serie NOVA TERRA nº 49

141 and wehrlites, pyroxenite cumulates, metagabbros and fl asergabbros, metadoleritic dikes, metabasalts, and metacherts ( Pedro, 2004 ). They occur on top of a sequence of mica schists and minor metabasites and marbles (Cubito-Moura Unit; Araújo et al., 2005; Pedro et al., 2005 ) af- fected by high-P metamorphism during the Late Devonian ( Booth-Rea et al., 2006; Rosas et al., 2008; Rubio Pascual et al., 2013 ). This high-P unit meets the main features to be a tectonostratigraphic correlative of the upper sequence of the basal allochthonous units of NW Iberia ( Díez Fernández and Arenas, 2015 ). Striking similarities between units are not only found at a large scale. All the microstructural characteristics observed during the thin-section analysis of our samples of Ediacaran metasedimentary rocks (see Section 3.2 , and further details in Chichorro, 2006 ) are rather similar to those described for the basal allochthonous units of NW Iberia in pre- vious works. The existence of aligned microinclusions (quartz + white mica + garnet ± chlorite ± rutile) within albite porphyroblasts is a distinctive feature of the metasedimentary rocks of the basal alloch- thonous units in NW Iberia ( Fig. 4 f). This ensemble formed under high-P conditions and represents the fi rst Variscan metamorphic im- print that can be recognized in the metasedimentary rocks ( Arenas et al., 1995 ). Subsequent exhumation favored a progressive retrogres- sion of the high-P fabric, which is now preserved as an either non- retrogressed high-P foliation inside albite porphyroblasts, or as a vari- ably retrogressed and crenulated foliation, preserved either within or outside albite porphyroblasts (e.g., Arenas et al., 1995; Díez Fernández, 2011; Díez Fernández and Martínez Catalán, 2012; Díez Fernández et al., 2011 ). A comparable mineral assemblage with identical micro- structural relationships exists in the basal allochthonous units of SW Iberia ( Fig. 4 b, c, and d), although in this region it has been tentatively proposed as a vestige of Cadomian deformation ( Chichorro, 2006 ). Retrogression affecting the internal fabrics is more intense in the ÉvoraMassif (e.g., Escoural Unit) if compared to NW Iberia. For instance, biotite growth largely overprints the internal fabric and rutile is usually transformed into ilmenite. Isotopic dating of these fabrics is lacking. However, the non-Precambrian age is clear for the basal allochthonous units of NW Iberia, as such internal fabrics are also found in metasedimentary rock series with Middle Cambrian maximum deposi- tional ages ( Arenas et al., 1995; Díez Fernández et al., 2010 ). Given that the basal allochthonous units of NW and SW Iberia experienced equiv- alent tectonometamorphic evolution during the Variscan orogeny, we propose that the internal fabrics they preserve account for a Variscan high-P event, at least the associations bearing quartz + white mica + garnet ± chlorite ± rutile. Yet, an imprint of Precambrian shearing cannot be fully discarded, since other exposures of Ediacaran metasedimentary rocks in SW Iberia were affected by Precambrian deformation (e.g., Quesada, 1990 ). Finally, even though other para- derived units of the Iberian Massif experienced high-P metamorphic conditions, fast exhumation, and severe retrogression during the Variscan orogeny, none of them exhibits such a microstructural record like the basal allochthonous units. This record seems somehow distinc- tive for themetasedimentary rocks of the latter units in response to Late Devonian high-P metamorphism. 5. Comparative lithostratigraphy and geochemistry The lower series of the basal allochthonous units of NW Iberia and the Évora Massif share two fundamental features that make themdiffer- ent from other series in their respective regions. (i) They are dominated by late Neoproterozoic metagreywackes, and (ii) they contain various Cambrian-Ordovician metagranitic and ma fi c rocks, the latter occurring in the metasedimentary rocks and within the metagranitoids (most likely basic dikes). From a chemical point of view (compare with Fuenlabrada et al., 2012 ), besides some shales, the majority of the sequence consists of wackes, both in NW Iberia and SW Iberia ( Fig. 5 a). Sediments are generally immature and derive from source areas subjected to low weathering. This is not only suggested by their chemical composition (see Section 3.3.1 ), but also by the fact that micas and feldspar can be still observed as detrital grains in the less deformed sections of the series (e.g., Díez Fernández, 2011 ). Source areas are dominated by felsic igne- ous upper crustal rocks in both cases. The variation in REE contents is limited and follows PAAS ( Fig. 7 a). Finally, trace elements show rather similar trends in both regions ( Fig. 7 b). A minor regional difference is the occurrence of black quartzites in the Évora Massif. Black metasedimentary rocks intercalated between Ediacaran metagreywackes have been also identi fi ed in the lower se- quence of the basal allochthonous units of NW Iberia ( Díez Fernández et al., 2010 ). The latter are essentially siliciclastic, as they have plenty 0.45 0.55 0.65 0.75 0.85 0.95 1.05 1.15 1.25 1.35 1.45 1.55 1.65 1.75 1.85 1.95 2.05 2.15 2.25 2.35 2.45 2.55 2.65 2.75 2.85 2.95 3.05 3.15 3.25 3.35 3.45 3.55 Age (Ga) Probability 0.001 0.002 0.003 Basal allochthonous units of NW Iberia n=611 90-110% conc. (Díez Fernández et al., 2010) Basal allochthonous units of the Évora Massif n=52 90-110% conc. (Pereira et al., 2008) 0.1 1 10 Sample/PAAS Cs Ba Rb Th U K Nb La Ce Sr Nd P Sm Zr Hf Eu Ti Dy Y Er Yb Sc Cr Ni PAAS NW Basal Allochthonous Units (Fuenlabrada et al., 2012) SW Basal Allochthonous Units (present work) 1 10 100 1000 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sample/Chondrite NW Basal Allochthonous Units (Fuenlabrada et al., 2012) SW Basal Allochthonous Units (present work) PAAS (c) (b) (a) Fig. 7. Diagrams for comparing the content in (a) REE and (b) trace elements of the Ediacaran series of the basal allochthonous units of NW and SW Iberia. (c) Composite probability density plot of detrital zircon ages from the basal allochthonous units of NW Iberia (grey line) and from the Évora Massif (black dashed line). Data have been extracted from Fig. 5 b and e, and from Pereira et al. (2008) , Díez Fernández et al. (2010) , and Fuenlabrada et al. (2012) . 295 R.D. Fernández et al. / Lithos 268 – 271 (2017) 285 – 301