Comments by reviewer Stefan Luth

Title: Insights into the tectonic evolution of the Svecofennian orogeny based on in situ Lu-Hf dating of garnet from Olkiluoto, SW Finland

Author(s): Jon Engström et al.

MS No.: egusphere-2024-2034

MS type: Research article

General comments

As the title clearly reflects, the manuscript addresses a relevant question on the tectonic evolution of the Svecofennian orogeny by applying a relatively novel approach: In situ dating of garnet and apatite by the Lu-Hf isotope system.  In that way, I consider the manuscript a classic geological approach by using data from the micro/atom-scale to resolve the bigger picture of tectonics.

Relatively little is known about the Svecofennian orogeny (c. 2.0-1.8 Ga) in comparison to the other younger orogens dominating Europe, such as the Alps, Pyrenees, Variscan and Calededonides. To solve the complicated puzzle of this much older and metal-rich orogen, dating in combination with structural geology as presented in the concerned manuscript, is indeed very needed. Today, by far most of the published geochronological data from Fennoscandia comes from U/Pb on zircons, providing mainly the age of crystallization. Dating metamorphism and deformation within a minor error bar, however, is only possible recently and can be performed in only a handful of laboratories. In that perspective, the paper is useful for a large audience as it showcases a good example on how in situ dating of garnet can be applied to resolve for regional to crustal-scale tectonics.

In terms of locality, the authors have chosen their study area very right. Complicated but well mapped geology and directly relevant to society. The Olkiluoto study area has been highly investigated for the construction of a nuclear waste repository. The area represent the southwestern tip of Finland, which makes the correlation with the Swedish geology - mostly masked by the Baltic Sea - very appropriate.  

The manuscript presents substantial conclusions which are in line with their own contribution and credited related work.  The conclusions provide a solid base for future investigations, such as pinpointing study areas on the Swedish side and better constrain the evolution of the abundant crustal-scale shear zones.

As a layman on garnet dating, I still consider the applied scientific methods and assumptions clearly outlined. Only some technical terms may be explained a little more (see technical comments).

In combination with the provided supplementary data (shooting points and tables) the results are traceable for any fellow scientist.  

When it comes to the title and abstract, I do miss the word “apatite”. It is only in the introduction that I read for the first time that the authors applied Lu-Hf dating on both garnet and apatite. Is the latter less important? Please also elaborate throughout the manuscript on explaining the difference and the added value of dating both mineral types, in particular in relation to the cooling path/rate.

In general, the manuscript is well structured, well written and organized (only some technical comments).

Specific comments and technical corrections

See attached pdf file.

Dear Editor and authors,

I appreciate the opportunity to review this manuscript, which demonstrates the application of the novel in-situ Lu-Hf dating technique of garnet and apatite in conjunction with U-Pb apatite geochronology to unravel the tectono-metamorphic evolution of the Olkiluoto site in southwestern Finland.

The authors mention only at the beginning of the manuscript that the findings of this study have the important goal to characterise the geological site which is planned to be used for the construction of a nuclear waste repository. However, it is not clear how such a site could be planned in a terrain of deformed migmatitic rocks and how the obtained findings can help characterising the site in light of this application.

Below are major comments that I believe will assist the authors in enhancing their manuscript. Additional comments, questions and notes are included in the attached PDFs (main text and supplementary figures).

Introduction:

The introduction could be made more relevant and focused by addressing the integration of in-situ geochronology techniques for the key petrochronometers (garnet and apatite).

I recommend reshaping the introduction to expand on the new petrochronological tools employed to analyse the complex (potentially polymetamorphic) Precambrian terrains. This could involve incorporating part of the third paragraph that introduces the in-situ Lu-Hf dating technique for garnet, along with expanding on recent studies where this method was applied to ancient rocks, possibly together with other dating techniques, including Lu-Hf and U-Pb geochronology of apatite.

The discussion of apatite as a geochronometer is only introduced after line 49. Why is that? I believe that the integration of more petrochronometers is a powerful tool, and the authors should highlight this. Both in-situ Lu-Hf dating of garnet and apatite were recently developed (Simpson et al., 2021), and their integration for investigating complex metamorphic histories, along with other systematics, should be highlighted in the title, abstract, and detailed further in the Introduction.

The tectono-metamorphic evolution of the Olkiluoto site could then be outlined in a subsequent paragraph. This can be streamlined by incorporating relevant information into the geological setting section.

Upon reviewing the manuscript, I noted the use of U-Pb dating of apatite and trace element analysis. These combined geochronological methods should be central to the Introduction, as they are crucial for unravelling the polymetamorphism of the studied site.

Geological setting of the study area:

2.1 I find this section partly repetitive, between the first two paragraphs. Those can be shortened and combined such that the reader can easily follow the already published geological overview of the region. I think that this section would benefit from combining the tectonic and the metamorphic events for the Svecofennian orogeny in one paragraph.

2.2 This section can benefit from being more concise and informative. Much of the information is vague and confusingly phrased.

Methods of the study:

This section should be titled “Methodology” or “Petrochronology Approach”, as it includes the sample descriptions. Alternatively, the investigated samples could be described before Section 3 which can then focus on the analytical methods employed.

Sample description: It would benefit from including a description of the relationships between the compositional layering and the leucosomes, helping the reader connect hand-sample information to the micro-textures. This is crucial because garnet crystals within the matrix and leucosomes were dated, and most interpretations rely on this data. The authors should refer to Fig. 4 in the text.

EPMA: The minerals analyzed for this study should be specified in this section. A table presenting representative mineral chemistry analysis should be included in the main body of the manuscript for better access to this information, which is relevant to this petrochemical and petrochronological study.

 In-situ Lu-Hf geochronology of garnet and apatite: Methodologies for calculating uncertainties should be incorporated in the main text, and all dates obtained for the reference materials must be reported.

Method – Bulk-rock geochemistry: This subsection is missing from the methods section.

Texture-geochronology-REE-PT modelling: U-Pb geochronology and trace element data of apatite were collected along with in-situ Lu-Hf dating of garnet and apatite. However, this method is not mentioned in the text until line 207. The authors should clarify why both Lu-Hf and U-Pb geochronology were employed to date apatite. This should also be reflected in the abstract and Introduction. More textural information about the analyzed apatite grains should be provided in the manuscript, accompanied by BSE images of the grains (at least in the supplementary materials), the number of grains analyzed, and the sample IDs from which the apatite grains originated. Apatite was described only as inclusions in the rim of a large garnet crystal (sample MM31), whereas the P-T modeling was conducted on sample MM30A, which does not describe any apatite crystals nor report them in this manuscript (Fig. 5B). I want to emphasize that the scattered analysis, observations, and descriptions between the two different samples (MM30A and MM31) are continuously mixed throughout the manuscript, both in the results and in the discussion sections, leading to interpretations that lack robust data support. Meso-scale and microscale textural observations of garnet textures and associated mineralogy appear different between the two samples; therefore, I find it hard to use the information from the two samples in an interchangeable way. Finally, whole rock geochemistry is presented for only one sample, which limits the geochemical comparison between the two.

Small garnet crystals are reported within the matrix, defining compositional layering in the studied TGG (sample MM30A), while a larger garnet crystal is enclosed within felsic leucosomes in sample MM31. It is crucial for the authors to better describe and report the relationships between the leucosomes and the dominant foliation, considering the interpretations are based on garnet data obtained from these two features. These two types of garnet crystals also exhibit different isotopic and REE analyses, indicating at least different ages, Lu content, and textural contexts. Therefore, while the uncertainties for Lu-Hf dates of garnet crystals do not allow for confident interpretations, data should not be consolidated into a single, individual age derived from different garnet crystals and samples, especially given their distinct textural contexts.

Discussion: Most of my comments on this section are included in the attached PDF. I find this section poorly structured; the textural and petrochronological observations, along with P-T modeling of the two samples, are merged and used for speculative interpretations unsupported by the data. Many paragraphs contain lists of metamorphic ages recorded elsewhere within the extensive 40,000 square kilometre region shown in Fig. 11, and where their relationships with the studied site are unclear.

Overall, I encourage the authors to be more specific and less vague throughout the text to improve clarity and readability. Samples are often referred to without their sample IDs, merely identified as "rock." I also advise employing a labelling system for the analyzed garnet crystals, such as "garnet 1" (the large crystal in sample MM31) and "garnet 2," "garnet 3," and "garnet 4" for the smaller garnet crystals in sample MM30. This labelling scheme will help the reader easily track which analyses correspond to which sample and grain.

Supplementary material: The supplementary material should be labelled as Table S1, Table S2, Table S3, etc., to clearly indicate to which Supplementary Table the authors are referring. I recommend removing the "Index" label in the Supplementary Data Table and instead labelling each table as S1, S2, etc. Each table should have a caption that describes its content.

I invite the authors to carefully revise the manuscript, particularly regarding interpretations based on merged dates obtained from different samples and petrochronometers with varying textural contexts.

For comments on figures, please refer to the attached technical notes in the PDF