serie NOVA TERRA nº 49

121 system. Rodríguez Alonso et al. (2004b) found clear evidence of magmatism coeval with Neoproterozoic – Cambrian sedimentation, strongly suggesting a tectonically andmagmatically active environment as the direct contributor to the metasedimentary rocks of the CIZ. Our geochemical results accord well with this active environment. Whereas tectonic setting discrimination diagrams indicate a deposition environ- ment linked to an active arc system, the lack of strong geochemical ev- idence of a ma fi c source for the Ediacaran greywackes suggests a mature active margin. TDM ages are notably older than the deposition age. This is in agree- ment with a mixed source for the sediments that would consist of a ju- venile (arc-derived) contribution and a much older (cratonic) input. Older TDM ages and lower ε Nd (T) values in the Paleozoic shales than in the Precambrian greywackes reveal that during the Early Cambrian the juvenile input either waned or was outpaced by more evolved and older crustal material, probably coming from the emergent Gondwana hinterland. Trace element discrimination diagrams ( Fig. 7 ) suggest a passive margin-like environment for the Early Cambrian series. In order to reconcile all these observations, we support previous models that proposed a wide back-arc as the most likely setting for the autoch- thonous series of the CIZ during the Early Paleozoic (e.g. Orejana et al., 2015 ). Accordingly, the Cambrian series must have been laid down clos- er to the mainland than to the main axis of volcanic arc activity. Such a scenario explains the more homogeneous and recycled character of the sediments and the controversial trace element results that indicate an active geodynamic setting. A wide back-arc basin is in agreement with the accepted transition from a convergent regime (Cadomian arc) to an extensional regime ( Nance et al., 2010 ). These arc-related basins along the northern margin of Gondwana would have been fi lled with different proportions of crustal recycled materials from different sources, more or less close to the areas of major magmatic activity. Despite the relative scarcity of Nd model ages from comparable lithologies of the same age in the Variscan and East Avalonia terranes, our results do not disagree with those from other regions of Europe ( Fig. 9 ). Other Variscan (Saxo-Thuringia, Bohemian Massif — Germany; Linnemann and Romer, 2002; Linnemann et al., 2004 ) and East Avalonian (Uriconian, southern Britain. Thorogood, 1990 ) tectonostratigraphic terranes show a relative increase in the Nd model ages in younger stratigraphic units, presumably as a result of a greater contribution from older sources. As in the case of the Ediacaran and Cambrian transition in the CIZ, ε Nd results show a relative drop in Edi- acaran Saxo-Thuringia and Uriconian Ediacaran relative to Cambrian sedimentary units, probably linked with the supply of different propor- tion of juvenile arc-relatedmaterial. Cambrian sediments show less pos- itive values of ε Nd, probably inherited from reworking of older sediments and long-established continental crust. Nd model ages for the CIZ here presented are in agreement with a similar cratonic source to Saxo-Thuringia sedimentary units ( Fig. 9 ), with a progressive west- ward decrease of the T DM along the Avalonian – Cadomian arc, according to Linnemann and Romer (2002) . An extensive analysis of U – Pb detrital zircon populations in the Iberian Massif ( Díez Fernández et al., 2010, 2012; Fernández Suárez et al., 2000, 2002, 2014; Orejana et al., 2015; Pereira et al., 2012; Ugidos et al., 2003b ) has shown a wide range of potential provenance sources for the Late Ediacaran – Early Cambrian sandstones. The pres- ence or absence of Stenian – Tonian zircon populations has provided an important clue in interpreting tentatively the paleogeographic positions of the different Iberian terranes. According to the Nd model ages here presented, the West Africa Craton is envisaged as the main cratonic source for the metasedimentary rock sequences of the CIZ. The geo- chemical and isotopic results here reported are in agreement with the development of paleo-basins along the northern margin of Gondwana -12 -8 -4 0 4 8 12 0 0.5 1.0 1.5 2.0 2.5 Age (Ga) CHUR DEPLETED MANTLE ε Nd Max Dep Ages T DM : 1256-1334 (n=12) Pusa Shales (Early Cambrian) Lower Alcudian metagreywackes (Late Ediacaran) T DM : 1444-1657 (n=12) East Avalonia (Uriconian, Southern Britain) Saxo-Thuringia (Bohemian Massif, Germany) Range of Nd Model ages for: Ediacaran Early Cambrian Reservoirdominatedby pre-Grenvillian (>0.9-1.1Ga) cratonic crust Reservoirdominatedby Grenvillian and (?) post-Grenvillian/ pre-Cadomian crust Ediacaran Early Cambrian Fig.9. TDMmodelages( DePaolo,1981 )oftheLateEdiacarangreywackesandtheEarlyCambrianshalesofthesouthernCIZ(IberianMassif).Diamondsshowthe ε Ndvaluesat565Maand 530 Ma, reference ages for the deposition of the Late Ediacaran greywackes and the Early Cambrian shales, respectively. Data source for comparative model ages from different regions taken from Linnemann and Romer (2002) and Thorogood (1990) . 27 J.M. Fuenlabrada et al. / Tectonophysics 681 (2016) 15 – 30