Petrogenesis of Early Cretaceous High-Mg Adakitic Pluton in the North Lhasa Block, Tibet Plateau: Implications for the Tethyan Orogeny

Round 1

Reviewer 1 Report

The authors carried out a study of zircon U-Pb geochronology and whole-rock geochemistry for  a  quartz diorite pluton in the northern margin of the Lhasa Block to constrain its petrogenesis and tectonic implications. They inferred that the studied projects belong to High Ma# adakite and a delamination of continental crust model was proposed. However, it is not the case for the adakite naming and there are several possibilities that should be taken into account. At present style, the manuscript is too dilute to be publication.   

Author Response

We thank the reviewer for considering our work and providing this encouraging assessment. We thank the reviewer for his/her careful reading of our manuscript and providing thoughtful criticism.We believe that these have greatly improved our paper.
Inconsistencies in the text have been thoroughly reviewed and corrected, and manuscript has been improved in accordance with reviewer’s suggestions. Figures and tables in the paper have also been revised to reflect reviewer’s comments.

Author Response File: Author Response.pdf

Reviewer 2 Report

Review of the manuscript Petrogenesis of Early Cretaceous High-Mg Adakite in the North Lhasa Block, Tibet Plateau: Implications for the Tethyan Orogeny

Authors:

Qianru Xiao, Yongfeng Wei, Zhongyuan Liu, Fuhao Xiong, Lei Fan, Jianxin Wu and Han Zhao

This paper presents geochemical and geochronological data of quartz diorite rocks of Buruzecuo pluton which is situated in the northern margin of the Lhasa block. The authors present petrogenesis of studied adakites and tectonic implications of the Early Cretaceous tectono-magmatic evolution in southern Tibet.

This paper could be an interesting regional contribution to the model of the paleography, collisional (connected with oceanic subduction and collision of the Tethys terranes) and post-collisional evolution of the Lhasa block. However it needs to be significantly reworked before publications.

The structure of the text is sometimes confusing, some ideas and interpretations should be better discussed and correlated with published data. The manuscript is unsatisfactory in several points, which have to be corrected in order to increase the quality and importance of paper.

Main concerns:

Introduction

The purpose of the research has not been clearly defined. In the introduction to the article, the authors declare the following goals: ‘comprehensive studies on the petrology, petrography, LA-ICP-MS zircon U-Pb chronology and bulk geochemistry of the Early Cretaceous diorite (in fact granodiorite) in the northern Lhasa block’ and next: ‘Combining these data with regional geological observations of the Late Mesozoic magmatic rocks, we discuss their petrogenesis and tectonic implications on the Early Cretaceous magmatic-tectonic evolution process and Tethys orogeny in the Lhasa block, southern Tibet’.

First of all, the research goal should be defined precisely.

I cannot agree that the studies are comprehensive, because they are based on geochemical analysis of 15th samples and just 2 results of SHRIMP zircon dating.

In addition, it is not known how large the investigated intrusion is?

Geological setting and petrography

 The text lacks the results of petrographic description! This part requires significant supplementation. We should know or the chemical composition of minerals have been analyzed? What is the texture of rocks? Are the samples similar in mineral composition or maybe they differ in some way? The chemical composition of the whole rock samples (Table 2) shows that the rocks are not identical. What kind of accessory minerals were observed? Are the minerals fresh or altered e. g. in post-magmatic processes? The last is especially important when using discrimination diagrams based on mobile elements.

The localization of Buruzecuo pluton on an enlarged map is essential! One should know what the position, relations, age and geochemical profiles (type of the adakites) of rocks presented in the text are. Their position in relation to ophiolites is also important.

 U-Pb zircon age

 Were the two dating rock samples similar? What is their relation to each other in the field?

There is no information about zircon morphotypes. Are they similar in both samples?

Bulk geochemical characteristics

 There is no reference to the content of H₂O and CO₂ in the text even though they were shown in the table (Table 2). Volatile constituents are important in interpreting of petrogenesis of this type of rocks.

Are there any ore deposits associated with studied adakite intrusion?

For a more complete clarification of the classification of the studied rocks as adakites, Table 2 should include: #Mg, La / Yb, Sr / Yb, K₂O + Na₂O, CaO + Na₂O. All values in the text should be given in % and ppm.

Discussion and conclusions

Several important adakite-related publications are missing from the discussion. 

A significant list of articles can be found in references of ‘A review of research on adakite’ Lingyu Zhang, Shichao Li & Qingying Zhao

Without defining the spatial and age relations between the adakites compared in the text, presented conclusions seems to be superficial.

If the article is to remain as ambitious as assumed, last chapter should synthetize the results (and literature data) into the geodynamic model. Maybe schematic figure showing the idea of geotectonic evolution of the Lhasa block should be attached. However it will be difficult without significantly supplementing the text with the suggested data.

More detailed comments are provided in the text!

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for considering our work and providing this encouraging assessment. We thank the reviewer for his/her careful reading of our manuscript and providing thoughtful criticism. We believe that these have greatly improved our paper.

To facilitate your review of our revisions, the following is a point-by-point response to the comments. Our responses are shown as text, and corresponding positions are shown as text.

Author Response File: Author Response.docx

Reviewer 3 Report

This work by Xiao and co-authors on the U-Pb zircon geochronology and petrogenesis of the Bieruzecuo quartz (North Lhasa Block) is well presented, presents original high-quality analytical data, and discusses the data appropriately. This work is of interest to better understand the magma source and tectonic setting of the Early Cretaceous adakitic magmatism of the Lhasa Block and thus, should be published. However, I have noticed that it could be substantially improved if some formatting details in the text and figures were corrected and some parts of the text content were clarified (see the attached pdf with my suggestions). 

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for considering our work and providing this encouraging assessment. We thank the reviewer for his/her careful reading of our manuscript and providing thoughtful criticism.We believe that these have greatly improved our paper.
Inconsistencies in the text have been thoroughly reviewed and corrected, and manuscript has been improved in accordance with reviewer’s suggestions. Figures and tables in the paper have also been revised to reflect reviewer’s comments.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have greatly revised their manuscript according to my comments and now it can be accepted for publication. 

Author Response

We thank the reviewer for considering our work and providing this encouraging assessment. We thank the reviewer for his/her careful reading of our manuscript and providing thoughtful criticism. We believe that these have greatly improved our paper.

Reviewer 2 Report

The manuscript has been slightly improved, but most of my comments were not taken into account.

The text needs to be corrected again. All tables require formatting, there are also many editing errors in the text.

A map Fig. 1 (c) has been added, but the location of the other adakites is still not clearly marked. It is not known where the intrusions mentioned in Figures 5-11 are? The insert legends attached to these figures (Fig. 5-11) should include geological names (not names of the authors of cited publications) that would be definitely compatible with the names in Fig. 1

What are the highlighted numbers 1-6 in Fig. 1 (c)?

Are the zircon morphotypes  analyzed on SHRIMP similar (after Pupin and Turco, 1972 diagram)? Instead of an answer on this question, there was a sentence: The zircons in the two samples show similar zircon texture, ……

The values of trace elements (Table 2) in the whole text should be cited in ppm (parts per million).

Fig. 7 (a) It’s still not explained why the spider diagrams are standardized to primitive mantle since their origin (suggested later in the text) might be related to the lower crust? 

Lines 85-93 The petrographic description is weak and contains incomprehensible mistakes:

‘Plagioclase is mainly feldspar (….?), plagioclase, semi-automorphic plate, particle size is generally 0.5mm ~ 1.6mm. Alkali feldspar is syenite with xenomorphic granular structures. Zircon, apatite, magnetite are rare, and other minerals are present only in trace amounts (Fig 2). The alterations cannot be observed in the fresh thin sections of the samples.’

There is sphene in the microphotos. What about texture of rocks?

Correct petrographic description is needed!

In my opinion, the chemical composition of minerals (e.g. andesine as the plagioclase phase) may indicate magma mixing rather than partial melting. There are not enough data for a such conclusion, but Figures 9, in particular, 9 (a) do not indicate clearly trends.

5.3 Chapter

There is too many repetitions of sentences talking in different ways about partial melting of the delaminated continental lower crust

Instead of the Pearce (1984) diagrams, I expected rather a graphic model of the geotectonic situation in which adakite magma melted – of course based on the regional geology.

Author Response

We thank the reviewer for considering our work and providing this encouraging assessment. We thank the reviewer for his/her careful reading of our manuscript and providing thoughtful criticism. We believe that these have greatly improved our paper.

Thanks for the patient comments and positive suggestions of the reviewers, we have made changes according to the opinions in details. To facilitate your review of our revisions, the following is a point-by-point response to the comments.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Dear Authors,   thank you for correcting the manuscript according to my comments. After the corrections, the text looks much better. I accept your explanations. Only the sentence (line 89) Alkali feldspar is syenite with xenomorphic granular structures please convert to Alkali feldspar has xenomorphic granular structure. Syenite is after all a type of rock.
  Thank you for your cooperation.          

Author Response

We thank the reviewer for providing this encouraging assessment and his/her careful reading of our manuscript and providing thoughtful criticism. We believe that these have greatly improved our paper.

The manuscript has been improved in accordance with reviewer’s suggestions. We have converted the sentence Alkali feldspar is syenite with xenomorphic granular structures to Alkali feldspar has xenomorphic granular structur in line 81-82. Thanks for the patient comments and positive suggestions of the reviewers.

Author Response File: Author Response.pdf