the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Apr 2025
Revisiting the geodynamic model for the Mako greenstone belt: new evidence for a Paleoproterozoic oceanic plateau in the West African Craton
Abstract. Geochemical and isotopic data from the Paleoproterozoic greenstone belt of Mako (Eastern Senegal) provide evidence for the existence of two tholeiitic and one calc-alkaline series. Tholeiitic series are composed of ultramafic and mafic rocks. Tholeiites 1 are characterized by flat Light Rare Earth Elements (LREE) and Middle Rare Earth Elements (MREE) patterns and by initial isotopic compositions of 87Sr/86Sr (2.1 Ga) from 0.70083 to 0.70368 and εNd (2.1 Ga) from +1.55 to +5.90 (except sample IL49). Tholeiites 2 are characterized by a more enriched LREE and MREE pattern, and by initial isotopic compositions 87Sr/86Sr (2.1 Ga) from 0.7009 to 0.70263 and εNd (2.1 Ga) from +2.14 to +7.35. In addition tholeiites 2 show a slightly negative Nb anomaly, while the tholeiites 1 have no Nb anomaly. Calc-alkaline rocks are characterized by more enriched LREE and MREE patterns and more depleted Heavy Rare Earth Elements (HREE), indicating more pronounced general LREE/HREE fractionation; they have a more pronounced negative Nb and Ti anomalies and initial isotopic signatures of 87Sr/86Sr (2.1 Ga) from 0.7009 to 0.70618 and εNd (2.1 Ga) from +3.12 to +6.40. Comparison of geochemical and isotopic data of the three magmatic series with those of other Paleoproterozoic domains of the West African Craton and juvenile volcanic rocks leads us to propose an evolutive model for the Mako series comprising an initial emplacement of an oceanic plateau, in which the first tholeiitic series was generated, followed by a subduction process that triggers the contamination of the mantle plume source and the generation of the second tholeiitic series at the earliest stages of the arc built; and ending with the maturation of an island arc and the calc-alkaline series emplacement, which has a chemical signature indicating that the source region was likely a metasomatized mantle probably mixed with some plume mantle source.
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Status: closed
- RC1: 'Comment on egusphere-2025-814', Anonymous Referee #1, 10 Jun 2025
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RC2: 'Comment on egusphere-2025-814', Anonymous Referee #2, 03 Jul 2025
Dear Editor and Authors,
I have completed my review of ‘Revisiting the geodynamic model for the Mako greenstone belt: new evidence for a Paleoproterozoic oceanic plateau in the West African Craton’ (MS No.: egusphere-2025-814) by Ibrahima Labou et al.
This paper offers a new set of trace element and radiogenic isotope data from a field area in eastern Senegal that provides further evidence of the tectonic setting that some of the plutonic and volcanic rocks of the southern West Africa Craton (sWAC) formed in. While I think this is a useful contribution to our understanding of the sWACs evolution, the manuscript requires some substantive work before it is suitable to be published.
My main points for consideration by the authors are as follows:
- The introduction requires modification to be clearer about the scope and impact of this study. The basic question is of the tectonic setting for early crustal growth in the Mako (and sWAC more widely?). Need more of an explanation as to why the Mako is a good study area and why your approach will result in improved outcomes for our understanding of the sWAC. Is it the Mako itself that is more complete a sequence / better exposed? Is it the geochemical techniques you are using? Can these be applied to other belts or compared with existing data to evaluate those other models?
- You mention the existing interpretations for 5-6 different tectonic settings elsewhere in the sWAC. This either speaks to highly ambiguous data or significant heterogeneity in the settings recorded by these rocks. Some evaluation of these interpretations is needed in order to make your discussion more robust. Are you providing an alternative proposal for the wider evolution of the WAC or is this more evidence of well documented heterogeneity in tectonic setting during early crustal growth?
- It is not clear that Archean and Proterozoic greenstone belts are equivalent to each other, though the introduction rather implies this. There are some pretty big differences, such as lack of komatiites and BIFs in the Proterozoic. Need to be clearer about this as it speaks to different tectonic / depositional processes operating the Proterozoic. Notably, the list of different tectonic setting interpretations from across the sWAC only includes modern tectonic settings, so we are essentially eliminating the possibility of the more exotic settings in the Archean here. This should be addressed and clarified, perhaps with some comparison to other Proterozoic greenstone settings.
- What age data are available here? Do we know the age of any of the rocks you are analysing or any of the later plutonic rocks that cross cut. Given the diachronous nature of the sWAC, it is important to have some estimate for the age of these rocks when compared to studies conducted in e.g., Ghana.
- The definition of the tholeiitic and calc-alkaline series is very unclear. Both seem to include ultramafic rocks (stated that the latter doesn’t earlier in the intro, which is contradictory). Further to this, it is unclear in the maps how the two geochemically defined tholeiitic series relate to each other in the field. Is this something that could be represented on the maps in some clearer way (see comments below).
- In the eastern KKI, we no longer recognise the SMSZ as a single large fault and so should not be drawn as such on maps – see Allibone 2020 for up to date maps of eastern KKI. In particular, Figure 2 should be updated to include information from more recent maps – see Allibone et al 2020 (https://doi.org/10.5382/SP.23.07 ) .
- Line 203 – you mention variable alteration of the rock – have you made any clear formal assessment of the levels of alteration in your samples? I see no LOI or use of other standard geochemical alteration indices. This is a crucial point as interpretation of the geochemistry relies on it not having been modified since the emplacement of these rocks…. It may also be useful to see more photomicrographs of the rocks to illustrate this, if alteration is present and visible.
Some more minor issues which should be addressed for clarity and improved structure and readability:
- Figure 3 is not very clear as the mix of ‘series’ terminology in the text, ‘massif’ in the caption and then individual lithologies in the map legend is very convoluted. Need to formalise this and be consistent between the figures, captions and text. Could add series to the legend on map, for example.
- In general, the discussion section makes logical sense but for me is too long with several points being overly laboured. It needs reducing for clarity and concision.
- First part of section 5.1 is too long and could be reduced significantly by eliminating repetition and condensing references.
- In line 180 – what are the +/- figures on the standard analyses? Are these %?
- Line 186 to 192 mostly repeats information from the introduction about rock series.
- Figure 4 is hard to read as the coloured symbols do not stand out against the geological map – better to use black symbols of different shapes – squares and triangles?
Kind regards,
Reviewer 2
Citation: https://doi.org/10.5194/egusphere-2025-814-RC2
Status: closed
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RC1: 'Comment on egusphere-2025-814', Anonymous Referee #1, 10 Jun 2025
Dear Editor and Authors,
I have reviewed the manuscript "Revisiting the geodynamic model for the Mako greenstone belt: new evidence for a Paleoproterozoic oceanic plateau in the West African Craton" (MS No.: egusphere-2025-814) by Ibrahima Labou et al.
While the study presents an original concept and contributes valuable geochemical and isotopic data (45 samples of basalts, gabbros, andesites, and dacite), the manuscript is poorly written and highly confusing. A major issue is the inconsistent classification of rocks into two series, making their types and characteristics difficult to discern. All figures should be organized and discussed by rock type (basalts, gabbros, andesites, dacite) for clarity.
Due to the concerns outlined below, I regret that the current version does not meet Solid Earth's publication standards, and I cannot recommend it in its present form. Significant revisions are required to address these issues.
- The authors did not specify the types of rocks studied in the tholeiitic and calc-alkaline series. While they mention Lherzolite, Harzburgite, and Gabbro under the tholeiitic series, only Gabbro is discussed in the manuscript. The use of ultramafic rocks seems incorrect—mafic rocks would be more appropriate. If both basalts (volcanic) and Gabbro (plutonic) were studied, the manuscript should explicitly differentiate between mafic volcanic and plutonic rocks. This ambiguity makes it difficult to understand the rock types analyzed. Although the abstract mentions both volcanic and plutonic rocks, the authors should discuss the characteristics of each rock type separately for clarity.
- Abbreviations should be defined upon first use. For example, "WAC" (West African Craton) appears multiple times but is never spelled out in the introduction.
- Line 36: Paleoproterozoic cratons are less known. The authors should briefly discuss well-known Proterozoic cratons (e.g., North China Craton, Lapland Greenstone Belt) alongside the WAC for clarity.
- Lines 58–67: The proposed geodynamic models of the WAC should be rewritten. The excessive number of citations makes it difficult to identify the scientific gaps or ongoing debates.
- Why revisit the tectonomagmatic evolution of the Mako greenstone belt? What is its age?
- How do Archean greenstone belts differ from Paleoproterozoic ones? A brief comparison should be added to the introduction.
- Research gap and objectives need to be clarified.
- Line 36: Use "evolutionary model," not "evolutive model."
- Line 159: All figures should be organized by rock type as described in the text and discussed consistently throughout the manuscript. This is a major issue—currently, the rocks are split into two series, making it difficult to identify their types and characteristics.
- Line 176: Specify which rocks were selected for isotopic studies.
- Lines 473–475: Do mantle plumes originate from the core-mantle boundary or other depths? For a detailed discussion on this topic, I recommend reviewing Derek Wyman’s 2020 article 'Komatiites From Mantle Transition Zone Plumes' and the 2024 study 'Archean High-Mg Magmas: Their Significance for Continental Growth at Cratonic Margins.' These works comprehensively analyze mantle plume characteristics, including their potential sources in the mantle transition zone rather than the deep core-mantle boundary.
The abstract, discussion, and conclusions may require revisions based on these suggestions. I strongly advise implementing the proposed improvements to strengthen the manuscript's quality and scientific rigor. Please also ensure consistent and precise terminology and abbreviation usage. Lastly, I recommend reducing excessive citations for better readability. I hope my feedback proves useful in enhancing the manuscript.
#Reviewer
Citation: https://doi.org/10.5194/egusphere-2025-814-RC1 -
RC2: 'Comment on egusphere-2025-814', Anonymous Referee #2, 03 Jul 2025
Dear Editor and Authors,
I have completed my review of ‘Revisiting the geodynamic model for the Mako greenstone belt: new evidence for a Paleoproterozoic oceanic plateau in the West African Craton’ (MS No.: egusphere-2025-814) by Ibrahima Labou et al.
This paper offers a new set of trace element and radiogenic isotope data from a field area in eastern Senegal that provides further evidence of the tectonic setting that some of the plutonic and volcanic rocks of the southern West Africa Craton (sWAC) formed in. While I think this is a useful contribution to our understanding of the sWACs evolution, the manuscript requires some substantive work before it is suitable to be published.
My main points for consideration by the authors are as follows:
- The introduction requires modification to be clearer about the scope and impact of this study. The basic question is of the tectonic setting for early crustal growth in the Mako (and sWAC more widely?). Need more of an explanation as to why the Mako is a good study area and why your approach will result in improved outcomes for our understanding of the sWAC. Is it the Mako itself that is more complete a sequence / better exposed? Is it the geochemical techniques you are using? Can these be applied to other belts or compared with existing data to evaluate those other models?
- You mention the existing interpretations for 5-6 different tectonic settings elsewhere in the sWAC. This either speaks to highly ambiguous data or significant heterogeneity in the settings recorded by these rocks. Some evaluation of these interpretations is needed in order to make your discussion more robust. Are you providing an alternative proposal for the wider evolution of the WAC or is this more evidence of well documented heterogeneity in tectonic setting during early crustal growth?
- It is not clear that Archean and Proterozoic greenstone belts are equivalent to each other, though the introduction rather implies this. There are some pretty big differences, such as lack of komatiites and BIFs in the Proterozoic. Need to be clearer about this as it speaks to different tectonic / depositional processes operating the Proterozoic. Notably, the list of different tectonic setting interpretations from across the sWAC only includes modern tectonic settings, so we are essentially eliminating the possibility of the more exotic settings in the Archean here. This should be addressed and clarified, perhaps with some comparison to other Proterozoic greenstone settings.
- What age data are available here? Do we know the age of any of the rocks you are analysing or any of the later plutonic rocks that cross cut. Given the diachronous nature of the sWAC, it is important to have some estimate for the age of these rocks when compared to studies conducted in e.g., Ghana.
- The definition of the tholeiitic and calc-alkaline series is very unclear. Both seem to include ultramafic rocks (stated that the latter doesn’t earlier in the intro, which is contradictory). Further to this, it is unclear in the maps how the two geochemically defined tholeiitic series relate to each other in the field. Is this something that could be represented on the maps in some clearer way (see comments below).
- In the eastern KKI, we no longer recognise the SMSZ as a single large fault and so should not be drawn as such on maps – see Allibone 2020 for up to date maps of eastern KKI. In particular, Figure 2 should be updated to include information from more recent maps – see Allibone et al 2020 (https://doi.org/10.5382/SP.23.07 ) .
- Line 203 – you mention variable alteration of the rock – have you made any clear formal assessment of the levels of alteration in your samples? I see no LOI or use of other standard geochemical alteration indices. This is a crucial point as interpretation of the geochemistry relies on it not having been modified since the emplacement of these rocks…. It may also be useful to see more photomicrographs of the rocks to illustrate this, if alteration is present and visible.
Some more minor issues which should be addressed for clarity and improved structure and readability:
- Figure 3 is not very clear as the mix of ‘series’ terminology in the text, ‘massif’ in the caption and then individual lithologies in the map legend is very convoluted. Need to formalise this and be consistent between the figures, captions and text. Could add series to the legend on map, for example.
- In general, the discussion section makes logical sense but for me is too long with several points being overly laboured. It needs reducing for clarity and concision.
- First part of section 5.1 is too long and could be reduced significantly by eliminating repetition and condensing references.
- In line 180 – what are the +/- figures on the standard analyses? Are these %?
- Line 186 to 192 mostly repeats information from the introduction about rock series.
- Figure 4 is hard to read as the coloured symbols do not stand out against the geological map – better to use black symbols of different shapes – squares and triangles?
Kind regards,
Reviewer 2
Citation: https://doi.org/10.5194/egusphere-2025-814-RC2
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Dear Editor and Authors,
I have reviewed the manuscript "Revisiting the geodynamic model for the Mako greenstone belt: new evidence for a Paleoproterozoic oceanic plateau in the West African Craton" (MS No.: egusphere-2025-814) by Ibrahima Labou et al.
While the study presents an original concept and contributes valuable geochemical and isotopic data (45 samples of basalts, gabbros, andesites, and dacite), the manuscript is poorly written and highly confusing. A major issue is the inconsistent classification of rocks into two series, making their types and characteristics difficult to discern. All figures should be organized and discussed by rock type (basalts, gabbros, andesites, dacite) for clarity.
Due to the concerns outlined below, I regret that the current version does not meet Solid Earth's publication standards, and I cannot recommend it in its present form. Significant revisions are required to address these issues.
The abstract, discussion, and conclusions may require revisions based on these suggestions. I strongly advise implementing the proposed improvements to strengthen the manuscript's quality and scientific rigor. Please also ensure consistent and precise terminology and abbreviation usage. Lastly, I recommend reducing excessive citations for better readability. I hope my feedback proves useful in enhancing the manuscript.
#Reviewer