serie NOVA TERRA nº 49

120 range between − 5.2 and − 4.0 ( Table 6 ), clearly lower than those of the Late Ediacaran greywackes, which vary between − 3.0 and − 1.4 ( Table 5 ). Nd TDM model ages are Mesoproterozoic and vary between 1444 and 1657 Ma ( Table 6 ) with an average value of 1516 Ma for the Pusa Shales; and between 1256 and 1334 Ma with an average value of 1288 Ma for the Ediacaran greywackes ( Table 5 ). 5. Discussion A series of magmatic arcs formed on the periphery of Gondwana during the Neoproterozoic (Cadomian – Pan-African cycle) and Cambri- an (c 750 – 500 Ma) are considered one of the main sediment suppliers for the sedimentary sequences that make the pre-Silurian basement units of Western and Central Europe ( Albert et al., 2015; Murphy and Nance, 2002; Murphy et al., 2006; Stamp fl i and Borel, 2002; Von Raumer and Stamp fl i, 2008; Von Raumer et al., 2015 ). The Iberian Mas- sif represents a good example for the evolution of a Late Ediacaran ac- tive margin in the northern margin of West-Central Gondwana and its subsequent transition to a passive margin in Cambrian – Ordovician times ( Murphy et al., 2006; Nance et al., 2008, 2010 ). The late stages of the Cadomian orogeny have been identi fi ed in the CIZ as responsible for an intra-Alcudian unconformity of Precambrian – Cambrian age ( Martínez Poyatos, 2002; Martínez Poyatos et al., 2014; Pieren and García Hidalgo, 1999; Pieren et al., 1987; Simancas et al., 2004; Talavera et al., 2015 ). This tectonic event could explain the differ- ent geochemical and isotopic records found in the two series studied in this work. In this regard, it has been proposed that the Neoproterozoic – Cambrian sequences of the CIZ record an evolving geodynamic setting ( Pieren, 2000; Rodríguez Alonso et al., 2004b ). Moreover, some authors have suggested a division of the CIZ into two different domains sharing a common depositional history ( Orejana et al., 2015; Villaseca et al., 2014 ). Our results con fi rm the aforementioned difference between the northern and southern sequences on the basis of the presence of a juve- nile contribution for the Late Ediacaran greywackes collected close to the southern margin of CIZ. But, more importantly, the data here pre- sented provide compelling geochemical evidence about the contrasted paleogeographic scenarios for the deposition of sediments in the CIZ during the Ediacaran and then the Cambrian. Our geochemical results point to an active margin setting as the most reasonable scenario for the deposition of the Ediacaran – Cambrian series of the CIZ. Trace element diagrams of the Ediacaran greywackes indicate a clear af fi nity to a continental island arc ( Fig. 6 ). On the other hand, although Early Cambrian sequences here exposed are not represented by wacke-type samples, limiting a complete comparison of source composition and tectonic setting, an approach has been achieved by correlating major and trace geochemical values with those reported for the four main geodynamic settings ( Fig. 7 ). Consider- ing their composition, the Cambrian shales display typical patterns for a passivemargin environment. Yet, according to Bhatia and Crook (1986) , the lower content of Zr in shales, and therefore lower Zr/Nb and Zr/Th ratios, and relatively higher Ba, Rb, Sr, V and Sc abundances are more compatible with an active geodynamic environment, which is con- fi rmed by La/Th ratios well above 2 (3.3 average value). The Nd model ages of the Ediacaran greywackes (1256 – 1334 Ma) are clearly younger than those obtained for the Early Cambrian shales (1444 – 1657 Ma) ( Fig. 9 ). Younger Nd model ages in greywackes can be explained by a larger supply of juvenile material, as also indicated by higher ε Nd (T) values ( − 3.0 to − 1.5) than in the Cambrian shales ( − 5.2 to − 4.1). In the latter, the older Nd model ages are compatible with the presence of more recycled crustal material. A larger juvenile (mantle- derived) contribution in the Neoproterozoic greywackes locates the ba- sins relatively closer to the major axis of magmatic activity of the arc Table 5 Whole rock Nd isotope data of Lower Alcudian greywackes (Late Ediacaran). Sm Nd 147 Sm/ 144 Nd 143 Nd/ 144 Nd SErr ⁎ 10 − 6 εΝ d (0) εΝ d (565) a T DM (Ma) b CIA-13 3.38 16.74 0.1220 0.512280 2 − 7.0 − 1.6 1269 CIA-14 3.78 18.81 0.1214 0.512269 2 − 7.2 − 1.8 1278 CIA-15 3.28 16.10 0.1230 0.512291 2 − 6.8 − 1.4 1263 CIA-16 3.18 15.57 0.1236 0.512293 2 − 6.7 − 1.5 1268 CIA-17 3.68 18.18 0.1223 0.512290 2 − 6.8 − 1.4 1256 CIA-18 3.24 15.79 0.1242 0.512293 1 − 6.7 − 1.5 1277 CIA-19 3.49 17.07 0.1236 0.512277 2 − 7.0 − 1.8 1295 CIA-20 3.40 16.77 0.1224 0.512266 1 − 7.2 − 1.9 1296 CIA-21 6.33 29.50 0.1298 0.512310 2 − 6.4 − 1.6 1329 CIA-22 6.69 31.95 0.1265 0.512305 2 − 6.5 − 1.4 1290 CIA-23 5.33 26.38 0.1221 0.512258 2 − 7.4 − 2.0 1305 CIA-24 4.86 25.32 0.1161 0.512188 2 − 8.8 − 3.0 1334 a ε Nd(t) calculated for 565 Ma. b Nd model ages calculated according to DePaolo (1981). Table 6 Whole rock Nd isotope data of Pusa Shales (Early Cambrian). Sm Nd 147 Sm/ 144 Nd 143 Nd/ 144 Nd SErr*10 − 6 ε Nd (0) ε Nd (530) a T DM (Ma) b CIA-1 4.74 21.78 0.1315 0.512147 1 − 9.6 − 5.2 1657 CIA-2 4.64 21.71 0.1290 0.512162 2 − 9.3 − 4.7 1583 CIA-3 4.38 21.71 0.1219 0.512172 1 − 9.1 − 4.0 1444 CIA-4 4.36 22.13 0.1191 0.512115 1 − 10.2 − 5.0 1493 CIA-5 4.80 23.67 0.1224 0.512144 2 − 9.6 − 4.6 1498 CIA-6 4.46 22.03 0.1222 0.512125 2 − 10.0 − 5.0 1527 CIA-7 4.61 22.68 0.1228 0.512148 1 − 9.6 − 4.6 1499 CIA-8 4.31 21.49 0.1213 0.512134 1 − 9.8 − 4.7 1496 CIA-9 4.86 23.23 0.1265 0.512159 1 − 9.3 − 4.6 1542 CIA-10 4.52 22.43 0.1218 0.512136 2 − 9.8 − 4.7 1501 CIA-11 4.32 21.34 0.1224 0.512166 1 − 9.2 − 4.2 1462 CIA-12 3.94 19.86 0.1200 0.512129 2 − 9.9 − 4.8 1485 a ε Nd (T) calculated for 530 Ma. b Nd model ages calculated according to DePaolo (1981) . 26 J.M. Fuenlabrada et al. / Tectonophysics 681 (2016) 15 – 30