A Late Cambrian Continental Convergent Margin in the North Qilian Orogenic Belt, Northwestern China: Geochemical and Geochronological Evidence from Hongtugou Mafic Rocks
Round 1
Reviewer 1 Report
The tectonic evolution of the North Qilian Orogenic Belt (NQOB) has long been controversial. In this paper, the authors reported some new geochronological and geochemical data of mafic rocks in the Hongtugou area within the South Ophiolite belt of the NQOB and discussed the tectonic evolution of the NQOB. Based on these data, the authors propose that the mafic rocks in the Hongtugou area should not be classified as ophiolite and explained that these rocks were formed in a continental convergent margin possibly shortly after subduction initiation.
Although this work provides some data on the ages and geochemical characteristics of mafic rocks, and propose some new thoughts on the tectonic evolution of the NQOB, some conclusions lack sufficient and strong evidence. I would like to recommend a major revision.
Major concerns:
1. The authors propose that the ultramafic-mafic rock assemblage in the Hongtugou area is not ophiolite as they think that the rock assemblage in the Hongtugou area is not consistent with the definition of ophiolite by Dilek and Furnes (2011). However, the evidence for this conclusion is weak. The authors need to give a detailed explanation of why the Hongtugou ultramafic-mafic rock assemblage is not ophiolite.
2. The evidence for the tectonic setting and subduction initiation is weak. Based on the geochemical data of mafic rocks, the basalt and gabbro samples in this study were interpreted to be formed in an SSZ fore-arc tectonic setting. However, the authors seem to ignore the possibilities of back-arc. They also need to collect the geochemical data of some typical back-arc basalt and add back-arc basalt samples in Fig. 4G, and give a detailed explanation to preclude the possibilities that the basalt and gabbro samples were formed in the back-arc setting. If these rocks were formed during subduction initiation, characteristic fore-arc rock assemblage, including fore-arc basalt (FAB) and boninite is expected to be formed. I don’t know if these rocks were identified in the Hongtugou area.
3. The zircon ages of gabbro (13HC-025-1) and diabase (13HC-015-1) are much younger than that of the isotropic gabbro (14HC-005-1). However, the authors only used the age of isotropic gabbro (~507 Ma) and interpreted it as the emplacement age. They should give more explanation about the younger ages. Does the younger diabase represent later veins? In addition, the 507 Ma should be interpreted as the crystallization age of the gabbro rather than the emplacement age.
General comments:
Figure 2: add the legend of diabase
In Figure 3A, it’s hard to tell the cumulate texture.
Figure 3: In general, the abbreviation of plagioclase is ‘pl’ rather than ‘pla’.
Figure caption: What’s the meaning of ‘XPL view’?
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
Hello folks,
The paper is rather simplistic, but based on the relevance of the topic and dating ophiolitic rocks, which is pretty difficult, I think the paper is quite relevant. English review is mandatory, because the text is too repetitive, present gramatical issues and so on. I would recommend a professional revieweing by a native speaker.
Figures could be improved. My main concer is the geological map, which is definetly bad. Colours, drawing, cover cross-cut by early structures and so on. You might take a look of it and spend sometime on it, It would improve a lot your paper presentation. Scales on the microphotographs must be expanded.
All the best!
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The revised manuscript looks better than the original version. Most of my concerns are addressed. Nevertheless, I still recommend a minor revision before acceptance for publication.
(1) Line 163: Basalt is not an intrusive rock. It is impossible that the basalt intruded into the gabbro. The field relationship between basalt and gabbro needs to be checked carefully.
(2) Line 305‒309: The authors claim that the younger ages (~130 Ma and ~236 Ma) probably represent metamorphic ages. Is there any evidence to support, such as zircon CL images? If the authors can’t provide robust evidence to support that the younger ages are metamorphic ages, the age of cumulate gabbro and diabase should be ~130 and ~162 Ma.
Author Response
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Author Response File: 
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