Tectonic Evolution of the West Bogeda: Evidences from Zircon U-Pb Geochronology and Geochemistry Proxies, NW China

Round 1

Reviewer 1 Report

The article is dedicated to the detailed study of Permian sandstones from the West Bogeda Shan, their U-Pb geochronology and WR-geochemistry. The importance of the provided data is undoubted; the research is conducted at high scientific level and corresponds to the world standards. The article covers important issues of tectonics in the Central-Asian Orogenic Belt (CAOB).

The authors correlate results of the detrital zircon U-Pb geochronology with conclusions on major and secondary elements, thus obtaining a clear view at the sandstones formation.

The article is written in a good scientific language and shows high expertise in the study subject. The impressive reference list emphasizes the authors' competence.

The reviewer believes that the manuscript can be accepted for publication after correcting some minor mistakes in the text. I recommend the authors to make all references consistent with the requirements.

Author Response

The article is dedicated to the detailed study of Permian sandstones from the West Bogeda Shan, their U-Pb geochronology and WR-geochemistry. The importance of the provided data is undoubted; the research is conducted at high scientific level and corresponds to the world standards. The article covers important issues of tectonics in the Central-Asian Orogenic Belt (CAOB).

 

We thank the reviewer for the positive comments on our manuscript. Your encouragement makes us do better.

 

Point 1:

The authors correlate results of the detrital zircon U-Pb geochronology with conclusions on major and secondary elements, thus obtaining a clear view at the sandstones formation.

Response 1:

Thanks.

 

Point 2:

The article is written in a good scientific language and shows high expertise in the study subject. The impressive reference list emphasizes the authors' competence.

Response 2:

Thanks.

 

Point 3:

The reviewer believes that the manuscript can be accepted for publication after correcting some minor mistakes in the text. I recommend the authors to make all references consistent with the requirements.

Response 3:

The references have been adjusted following the requirements of the Journal.  

Reviewer 2 Report

The authors of the manuscript present new data for Permian siliciclastics. Generally, the study is suitablefor publication, however, some major revisions have to be done.

At the current status I highly recommend to shorten the text and to eliminate the multiple doublings to enable a straight forward reading (and understanding) of the text.

The methods part, particularly the description of the detrital zircon age determination needs a complete revision, as main information is missing.

I have some concerns about the narrow grain-size chosen by the authors. This may introduce some bias to the detrital zircon population. Please explain why you have chosen such a narrow range of grain sizes.

Please find the complete comments and suggestions in the attached PDF.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 2 Comments

 

The authors of the manuscript present new data for Permian siliciclastic. Generally, the study is suitable for publication, however, some major revisions have to be done.

 

We express our thanks for your detailed modification and suggestions which highlighted on the annotated PDF. Based on your detailed suggestions, we have made relative revisions and modifications.

 

Point 1:

At the current status I highly recommend to shorten the text and to eliminate the multiple doublings to enable a straight forward reading (and understanding) of the text.

Response 1:

Thanks. The manuscript has been refined by the authors.

 

Point 2:

The methods part, particularly the description of the detrital zircon age determination needs a complete revision, as main information is missing.

Response 2:

Thanks to your constructive advice. The methods of zircon collect and U-Pb age determination have been completed in the revised manuscript.

 

Point 3:

Line 21: has complex zircon ages. What are complex zircon ages? There are complex zircon grains...

Do you mean many different age populations indicating a complex sedimentation history?

Response 3:

Thanks. What we mean is many different age groups indicating a complex sedimentation history and mountain building evolution.

 

Point 4:

Line 22: 335 Ma and 455 Ma. Either ca. 355 Ma and ca. 455 Ma or you give errors. Please check for the entire text

Response 4:

Thanks to your professional comments and advice. We have changed all of them to ca. age (like ca. 355) in the whole text.

 

Point 5:

Line 38-39. Maybe you can split the sentence here into two sentences. That would make it easier to read and to get the facts.

Response 5:

Thanks. We have already split it into two sentences in the manuscript. Such as “The Central Asian Orogenic Belt (CAOB) is located between the Siberia Craton to the north and the Tarim and North China Craton to the south. It is regarded as the largest (extending 7000 km from west to east) accretionary orogenic belt on the Earth”.

 

Point 6:

Line 65. Chen et al., 2015b. Where is Chen et al. 2015a?

Response 6:

Thanks. It is our error and careless, and there is no Chen et al. 2015a. We have already modified it in the manuscript.

 

Point 7:

Early to Late. But if you follow the International Chronostratigraphic Chart it is Lower and Upper. Please consider a change to this nomenclature and/or check throughout the text.

Response 7:

Thanks. We have changed them according to international Chronostratigraphic Chart, such as early Permian to Lower Permian, Late Permian to Upper Permian in the manuscript.

 

Point 8:

Line 81-82: sedimentary stratigraphy. Sedimentary rocks. Which type? Siliciclastic? Carbonates?

Response 8:

Thanks. The rock type of Carboniferous has been added in the manuscript. Such as “Lower Carboniferous composed by marine volcanic ignimbrite and bimodal volcanic lava, while Upper Carboniferous is dominated by felsic ignimbrite and marine basaltic lava”.

 

Point 9:

Line 119: 63-125 μm. Why did you choose such a narrow range of sizes? You lose a lot of bigger and smaller zircon grains, which introduces a bias to the sample.

Response 9:

Thanks. The reason for this is that our sample are dominated by quartz-rich medium-fine sandstone. According to Jin et al., 2019 (Minerals), zircon is mainly concentrated in fine sediment. And we tried to collect zircons from coarse-grained sediments, but it is quite rare. Thus, detrital zircons with in the 63-125 μm size fraction are chosen from sediment samples for U-Pb dating analysis.

 

Point 10:

Line 131: Ages with discordances over 10%. Please give your equation for calculating the degree of discordance.

Response 10:

Thanks. The discordance in this study is defined as [100%-100% × abs [1 − (²⁰⁶Pb/²³⁸U age)/(²⁰⁷Pb/²³⁵U age)]]. The equation has been added in the revised manuscript (Line 127-128).

 

Point 11:

Figure 6: Triassic and Jurassic ages in Permian sandstone?

Response 11:

Thanks. We got 577 zircon U-Pb ages for Upper Permian strata, and only one age was younger than 251 Ma (Permian). The reason of this maybe largely due to error，and we have already removed it in Figure 5 and Figure 6 and later discussions.

 

Point 12:

Figure 7: how many grains measured? how many grains 90-110% conc.?

Response 11:

Thanks. The measured grains (N) and grains with 90-110% concordance (n) have been added in the Figure 7. N and n also have been explained in the figure caption.

 

Point 13:

Line 165: concordance <90%. what about the analyses with >110%? Results of all samples within 90-110% concordance (or less than 10% discordance) are plotted in the U-Pb concordia diagrams (Appendix A).

Response 13:

Thanks. The data with concordance <90% and > 110% was excluded during the following provenance analysis. We have already changed Lin165 to “Results of all samples within 90-110% concordance (or less than 10% discordance) are plotted in the U-Pb concordia diagrams (Appendix A), and the data with concordance <90% and > 110% are excluded during subsequent analysis”.

 

Point 14:

Line 177: Carboniferous account for 94.3%. If 94.3 %94.3 % are older than Carboniferous you cannot have another 69.4% of 295-354 Ma ages. Do you mean that 94.3% are pre-Permian?

Response 14:

Thanks. What we mean is 94.3 % of zircon U-Pb ages are older than Carboniferous (>295 Ma). Among all the zircon U-Pb ages are older than Carboniferous, 69.4 % of are belong to Carboniferous (295-354 Ma). We have already modified it in the manuscript.

 

Point 15:

Figure 9: Which diagram shows which sample? Please add a legend.

Response 15:

Thanks. The legend of our own samples has been added in the Figure 9 and replaced.

 

Point 16:

Line 301: The lithology and depositional environment of West Bogeda area experienced a complicated evolution during the Permian. what does complicated mean? complex? multiphase?

Response 16:

Thanks. What we want to express is multiphase evolution stages. Thus, we have already changed “complicated” to “multiphase” in the manuscript.

 

Point 17:

Line 314: outcrop measurement. What does that mean?

Response 17:

Thanks. We mean the field works. We have changed it in the manuscript.

Reviewer 3 Report

Minor spelling and grammar changes are provided in the attached PDF. I would caution using REE values like LREE/HREE rations and Eu/Eu* in sedimentary rocks that havent been identified by parent rock material for comparison. Even the, these ratios could have been effected by open system process, and redox conditions that effect ratios, with little or no way to evaluate. Classification and characterization of provenance seems fine, but geochemical modeling and assumption based on Harker diagram trends should be viewed conservatively.

Comments for author File: Comments.pdf

Author Response

Minor spelling and grammar changes are provided in the attached PDF. I would caution using REE values like LREE/HREE rations and Eu/Eu* in sedimentary rocks that haven’t been identified by parent rock material for comparison. Even the, these ratios could have been effected by open system process, and redox conditions that effect ratios, with little or no way to evaluate. Classification and characterization of provenance seems fine, but geochemical modelling and assumption based on Harker diagram trends should be viewed conservatively.

 

We express our thanks to your comments and suggestions which have been incorporated in this revised version.

 

 

Point 1:

Line 21: NTS and YCTS.  Acronyms not defined.

Response 1:

Thanks. We have already changed them to North Tian Shan (NTS) and Yili-Central Tian Shan (YCTS) in the manuscript in Line 21.

 

Point 2:

Line 118: The samples for detrital zircon U-Pb dating were prepared following the previous procedures. What previous procedure?

Response 2:

Thanks. The procedures of zircon collection and U-Pb age determination according to previous work have been wrote in Line 114-132.

 

Point 3:

Table 1: Adjust table columns to account for text.

Response 3:

Thanks. Table 1 has been adjusted in the manuscript to account for text.

 

Point 4:

Line 196: SiO₂/Al₂O₃ ratios tend to increase with increasing SiO2 from early to late Permian. Why is this important? SiO₂/x function vs. SiO₂ is not going to be a useful indicator unless x values are so small or large that you need some factor to be able to graph it appropriately, I suggest deleting this sentence, since its not graphed or in a Table anywhere.

Response 4:

Thanks. This sentence has been removed in the manuscript.

 

Point 5:

Line 406: especially Eu anomalies also indicate the changes of sediment provenance in the late Permian. The nature of the original material, redox state and availability of Eu could play a large role in these numbers - they aren't significantly different to suggest otherwise.

Response 5:

Thanks. We agree with your comment. The zircon U-Pb ages are used as robust proxy for provenance discrimination in our manuscript. As you said, the REEs may be affected by many factors, but it still could be used as a reliable proxy for provenance analysis if all the major controlling factors are fully considered.

Round 2

Reviewer 2 Report

Dear authors,

the manuscript is much easier to understand in the current than in the last version. Please find minor comments, questions, and suggestions in the annotated PDF (attached).

Please give some more information in the methods part.

Why did you chose the very narrow size range of 63-125 µm?

How is error propagation etc. managed?

 

Sincerely,

reviewer

Comments for author File: Comments.pdf

Author Response

The manuscript is much easier to understand in the current than in the last version. Please find minor comments, questions, and suggestions in the annotated PDF (attached).

 

We thank the reviewer for the comments on our manuscript. We express our thanks to you for your detailed modification suggestions which highlighted on the annotated PDF. Based on your detailed suggestions, we have made relative modifications.

 

 

Point 1:

Line 100: normally, numbers up to twelve are written, from 13 on they given as numbers (except values of results etc.).

Response 1:

Thanks. All numbers up to twelve have been written in the manuscript. Such as 11 has already changed to eleven.

 

Point 2:

Line 104: medium-fine→ medium- to fine-grained?

Response 2:

Thanks. The medium-fine in the manuscript has been modified into medium to fine grained.

 

Point 3:

Line 115: Why did you chose such a narrow range of sizes? You loose a lot of bigger and smaller zircon grains, which introduces a bias to the sample.

Response 3:

Thanks. The reason for this is that our sample are dominated by quartz-rich medium-fine sandstone. According to Jin et al., 2019 (Minerals), zircon is mainly concentrated in fine sediment. And we tried to collect zircons from coarse-grained sediments, but it is quite rare. If the particle is too fine, then it is difficult to be ablated with laser. Thus, detrital zircons with in the 63-125 μm size fraction are chosen from sediment samples for U-Pb dating analysis.

Point 4:

Line 171: 193 Ma does not fit to Permian rocks that did not suffered a severe metamorphic overprint. So how do you explain such young ages?

Response 4:

Thanks. We got 577 zircon U-Pb ages for Upper Permian strata, and only one age was younger than 251 Ma (192.86 Ma). The reason of this maybe largely due to error，and we have already removed it in later discussions. The Min value of zircon U-Pb dating is 262.14 Ma and we have already changed it in the manuscript.

Point 5:

Figure 9: something does not work here. you have eleven samples in the legend and below, you state that a, b, and c are from different times. How can you then have more than eleven lines in the plots?

Response 5:

Thanks. We have eleven samples for Upper Permian and plotted in the Figure 9c. The legend in Figure 9c is only for these eleven samples. The data of Lower and Middle Permian are collected from previous researches and without legend in the manuscript.

 

Point 6:

Please give some more information in the methods part.

Response 6:

Thanks. We have already modified the method part of zircon U-Pb dating in the manuscript. From line 122 to 139.