The Formation of the North Qilian Shan through Time: Clues from Detrital Zircon Fission-Track Data from Modern River Sediments

Round 1

Reviewer 1 Report

In this study entitled “The Formation of the North Qilian Shan through Time: Clues  from Detrital Zircon Fission-Track Data from Modern River  Sediments “ by Xu Lin et al.,  a detrital ZFT study from modern river sands  of the upper Hei River is presented.

Authors collected 10 samples but they manage to date only a relatively (for a detrital study) number of grains (see below the comment on the etching practise), thus they decide to merge all the single-grain age data together (total 169) and discuss the obtained age distribution. Authors are comparing their ZFT age peaks with the ZrU-Pb age peak from detrital studies from Hei and other rivers draining different portion of  the Qilian Shan and ZrU-Pb age peak of the bedrock. The underlain idea is that if a peak is present in all the data-set is indicative of the zircon population magmatic origin. On the contrary, if a peak is present in the ZFT age distribution is indicative of a phase of metamorphism related to tectonic activity.

Through the text there is a certain confusion about the common use of the terms geochronology and thermochronology. Geochronology is when you are dating the formation (magmatic) age of the rocks as it is for the Zr U-Pb datings. Thermochronology is used for the other lower temepearture techniques that give clues on  rock heating/cooling events, successive to the formation age.

Zircon etching: one distinctly different approach for Detrital ZFT typically involves making several mounts that then receive different etch times. But at least two mounts are recommended per sample that are then etched for different lengths of time (see Naeser et al. 1987; Garver et al. 2000b; Bernet et al. 2004b). The reason for the different etch times is that a detrital sample contains a mixture of zircons with different amounts of radiation damage, and therefore different chemical reactivity. It is possible that dealing with very old age population an etching of 18 hrs it is too long and can destroy or ruin the grains.

Line 236: The laser ablation spot size was chosen to cover the maximum 236 area over which spontaneous fission tracks were counted. May you be more specific? It is not clear.

Line 241: in Table 1 there aren't reported the sample's lithological characteristics....

 

Some perplexities on data discussion:

 

At line 306 when authors describe the origin of the ZFT 800 Ma population that it is not present in the ZrU-Pb age distributions, at least not as a peak but just as some grains, they again invoke a magmatic origin. I think that more attention  and may be more discussion is needed to explain this.

 

At line 343 authors stated: A similar zircon U-Pb peak age (380 Ma) has been reported from the Hei River basin (Gong et al., 2017, Figure 8 B), which indicate that ZFT ages again correspond to granitoid emplacement and not to a strong exhumation event. What do you mean when you say from the Hei river basin? Modern sand sample or substratum? And I don't understand how the fact that this peak is found in the Hei river sediments, too, supports in some ways that it has a magmatic origin.

 

 

Fig. 1D needs to be improved

 

A number of minor comments are listed throughout the text in the attached file.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer1：

Many thanks to you for your careful revision and suggestions. We agree with the modifications of the manuscript. Below are some detailed answer to your questions.

 

usually ZFT method is considered a thermochronological method....not a geochronological method, in this case I would suggest: thermo-/geochronological ages because you are using also the ZrU_Pb ages

Reply: This is a good remark and we have paid attention in the text to use the words geochronology and thermochronology accordingly.

These are not closure temperatures...but the thermal sesitivity ranges of the systems i.e. the APAZ and the HePRZ.

Reply: Thank you very much for your comment. We have revised it. 

geochronological is usually used for methods as Zr U-Pb that commomnly give magmatic or high-grade metamorphic ages. AFT, AHe and ZFT usually are defined as thermochronological age, giving information on the source area exhumation.

Reply: Once again, thank you for your comments.

One distinctly different approach for Detrital ZFT typically involves making several mounts that then receive different etch times. At least two mounts are recommended per sample that are then etched for different lengths of time (see Naeser et al. 1987; Garver et al. 2000b; Bernet et al. 2004b).

Reply: Yes, this is a good remark and we thank you for the references that we added to the new version of the text, explaining that we did not get enough zircons to mount several aliquots for each sample. We thus preferred to use the initial method developped by Gleadow et al. To keep consistency between the samples.

I note that this peak is not present in the bedrock and in the other modern river sand detrital studies....

Reply: Yes, there really wasn't a significant age peak, but there is a zircon U-Pb age of 800 Ma in bedrock and fluvial detritus.

I don't understand how the fact that this peak is found in the Hei river sediment, too, support in some ways that it has a magmatic origin

Reply: Considering that the ZFT and U-Pb ages of these zircons are very similar, we believe that they represent  the age of magmatic intrusion. One could also envisage a metamorphic event but we think cooling following such a high-temperature metamorphic event would lead to more difference between ZU-Pb and ZFT ages.

This plot is in the reverse sense respect to all the other, I suggest to conform and have the younger ages going to the left.

Reply: Yes, it does look awkward. We have amended it as you suggested.

 

Thank you very much for taking the time to review our article. Wish you a happy life, work smoothly!

 

All the best,

Lin Xu, in behalf of all the co-authors

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear authors

The following paragraphs contain the review of the article

“The Formation of the North Qilian Shan through Time: Clues from Detrital Zircon Fission-Track Data from Modern River Sediments”

By Xu Lin,g, Jolivet Marc, Jing Liu-Zeng, Feng Cheng, Zhonghai Wu, Yuntao Tian, Lingling Li, and Jixin Chen

The topic of this paper is the formation of the North Qilian Shan in the NE Tibetan Plateau through detrital zircon fission-track (ZFT) analyses of river sediments

The authors present a set of single-grain detrital ZFT data from river-bed sediments of the upper Hei River catchment in North Qilian Shan. They integrate their results with published results and further elucidate the exhumation history of the area from the Mid to late Proterozoic until the Cenozoic.

I believe that this paper will undoubtedly be of interest to international researchers working in this region.

The article is appropriately structured, concise, cohesive, and thoroughly referenced, and the figures are of excellent quality.

I believe, it will make a valuable contribution and I suggest that it should be accepted after minor revisions on the following issues:

1. Although I am not an expert on regional geology, from the presented results (literature and this study), it seems that the ~750-725Ma age peak could be related to a distinct episode. It is worth further exploring and commenting on.
2. I understand that none of the authors are native English speakers, but the English text needs minor improvement.

Also, I have noticed a few typos highlighted in the pdf file.

Sincerely

Comments for author File: Comments.pdf

Author Response

Dear Reviewer2:

Thank you for your careful review and comments. We agree with your suggestions and have answered the questions. Below are some more complete answers to your main questions.

 

The age peaks are largely similar as mentioned in the text. The 756 age peak is 15-200Ma younger than the 900-970Ma peaks. This is a significant difference. The bedrock samples also have a ~750Ma peak. This is worth further exploring and commenting.

Reply: Yes, that's a good observation. Indeed the ZU-Pb peak at 756 Ma is only present in the western North Qilian Shan river. We added a few sentences in section 2.2 to acknowledge the fact that the Hei He river was not reaching that source.

 

Thank you very much for your time to revise the article. I wish you every success in your work !

 

All the best,

Lin Xu, on behalf of all the co-authors

Author Response File: Author Response.pdf