Nova Terra 52

121 O p ening and closure of C adomian p eri - Gond w anan oceans : age and evolution of the Mérida O p hiolite ( S W I beria ) R. Arenas a , E. Rojo-Pérez a , R. Díez Fernández b , R. Albert c,d , I. Novo-Fernández a , S. Sánchez Martínez a , J. M. Fuenlabrada e , P. Andonaegui a , D. Moreno-Martín a , A. Gerdes c,d and A. Garcia-Casco f a Departamento de Mineralogía y Petrología and Instituto de Geociencias (UCM, CSIC), Universidad Complutense. Madrid, Spain; b Instituto Geológico y Minero de España – CSIC. Salamanca, Spain; c Institute of Geosciences, Goethe University Frankfurt, Frankfurt am Main, Germany; d Frankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, Frankfurt am Main, Germany; e Unidad de Geocronología (CAI de Ciencias de la Tierra y Arqueometría), Universidad Complutense. Madrid, Spain; f Departamento de Mineralogía y Petrología and Instituto Andaluz de Ciencias de la Tierra(UGR, CSIC), Universidad de Granada. Granada, Spain ABSTRACT In the SW Iberian Massif, the Ossa-Morena Complex contains a stack of units of different origin and tectonothermal evolution. Individual terranes of the complex record a Cadomian history, tradi- tionally interpreted in relation to the dynamics of a peri-Gondwanan volcanic arc. The interpreta- tion of the Cadomian terranes is a key issue in the reconstruction of the Ediacaran margin of Gondwana in NW Africa. In the Mérida Massif, a mafic – ultramafic unit stacked between two units of continental or arc affinity is interpreted as a Cadomian ophiolite, the Mérida Ophiolite. Within it, mafic lithologies dated at c . 596 Ma (U-Pb in zircon) have calc-alkaline or tholeiitic affinity, indicating generation in a supra-subduction zone setting. The ophiolite shows a main event of Cadomian metamorphism ( c . 590 Ma, U-Pb garnet dating) with a prograde P-T path and peak conditions at c . 700°C and 8 kbar. The mafic rocks show Nd isotopic compositions clearly different from those characteristics of the mafic rocks formed in the axial zone of the volcanic arc. Hence, the Mérida Ophiolite probably represents oceanic lithosphere of a back-arc basin, opened between the volcanic arc and the outer margin of Gondwana. Closure of the back-arc occurred during collision of the volcanic arc with the margin of Gondwana (starting at c . 590 Ma), which caused rapid accretion of the back-arc oceanic lithosphere beneath the arc and prograde metamorphism typical of subduction inception. The Mérida Ophiolite was finally obducted above the continental margin of Gondwana, presently preserved in gneissic unit piled beneath the ophiolite. Cadomian evolu- tion of this peri-Gondwanan arc section continued with new tectonothermal events dated in the range 560–540 Ma that alternated with episodes of intense magmatic activity and preceded the collapse of the magmatic arc and its erosive levelling, followed by the generation of a new episode of intense magmatic activity in the arc and the unconformable deposition of the Ediacaran- Cambrian Malcocinado Formation. ARTICLE HISTORY Received 9 April 2022 Accepted 25 September 2022 KEYWORDS Ophiolites; Ediacaran oceans; peri-Gondwana; Cadomian Orogeny; subduction inception; Iberian massif 1 . Introduction It is difficult to determine details of the dynamic evolu- tion of the northwestern African margin of Gondwana during the Ediacaran and Early Cambrian. The problem arises in part from the participation of this margin in the final events of Pangea assembly, since it is currently part of the Variscan Orogen (Figure 1a; Eguiluz et al . 2000), having generally been highly deformed during the Devonian and Carboniferous. In this context, the oldest geodynamic events are attributed to the Pan-African Orogeny in Africa and Cadomian Orogeny in Europe (D’Lemos et al . 1990,; Quesada 1990,; Liégeois et al . 1994; Von Raumer et al . 2015). The nature of this oro- geny has long been enigmatic, since it has not been possible to clearly recognize its causes and geodynamic setting. That the Cadomian Orogeny is related to the convergence and interaction of large continental masses can be ruled out. Instead, the Ediacaran dynamics of the African margin of Gondwana seem to be related to the building and dismantling of a long-lived volcanic arc along this margin, with the characteristics of subduction and the eventual opening of fore-arc and/or back-arc basins (Pereira et al . 2006; Linnemann et al . 2008, 2014). In order to understand the margin’s Ediacaran evolution, it is therefore necessary to identify the terranes involved and clearly establish their nature and significance. In order to determine the identity and chronology of the participating oceanic domains, it is essential to trace the C ON TACT R. Arenas rarenas@ucm.es Departamento de Mineralogía y Petrología and Instituto de Geociencias (UCM, CSIC), Facultad de Geología, Universidad Complutense de Madrid, José Antonio Novais, no 12, 28040 Madrid, Spain Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2022.2129475 INTERNATIONAL GEOLOGY REVIEW https://doi.org/10.1080/00206814.2022.2129475 © 2022 Informa UK Limited, trading as Taylor & Francis Group