serie NOVA TERRA nº 49

91 2. Geological setting The NW Iberian section of the Variscan Belt contains a set of alloch- thonous terranes thrust over an autochthonous domain representing the margin of Gondwana ( Fig. 1 a). It is generally accepted that the low- ermost allochthonous terrane (Parautochthon), mainly constituted by metasedimentary rocks, represents an external section of the Paleozoic Gondwana margin. However, the interpretation of those terranes locat- ed either in the hanging wall or in the footwall to the Variscan suture zone is more complex, as they are probably affected by important dis- placements and may have an exotic nature ( Martínez Catalán et al., 2007, 2009 ). In the NW Iberian Massif, the suture zone is marked by several ophiolitic units ( Fig. 1 b and c) different in age and likely related to different geodynamic settings, but considered as a whole formed dur- ing different evolutionary stages of the Rheic Ocean. These units were obducted over the most external margin of Gondwana at the onset of the Variscan collision, after closure of this ocean at the end of Devonian times ( Arenas et al., 2007; Díaz García et al., 1999; Gómez Barreiro et al., 2010; Sánchez Martínez et al., 2007 ). The upper section of the allochthonous pile (upper units) rests over the ophiolites and contains an intricate succession of siliciclastic metasedimentary rocks intruded by large massifs of granitoids and gabbros ( Fig. 1 b and c). This terrane has a poly- metamorphic tectonothermal evolution developed during two consecutive Middle Cambrian and Late Silurian – Middle Devonian events ( Abati et al., 1999, 2007; Fernández-Suárez et al., 2002, 2007 ). The upper units are considered a section of a Cambrian peri-Gondwanan magmatic arc, some of these units show a counter-clockwise P – T evolution associated to magmatic under- plating ( Abati et al., 2003 ). In a subsequent event, the arc drifted away from Gondwana leaving the Rheic Ocean at its tail, being accreted to the southern margin of Laurussia afterwards ( Gómez Barreiro et al., 2007 ). The uppermost sequence of this terrane is constituted by a low grade thick sequence of metagreywackes in- truded by a network of diabasic dikes. These metagreywackes are Middle Cambrian in age, and their tectonic setting was revealed using major and trace element geochemistry and Sm – Nd system- atics ( Fuenlabrada et al., 2010 ). The basal units of the allochthonous complexes de fi ne a crustal accretionary complex stacked below the ophiolites ( Fig. 1 c). They are formed by thick metasedimentary rock sequences in- truded by calc-alkaline granitoids (c. 493 Ma; Abati et al., 2010b; Díez Fernández et al., 2012 ) and minor alkaline to peralkaline massifs (c. 475 – 470 Ma; Díez Fernández et al., 2012; Rodríguez Aller, 2005; Rodríguez et al., 2007 ), both transformed in variably deformed orthogneisses, as well as ma fi c rocks transformed in amphibolites, blueschists and eclogites ( Gil Ibarguchi and Ortega Gironés, 1985 ). These units are considered to represent a section of the most external margin of Gondwana ( Arenas et al., 1986; Díez Fernández et al., 2011; Martínez Catalán et al., 1996 ). According to the chemical composition of the orthogneisses, the protoliths of the basal units were formed in the context of a peri-Gondwanan magmatic arc (calc-alkaline granitoids), affected by subsequent rifting during the opening of the Rheic Ocean (alkaline-peralkaline granitoids; Abati et al., 2010b ). However, any de- scription of the compositional characteristics of the metasedimentary rocks did not exist until now, although these are the oldest lithologies and they can be used to deduce the initial tectonic setting. This margin was subsequently affected by eo-Variscan high-P low-to- intermediate-T metamorphism, developed at c. 370 Ma during sub- duction beneath Laurussia at the onset of the Variscan collision, in the main stage of the Pangea assembly ( Abati et al., 2010b; Rodríguez et al., 2003 ). 3. The sedimentary record of the basal units Two different and superimposed metasedimentary rock se- quences can be distinguished in the basal units. The deformation shows an heterogeneous character with important partitioning, which allows the preservation of low to very low deformed sections, Fig. 2. Idealized stratigraphic columns summarizing the main sedimentological fea- tures observed in the lower (a) and upper (b) sequences. The position of the analysed samples is also shown. Due to strong Variscan deformation, the vertical distribution of the facies of the lower sequence shown in this fi gure is approximate. 198 J.M. Fuenlabrada et al. / Lithos 148 (2012) 196 – 208