serie NOVA TERRA nº 49

99 from Morocco include these zircons only since Middle Cambrian times, without any signi fi cant presence before this period (see also Ábalos et al., 2012; Abati et al., 2010a; Boher et al., 1992; Potrel et al., 1998 ). The presence of Stenian – Tonian detrital zircons is more common in the Saharan Craton, also in Lower Cambrian – Neoproterozoic sedimen- tary series ( Avigad et al., 2003, 2012 ). Anyway the existence of large outcrops of primary Stenian – Tonian materials is unknown in North Af- rica, and thence the actual source of these zircons is enigmatic suggesting long transport from source areas located far away. Using this information, Díez Fernández et al. (2010) suggested that sediments from the basal units were deposited in basins probably located in an in- termediate position between the West African Craton and the Saharan Craton. This interpretation is very sensitive to the actual age of the stud- ied series, to which only a maximum depositional age approach based in detrital zircons is available, but it is coherent with a provenance not too far from the present location of the investigated allochthonous sed- imentary rock series. The Nd model ages of the sedimentary sequences of the basal units are considerably older than those of the Cambrian greywackes (530 – 500 Ma) of the upper units of the same region, which range be- tween 720 Ma and 1215 Ma ( Fuenlabrada et al., 2010 ; Fig. 6 ). Separat- ed from the basal units by ophiolites, the upper units have been repeatedly interpreted as a well-preserved section of a peri- Gondwanan arc ( Abati et al., 2003, 2007; Díaz García et al., 2010; Fuenlabrada et al., 2010 ), what suggests that these relatively young Nd model ages are in fl uenced by abundant supplies of juvenile mate- rial generated in relation to the magmatic activity of the arc (see Fig. 7 c). Drifting arc S N back-arc basin (c) Cambrian Passive margin Upper Sequence GONDWANA IAPETUS -TORNQ UI ST C R U S T Cambro-Ordovician active arc Cambro-Ordovician mafic magmatism (Upper seq.) Cambrian sediments (Upper seq.) Cambro-Ordovician arc-related granitoids Late Neoproterozoic - Early Paleozoic? sediments (Lower seq.) Relicts of the Cadomian-Avalonian arc Neoproterozoic Granites Pre- to Syn-Cadomian continental crust Acretted oceanic crust and sediments Oceanic crust Mantle lithosphere ? retro-arc basin (b) Late Neoproterozoic - Early Paleozoic? Passive margin S N Lower Sequence Late Neoproterozoic - Early Paleozoic? sediments Cadomian-Avalonian active arc Neoproterozoic Granites Pre- to Syn-Cadomian continental crust Acretted oceanic crust and sediments Oceanic crust Mantle lithosphere GONDWANA IAPETUS-TORNQUIST CRUST (a) Late Neoproterozoic Passive margin S N Lower Sequence Late Neoproterozoic sediments Cadomian-Avalonian active arc Neoproterozoic Granites Pre- to Syn-Cadomian continental crust Acretted oceanic crust and sediments Oceanic crust Mantle lithosphere back-arc basin GONDWANA IAPETUS-TORNQUIST CRUST Fig. 7. Cartoon showing the tectonic setting for the lower (Late Neoproterozoic) and upper (Middle – Late Cambrian) sequences of the basal units of the allochthonous complexes of NW Iberia. In (a) and (b), the lower sequence is a greywacke series deposited near the main continent, where the most important supplies come from the interior of Gondwana. In (c), the Cambrian sediments fi ll a back-arc related to the drifting of an ensialic arc; they are being deposited close to the continental edge. Cambro-Ordovician calc-alkaline magmatism is intruding the Neoproterozoic sediments, while the roll-back of the oceanic slab is forcing the active arc to migrate oceanward, leaving behind synchronous deposits in the back ‐ arc and a tail of Cambro-Ordovician plutons. The Nd model ages of the clastic sediments derived from this rifted arc (the greywackes from the uppermost Allochthon in Fig. 6 ) indicates contribution from juvenile sources. These models re fi ne previous interpretations presented by Díez Fernández et al. (2010) . 206 J.M. Fuenlabrada et al. / Lithos 148 (2012) 196 – 208