Tracking 40 Million Years of Migrating Magmatism across the Idaho Batholith Using Zircon U-Pb Ages and Hf Isotopes from Cretaceous Bentonites

Round 1

Reviewer 1 Report

Dear Authors,

This paper deals with bentonite deposits formed after the transport and alteration of felsic ash of volcanic eruptions, mainly derived from Idaho batholith magmatism (USA) during a near-continuous 40 m.y. magmatism, providing information on the Cretaceous evolution of the Laurentian margin. The Authors used Using LA-ICP-MS, and have analyzed the U-Pb ages and Hf isotopic compositions of nearly 700 zircon grains from forty-nine bentonite beds from different areas. The manuscript presents an interesting topic, which should catch the attention of the readers of Minerals.

As far as I know, the manuscript has not been published previously.

The title is conforming with the contents of the Ms and the approach and results and conclusions intelligible from the abstract alone. The structure of the paper needs to be reinforced in some parts.

In the introduction, beside the regional interest, it is missing an international point of view on the subject, which could be placed at the beginning of the introduction. Possibly, adding some references to similar work elsewhere in the world could help sustaining the arguments of the first lines. In the review process, I missed the focus that should lead the reader to identify the edge between what is known and what is yet to be understood, which is basically what you want to tell in this paper. So, later in the introduction the aim should be explicitly filling this gap in knowledge.

Geological background should be reinforced as suggested.

The Methods and the Results are quite fine but need integrations to be made. Discussion should benefit of a reorganization. Conclusions need to highlight what is new.

Figures are fine but can be improved.

Suggestions for improving technical points have been provided in the attached PDF file with detailed comments that will help preparing the final version of the manuscript.

After random checks if the literature, I found that the reference list corresponds to the cited papers in the text. No excess of self-citations was found.

Notwithstanding these potential limitations, and considering that addressing these would lie beyond the intended scope of the manuscript, I recommend to moderately review the whole MS for making an informative and well-balanced account before publication in Minerals.

Best wishes

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

In the manuscript “Tracking 40 million years of migrating magmatism across the Idaho batholith using zircon U-Pb ages and Hf isotopes from 3 Cretaceous bentonites” Jeffrey S. Hannon and others dated zircons from Creatceous bentonite beds using zircon U-Pb CA-ID-TIMS. These bentonites are supposed to be a product of felsic ashes and were used to reconstruct the provenance and from this the migration of magmatism. Additionally, the authors give Hf isotope ratios of the same zircons and used these data to define periods of plutonic emplacement. Finally, the authors develop a model that presents the changing position of magmatism around the basin from 100 to 72 Ma.

Altogether, I found this study well planned and executed. I think this manuscript is original, new and of interest for many researchers working on the magmatic and tectonic evolution of northern America. The structure of the manuscript suits the contents well and does not need to be changed. The manuscript is well written and concise.

In summary, I have four major concerns about this manuscript.

1. It is not possible to understand the process of interpretation from the raw data to the discussion and conclusions. An intermediate step of data interpretation should be shown to understand, which data have been used to define mean ages. This step should be presented as graphics (possibly in the supplement) to allow the reader to see the data and judge about data quality. Another step that needs to be presented in more detail is the definition of age groups in Figure 4. This step is very important for the discussion and conclusions and should be comprehensible for the reader.
2. The discordant data should not be used to discuss the magmatic evolution of the area of interest. These data are probably affected by Pb loss and the ages are meaningless. To discard discordant data, the authors have to calculate concordance in the supplementary table. To define a range of concordance that they define as “concordant” (typically something like 95-105 %). Afterwards, only concordant data should be used to produce the age distributions of Figure 4.
3. A figure is missing that shows a geological map of the Bighorn Basin with sampling points and a stratigraphic column that also shows the sampling points and how the single samples were summarized. This information is necessary to understand the dataset and to follow the text and figures.
4. The final model (Figure 6) is almost the same as in Hannon et al., (2020), just a bit more detailed. Are these details given by the zircon data? It is necessary to compare both models in the text, name differences and similarities and specify the additional input of the new zircon data.

In the following, I want to give some detailed comments to specific parts of the manuscript.

Abstract

Line 18: pluses should be pulses

Line 22: Add a hyphen in “Mid- to Late Cretaceous”

1 Introduction

Line 38: I am not sure, if the word ge-ochemistry can be divided like this. Please check.

Lines 38-41: If the magma sources have already been identified in another study, you have to explain, why your new zircon data are necessary for further understanding the region. In the moment it sounds like you did it just because it was possible. Define the goal of your study and the gap that exists in knowledge and that you are going to fill.

1.1 Geological Background

Lines 95ff: Obviously, the two shear zones (Salmon River Suture Zone (SRSZ), Western Idaho Shear Zone (WISZ)) are important enough to mention them in the text. Please show their position in a figure. Otherwise, their importance is hard to understand. All geological units, faults, or basins that are discussed in the text should appear in a figure and vice versa.

2 Materials and Methods

Lines 118-120: Please give a stratigraphic column, and show your sampling points in this column and how you summarized the data. For readers that are not used to the area of interest, the formation names are not sufficient to follow the discussion.

Line 125: Wetherill Concordia, not Westerill.

3 Results

Line 180: Half of the grains have concordant ages? This is not visible in Figure 3A. You have to calculate the concordance of the single grain ages and present it in the table with the analytical results. Once you have calculated the concordance, you need to define from these numbers, what means concordant to you (e.g. 95-105 % concordance).

Line 181ff: For all of your samples:  You present weighted mean average ages of groups of data. But the reader can not at all follow the production of these weighted mean averages. A plot for each average age has to be presented (maybe in the supplement) to make your results comprehensible.

I suggest to calculate not weighted mean ages, but Concordia ages. If you calculate the ages from concordant grains only, this should be possible. Concordia ages are more trustworthy than weighted mean ages, because both ages (206-238 and 207-235) are used for their calculation.

Line 185ff: Give all ages with their errors, not just the numbers.

Line 199, 207: Make sure that each zircon you refer to in the text is actually shown in Figure 2. Check this here and at any other text of the manuscript.

4 Discussion

I cannot judge if the statements about provenance of the zircon ages is right or wrong. This is because the definition of the age groups for potential provenance regions seems to be somewhat arbitrary. Another problem is the use of discordant ages for the interpretation and discussion. Please improve this part of the manuscript.

Figures

Figure 1:

Show the exact position and shape of the Bighorn Basin. I think it is not sufficient to present your area of interest as an ellipse only.

What exactly is this fault in the southeast? Please add a name and define what it is. A fault or a suture zone?

Figure 2:

The quality of the CL-images is not very good. Is it possible to increase the contrast of the pictures, to make the internal structures more visible? Some of the zircons seem to have black-CL rims, as you also dicuss in the text. These rims are hardly visible. Please surround the zircons with a lighter color (not black) to make these rims visible.

There is a problem in the figure caption. I was not able to follow your explanations, because some of the zircons you refer to are not there, e.g.  B, grain vii, C, grain vii. Please check and correct the figure caption.

B, grain i (3.3 Ga) is an antecryst? Please check the figure with its caption and with the text in the Results section.

Line 153: B, grains ii vi, and x: a comma is missing after ii

There is some pink color in the CL-images C: what is this?

A stratigraphic column with formation names is absolutely necessary to understand this figure. I assume “Gloverly”, “Mesaverde” and “Mowry” are formations? You did not introduce these names in the text or in a figure.

What about the other formations (from Figure 3, for example). You do not have CL-images here? Please explain.

Figure 4:

The data that can be presented in such a plot have to be concordant (see my comment to the definition of concordance from the data!). All discordant data that are shown in such a plot are meaningless and smear the age distributions.

How did you calculate the age peaks? Use appropriate software and specify in the text or do not give peak ages.

Where do the age groups come from? I understand for the data of Gaschnig (2013), but not the colored bars (Boulder B., Suture, Island Arc, Antler…and so on.) Please specify in the figure caption, where these age ranges are taken from.

Caption: use superscripts for isotopes, here and in the rest of the text and figures.

Show the stratigraphic age of each formation in this plot to illustrate the text.

Supplement:

Column Q is called “error”. I think this should be “rho” (the error correlation!), right?

This table is confusing because of redundant information (e.g. columns E, R, X), missing information (are the errors 1σ or 2σ?, Numbers in columns N and P are not explained), and missing data (where is the concordance?).

Use the suggestions of Horstwood et al. (2016) to present your data: their page 321. Please recognize the footnotes also. I am almost sure that you need to add some information about fractionation correction and so on.

What about the data in Sheet 1 and Sheet 2. Do you want them to be there? Column headings are missing in Sheet 1.

Please reflect on significant digits. For most of your data, you give far too many digits, what makes it difficult to read the table.

Split the data and present them in single excel sheets for single formations. This will help to read the table. In the moment, the reader has to search for the formation that belongs to a single data point.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

This is an interesting data set but there a number of major and minor issues to the point where I think it should be rejected (with encouragement to resubmit after considerable overhaul).  Detailed comments are provided on the text and figures in the attached annotated pdf.  I will only list some key issues here: 

• There are apparently some zircons with older cores but rims that are equivalent to the stratigraphic age of the unit.  These seem like unambiguous cases of actual air-fall ash zircons, but there are also whole zircons that are older than the stratigraphic units.  What is the proof that these are not just detrital zircons mixed in with the ash from overlying or underlying units?  This is a lingering question through much of the paper and there needs to be serious documentation of things like physical textures and sizes of these if these older grains are to be accepted as nearly all being derived from volcanic eruptions.  One option is to just focus on the clear-cut eruption zircons (which provide enough of a story by themselves).  
• There appears to be some major confusion about either the literature or how Hf isotope calculations and interpretations work in the Discussion sections on the Hf.  See my comments on the pdf.  There are missing citations and the literatures ranges given are way off (and its not even clear what "t" these eHf values are calculated to.  A somewhat related issue is the use of the older 176Lu decay constant, which is going to have at least some effect when you are comparing to literature data from the last 20 years.  
• Some issues specific to the data appendix are listed here: 
  • • - There are a number of issues with the number of decimal places and/or significant digits.  Just as an example, U content should have either one or no decimal places and 206/204 should probably be an integer.  
      - Some of the column labels have numbers that I guess are supposed to be linked to foot notes but I do not see any foot notes present.
      - Uncertainties on 176Hf/177Hf and eHf should be at the 2 standard error level, not one
      - Please include a column with the 176Yb/177Hf
      - There are two additional worksheets.  One has no column labels!  Are these supposed to be here?  (Sheet1 and Sheet2)  If they belong, fix!
      - in Row 80 and 152 you've got impossibly high and impossibly low Hf isotope values.  What happened?  That this escaped notice doesn't create confidence in the rest of the dataset

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

You've addressed my other concerns but issues remain with Figure 5c, although I now understand the source of confusion.  The shaded eHf ranges appear to reflect the initial Hf values shown in Figure 7 of Gaschnig et al. (2013; GSAB).  However, the key point is that those values in the older paper are eHf at the times in the Precambrian (and Paleozoic in a few cases) that those zircon cores crystallized.  You cannot then go and translate those ranges to the Cretaceous as you do in Figure 7c because the CHUR framework is in constant motion even if the zircon 176Hf/177Hf barely changes.  For example, the Archean inherited cores of the southern Atlanta lobe have Archean eHf values from about -15 to 0 but if you calculate what these would have been in the Cretaceous, they will give you values that are in the -60s or thereabouts.  The fact that they were -15 to 0 in the Archean is totally separate from where things fall in the Cretaceous.  This will be true to a lesser extent for the NAL/BL.  So this still needs to be fixed.  Honestly, it makes more sense to just compare the ash zircon results in this figure to zircon magmatic rim Hf from the Idaho batholith and other things (and calculate equivalent Hf values from published WR Nd using the terrestrial arrray in cases where Hf is not available, such as for some of the Montana igneous rocks).  Those igneous Hf and Nd values do have some bearing on geographic terranes (as, for example, Nd and Hf tend to be more negative in the southern atlanta lobe than the northern) but they also reflect variable degrees of hybridization between magmas with different Hf isotope compositions.   

Author Response

Response to Reviewer 3 Comments

You've addressed my other concerns but issues remain with Figure 5c, although I now understand the source of confusion.  The shaded eHf ranges appear to reflect the initial Hf values shown in Figure 7 of Gaschnig et al. (2013; GSAB).  However, the key point is that those values in the older paper are eHf at the times in the Precambrian (and Paleozoic in a few cases) that those zircon cores crystallized.  You cannot then go and translate those ranges to the Cretaceous as you do in Figure 7c because the CHUR framework is in constant motion even if the zircon 176Hf/177Hf barely changes.  For example, the Archean inherited cores of the southern Atlanta lobe have Archean eHf values from about -15 to 0 but if you calculate what these would have been in the Cretaceous, they will give you values that are in the -60s or thereabouts.  The fact that they were -15 to 0 in the Archean is totally separate from where things fall in the Cretaceous.  This will be true to a lesser extent for the NAL/BL.  So this still needs to be fixed.  Honestly, it makes more sense to just compare the ash zircon results in this figure to zircon magmatic rim Hf from the Idaho batholith and other things (and calculate equivalent Hf values from published WR Nd using the terrestrial arrray in cases where Hf is not available, such as for some of the Montana igneous rocks).  Those igneous Hf and Nd values do have some bearing on geographic terranes (as, for example, Nd and Hf tend to be more negative in the southern atlanta lobe than the northern) but they also reflect variable degrees of hybridization between magmas with different Hf isotope compositions.   

I see the problem here, thanks for pointing this out. You are correct, the values I used were reflective of the age-corrected Hf values from the Precambrian. I took your suggestion and found magmatic zircon compositions (with ages) from various sources, and include these ranges on fig 5C. This is a much more useful comparison for the ash zircon. I have added a few lines of text to address these changes (554-558, 598-600). Hopefully this seems more useful to the reader, thank you for the suggestion!