Nova Terra 52

precision of ± 0.00002 (2σ). Analytical errors on the 147 Sm/ 144 Nd and 143 Nd/ 144 Nd ratios were estimated to be <0.1% and 0.006%, respec- tively. The new Sm-Nd isotopic data from the Serie Negra (Ossa Morena Complex) will be now compared with equivalent data from the Lower Alcudian Series (Central Iberian Zone) published by Fuenlabrada et al. (2016) The isotopic Sm-Nd values of these 28 samples are included in Table 2. 147 Sm/ 144 Nd values range from 0.1049 to 0.1396 (Table 2), with an average value (0.1175) close to those of the upper crust (0.12) and always under 0.165, which has been considered as an upper filter for TDM calculations (Stern, 2002). The study of the Nd isotope composition of siliciclastic rocks has contributed to understanding crustal evolution (McLennan et al., 1990; McLennan and Hemming, 1992). Crustal models consider that the Nd model ages for siliciclastic rocks, typically older than their maximum depositional ages, are an average for the moment of melt extraction for each individual component from its source (McCulloch and Wasserburg, 1978; Allegre and Rousseau, 1984; and others). Those ages represent a complex mixture of crustal terrigenous components re- sulting from preexisting sedimentary rocks, variably aged upper-crustal igneous rocks, and other juvenile components (McLennan and Hemming, 1992). 4.2. Results Although the precise age of azoic sedimentary series may be con- troversial, the most probable depositional age for both Ediacaran series here compared is quite similar, as constrained by the age of underlying and overlying series and the maximum depositional age deduced from detrital zircons they contain. The reference ages for the calculation of the ε Nd (i) values have been set to 565 Ma for both Ediacaran series. The Serie Negra samples show a range of highly negative values, with ε Nd (0) varying between −9.8 and −20.5, and ε Nd (565) values between −4.0 and −13.9, with an old TDM age that ranges from Mesoproterozoic to Paleoproterozoic (1421–2040 Ma) (Table 2). The siliciclastic rocks from the Lower Alcudian exhibit quite different values of ε Nd (0) (from −6.4 to −8.8) and ε Nd (565) (from −1.4 to −3.0), as well as younger Nd model ages (1256–1334 Ma) (Table 2). None of the ranges for the aforementioned values in the Central Iberian Zone even overlaps a little with any of those for the Ossa-Morena Complex. Fig. 6 shows an ε Nd vs age (Ga) diagram for both groups of Edia- caran siliciclastic rocks. Besides indicating the variation limits of their TDM values, that figure also includes a collection of Nd model ages from different regions (Linnemann and Romer, 2002) that are con- sidered relevant to discussing paleogeographic affinity between the Ediacaran series of the African paleo-margin of Gondwana. The dia- gram includes old isotopic signatures from the West African Craton and from the Amazonian Craton, along with other Nd model ages from more juvenile terranes. Intermediate ages, more common in other terranes, could reflect isotopic mixing between both sources. The provenance and isotopic sources of the Serie Negra (Obejo-Valsequillo Domain) and Lower Alcudian siliciclastic rocks (Central Iberian Zone) are a mixture between the West African Craton and other western terranes, such as West Avalonia in Atlantic Canada and Carolina. The Sm-Nd isotopic compositions of studied series suggest a provenance from areas domi- nated by pre-Mesoproterozoic cratonic crust, therefore lacking of Me- soproterozoic igneous activity. A provenance from the African paleo- margin of Gondwana is in good agreement with this set of observations. The same isotopic features are also a tool for tectonic discrimina- tion, when considering the Sm/Nd fractionation from CHUR for crustal rocks (f Sm/Nd ; DePaolo, 1988). Fig. 7 shows a f Sm/Nd vs ε Nd (T) diagram that discriminates between passive and active margin settings (McLennan and Hemming, 1992). Siliciclastic rocks deposited in pas- sive margins or derived from cratonic areas tend to plot towards more negative ε Nd (T) values compared to those derived from source areas connected to active volcanic arcs, where the input of juvenile material is usually larger (McLennan and Hemming, 1992). Most of the studied samples plot in the overlapping region between active and passive margin, although the Ediacaran series here compared show different isotopic features. The samples of the Serie Negra yielded f Sm/Nd values ranging between −0.30 and −0.47, while the Lower Alcudian samples are clearly distinguishable, since these plot within a more restricted range (f Sm/Nd , from −0.34 to −0.41) (Fig. 7). The Serie Negra samples show larger affinity with cratonic regions, what alongside the volcanic arc setting for the deposition of both series, suggest that old cratonic sources were involved, being either part of the basement of the volcanic arc and/or from the neighboring continental region. 5. Discussion In the SW of the Iberian Massif, the South-Portuguese Zone re- presents the southern margin of Laurussia, i.e. the Avalonian foreland collided with Gondwana during the assembly of Pangea in Devonian times (Figs. 1 and 2) (Díez Fernández et al., 2016). The Ossa-Morena Complex, in the Galicia – Ossa-Morena Zone (Arenas et al., 2016a), together with the other allochthonous complexes of the Iberian Massif (Díez Fernández and Arenas, 2015; Arenas et al., 2016b), define the most external margin of Gondwana, the region affected by the most intense dynamic in Variscan and pre-Variscan times. This interpretation is clearly confirmed at present by the existence in those complexes of a group of different ophiolitic units (Arenas and Sánchez Martínez, 2015; Arenas et al., 2018) and a pair of successive high-P metamorphic events dated at c. 400 and 370 Ma (Ordóñez Casado et al., 2001; Fernández- Suárez et al., 2007; Abati et al., 2010; Arenas et al., 2014; Díez Fernández et al., 2016). Several works have confirmed that the Ava- lonian-African margin of Gondwana has been an active margin for a long time, at least between 750 and 500 Ma, the Avalonian-Cadomian arc (Rodríguez-Alonso et al., 2004; Pereira et al., 2006; Linnemann et al., 2008, 2014; von Raumer and Stampfli, 2008; Fuenlabrada et al., 2010; Albert et al., 2015a, 2015b, von Raumer et al., 2015; Andonaegui et al., 2016). According to this interpretation, the Central Iberian Zone very likely represents a back-arc basin located behind the most active part of the margin (Fernández-Suárez et al., 2014; Albert et al., 2015a; Orejana et al., 2015; Díez Fernández et al., 2016). The contrasting lo- cation of both Ediacaran series, Serie Negra and Lower Alcudian, de- posited respectively in the frontal arc and in the back-arc, explains the differences observed in the stratigraphic columns and in the intensity of the Ediacaran-Cambrian magmatism (Fig. 4). It has been even sug- gested that the Serie Negra was deposited in a fore-arc basin opened at c. 600 Ma during a short event of subduction roll-back accompanied by boninitic magmatism (Arenas et al., 2018). The major and trace element geochemistry and the Sm-Nd isotopic data presented in this work are compatible with this peri -Gondwanan dynamic context. The siliciclastic rocks of the Serie Negra and Lower Alcudian have compositions suggesting a deposition in an active margin setting (Figs. 5 and 7). TDM values of the Serie Negra are significantly older (1421–2040 Ma) than those of the Lower Alcudian (1256–1334 Ma), which would situate its source areas in the periphery of a cratonic region (Figs. 6 and 7), very likely in the realm of the West African Craton. This interpretation is at odds with sedimentation of the studied section of the Serie Negra very close to the most active part of the Ediacaran magmatic arc, as old TDM values would require scarce supply of juvenile material coming from the arc. However, it can also point to the presence of a rather narrow active arc at that time, built on an external section of the margin of Gondwana detached from the West African Craton itself. This way, the basement of the magmatic arc would probably be Archean and/or Paleoproterozoic in age, as those are the ages for the oldest basement rocks found in the (submerged) Iberian continental margin of the Cantabrian coast (Gardien et al., 2000) and in equivalent regions of the Channel Islands (Samson and D’Lemos, 1998). Nd model ages (Fig. 6) indicate that the source areas for the Lower E. Rojo-Pérez et al. 3UHFDPEULDQ 5HVHDUFK ² Contrasting isotopic sources: CIZ - OMC boundary