Changing Carboniferous Arc Magmatism in the Ossa-Morena Zone (Southwest Iberia): Implications for the Variscan Belt
Round 1
Reviewer 1 Report
The authors presented new geochemical and Nd isotopic and zircon U-Pb geochronological data for magmatic rocks the Ossa-Morena zone (SW Iberia) during the main (Visean-Serpukhovian) and latest (Bashkirian-Moscovian) stages of arc magmatism, in an attempt to constrain the Carboniferous tectonic evolution of this region. The data are of good quality and the work is of interest. However, some aspects of the manuscript must be revised carefully before acceptance is considered.
A major concern of mine is that the possibility of ridge subduction during the Visean-Serpukhovian stages should be evaluated and enhanced. For example, the magmatic associations related to ridge subduction are very complicate (Thorkelson, 1996; Benoit et al., 2002; Thorkelson and Breitsprecher, 2005; Zhang, 2014); are the magmatic associations in the study area comparable to the typical ones produced by the ridge subduction? You can tabulate the facts.
In addition, the ridge subduction generally is a flat-slab subduction, which is the basic mechanism that shapes the Cordillera-/Andean-style orogen. Is there any orogenic record related to the ridge subduction? Specifically, a magmatic gap along the continental margin occurs when the ridge enters the subduction zone (Zhang, 2014). Is such a gap documented in this area? Please address these aspects carefully.
Thorkelson, D.J., 1996, Subduction of diverging plates and the principles of slab window formation: Tectonophysics, v. 255, p. 47–63.
Thorkelson, D.J., and Breitsprecher, K., 2005, Partial melting of slab window margins: Genesis of adakitic and non-adakitic magmas: Lithos, v. 79, p. 25–41
Zhang, K.J., 2014. Genesis of the Late Mesozoic Great Xing’an Range Large Igneous Province: A Mongol–Okhotsk slab window model. International Geology Review, v. 56, p. 1557–1583.
Specific comments
Fig.1: Add a diagram to show the tectonic framework; specifically, indicate the tectonic suture and affinities of different tectonic units in the diagram. In particular, you had better show in the figure the local names that appear in the text so that the international readers can appreciate your work.
Fig.2: Too many field pictures that did not pass on significant information are unnecessary. Please add micrographs for these magmatic rocks.
Fig.5: Add diagrams for primitive mantle-normalized trace elemental patterns. You can put the diagrams for chondrite-normalized rare-earth elemental patterns and primitive mantle-normalized trace elemental patterns as a single figure.
Fig.10: 1) mark the tectonic units in all the maps and sections. 2) the oceanic lithosphere is about 60-70 km thick whereas the continental lithosphere is up to 150 km thick or more; so please revise the oceanic and continental lithospheres. 3) the evolvement of the oceanic ridge in a subduction zone will result in flattening of the oceanic slab; please revise the figure b.
Table S1: major elements, wt. %
Tables S1 and S2: give the source reference for the age of 318 Ma.
Author Response
“A major concern of mine is that the possibility of ridge subduction during the Visean Serpukhovian stages should be evaluated and enhanced. For example, the magmatic associations related to ridge subduction are very complicated (Thorkelson, 1996; Benoit et al., 2002; Thorkelson and Breitsprecher, 2005; Zhang, 2014); is the magmatic associations in the study area compared to the typical ones produced by the ridge subduction? You can tabulate the facts.”
- The characterization of the magmatic associations relate to ridge subduction is complicated and poorly constrained. In general, high thermal gradients and low-pressure melting are the expected conditions. A wide variety of magmas may be generated under such conditions. Depending on the mantle composition, more or less affected by previous metasomatic processes, magmas may have varied compositions. In sum, a specific magmatic association of ridge subduction is not established.
“In addition, the ridge subduction generally is flat-slab subduction, which is the basic
the mechanism that shapes the Cordillera-/Andean-style orogen. Is there any orogenic record related to the ridge subduction? Specifically, a magmatic gap along the continental margin occurs when the ridge enters the subduction zone (Zhang, 2014). Is such a gap documented in this area? Please address these aspects carefully.”
- In accretionary Cordillera-/Andean-style orogens it is much easier to gather information about the temporal and geographical distribution of the different stages of synorogenic magmatism than in collisional orogens. In the case of Iberia, the situation is even more complicated because the Carboniferous have assisted the simultaneous evolution of a collisional orogen and an accretionary orogen. As a result, the temporal and geographical distribution of the different stages of synorogenic magmatism associated with the subduction development of the Paleotethys were tectonically reworked by strike-slip motions that resulted from the progressive collision between Gondwana and Laurussia. This discussion is well documented in a recent paper by Rodriguez, C., Pereira, M.F., Castro, A., Gutiérrez-Alonso, G., Fernández, C. (2021); Variscan intracrustal recycling by melting of Carboniferous arc-like igneous protoliths (Évora Massif, Iberian Variscan belt). GSA Bulletin. Anyway, we decided to touch briefly on this topic in the discussion of the present paper.
“Fig.1: Add a diagram to show the tectonic framework; specifically, indicate the tectonic suture and affinities of different tectonic units in the diagram. In particular, you had a better show in the figure the local names that appear in the text so that the international readers can appreciate your work.”
- Figure 1 has been modified and a sketch showing the location of Iberia on the Appalachian-Variscan belt has been included.
“Fig.2: Too many field pictures that did not pass on significant information are unnecessary. Please add micrographs for these magmatic rocks.”
- Figure 2 has been modified and new photographs of outcrops and microphotographs have been included.
“Fig.5: Add diagrams for primitive mantle-normalized trace elemental patterns. You can put the diagrams for chondrite-normalized rare-earth elemental patterns and primitive mantle normalized trace elemental patterns as a single figure.”
- Figure 5 has been modified and primitive mantle-normalized trace elemental patterns have been included.
“Fig.10: 1) mark the tectonic units in all the maps and sections. 2) the oceanic lithosphere is about 60-70 km thick whereas the continental lithosphere is up to 150 km thick or more; so please revise the oceanic and continental lithospheres. 3) the evolvement of the oceanic ridge in a subduction zone will result in flattening of the oceanic slab; please revise the figure b.”
- Figure 10 has been modified and the reviewer's suggestions were included.
“Table S1: major elements, wt. %”
- Table S1 has been modified.
“Tables S1 and S2: give the source reference for the age of 318 Ma.”
- References have been included in tables S1 and S2.
Reviewer 2 Report
The manuscript of Pereira et al. carried out an integrated study of Carboniferous magmatism in southwestern Iberia and presented comprehensive data on the petrology and zircon U-Pb ages. I think this is a good work presenting important information on the continental crust in the Ossa-Morena zone and on Carboniferous magmatism. I suggest only minor to moderate revisions before publication in Minerals.
- Description of analytical methods is long and it may be better to put it into appendix.
- Line 152-153 – re-write sentence water .. polish
- Line 185 remaining should probably read remainder
- Line 250 “volcano-complex? Rephrase
- Line 275 – if rocks were metasomatised, there should be an attempt to evaluate the effect of metasomatism on chemical composition. Could silica gap be the result of it?
- Line 411 the sentence is convoluted; difficult to follow; rewrite
Author Response
“Description of analytical methods is long and it may be better to put it into an appendix.”
- The text about the analytical methods has been removed and a new file has been created to include as supplementary material.
“Line 152-153 – re-write sentence water .. polish”
- This sentence has been modified.
“Line 185 remaining should probably read remainder”
- This word has been replaced.
“Line 250 “volcano-complex? Rephrase”
- This word has been replaced.
“Line 275 – if rocks were metasomatized, there should be an attempt to evaluate the Effect of metasomatism on chemical composition. Could silica gap be the result of it?”
- Not necessary. Silica gaps may have varied origins. Source metasomatism is not among the likely causes. The silica gap may represent the parental magma from which the liquids and residues fractionate.
“Line 411 the sentence is convoluted; difficult to follow; rewrite”
- This sentence has been modified.
Reviewer 3 Report
This MS deserve to be enlarged to the adjacent Variscan belts, at least in the presentation and in the discussion. See the attached file
Comments for author File: 
Comments.pdf
Author Response
“Title: I suggest adding « implications for the Variscan orogen » to be more attractive for foreign geologists.”
- Title has been modified.
“Abstract: Only one geochronological age est cited: 313+/-3Ma. This abstract is very useful for local geologists but not for foreign readers. Give more information on the Carboniferous environment, at least in Iberia.”
- Abstract has been modified and the reviewer's suggestion was included.
“Keywords: ion adds « paleoTethys margin ».”
- This keyword was added.
“Introduction: The local geology is well exposed with local geological scheme and crosssection. But to locate this Ossa-Morena zone in the adjacent variscan zone is needed. There are many geological schemes devoted to the Variscan system (Nance, Keppie, DiezFernandez, Quesada, Martinez-Catalan, Simancas, Linnemann, etc.) which allow you to include the Ossa-Morena zone within a large Variscan system.”
- Figure 1 has been modified and a sketch showing the location of Iberia on the Appalachian-Variscan belt has been included.
“Discussion: It is very interesting but we have some difficulties locating the Paleotethys concerning the Ossa-Morena zone. A cartoon with the location of the Rheic ocean closure (Upper Devonian), location of paleoTethys (Carboniferous arcs), closure of the PaleoTethys (late Carboniferous ??), and opening of the Téthys, will be more useful to understand the different stages of the Variscan building.”
- Figure 1 has been modified and a sketch showing the location of Iberia on the Appalachian-Variscan belt, Rheic and Paleotethys sutures, Gondwana, and Laurussian continents have been included.
“Conclusion: This conclusion is interesting for the local evolution but does not open the
subject to other variscan belts, even in Iberia.”
- Discussion and Conclusion have been modified to include this suggestion from the reviewer.
“Fig 1.-Well done but a general scheme is missing.”
- Figure 1 has been modified and a sketch showing the location of Iberia on the Appalachian-Variscan belt has been included.
“Fig.3-4 –Well, but then I am wondering if the Evora gneiss plots are useful.”
- Igneous rocks form Évora gneiss dome are represented for comparison in the regional context.
“Fig.10- It is difficult to locate these schemes concerning the Gondwana.”
- Figure 10 has been modified indicating the location of Gondwana and Laurussia.
Round 2
Reviewer 1 Report
The title looks bizarre and can be revised as:
Changing Carboniferous arc magmatism in the Ossa-Morena Zone (Iberian Variscan belt): Implications for the evolution of the Variscan belt
Author Response
Following Reviewer#1 suggestion we have changed the title to: "Changing Carboniferous arc magmatism in the Ossa-Morena Zone (southwest Iberia): Implications for the evolution of the Variscan belt".
Author Response File: 
Author Response.docx
