serie NOVA TERRA nº 49

40 et al., 2012 ). The stratigraphic record of the upper units of NW Iberia is not well known, although U – Pb and Lu – Hf isotopic ratios obtained from detrital zircon grains indicate a distinct Gondwanan provenance in the periphery of the WAC ( Albert et al., 2015 ). This origin, together with their lithological constitution and pre-Variscan evolution, indicate that the upper units represent a section of a peri-Gondwanan magmatic arc ( Abati et al., 1999 ). The tectonic fabrics and the tectonothermal events related to the evolution of the arc are dominant features in the higher structural level of the upper units, which shows less penetrative Variscan deformation and metamorphism generally concentrated in discrete shear zones ( Abati et al., 1999, 2007; Gómez-Barreiro et al., 2006; González Cuadra, 2007 ). In contrast, the lower part of the upper units is affected by an eo-Variscan high-P and high-T (HP-HT) meta- morphic event, dated at c. 400 – 390 Ma ( Ordóñez Casado et al., 2001; Fernández-Suárez et al., 2007 ). This high-P event, probably the fi rst and oldest tectonothermal event related to the Variscan cycle, occurred in response to the initial collision between Gondwana and Laurussia ( Arenas et al., 2014b ). The ophiolites of NW Iberia have received considerable attention in recent years, when numerous papers have described in detail their li- thologies, chemical composition and isotopic geochronology. It is now well-established that their igneous protoliths have a wide age range, what hinders their assignment to a single oceanic domain. Their protoliths do not represent common MORB types, their origin being usually associated with a supra-subduction setting. Two groups of ophiolites have been described ( Arenas and Sánchez Martínez, 2015 ): an older group (lower ophiolitic units) that contains metaigneous rocks of Middle-Late Cambrian age (c. 500 – 495 Ma), and a younger group (upper ophiolitic units) that includes ultrama fi c rocks and Devonian gabbros (Emsian-Eifelian; c. 395 Ma; Díaz García et al., 1999; Pin et al., 2002 ). The lower ophiolites are interpreted to represent the opening of a back-arc basin in the periphery of Gondwana (Vila de Cruces Ophiolite; Arenas et al., 2007 ), or remnants of a Cambrian peri- Gondwanan ocean tract (Iapetus-Törnquist) accreted to the base of a magmatic arc (Bazar Ophiolite; Sánchez Martínez et al., 2012 ). Recent isotopic data (Lu – Hf in zircon, whole rock geochemistry and Sm – Nd systematics) indicate that the protoliths of the upper ophiolites (Careón, Purrido and Moeche ophiolites) interacted with isotopic sources of con- tinental nature ( Sánchez Martínez et al., 2011; Arenas et al., 2014a ). Consequently, their origin is hard to reconcile with wide oceanic do- mains. The origin of these ophiolites is a key aspect to understand the fi rst tectonothermal events related to the assembly of Pangea. They have been recently interpreted as generated during the opening of an ephemeral pull-apart basin. The inception of this basin occurred in the way of two events of dextral convergence between Gondwana and Laurussia. These two collisional episodes led to different high-P meta- morphic events that affected two different sections of the margin of Gondwana at c. 400 Ma (upper units) and c. 370 Ma (basal units) ( Arenas et al., 2014b ). 3. The SW section of the Iberian Massif: The Ossa-Morena Zone The SW of the Iberian Massif has been classically divided in two different zones separated by an ophiolite dated at c. 340 – 332 Ma (Beja-Acebuches Ophiolite; Castro et al., 1999; Díaz-Azpiroz et al., 2006; Azor et al., 2008 ), the South-Portuguese Zone and the Ossa- Morena Zone ( Fig. 1 ). The South-Portuguese Zone is correlated along the Variscan Orogen with the Rhenohercynian Zone, considered part of Avalonia and hence a section of the southern margin of Laurussia ( Franke, 2000; Matte, 2001; Murphy et al., 2011; Linnemann et al., 2012 ). The Ossa-Morena Zone is part of the peri-Gondwanan realm ( López-Guijarro et al., 2008; Pereira et al., 2012 ), and has been usually considered an autochthonous domain of the Variscan Orogen. New geo- logical data and correlations indicate that rather it should be considered a composite ensemble formed by several different terranes with distinct origin and tectonothermal evolution. In this view most of the previous Ossa-Morena Zone can be interpreted as an allochthonous complex of the Iberian Massif, the Ossa-Morena Complex ( Díez Fernández and Arenas, 2015 ). The arguments for this interpretation are similar to those already used in other parts of the orogen, such as in the NW of the IberianMassif: the presence of ophiolitic units located on top of con- tinental terranes bearing the imprint of high-P metamorphism and thrust on top of continental units not affected by high-P events. This new interpretation of the geology of the Ossa-Morena Zone presented by Díez Fernández and Arenas (2015) , has been the subject of a recent and detailed discussion ( Simancas et al., 2016; Díez Fernández and Arenas, 2016 ). The reader may fi nd in those papers additional informa- tion regarding the structure and tectonothermal evolution of the units making the Ossa-Morena Complex. In some sectors of the SW Iberian Massif rocks located below high-P units lack evidence of high-P meta- morphism and, consequently, should be considered autochthonous do- mains in relation to the high-P units ( Fig. 1 ). This SW Iberia autochthon probably represents the continuation of the Central Iberian Zone below the Ossa-Morena allochthonous complex. The Ossa-Morena Zone contains an uppermost pre-orogenic pile of metasedimentary and metaigneous rocks with ages ranging from Ediacaran to Devonian. Variscan and eo-Variscan deformation shows var- iable intensity and tectonic polarity ( Simancas et al., 2001 ). This zone also records previous tectonothermal and igneous activity typically referred to as Cadomian, and probably linked to the dynamics of a peri- Gondwanan arc-system that was active during Neoproterozoic and Cam- brian times ( Linnemann et al., 2008 ). The Ossa-Morena Zone includes two HP-LIT metamorphic units, the Central Unit (Blastomylonitic Corri- dor or Coimbra-Córdoba Shear Zone) to the North ( Azor et al., 1994 ) and the Cubito-Moura Unit to the South ( Araújo et al., 2005; Booth-Rea et al., 2006; Rubio Pascual et al., 2013 ) ( Fig. 1 ). The high-Pmetamorphism is dated at c. 370 Ma in the Cubito-Moura Unit ( Moita et al., 2005 ), and hence its Variscan nature is clear. A less precise Variscan age has been ob- tained also for the high-P event in the Central Unit (c. 380 – 340 Ma; Ordóñez Casado, 1998; Pereira et al., 2010 ). The distribution and extent of the ophiolites of the Ossa-Morena Zone (to the north of Beja- Acebuches) is not well known. However, ma fi c-ultrama fi c sequences are located structurally above the high-P units ( Fig. 1 ). These ophiolites are referred to as the internal ophiolitic units of the Ossa-Morena Zone, and some of them have been dated at Cambrian – Ordovician ( Pedro et al., 2010 ). Other ophiolites exposed throughout the Ossa-Morena Zone include the Carvalhal Amphibolites and the Calzadilla Ophiolite ( Fig. 1 ). The Ossa-Morena Zone is usually considered separated from the Central Iberian Zone by the intermediate Obejo-Valsequillo Domain ( Apalategui and Pérez-Lorente, 1983; Martín Parra et al., 2006 ), with sup- posedly transitional stratigraphic characteristics ( Martínez Poyatos, 2002; San José et al., 2004 ). However, its paleontological record is some- what enigmatic and also different of that of the Central Iberian Zone, what has casted doubt on such transitional character ( Gutiérrez-Marco et al., 2014 ). The interpretation of the Ossa-Morena Zone as an allochtho- nous domain of the Iberian Massif proposed by Díez Fernández and Arenas (2015) overcomes this problem, because the Central Iberian Zone and the Obejo-Valsequillo Domain would be originally located far apart each other. The northern limit of the Ossa-Morena Zone is de fi ned by a large extensional fault, the Puente Génave-Castelo de Vide detachment ( Fig. 1 ; Martín Parra et al., 2006 ), which subtracts a signi fi cant portion from both the overlying Obejo-Valsequillo Domain and the underlying Central Iberian Zone, thus making it dif fi cult to identify a suture zone in this boundary. 4. Equivalence between the NW and SW of the Iberian Massif: the Galicia – Ossa-Morena Zone The af fi nity between the continental crusts of NWand SW Iberia was fi rst suggested by Castro (1987) . The Ossa-Morena Zonewas considered a microcontinent accreted to the Gondwanan margin, represented by 138 R. Arenas et al. / Tectonophysics 681 (2016) 135 – 143