Nova Terra 52

(avg. 3.96 and 0.72, respectively; Table 1) match those of the Lower Alcudian metasedimentary rocks (avg. 3.34 and 0.79; Fuenlabrada et al. 2016). Those ratios are slightly higher when compared to those of PAAS (2.6 and 0.9, respectively), and to the UCC (2.7 and 0.7, respectively; Condie, 1993), what favours an intermediate-felsic provenance (Cullers, 2002). This interpretation is in agreement with the lower content in Sc, Cr, and Ni observed in both metasedimentary series. Fig. 5B presents multivariation diagrams (following Thompson, 1982), normalized to PAAS, for both metasedimentary series. Resulting patterns show depletion in most LILE elements (Cs, Rb, and K), and larger variabilibity in the case of the metagreywackes of the Obejo- Valsequillo Domain, likely indicative of limited recycling. LILE ele- ments display a positive slope up to HFSE elements (Zr, Hf, Sm, HREE and Sc), which show values close to those of the PAAS. Both features, together with a Ti negative anomaly, are consistent with an active margin setting for the deposition of the two metasedimentary series (Winchester and Max, 1989). The immature character and limited weathering/recycling deduced for the samples, allow the use of tectonic setting discrimination dia- grams. Ternary diagrams (La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10; Fig. 5C), proposed by Bhatia and Crook (1986), suggest a similar geo- dynamic environment during the sedimentation of both Ediacaran series. All the samples plot tightly within the field characteristic for a basin connected to a volcanic arc built over a thinned continental crust (field B; Fig. 5C). The values of La/Th, La/Sc, Th/Sc and Ti/Zr for both metasedimentary series reinforce this interpretation (Table 1 and Fuenlabrada et al., 2016), since the ratios are close to the range defined by Bhatia and Crook (1986) (2.36, 1.82, 0.85 and 19.70, respectively) as typical for this tectonic setting in particular. According to the major and trace element geochemical data and comparisons presented in this chapter, it can be concluded that the whole rock geochemical composition of the Lower Alcudian and Serie Negra siliciclastic series are similar. Both series were deposited in a similar geodynamic setting, i.e. in the vicinity of an active volcanic arc built on a thinned section of a continental margin, probably the African margin of Gondwana. 4. Sm-Nd isotope geochemistry Major and trace element geochemistry alone are not enough to constrain the paleogeographic position of the basin or basins in which Ediacaran sedimentation took place, neither to estimate the contribu- tion of major cratonic sources to the infill of the basins. Sm-Nd sys- tematics, on the other hand, can supply significant information in re- lation to these questions. 4.1. Methodology In the new 16 samples, isotopic analyses were performed on pow- dered rocks milled at Universidad Complutense de Madrid. Sm–Nd isotopic analyses were performed at the Geochronology and Isotope Geochemistry Service of Universidad Complutense de Madrid, using Isotope Dilution Thermal Ionization Mass Spectrometry (IDTIMS). Samples were spiked with a mixed 149 Sm– 150 Nd tracer and analysed in an IsotopX-Phoenix spectrometer (TIMS), using a single collection and a dynamic multicollection mode for Sm and Nd, respectively. The 143 Nd/ 144 Nd ratios were corrected for 142 Ce and 144 Sm interferences and normalized to 146 Nd/ 144 Nd =07219 value (O’Nions et al., 1979) in order to correct procedural and instrumental mass fractionation. Drifts from La Jolla reference value (Lugmair et al., 1983) were corrected by analyzing the standard along with the samples, yielding an average value of 143 Nd/ 144 Nd= 0.511851 for 6 replicates , with an internal Table 2 Whole rock Nd isotope data of Obejo-Valsequillo (Serie Negra) and Lower Alcudian greywackes (Late Ediacaran). Sample Sm Nd 147 Sm/144 Nd f sm/Nd 143 Nd/ 144 Nd ±StErr*10 –6 ε Nd (0) ε Nd (T) a TDM(Ma) b OV-1 5,24 26,25 0,1206 −0,39 0,512008 1 −12,3 −6,8 1692 OV-2 4,08 21,40 0,1151 −0,41 0,511955 1 −13,3 −7,4 1679 OV-3 5,80 28,22 0,1242 −0,37 0,512037 1 −11,7 −6,5 1711 OV-6 4,61 25,26 0,1103 −0,44 0,512050 2 −11,5 −5,2 1463 OV-7 4,29 24,23 0,1069 −0,46 0,511776 1 −16,8 −10,3 1807 OV-8 5,33 27,62 0,1167 −0,41 0,512137 1 −9,8 −4,0 1421 OV-9 4,56 22,87 0,1204 −0,39 0,511953 2 −13,4 −7,9 1778 OV-10 4,54 23,11 0,1187 −0,40 0,511956 1 −13,3 −7,7 1741 OV-11 4,49 25,86 0,1049 −0,47 0,511588 1 −20,5 −13,9 2040 OV-12 4,27 23,35 0,1105 −0,44 0,511906 2 −14,3 −8,1 1678 OV-13 4,25 21,27 0,1209 −0,39 0,512065 2 −11,2 −5,7 1603 OV-14 3,56 17,76 0,1213 −0,38 0,512032 1 −11,8 −6,4 1664 OV-15 3,25 16,28 0,1205 −0,39 0,511877 1 −14,8 −9,4 1907 OV-16 2,34 10,24 0,1382 −0,30 0,512049 2 −11,5 −7,3 2006 OV-17 4,52 23,62 0,1157 −0,41 0,512038 1 −11,7 −5,9 1560 OV-18 4,10 21,68 0,1142 −0,42 0,511959 1 −13,2 −7,3 1659 CIA-13 3,38 16,74 0,1220 −0,38 0,512280 2 −7,0 −1,6 1269 CIA-14 3,78 18,81 0,1214 −0,38 0,512269 2 −7,2 −1,8 1278 CIA-15 3,28 16,10 0,1230 −0,37 0,512291 2 −6,8 −1,4 1263 CIA-16 3,18 15,57 0,1236 −0,37 0,512293 2 −6,7 −1,5 1268 CIA-17 3,68 18,18 0,1223 −0,38 0,512290 2 −6,8 −1,4 1256 CIA-18 3,24 15,79 0,1242 −0,37 0,512293 1 −6,7 −1,5 1277 CIA-19 3,49 17,07 0,1236 −0,37 0,512277 2 −7,0 −1,8 1295 CIA-20 3,40 16,77 0,1224 −0,38 0,512266 1 −7,2 −1,9 1296 CIA-21 6,33 29,50 0,1298 −0,34 0,512310 2 −6,4 −1,6 1329 CIA-22 6,69 31,95 0,1265 −0,36 0,512305 2 −6,5 −1,4 1290 CIA-23 5,33 26,38 0,1221 −0,38 0,512258 2 −7,4 −2,0 1305 CIA-24 4,86 25,32 0,1161 −0,41 0,512188 2 −8,8 −3,0 1334 OV - Ediacaran Serie Negra greywackes (Obejo-Valsequillo Domain). CIA - Ediacaran Lower Alcudian greywackes (Central Iberian Zone; Fuenlabrada et al., 2016). a eNd (T) calculated for 565 Ma. b Nd model ages after DePaolo (1981). Decay constant for 147 Sm: 6.54 × 10 −12 y 1 (Lugmair and Marti, 1978). Present-day CHUR parameters: 147 Sm/ 144 Nd= 0.1967; 143 Nd/ 144 Nd = 0.512638 (Jacobsen and Wasserburg, 1980). E. Rojo-Pérez et al. 3UHFDPEULDQ 5HVHDUFK ² &KDSWHU