serie NOVA TERRA nº 49

119 Although tectonic setting discrimination diagrams should be used carefully, those developed by Bhatia and Crook (1986) have proved to be an invaluable tool to distinguish between four tectonic settings for sandstone deposition: (A) oceanic island arc, (B) continental island arc, (C) active continental margin, and (D) passive margin. A plot of Ti/Zr versus La/Sc clearly reveals the Late Ediacaran greywackes as de- posited in an active margin ( Fig. 6 a). La/Sc ratios range from 2.3 to 3.3, whereas Ti/Zr ratios are all above 10 (14 – 21.6), fairly far from expected values of a passive geodynamic setting. Ternary diagrams (La – Th – Sc, Th – Co – Zr/10 and Th – Sc – Zr/10) suggest a similar environment of sedimentation for the greywackes, as they all plot well inside fi eld B (active continental margin; Fig. 6 b, c and d, respectively). Such results constrain the deposition of the Late Ediacaran greywackes to basins related to convergent dynamics, most likely in a continental volcanic arc built on a thinned continental margin ( Bhatia and Crook, 1986 ). A set of major and trace elements was normalized to PAAS following Thompson (1982) ( Fig. 7 ). The pattern for the Ediacaran greywacke samples is consistent with the main features observed by Winchester and Max (1989) in sediments accumulated in active margins. The diagram shows a decrease in most LILE elements (Cs, Rb, Th, U, La, Ce and K 2 O), generating a fractionation of positive slope, while values for HSFE elements (Zr, Hf, Sm, HREE and Sc) are close to unit ( Fig. 7 a). A negative TiO 2 anomaly can be observed. There is no signi fi cant positive Sr anomaly, possibly due to weak alteration in post-depositional pro- cesses, in agreement with low CIA values (63, on average) ( Table 1 ). Signi fi cant lower Cr and Ni values suggest a mixed source, related to an evolved (felsic) volcanic arc. In the shales, the patterns of major and trace elements contents nor- malized to PAAS are similar to those described by Winchester and Max (1989) for passive continental margins ( Fig. 7 b). This is typi fi ed by normalized values of Rb, Th, U, K, La, Ce and Y close to the unit, and by the existence of negative anomalies of variable intensity in the case of Sr, P and Ti. According to those patterns, the Cambrian shales may have been deposited away from an arc edi fi ce, and thus relatively far from the areas of greatest magmatic activity and closer to emerged sec- tions of a mainland. In a plot of Th/U versus Th ( McLennan et al., 1993 ), the two sedimen- tary sequences present clearly different trends ( Fig. 8 ). The Cambrian shales display Th/U values above 3.5 – 4.0, typical for (i) igneous rocks emplaced at upper continental crust, (ii) cratonic provenance, and (iii) certain alteration and recycling. On the contrary, the Ediacaran greywackes show lower Th/U values, lower Th, and slightly lower U abundances ( Table 1 ), all of which are probably inherited from the source rocks and associated with a depletedmantle (juvenile) contribu- tion ( Fig. 8 ). 4.3. Sm – Nd isotope geochemistry Sm – Nd isotopic data obtained from the greywackes and shales are given in Table 5 and 6 and plotted in Fig. 9 . Geochronological data for calculation of ε Nd (T) were estimated according to stratigraphic and structural features of the selected samples. The reference age considered for the sedimentation of the Ediacaran greywackes and Cambrian shales is 565 Ma and 530 Ma, respectively (see section Geological setting ). The analyzed rocks show 147 Sm/ 144 Nd ratios varying from 0.1161 to 0.1315, which are normal values for clastic sediments (0.1 to 0.13; Zhao et al., 1992 ), and similar to those of continental crust (~0.12). These ratios are also lower than the upper limit ( 147 Sm/ 144 Nd = 0.165) proposed by Stern (2002) as suitable to perform Nd TDM calculations. The samples show relatively uniform ε Nd( 0 ) values within each group (an average of − 9.7 and − 7.1 for the shales and greywackes, re- spectively). In the case of the Early Cambrian shales, ε Nd( 530 ) values a b Fig. 7. PAAS-normalized trace elements diagrams. (a) Late Ediacaran greywackes. (b) Early Cambrian shales. PAAS after Taylor and McLennan (1985) . Fig. 8. Th/U versus Th plot after McLennan et al. (1993) . See text for discussion. 25 J.M. Fuenlabrada et al. / Tectonophysics 681 (2016) 15 – 30