Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block: Evidence from Zircon Ages and Geochemistry

Round 1

Reviewer 1 Report

Dear Dr. Wang and Co-Authors,

I reviewed your manuscript "Multiple-stage Neoproterozoic magmatism recorded in the Zhangbaling uplift of the northeastern Yangtze Block: evidence from zircon ages and geochemistry" you submitted for publication in Minerals.

Your manuscript is well-written and appears sound in its reasoning and conclusions. I have a few, minor suggestions for modification that mainly pertain to spelling and language style. Please check the data points/zircon analyses you used to calculate age populations. Their number does not always add up to the amount of grains mentioned in the text. These comments and suggestions for modification are indicated in the attached PDF version of your manuscript.

You could also consider adding a supplementary file that shows images of representative zircon grains from your samples and their age populations.

I think the necessary revisions are minor in nature and I recommend publication of your manuscript.

Best regards,

Axel Wittmann, Arizona State University

Comments for author File: Comments.pdf

Author Response

Reviewer 1: Comments and Suggestions for Authors

Marked in the text of “peer-review-27665674.v3.pdf”

1. All the suggestions and corrections marked in the manuscript have been done in the reversion, please see the marked text.
2. Cathodoluminescence images of representative zircon grains are given as supplementary material in Figure A1.
3. A reference (Griffin et al., 2008) for the GLITTER program has been added in the reference list.
4. The numbers of analyzed zircon grains for calculation of mean ages have been checked and corrected if mistake.
5. Text of Line 324-328 is revised as “These old 790 Ma zircon grains have significantly different Hf isotopic compositions from the zircon grains of the early-stage volcanic rocks. This implies that the magma sources of the late-stage volcanic rocks might have some contribution from juvenile materials, prob-ably of ~790 Ma mafic rocks” following the suggestion.

Reviewer 2 Report

The manuscript “Multiple-stage Neoproterozoic magmatism recorded in the Zhangbaling uplift of the northeastern Yangtze Block: evidence from zircon ages and geochemistry”, is of general interest, and the analytical methods employed should be adequate to address the declared objectives. There are some language problems (not serious ones) that I’ve done my best to help improving in the attached .pdf document.

 

However, I am not convinced regarding the meaning of the ages reported. The huge variability of ages calculated is striking, and while the general conclusions reached might well be correct, I am not so sure that those are properly sustained by the data reported. In my opinion, a much more in-depth discussion and justification of the choice of zircons analyzed, and their meaning within the rocks being characterized, is a must. In other words, I am not convinced at all that zircons reported are truly primary magmatic at the time of events assumed to have occurred. If potential readers of the eventually published article were to have similar feelings, then the article would not represent real advance in knowledge.

 

The above being the main criticism, there are other issues that need to be dealt with; please see the attached document.

Comments for author File: Comments.pdf

Author Response

Reviewer 2: Comments and Suggestions for Authors

Marked in the text of “peer-review-27665894.v1.pdf”

1. All the suggestions and corrections marked in the manuscript have been done in the reversion, please see the marked text.
2. Key words have been replaced with “Rodinia; Volcanism; Geochemistry; Zircon age; Nd-Hf-Pb isotopes”.
3. In Section 4.1, age data of volcanic rocks from different localities with Nd-Hf isotopic data are summarized in Table 1.
4. The word “intrusion” has been replaced by “extrusion” for the volcanism in Line 196 and other sites of the text.
5. Reference 36 has been replaced by “Miyashiro, 1974”.

Reviewer 3 Report

The manuscript by Wang et al. is a contribution to a lost-lasting debate on the nature of Neoproterozoic volcanism, including occurrences of felsic rocks, which arguably relates to large-scale rifting or subduction processes. While the research strategy used for this study is pretty conventional and widespread, there might be some serious flaws in zircon studies and their interpretation. My major concern in the latter is very formal treatment of U-Pb and trace-element data for zircons, which, to me, are critical for direct statements about the duration and protracted versus multi-stage magmatism in the area. In particular, the mentioned spreads of U and Th contents might result from intracrustal fractionation processes and/or zircon affinity for distinct felsic melt batches/cameras/unexposed crustal intrusions. Furthermore, systematics finding of Mesozoic zircons are indicative of possible artificial contamination, as there is no alternative hypothesis on their nature based on the data. Lastly, whole-rock Nd and Pb data should be carefully linked to zircon data and stress similar conclusions. I’m sure there may be a stronger involvement of the data on the observed isotopic differences, other than a “contribution of juvenile crustal rocks”. The ms is thus not ready for publication and requires a significant revision starting from the data treatment followed by their careful and straightforward  interpretation.

Line 12. plays an important role

Lines 17-18. It is unclear to me whether a 60 Ma spread here reflects a real duration of volcanism, or just represents an analytical artefact, or results from contamination of relatively young volcanic rocks with zircons from older crustal rocks.

Lines 20-22. Please indicate the geochemical affinity of both “old” and “young” volcanics.

Lines 23. εNd values of -4.6 and εHf -0.4 are barely “juvenile” signatures. Also, suggested reasons for Nd-Hf decoupling for younger rocks should be discussed in the ms and highlighted here.

Line 26. What is the “also” placed here for? The age range is evidently different from the ages listed here, and it remains unclear to a reader how these detrital zircons in metasediments relate to the above.

Line 27-28. This last sentence is too ambiguous. One should first prove the 790-700 Ma range to result from over a 100 Myr protracted subduction (which is unlikely), and then switch to conclusions like that. Furthermore, nothing is said here about the implications of such a dramatic shift in the isotopic composition of volcanics within the mentioned range.

Lines 150-151 and further. It is not indicated here or elsewhere how the discordance of age estimates was treated, e.g., what was the discordance cutout filter for the obtained results? “High” is not appropriate to use in this context.

Also, here is my major concern about the U-Pb data on zircons. The Early Precambrian zircons are trustworthy regarded as a product of contamination from the basement crystalline rocks. However, less than 20 other grains have their U-Pb dates spread over 250 Myr, which is hard to believe. There is now way the mean ages are calculated from such a spread. The issue of Mesozoic zircons is still to be resolved as they also could represent a systematic artificial contamination.

Lines 160-165. Again, there is a serious flaw leading to misinterpretation of the age data. Two age ranges (830-720 Ma and 757-723 Ma), which, from the authors’ point of view, represent trapped xenocrysts and zircon crystallized from melt itself, in fact intersect!

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Relative to the original submission, the formal aspects raised have been largely improved, although not completely, in my opinion. In any case, from the formal point of view, the manuscript is probably fit for publishing, except for minor questions that will be addressed below.

 

The scientific interpretation, however, is still a matter of concern. The meaning of the zircon ages reported, and the interpretation of these by the authors, is still not convincing. Relative to the original submission, supplementary Figure A1 has been provided, showing CL images of some of the zircons analyzed. It is not clear to me (there is no figure caption!) the actual meaning of the red circles. If the area marked represents the ablated area, then multiple growth zones have been sampled together, and if that is the case, values obtained are meaningless. This is important enough as to require undoubted clarification by the authors.

Alternatively, the red circles may contour actual ablation pits. In this case, the pictures are not informative enough, having been taken before ablation. The actually informative image is that of the samples after ablation, in order to evaluate if ablation pits were burnt into individually homogeneous areas, or otherwise affected multiple growth zones. Obviously, this is of paramount importance for correct interpretation of the results.

As such, Section 4.1 Zircon U-Pb isotopic ages and Hf isotopic composition might need to be revised (by the way, I still find this section to be cumbersome and difficult to read).

 

The above being my main criticism, should the Editor decide to go ahead with publishing, the corrections marked in the attached file need to be considered.

Comments for author File: Comments.pdf

Author Response

Please see the attachment

Author Response File: Author Response.docx