Zircon, Monazite SHRIMP U-Th-Pb and Quartz Oxygen Isotopic Results from the Higher Himalayan Crystallines (HHC) of the Sikkim Himalayas

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review of: Zircon, Monazite SHRIMP U-Th-Pb and Quartz Oxygen Isotopic Results from the Higher Himalayan Crystallines (HHC) of the Sikkim Himalayas, India

This paper presents zircon and monazite dates and stable isotopic compositions from migmatites from the Himalayas. Ultimately, the zircon dates are too old to time Himalayan orogenesis, and the monazite dates align with other time constraints commonly reported from the HHC rocks. I am not sure what is new regarding the contributions, but the paper is well written, and the data add to our understanding of the development of the HHC unit. The authors report two thrust sheets in the HHC; these would be helpful to map in Figure 1. 

Introduction: "The Himalayan mountain chain evolved due to a continent-continent collision of the indenting Indian plate with the Eurasian plate not later than 57 Ma ago [1,2], whereas, based on various popular models of the evolution of the Himalayas [3], interpreted that the onset took place at 59 ± 1 Ma." This is an oversimplification of the collisional processes and numerous researchers who aim to understand the timing and dynamics of collision. It is most definitely not so well constrained to that timeframe. Please see Catlos (2022) for a review of what is known about the timing of collision (https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/9781119773856.ch6)

The introduction section reads like a geological background section. It should identify a problem statement or a hypothesis and highlight the significance of the research and how it contributes to a knowledge gap. Most of the text belongs in the geological background section.

Also, this introduction and geological background sections include details about the geological background of the metamorphic rocks of the Himalayas and their general geology. It might be more helpful to reframe in terms of understanding the partial melts (migmatites) they focus on. It would be good to have a more thorough review of these rocks across the Himalayas than general geology.

Figure 2. Please see Catlos et al. (2022) for a more recent map of the Sikkim region, including various lineaments (sometimes interpreted as strike-slip faults). What are the green dashed lines? Also, this paper provides some new P-T paths that may be interesting for these researchers regarding the tectonic evolution of the region.

Line 358-359: “Zircon from the north Sikkim samples marks the maximum age of deposition of the upper package up to 500 Ma.” I don’t understand how the zircon, dated from a partially melted metamorphic rock, could constrain a sedimentary depositional age.

Figure 6. The bin size and other constraints involved in developing these KDE plots should be provided. Also, the dates for the peaks would be helpful to include. Concordia would also be helpful to see as a supplementary file.

One might consider how the lineaments (increasingly interpreted as faults) present throughout this region and transect their sampling sites could influence their results.

 

 

 

   

 

 

Comments on the Quality of English Language

The paper is well-written and understandable.

Author Response

Reviewer 1: Comments and Suggestions for Authors

 

Thank you very much for taking the time to review this manuscript. Please find the detailed responses to the comments also find changes in the revised document (striking off the content that is not required and changes highlighted in yellow) in the re-submitted files

This paper presents zircon and monazite dates and stable isotopic compositions from migmatites from the Himalayas. Ultimately, the zircon dates are too old to time Himalayan orogenesis, and the monazite dates align with other time constraints commonly reported from the HHC rocks. I am not sure what is new regarding the contributions, but the paper is well written, and the data add to our understanding of the development of the HHC unit. The authors report two thrust sheets in the HHC; these would be helpful to map in Figure 1. 

Introduction: "The Himalayan mountain chain evolved due to a continent-continent collision of the indenting Indian plate with the Eurasian plate not later than 57 Ma ago [1,2], whereas, based on various popular models of the evolution of the Himalayas [3], interpreted that the onset took place at 59 ± 1 Ma." This is an oversimplification of the collisional processes and numerous researchers who aim to understand the timing and dynamics of collision. It is most definitely not so well constrained to that timeframe. Please see Catlos (2022) for a review of what is known about the timing of collision (https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/9781119773856.ch6)

The paper by Catlos (2022) highlights the review, which has also been narrated by many workers and even by Sngh (2020) [15], and the geochronological data from subduction zone magmatism and Eclogite facies rocks of continental crust clearly suggest the listed collisional timings. The manuscript is not for the collisional timing that is why it has not been discussed in detail.

The introduction section reads like a geological background section. It should identify a problem statement or a hypothesis and highlight the significance of the research and how it contributes to a knowledge gap. Most of the text belongs in the geological background section.

Also, this introduction and geological background sections include details about the geological background of the metamorphic rocks of the Himalayas and their general geology. It might be more helpful to reframe in terms of understanding the partial melts (migmatites) they focus on. It would be good to have a more thorough review of these rocks across the Himalayas than general geology.

Kindly see the last paragraph of the introduction (line 106 to 109) where it is clearly highlighting the purpose of the manuscript

Figure 2. Please see Catlos et al. (2022) for a more recent map of the Sikkim region, including various lineaments (sometimes interpreted as strike-slip faults). What are the green dashed lines? Also, this paper provides some new P-T paths that may be interesting for these researchers regarding the tectonic evolution of the region.

The present work is not to undertake mapping, but to look at the relationship between migmatites and in-situ melt generation with collision-related tourmaline-bearing leucogranite in the Higher Himalayan Crystallines (HHC). Figure 2 includes the structural data and also The map incorporates structural data as well as the metamorphic isograd which is appropriate for the manuscript.

Line 358-359: “Zircon from the north Sikkim samples marks the maximum age of deposition of the upper package up to 500 Ma.” I don’t understand how the zircon, dated from a partially melted metamorphic rock, could constrain a sedimentary depositional age.

The data are from the cores representing the protolith ages of the migmatites. It indicates the recycling of the metasediments which are finally converted into migmatites. The ages can constrain the maximum depositional age, which is what has been interpreted in lines 358-359.

Figure 6. The bin size and other constraints involved in developing these KDE plots should be provided. Also, the dates for the peaks would be helpful to include. Concordia would also be helpful to see as a supplementary file.

The bin sizes are different for different figures that have been mentioned in the figure captions and have been highlighted. Data has been listed in Supplementary Data Tables 1, 2, 3, & 4. Concordia diagrams are also included in the Supplementary figure as suggested by reviewers.

One might consider how the lineaments (increasingly interpreted as faults) present throughout this region and transect their sampling sites could influence their results.

We looked at the lineament map of Catlos et al. (2022) and could not establish the interrelationship.

Reviewer 2 Report

Comments and Suggestions for Authors

1. Remove "India" from the title.

2. Line 45: Post-collisional metamorphism is very scarce in the Eurasian continent.

3. Line 61: revise "are" as "is".

4. Line 63: delete "also".

5. The Figure 2 is not consistent with that shown in Fig. 1.

6. Line 164: 5 mm

7. Figure 4: mark the representative minerals on all the pictures.

8. Line 266: zircons were

9. Line 288: monazites were

10. Figure 5: mark all the analyzed spots for U-Pb and O isotopes on the picures. Note that the spots should be 20 μm in diameter as you described in the text.

11. Please explain the abbreviations at their first appearances; such as BSE.

Comments on the Quality of English Language

Minor revisions are needed.

Author Response

Reviewer 2:Comments and Suggestions for Authors

 

Thank you very much for taking the time to review this manuscript. Please find the detailed responses to the comments also find changes in the revised document (striking off the content that is not required and changes highlighted in yellow) in the re-submitted files

 

1. Remove "India" from the title.
2. Line 45: Post-collisional metamorphism is very scarce in the Eurasian continent.
3. Line 61: revise "are" as "is".
4. Line 63: delete "also".
5. The Figure 2 is not consistent with that shown in Fig. 1.
6. Line 164: 5 mm
7. Figure 4: mark the representative minerals on all the pictures.
8. Line 266: zircons were
9. Line 288: monazites were
10. Figure 5: mark all the analyzed spots for U-Pb and O isotopes on the picures. Note that the spots should be 20 μm in diameter as you described in the text.
11. Please explain the abbreviations at their first appearances; such as BSE.

The needful has been done in the revised manuscript  as suggested striking off the content that is not required and changes highlighted in yellow. Also see the revised Figure 4

Reviewer 3 Report

Comments and Suggestions for Authors

 

Paper by Shashank and coauthors  aims to identify sources and equilibrium conditions of migmatites cropping out in the Higher Himalayan Crystalline in Sikkim through zircon, monazite and quartz isotopic ratios. They analysed zircon and monazites from different samples and after describing their inner structures, they dated these minerals through U-Th-Pb methodology.

They also investigated oxygen isotopic composition of zircon, monazite and quartz

 

 

The Author assess that the HHC in Sikkim is a unique coherent package of metamorphic rocks.  I know there is a debate regarding the different positions of the Main Central Thrust in the study area presented in this paper and some discontinuities could be critical. In addition there is also the presence of another inter HHC  discontinuity, dividing the HCC in two different metamorphic units that experienced  different  P-T paths in different time , see for example paper by Chakraborty et al 2019 Tectonophysics. These Authors recognized a ductile top-to-the South shear zone (Chungthang Thambi shear zone) in the HHC  and correlated it with the High Himalayan Discontinuity recognized along the belt (see for example Montomoli et al., 2015 or Larson et al 2015, Wang et al 2018), in Sikkim, dividing the HHC itself in different tectonic slices. I think all these data should be presented in the “Geological framework” paragraph and discussed later in the frame of the acquired data

 

Regarding the study of zircon and monazite one main observation that arises from the presented data is that the Authors try to investigate zoning of monazite through BSE images. It is well known that monazite zoning is very active during different metamorphic stages, including melting, but the zoning should be investigated through chemical maps of some main elements (i.e. Y, Th,…) with a  microprobe. It is nowadays well known that chemical zoning of monazite could be easily related to different stages of rocks history, see for a wider explanation  papers by Cottle or book by Engi et al (2017 Reviews in Mineralogy & Geochemistry)

In the discussion Authors should discuss their age data in the frame of both existing data for the study area and also in a wider regional frame. The discussion as it is is not sufficient from my point of view.

I add some more minor suggestions and some integrations in the references in the annotated text

Hoping this suggestions could help in improving the manuscript

 

 

 

 

 

 

Comments for author File: Comments.pdf

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses to the comments also find changes in the revised document (striking off the content that is not required and changes highlighted in yellow) in the re-submitted files

 

Reviewer 3: Comments and Suggestions for Authors

 

he Author assess that the HHC in Sikkim is a unique coherent package of metamorphic rocks.  I know there is a debate regarding the different positions of the Main Central Thrust in the study area presented in this paper and some discontinuities could be critical. In addition there is also the presence of another inter HHC  discontinuity, dividing the HCC in two different metamorphic units that experienced  different  P-T paths in different time , see for example paper by Chakraborty et al 2019 Tectonophysics. These Authors recognized a ductile top-to-the South shear zone (Chungthang Thambi shear zone) in the HHC  and correlated it with the High Himalayan Discontinuity recognized along the belt (see for example Montomoli et al., 2015 or Larson et al 2015, Wang et al 2018), in Sikkim, dividing the HHC itself in different tectonic slices. I think all these data should be presented in the “Geological framework” paragraph and discussed later in the frame of the acquired data

 Chakraborty et al., 2019 are interpreting the Zema Fault (ZF), which is not seen in the field and they saying that it is not “However, the ZF has not been mapped in the field.” I don't think that is right to ask to put up something that is interpretative and we have not seen that in the field which is why it has not been incorporated in the text.

We have said that “The present work is not to undertake mapping, but to look at the relationship between migmatites and in-situ melt generation with collision-related tourmaline-bearing leucogranite in the Higher Himalayan Crystallines (HHC). Line 124-126”.  We have not incorporated it in our discussion or interpretation have not seen it.

Regarding the study of zircon and monazite, one main observation that arises from the presented data is that the Authors try to investigate zoning of monazite through BSE images. It is well known that monazite zoning is very active during different metamorphic stages, including melting, but the zoning should be investigated through chemical maps of some main elements (i.e. Y, Th,…) with a microprobe. It is nowadays well known that chemical zoning of monazite could be easily related to different stages of rocks history, see for a wider explanation  papers by Cottle or book by Engi et al (2017 Reviews in Mineralogy & Geochemistry)

It is not possible to put up the data asked by the reviewer at this point of time.

In the discussion Authors should discuss their age data in the frame of both existing data for the study area and also in a wider regional frame. The discussion as it is is not sufficient from my point of view.

We have discussed the relevant data which we have generated and interpreted.

I add some more minor suggestions and some integrations in the references in the annotated text

We have incorporated the suggestions which are appropriate and relevant.

 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Basically the Authors did not consider any of my comments

I would like to suggest again to improve the discussion comparing their results in a more regional frame, otherwise the paper is not relevant at all to an international audience. In addition if they are not able to charcaterize the monazite in function of their chemical component through EMPA analyses it is not worth while to take these data in the paper as they could testify mixed ages, or no significant ages , so I suggest to delete monazite results.

 Authors should take into consideration  the previous literature as I already suggested. Please see the paper

Rubatto et al. 2012 Contribution to mineralogy and petrology that discusses in Sikkim  the timescales of crustal melting studying samples not so far from the samples presented in this paperI am sorry to say that in the persent form the paper is not suitable for publication.

Author Response

Reviewer 3: Revised Comments and Suggestions for Authors

Basically, the Authors did not consider any of my comments

I would like to suggest again to improve the discussion by comparing their results in a more regional frame, otherwise, the paper is not relevant at all to an international audience. In addition, if they are not able to charcaterize the monazite in the function of their chemical component through EMPA analyses it is not worthwhile to take these data in the paper as they could testify mixed ages or no significant ages, so I suggest deleting monazite results.

Complex compositional zoning of metamorphic monazite even in the migmatites have been reported by many workers (Fitzsimons et al., 1997; Rubatto et al., 2001; Foter et al., 2002, 2004; Gibson et al., 2004; Kelsey et al., 2008) Gasser et al. (2012) observed two domains in monazite within gneissic zone leading to melt generation having two compositional domains; (i) Th and LREE-rich and Y-poor and Th and M-REE rich and Th and LREE-ppor domains. Although having two distinct domains the U-Pb data have no differences.

As per the literature available, it is important to establish the elemental variation for monazite dating using the electron microprobe (Williams et al., 2011), however, U-Pb ages, ideally by SHRIMP, from two different compositional domains have no difference (Gasser et al., 2012). Similarly, according to Engi et al. (2017) monazite poses fewer problems in isotopic analysis (page 389), the only thing that is required is a successful analytical protocol.

Therefore, we fail to understand why the reviewers want to delete the monazite results generated by SHRIMP.

Metamorphic zircon rim ages have been interpreted as a consequence of partial melting events and looking at similar monazite ages, which have also been assumed to crystallize in the metamorphic rock directly from melt (Herman and Rubatto, 2003), it appears that monazite formed during the similar time as of zircon rims close to peak metamorphic event. Together zircon and monazite ages indicate an episodic growth period at the time of a partial melting event.

Authors should take into consideration the previous literature as I already suggested. Please see the paper. Rubatto et al. 2012 Contribution to mineralogy and petrology that discusses in Sikkim the timescales of crustal melting studying samples not so far from the samples presented in this paper, I am sorry to say that in the present form, the paper is not suitable for publication.

We have included a table and also previous ages in the discussion part.