Nova Terra 52

477 International Journal of Earth Sciences (2021) 110:467–485 13 significant enrichment in Sr, linked to the high content in CaO described above. In both formations, LILE elements exhibit a slightly positive slope, while the HSFE element (Zr, Hf, Sm, HREE, and Sc) display flat patterns close to unity (Fig. 4d). A Ti negative anomaly is present in both formations, as expected for a dominant felsic provenance. The patterns of Montemolín and Tentudía formations are consistent with the features described for sediments deposited in an active margin setting (Winchester and Max 1989). The Montemolín and Tentudía formations lack of large deviations of their geochemical composition from references such as PAAS and UCC. Low Na, Ca, K and Sr contents may indicate weak post-depositional weathering/recycling alteration, but with no significant consequences for using immobile trace elements on provenance and tectonic set- ting discrimination diagrams. Ternary diagrams (Bhatia and Crook 1986; La–Th–Sc, Th–Co–Zr/10 and Th–Sc–Zr/10) (Fig. 5) provide a valuable tool to distinguish between four depositional settings for sedimentary series. All samples plot into the same field ( B ), suggesting a similar geodynamic environment for their sedimentation during Ediacaran times (Fig. 5). This tectonic setting would be characterized by the presence of a magmatic arc, built over a thinned continental crust. The La/Sc ratios for both Ediacaran formations (2.65 and 2.58 for Montemolín and Tentudía Formations, respec- tively) close to the PAAS value (2.38; Taylor and McLennan 1985), and Ti/Zr ratios all above 20, strengthen the afore- mentioned geodynamic environment interpretation, since the values are close to the range for the continental island arc setting defined by Bhatia and Crook (1986). Sm–Nd isotope geochemistry To constrain the paleo-location of the basins where the Neo- proterozoic sedimentary series were deposited, additional information to the major-trace element geochemical features is provided by the Sm–Nd system. These two elements are strongly immobile, thus reflecting an average of the origi- nal isotope composition from the mantle/crust source rocks. In this context, Nd model ages ( T DM ) in siliciclastic rocks may be regarded as a proxy for the average age of extraction of their constituents from a depleted mantle, and therefore, the study of the Nd isotope composition of sedimentary sequences provides an excellent tool to understand crustal evolution (McCulloch and Wasserburg 1978; Allegre and Rousseau 1984; McLennan et al. 1990), as well as to con- strain the mantle/crust provenance of terrigenous sediments. Sm–Nd isotope data from the metagreywackes of the Tentudía and Montemolín formations are given in Table 3 and plotted in Fig. 6. Age assignment for the calculation of εNd (T) is based on the stratigraphic and structural fea- tures of the selected samples (see “Geological setting”). The reference depositional age considered for the Montemolín Formation (older part of the Ediacaran Serie Negra Group) is 600 Ma, and 565 Ma for the Tentudía Formation (upper part). The metagreywackes from Montemolín Formation show homogeneous 147 Sm/ 144 Nd ratios, which vary between Fig. 5 Trace element diagrams with tectonic setting discrimination fields (after Bhatia and Crook 1986) Iberian–Bohemian correlations