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(Figure 7; Pearce et al . 1984). These types of magmas are uncommon in back-arc settings since few magmatic bodies derived from the axial zone of the arc reach that region. They are even more unusual in fore-arc sections, away from the subducting slab and mantle wedge regions where these magmas are generated (Castro 2014; Castro et al . 2021; Rojo-Pérez et al . 2022). Using analogous arguments, it is also very unlikely that the mafic unit represents the base of the volcanic arc, where it would be extensively intruded by granitic mag- mas. Rojo-Pérez et al . (2022) have shown that the igneous rocks of the upper unit of the Mérida Massif (Upper Schist-Metagranitoid Unit) were formed in the axial zone of a magmatic arc by partial melting of ascending melange diapirs (see also Castro 2014; Castro et al . 2021). The development of these diapirs involves mixtures of mafic and metasedimentary com- ponents, with the participation of an infra-arc mantle wedge extensively modified by fluid infiltration from the slab. Given the origin and distribution of this mag- matism in the axial zone of the volcanic arc, it is extre- mely unlikely that the intermediate mafic unit of the Mérida Massif was formed in that region of the arc. Hence, it is unlikely that this mafic unit represents the base of the magmatic arc that constitutes the upper unit of the Mérida Massif (Upper Schist-Metagranitoid Unit). However, the rare presence of small massifs of granitic rocks intruding the mafic unit could support its origin in a back-arc basin. In favour of this dynamic setting is the position occupied by the mafic unit and the Mérida Massif itself, far from the SW boundary of the Ossa- Morena Complex and closer to the Iberian Autochthonous Domain represented by the Central Iberian Zone, which in Ediacaran times formed a wide peri-Gondwanan back-arc section (Rodríguez-Alonso et al . 2004; Villaseca et al . 2014; Fuenlabrada et al . 2016, 2020). 9.3. Interpretation of the ultramafic rocks in the ophiolite Ultramafic rocks are scarce in the Mérida Ophiolite, and they are systematically highly sheared and serpenti- nized. A thin layer of serpentinites was identified at the base of the upper slice (Figure 2). The presence of these metaperidotites confirms the imbricate nature of the ophiolite, but it also implies the development of at least one suture zone with a root that penetrated into the mantle. This is an unlikely structure in the stacking of the base of a magmatic arc, but it is much more compa- tible with the closure of a back-arc domain. The devel- opment of more or less extensive peri-Gondwanan back- arc basins indicates that at least some sections of the peri-continental volcanic arc system probably formed part, at least temporarily, of a different lithospheric plate from the one that includes continental Gondwana. This peri-Gondwanan tectonic scenario did not terminate in the Ediacaran but continued into Cambrian, giving rise to new back-arc domains. Palaeozoic extension in some of these domains led to the formation of oceanic or transitional crusts (SSZ ophiolites), which were accreted during Variscan con- traction in the late Palaeozoic (Arenas et al . 2007, 2021b; Arenas and Sánchez Martínez 2015; Sánchez Martínez et al . 2021). Back-arc tectonics within the peri- Gondwanan arc system may also be linked to the rifting and drifting of larger terranes during the Palaeozoic, such as Avalonia, and the opening of larger oceanic basins, such as the Rheic Ocean (Nance et al . 2010). 9.4. Sm-Nd and Hf isotopic sources The whole-rock Nd isotopic data provide valuable infor- mation for defining the tectonic setting in which the protoliths of the Mérida Ophiolite originated, since these data can be compared with those obtained from the lithologies of the Upper Schist-Metagranitoid Unit (Rojo-Pérez et al . 2022). According to these data, the isotopic sources of the mafic rocks of the ophiolite are very different from those of the metabasites of similar age in the upper unit (Figure 8). Hence, it is not possible that both groups of mafic rocks were generated by partial melting of the same mantle wedge during the Ediacaran. The mafic rocks of the upper unit have iso- topic sources much older than the age of the protoliths ( c . 600 Ma vs. 1316–2110 Ma; Figure 8), suggesting that they formed from the partial melting of a mantle wedge infiltrated by fluids from metasedimentary rocks dragged down by the slab, since these sources isotopi- cally match those recorded by the siliciclastic metasedi- ments of the Serie Negra Group (Rojo-Pérez et al . 2019, 2021, 2022). The mafic rocks of the ophiolite have TDM ages very close to the age of the protoliths, almost corresponding to the DM of that age (Supplementary Table 2). This suggests that they were formed by partial melting of an unmodified mantle domain, away from the subducting slab. These data reinforce the interpretation that the mafic rocks of the ophiolite were not generated in the axial region of the peri-Gondwanan volcanic arc, but during the opening of a back-arc basin. The isotopic sources of the small bodies of metatonalites in the ophiolite have Nd isotopic sources comparable to those of the granitic rocks of the Upper Schist- Metagranitoid Unit. This supports the interpretation 24 R. ARENAS ET AL. &KDSWHU