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location of serpentinized ultramafic rock layers and/or the mapping of mylonites/ultramylonites within the mafic-ultramafic unit. The most complete section of the ophiolite is the one that occupies the intermediate struc- tural position, which can therefore be considered the type section. It is mainly composed of medium to coarse grained metagabbros with moderate deformation, which alternate with sections of pegmatoid gabbros and some horizons of hornblendites. The gabbros acquire a banded character towards the base of the ophiolite, where a mantle section is exposed consisting of a thin layer of peridotite, locally with spinel, strongly deformed, and completely serpentinized. Dikes of fine- grained dolerites intruding the metagabbros and two highly sheared metre-thick horizons of granitic orthog- neisses of tonalitic composition have also been recog- nized within the unit. The metaigneous rocks generally do not preserve primary minerals, which have been replaced by meta- morphic phases. Although deformation is heteroge- neous, the entire ophiolite section shows penetrative foliation and metamorphism that generally reach amphi- bolite facies conditions, although there is a local transi- tion to more extreme conditions with the onset of migmatization. Horizons with the highest intensity of deformation are marked by the development of nema- toblastic amphibolites, which can form centimetric banding. In the vicinity of the Albarregas River, near the city of Mérida, frequent horizons of amphibolites with centimetre-sized garnets occur. These garnet- bearing amphibolites occur at the top of the structural section of the lower slice, near the contact with the intermediate slice. It consists of nematoblastic amphibo- lites with large garnet porphyroblasts up to 2 cm in diameter (Figure 3). These garnets are in general synki- nematic and crystallized during the development of the regional foliation in these metabasites, the only pene- trative ductile fabric that was observed in the ophiolite. Common synkinematic garnets are subidioblastic, but later-developed crystals may be idioblastic. The foliated matrix of the amphibolites is made up of hornblende, plagioclase, clinozoisite, and ilmenite, with chlorite, ser- icite, sphene, and occasional actinolite as retrograde phases of generally limited development (Figure 3). The garnets contain inclusions of the same minerals that constitute the matrix, which also show similar com- positions (see below). It is uncertain whether the devel- opment of these garnet-bearing horizons is favoured by the development of reversed metamorphic gradients under the initial mantle sections of the ophiolite slices, in which case they could be considered a type of meta- morphic sole, analogous to those described in other ophiolites from the Iberian Massif (Díaz García et al . 1999). In any event, they are helpful in characterizing the metamorphic conditions that affected the ophiolite, as we will show in the next section. The mafic-ultramafic section of the Mérida Ophiolite was initially described as forming part of the Upper Schist-Metagranitoid Unit. This section was interpreted as a magmatic arc, of which the mafic-ultramafic section would be the base (Bandrés et al . 2004). The same authors also obtained an age of c . 577 Ma in a garnet- bearing amphibolite, interpreted as the protolith age (U-Pb in zircon) and an age of c . 555 Ma interpreted as the crystallization age of the large garnets (Sm-Nd, two point internal isochrons), which were considered igneous cumulates generated during the crystallization of mafic magmas. In this work, as mentioned before, we consider that the mafic-ultramafic section rather corre- sponds to an ophiolitic unit that does not represent the base of the peri-Gondwanan volcanic arc where the Upper Schist-Metagranitoid Unit was formed. Our inter- pretation is based on the almost exclusively mafic–ultra- mafic composition of the unit, which is almost devoid of the granitic rocks that characterize the arc and whose origin appears to be related to deep melting of material incorporated into the subducting slab (Rojo-Pérez et al . 2022). The isotopic characteristics of the mafic rocks in the ophiolite and those that form part of the Upper Schist-Metagranitois Unit (see below) are also different. The presence of ultramafic layers of mantle origin also clearly indicates the existence of a discontinuity between the Upper Schist-Metagranitoid Unit and the mafic-ultramafic unit that involves the upper mantle. In addition, the characteristic tectonometamorphic fabrics of the ophiolite also suggest a metamorphic origin for the garnets, which would have been formed during the accretion of the ophiolite under the Upper Schist- Metagranitoid Unit (Díez Fernández et al . 2022). In the next section, new U-Pb zircon ages of the metagabbros, the granitic rocks included in the Mérida Ophiolite and the large metamorphic garnets are also presented. 4 . M ethodo l ogy 4.1. Whole-rock geochemistry. Eleven representative well-preserved mafic and felsic rocks from the Mérida Ophiolite were selected for whole-rock chemical analysis. Nine samples correspond to the metagabbros that represent most of the ophiolitic unit, whereas the other two sampled the two small metagranite bodies included in the ophiolite (ME sam- ples in Supplementary Table 1). Major and trace element analyses were obtained at the ActLabs laboratories of Ontario (Canada) (Supplementary Table 1). Crushing and 6 R. ARENAS ET AL. &KDSWHU