Nova Terra 52

in the section of the margin located between the mag- matic arc and the mainland. These series are now found throughout the Iberian Autochthonous Domain, from the Central Iberian Zone to the Cantabrian Zone. However, these series have different stratigraphic char- acteristics from those of the Serie Negra Group (Fuenlabrada et al . 2016; Rojo-Pérez et al . 2019). Around c . 596 Ma, prolonged extension affecting the Gondwanan margin favoured the opening of a back-arc basin of unknown size, although considering the current dimensions of the Ossa-Morena section, neither this basin nor the previous fore-arc basin could have been very broad (Figure 16b). This back-arc basin has been identified as the tectonic setting for the generation of the mafic protoliths of the Mérida Ophiolite (Díez Fernández et al . 2022; this work), which have a characteristic SSZ composition. The isotopic sources of the mafic rocks generated in this back-arc (whole-rock Sm-Nd, Hf in zircons) differ greatly from those of the mafic rocks generated in the axial zone of the arc and have a marked juvenile character. For this reason, as well as its differentiated nature from the arc itself, the open- ing of a back-arc basin is considered a necessary prere- quisite. During this period, four different terranes formed the Gondwana margin based on their tectonothermal origin and evolution (fore-arc, arc, back-arc, and conti- nental margin), and at least two different lithospheric plates were involved. Back-arc basin spreading then ter- minated and the Gondwana margin switched to a contraction-dominated stage that culminated with the rapid collision of the arc and the outer margin. The lithosphere of the back-arc basin was accreted under the arc at c . 590 Ma (age of the main metamorphism in garnet amphibolites of the Mérida Ophiolite), reaching pressures of c . 8 Kbar (c. 25–30 Km). The lower unit of the Mérida Massif represents a crustal fragment of continen- tal affinity that has been correlated with the outer mar- gin of Gondwana (Magdalena Gneisses; Díez Fernández et al . 2022). The accretion of this section of the continen- tal margin under the imbricated ophiolitic slices prob- ably induced the onset of exhumation of the Mérida Ophiolite (Figure 16c). The arc-continent collision appar- ently progressed towards the trench until c . 540, favour- ing the closure of the fore-arc basin and the final obduction of the Calzadilla Ophiolite above the arc (Arenas et al . 2018; Díez Fernández et al . 2019, 2022). However, the characteristics of the metamorphic evolu- tion of the Magdalena Gneisses are not known, so the extent of burial of the Gondwana margin under the ophiolite and arc remains undetermined. The next event recorded in this peri-Gondwanan domain during the Ediacaran–Early Cambrian transition, involves the collapse and dissection of the magmatic arc, which is first recorded by the basal unconformity of the Malcocinado Formation (Figure 16d). However, this was not the end of activity within the arc, since the Malcocinado Formation contains not only siliciclastic sedimentary rocks but also abundant volcanic and plu- tonic rocks of calc-alkaline affinity, with predominance of andesitic lavas (Sarrionandia et al . 2020). The next epi- sode in the evolution of the arc was a new erosive cycle, which culminated in the deposition, again markedly dis- cordant, of the Torreárboles Formation during the Early Cambrian. Hence, a succession of constructive and destructive events, which are characteristics of large volcanic arc systems, can also be identified in this peri- Gondwanan arc. 10 . Conc l usions The Mérida Massif (SW Iberia) contains a stack of units with different origin and tectonothermal evolution. Occupying an intermediate structural position between two peri-Gondwanan units of continental or magmatic arc affinity and separated from them by tectonic contacts exists a mafic-ultramafic unit that is interpreted as an ophiolite, the Mérida Ophiolite. Its mafic protoliths have composition characteristics of calc-alkaline and tholeiitic rocks generated in a supra-subduction zone setting and have been dated at c . 596 Ma. The mafic unit is intruded by scarce and small tonalite bodies, dated at c . 566 Ma. Nd isotopic geochemical data for the mafic rocks from the upper unit (arc-derived) and the ophiolite suggest different isotopic sources (TDM = 1316–2110 vs. 548–960, respectively), indicating that they were sourced from mantle wedges of different nature. Accordingly, we con- sider the Mérida Ophiolite protoliths to have formed during the opening of a back-arc basin that separated a continental volcanic arc from the Gondwana margin. The ophiolite records a major tectonothermal episode dated at c . 590 Ma (U-Pb in garnet). The metamorphic characteristics of this event (c. 700°C and 8 kbar) suggest that it developed during rapid accretion of the back-arc lithosphere beneath the arc. Collision of the peri- Gondwanan margin against the arc caused early exhuma- tion and emplacement of the ophiolite above the outer section of Gondwana. The recognition of the Mérida Ophiolite as an Ediacaran ophiolite adds to the previous identification of the Calzadilla Ophiolite ( c . 600 Ma) and allows knowledge of the different terranes involved in Cadomian geodynamics to be further advanced. The investigation of these Ediacaran terranes is key to the reconstruction of the Gondwana margin during this per- iod and the characterization of their later participation in Variscan geodynamics. 26 R. ARENAS ET AL. &KDSWHU