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ages ranging between c . 500 and 390 Ma (Arenas and Sánchez Martínez 2015). These ophiolites are poorly represented in SW Iberia and are little known, although the Carvalhal Amphibolites (Pedro et al . 2010) and the Porvenir Serpentinites (Díez Fernández et al . 2021) could be included within this group. 2.2. The Mérida Massif Located in the northern sector of the Ossa-Morena Complex, the Mérida Massif forms part of the so-called Obejo-Valsequillo Domain. This massif was initially inves- tigated by Gonzalo (1987), Bandrés (2001) and Bandrés et al . (2004). More recently, Díez Fernández et al . (2022) have presented a new map of the massif that allows three tectonostratigraphic units to be distinguished beneath the Early Cambrian series, which can be distin- guished on the basis of their origin and evolution. From bottom to top, these include (Figures 2a and 2b): (i) a unit of paragneisses and orthogneisses with high- grade metamorphism and local migmatization (Magdalena Gneisses), (ii) a mafic-ultramafic unit inter- preted to be an ophiolite (Mérida Ophiolite; see below), and (iii) a metasedimentary unit intruded by granites, gabbros, and diorites (Upper Schist-Metagranitoid Unit). The contacts between these units show strong deforma- tion accompanied by the generation of mylonitic and ultramylonitic horizons. The contact between the Magdalena Gneisses and the Mérida Ophiolite is inter- preted to be a Cadomian thrust (Figure 2b). The primary contact between the ophiolite and the Upper Schist- Metagranitoid was also a thrust. However, this contact is now a Cadomian extensional detachment with strong omission and a significant metamorphic jump (Figure 2b), which juxtaposes an upper unit affected by greenschist facies metamorphism against the Mérida Ophiolite, affected by amphibolite to granulite facies metamorphism, with local incipient migmatization (Díez Fernández et al . 2022). The significance of the Magdalena Gneisses is uncer- tain and their age unknown. The lack of evidence for high-P metamorphism distinguished it from an equiva- lent Basal Allochthonous Terrane. However, the unit has a clear continental affinity and is considered a section of the Gondwana margin involved in the Ediacaran dynamics (Díez Fernández et al . 2022). The c . 4000 m thick Upper Schist-Metagranitoid Unit contains a section of the Serie Negra Group, which in this region includes metabasite-rich horizons ascribed to the Montemolín Formation (Bandrés 2001). The protoliths of the metase- dimentary series were intruded by several large plutons of granites, granodiorites, tonalites, and gabbros (Figures 2a and 2b), the compositions of which are characteristic of adakitic and calc-alkaline magmas gen- erated in a volcanic arc setting. The ages of the igneous protoliths of these plutons vary between 602 and 541 Ma (U-Pb in zircons; Rojo-Pérez et al . 2022). The age of the oldest plutons coincides with that considered most probable for the base of the Montemolín Formation, indicating that the sedimentation of the sili- ciclastic series and magmatism were coeval in the peri- Gondwanan arc. Based on these data, it has been pro- posed that the deposition of the Serie Negra Group began in a fore-arc basin during a slab roll-back episode (Arenas et al . 2018). A set of field photographs of all the units that are part of the Mérida Massif can be found in Díez Fernández et al . (2022). 3 . Mé rida O phio l ite Within the Ossa-Morena Complex, sequences of mafic and ultramafic rocks cropping out in the Évora Massif were first interpreted as ophiolites and described as the Internal Ossa-Morena Zone Ophiolites (Ribeiro et al . 2010). These rocks core synform structures above the Basal Allochthonous Terrane (Figure 1b). A poorly con- strained age of c. 480 Ma was reported for the mafic rocks (U-Pb dating of zircon; Pedro et al . 2010), suggest- ing that they represent some type of oceanic or transi- tional crust far removed from the Cadomian cycle and linked instead to the opening of the Rheic Ocean (Ribeiro et al . 2010). In the central sector of the Ossa- Morena Complex, another mafic-ultramafic unit that is highly deformed and imbricated with metasedimentary rocks of the Serie Negra Group has been described as the Calzadilla Ophiolite (Figure 1b; Arenas et al . 2018; Díez Fernández et al . 2019). The mafic rocks of this ophiolite have characteristic compositions of boninitic magmas, so the formation of the ophiolite protoliths is considered to be related to the opening of a fore-arc basin in the peri-Gondwanan domain. These mafic rocks have been dated at c . 600 Ma, while the deformation and meta- morphism of the mafic-ultramafic unit, and its imbrica- tion with the Serie Negra Group, apparently took place at c . 540 Ma (U-Pb dating of zircon; Arenas et al . 2018), probably during a late stage of the Cadomian deforma- tion event (see below) that affected the peri-Gondwanan margin and volcanic arc at c . 560–540 Ma (Blatrix and Burg 1981; Dallmeyer and Quesada 1992; Arenas et al . 2018; Díez Fernández et al . 2019, 2022). The mafic-ultramafic unit described as the Mérida Ophiolite is stacked between two terranes of continental affinity. It currently has an approximate maximum thick- ness of c. 3000 m, although it contains several slices that repeat its real thickness (Figures 2b and 2c). Three main slices can be distinguished according to either the INTERNATIONAL GEOLOGY REVIEW 5 Mérida Ophiolite (SW Iberia)