serie NOVA TERRA nº 49

142 of detrital grains, but they never de fi ne quartz-rich competent layers like in the Cabrela-Carvalhal synform. The siliciclastic nature of the pri- mary protoliths of the black quartzites of SW Iberia seems well established ( De Oliveira et al., 2003 ). But the origin of the silica-richma- trix characterizing some of those layers is yet under discussion, since a post-depositional derivation (diagenetic silici fi cation of dominantly ter- rigenous sediments) is a rather likely option for the case of late Neoproterozoic successions deposited in the periphery of Gondwana ( Dabard, 2000; Dabard and Loi, 1998 ). Following this line of evidence, the previous authors indicated that such silici fi cation is episodic, not uniform, and largely controlled by local conditions in the environment, thus calling “ into question the use of graphitic cherts as markers for lithostratigraphic correlation ” . Their conclusions seem well suited for the case of the Iberian Massif, and particularly for the lower part of the basal allochthonous units discussed here, where, from a lithostratigraphical point of view, they only differ from each other in the presence or absence of quartz-rich black layers. Besides, most of the grey color dominating these series comes from the moderate to high content in carbonaceous material and heavy minerals, particularly high in the blackish terms (e.g., De Oliveira et al., 2003 ). This adds to the dominantly immature character of the protoliths and common sedi- mentary facies (Bouma cycles) observed in both NW and SW Iberian basal allochthonous units. Apparently, there seems to exist a common petrological origin for most of the sedimentary successions, despite some of the less abundant lithologies are exclusive to some regions. Quartzites, however, are also found in the Ediacaran series of the basal allochthon of NW Iberia ( Díez Fernández et al., 2010 ), although they contain much less carbonaceous material and heavy minerals and thus have no blackish appearance. Another lithostratigraphic difference is the lack of marbles in the basal allochthonous units of NW Iberia. Although carbonate- richer horizons are frequent in NW Iberia, they occur only as thin calc-silicate layers in the whole series, never thicker than 20 cm. Marbles of the basal allochthonous units of the Évora Massif occur in a metasedimentary and metavolcanic rock series dated at Early Cambrian (Monfurado Unit; Chichorro et al., 2008 ). Metasedimentary rocks of that particular age have not been found in the basal allochthonous units of NW Iberia. There, the set of Ediacaran metagreywackes rests right below a volcano-sedimentary succession with Middle Cambrian maxi- mum depositional age ( Díez Fernández et al., 2010 ). In NW Iberia, the two latter sequences are separated by a major extensional fault (Bembibre-Ceán detachment; Díez Fernández et al., 2012c; Gómez Barreiro et al., 2010 ), which must have removed any potential lithostratigraphic unit correlative to the Early Cambrianmetasedimentary rocks of the Évora Massif. Therefore, the lack of Early Cambrian series in the basal allochthonous units of NW Iberia precludes any correlation based on geochemical proxies applied to successions of that age. 5.1. Comparison of tectonic setting and isotopic contributions during the Ediacaran The Ediacaran series of the basal allochthonous units of NW Iberia were deposited in a back-arc or retro-arc basin related to a Cadomian peri-Gondwanan arc system built on a thinned continental margin. This is typi fi ed by (i) the association of sedimentary facies (identi fi ed in the less deformed protoliths), (ii) major and trace element geochem- istry, and (iii) by the dominant input of Cadomian detrital zircons de- rived from present northern Africa ( Díez Fernández et al., 2010; Fuenlabrada et al., 2012 ). These same proxies point to a similar geodynamic setting for the late Neoproterozoic rocks that constitute the basal allochthonous units of the Évora Massif. (i) Their immature character ( Fig. 5 a and Tables 1 and 2 ) along with their primary struc- ture, formed after successive turbidity currents, suggest a tectonically controlled deposition close to the source area. (ii) Tectonic discrimina- tion diagrams support an active margin setting for deposition ( Fig. 5 c and d, see also M.F. Pereira et al., 2006, 2007 ). (iii) The detrital input of this succession, as constrained from zircon analysis, is dominated by Neoproterozoic (Cadomian) grains (ca. 0.75 – 0.55 Ga), followed by Paleoproterozoic grains (ca. 2.15 – 1.8 Ga), and then several Archean and Tonian clusters ( Pereira et al., 2008 ). These age populations match those of the basal allochthonous units of NW Iberia ( Díez Fernández et al., 2010 ), both in age range and relative abundance ( Fig. 7 c). Sm – Nd model ages of the Ediaracan metasedimentary rock sequences of the basal allochthonous units of the Évora Massif (1.50 – 1.85 Ga) are nearly similar to those of the metasedimentary rocks of the Central Unit (1.64 – 1.90 Ga; López-Guijarro et al., 2008 ), which is another basal allochthonous unit of SW Iberia ( Díez Fernández and Arenas, 2015 ). A direct comparison of Sm – Ndmodel ages of the Ediacaran successions of the basal allochthonous units of Iberia reveals slightly older model ages for the NW Iberian rocks, which still partly overlap with the rest of the values obtained from correlative units located in SW Iberia, i.e. Escoural Unit and Central Unit ( Fig. 6 ). The three main exposures of basal alloch- thonous units considered here de fi ne a clear rejuvenating trend from NWto SW. In absolute terms, differences betweenmodel ages are limited. However, if the range of Sm – Nd model ages of the basal allochthonous units is compared with that of the Ediacaran autochthonous series of the Central Iberian Zone (1.25 – 1.33 Ga; Fuenlabrada et al., 2016 ), located structurally underneath and geographically in between ( Fig. 1 ), a sharp contrast emerges. The range of ages for the Ediacaran autochthonous se- ries is clearly younger, does not overlap with that of the basal allochthon, and suggests a signi fi cant contribution from more juvenile isotopic sources ( Fig. 6 ). Consequently, the basal allochthonous units were depos- ited in a rather different location across the continental margin relative to the autochthonous series, as it can also be deduced from the contrasting structural position they acquired during the Variscan collision (lower structural position in the Variscan tectonic pile implies inner paleogeo- graphic location across Gondwana; Martínez Catalán et al., 2009 ). Detrital zircon ages and geochemical data alike indicate that the West African Craton was one of the fundamental sources for the metasedimentary rocks of the Iberian Massif during the Neoproterozoic and Paleozoic (e.g., Albert et al., 2015a; Díez Fernández et al., 2012b; Fernández-Suárez et al., 2002, 2013; Martínez Catalán et al., 2004; Pereira, 2015 ). Accordingly, the contribution of juvenile material seemsminor in any of the sets of basal allochthonous units of the Iberian Massif, as their Sm – Nd model ages approach or even match the range of protolith ages found in the aforementioned cratonic area (e.g., Ennih and Liégeois, 2008; Nance and Murphy, 1994 ). A potential isotopic rejuvenation effect, expected for a magmatic arc system on its adjacent basins, is missing or weak in the Ediacaran series of the basal allochthonous units. This cannot be explained by a lack of material derived from erosion of igneous rocks formed in the context of a Neoproterozoic (Cadomian) magmatic arc. Populations of detrital zircon grains in the Ediacaran series peak consistently at ca. 0.65 – 0.55 Ma (Cadomian ages), and are in fact the dominant group of igneous-derived grains ( Fig. 7 c). It is the particular nature of the source area, probably made of a cratonic block (ca. 2.2 – 1.8 Ga old) and/or by recycled material derived from Eburnean crust (e.g., Abati et al., 2012 ), and then intruded by abundant, not-juvenile Neoproterozoic material, that confers a rather distinctive signature to the Ediacaran metasedimentary rocks of the basal allochthonous units of the Iberian Massif. Such signature makes them genetically correlatives by means of isotopic and geochemical composition and detrital zircon input. Accordingly, a fragment of the West African Craton intruded by Neoproterozoic felsic magmas, mostly sourced from the same craton in the context of a continental volcanic arc, is the most likely source for the Ediacaran metasedimentary rocks. The lithostratigraphical, geochronological, and geochemical charac- teristics of the Ediacaran series studied in this work are, in general terms, equivalent to those exposed in the Saxo-Thuringian Zone of the Bohemian Massif. Similarities in the sedimentary record include (1) major and trace-elements signatures re fl ecting an active margin set- ting during deposition, (2) REE-patterns pointing to the continental 296 R.D. Fernández et al. / Lithos 268 – 271 (2017) 285 – 301