible? Do you mean that the metamorphic zircon grains or detrital? This is a bit farfetched. A protolith is the original, unmetamorphosed rock from which a given metamorphic rock is formed, so it is not possible that the metamorphic event was before the protolith was formed!!!
Line 379 – 1.97–1.80 Ga – This is a long period of time, also for geological processes. This should not be connected as one phase. Perhaps Phase 2A and 2B, but it is highly unlikely that the metamorphic ages are related to a single collisional event.
Line 381 – This research or these studies
Line 385 – This evidence or data
Line 389 - The reason probably laythat – Bad English
Line 391 – in=at
Lines 292 - 394 - or not documented. Metamorphic zircon does not always grow if the conditions or rock type are not right.
Author Response
Dear Professor,
Hope everything goes well with you.
Many thanks for the insightful comments and suggestions on our manuscript. We have made all the necessary revisions according to your advices. Please see the attachment for the details.
Best regards,
LingLing XIAO
Corresponding author, on behalf of all the authors
December 1, 2019
Author Response File: Author Response.pdf
Reviewer 3 Report
Xian and Chen have written an interesting and important geochronology of the Zuoquan and Zanhuang complexes in the North China Craton but improvements should be made. The data is good and credible but the interpretations are based on some speculation and are completely restricted to the North China Craton. Alternative interpretations are not carefully considered and it would be a more interesting manuscript if the North China Craton was placed in a global context. According to Zhao et al 2002 the supercontinent Columbia was assembled 2.1 to 1.8 Ga during the Orosirian Period. The development of a pre-Rodinian supercontinent is an exciting idea but nothing pertaining to the development of Columbia (or Nuna) is offered. Your tectonic implications should be expanded because they are a bit sketchy. Regional metamorphism between 1.97 Ga and 1.8 Ga during a continental collision event (Phase II) is proposed to have occurred at different crustal depths during subduction. The evidence for this is not clearly stated. However, 170 Million years is a long time for a single Phase II subduction event to have taken place between two small continental plates. Evidence for the proposed Phase I “… metamorphism related to underplating of large amounts of mantle derived magma” at 2.5 Ga is also in need of supporting evidence. Specific line by line suggestions follow:
Lines 14 to 29 – The abstract should be revised to make it clear which of the many statements are based on the new data of Xiao and Chen and which are based on previous research.
Line 39. “In this …” Is this Xiao and Chen or Zhao el al?
Lines 58 to 60. This should be part of the figure caption.
Line 102. What is the supracrustal assemblage? Is it Khondalite?
Lines 119 to 122. Are there any related S-type granites that could support a subduction interpretation?
Line 147. You describe several rocks interpreted as metapelites but how are you sure that they are not metavolcanics? During the Paleoproterozoic pelites were probably much less common than volcanics with similar bulk chemistry.
Line 152. Very good photomicrographs.
Lines 190, 206, 228, and 242. It should be “peak metamorphic age”.
Line 308. It should be “igneous origin”
Line 375. Please review the evidence related to underplating.
Line 388 to 390. Please review the evidence for a single prolonged Phase II subduction event.
Author Response
Dear Professor,
Hope everything goes well with you.
Many thanks for the insightful comments and suggestions on our manuscript. We have made all the necessary revisions according to your advices. Please see the attachment for the details.
Best regards,
LingLing XIAO
Corresponding author, on behalf of all the authors
December 1, 2019
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Revised manuscript addresses many of the issues from the previously reviewed version. I see some minor grammatical errors in the manuscript. I encourage the authors to do a final edit for English.
Author Response
【Reply】Many thanks again for the insightful comments and suggestions on our manuscript. We have the manuscript checked by using a professional English editing service (https://www.mdpi.com/authors/english). The manuscript was revised using the "Track Changes" function. Please see the text for the details.
Reviewer 2 Report
Thank you for following my comments and suggestions. Please double check spelling and grammar.
Author Response
【Reply】Many thanks again for the insightful comments and suggestions on our manuscript. We have the manuscript checked by using a professional English editing service (https://www.mdpi.com/authors/english). The manuscript was revised using the "Track Changes" function. Please see the text for the details.
Reviewer 3 Report
The authors have responded to all my suggestions and have developed an interesting and important manuscript that will be of interest to most petrologists and mineralogists. Congratulations.
Author Response
【Reply】Many thanks again for the insightful comments and suggestions on our manuscript.
