serie NOVA TERRA nº 49

118 plagioclase during Early Cambrian times, con fi rmed by values of the Al 2 O 3 /(CaO + Na 2 O) ratio, ranging from 6.3 to 10.3. However, deep changes in composition are ruled out by the Th/U values (average 4.2), which are similar to the upper crust (3.8). High MgO, Fe 2 O 3 and Al 2 O 3 along with lower SiO 2 content point to a mixture of sources. Very low values of CaO relative to PAAS (1.30 wt.%) on the other hand suggest a dominant pelitic contribution. Despite the high variability in the Al 2 O 3 contents, positive correlations between this oxide and Fe 2 O 3 and MgO indicate predominance of clay minerals and micas, con fi rmed by K 2 O/Al 2 O 3 values (0.18 average), between 0.0 and 0.3 ( Cox et al., 1995 ). The shale samples show very similar REE contents ( Table 4 ), with REE values ranging from186.4 to 206.6 ppm. They also display very sim- ilar chondrite-normalized ( Nakamura, 1974 ) fractionation patterns ( Fig. 5 b), with (La/Yb) N values between 8.18 and 9.15. LREE (La – Sm) show moderate enrichment relative to HREE, with almost fl at patterns, and (Gd/Yb) N values between 1.38 and 1.91. All samples present a slight negative Eu anomaly, with very homogeneous values between 0.64 and 0.70 (calculated following Taylor and McLennan, 1985 ), usually interpreted as an inheritance from an igneous source ( Awwiller, 1994; Compton et al., 2003; McLennan and Taylor, 1991; McLennan et al., 1980 ). Despite some differences in abundance between samples, the REE patterns of the shales are similar to those of PAAS. The content of trace elements is rather homogeneous and only shows very slight variations with respect to PAAS. Elements that are typically associated with heavy minerals, such as Zr, Nb, Th and U, have lower values compared to PAAS, except for Y, which is slightly higher. Co, Cu and Rb contents are similar to those of PAAS. The different Sr and Ba contents, related to PAAS, may be due to selective alteration of plagioclase relative to other feldspars. The high coef fi cient of correlation between Rb and K 2 O (r = 0.88) favors the same behavior in the case of K-feldspar. Those two elements, together with Ba, seem to have remained unaffected after chemical alterations. Shale samples show a Th/Sc ratio ( Table 2 ) slightly lower than PAAS, indicating a mixture of sources. This statement is con fi rmed by values of La/Sc and Ti/Zr equal to PAAS (La/Sc = 2.4, Ti/Zr = 29.1; averages); and values of Zr/Sc slightly lower than PAAS. The concentrations of Cr (average 97.5 ppm) and Ni (average 49.2 ppm) are very similar to PAAS, but the values of Ti/Nb (297 on average) are low. Altogether these ratios con fi rm a contribution from different sources, although dominated by a felsic input and affected by recycling processes to some extent. 4.1.3. Th – Hf – Co diagram A Th – Hf – Co ternary diagram ( Fig. 4 b) shows different patterns for both groups of rocks, with a greater scatter in the case of the greywackes, likely inherited from a more dynamic environment and fractionation effects by slight weathering and/or recycling. This diagram supports a mixed source for both the greywackes and shales, with a slight trend in the case of the shales to a more granitic average, clearly close to PAAS values. Co/Th vs. La/Sc diagram ( Fig. 4 c) reinforces the dominantly felsic provenance for the shales too (close to the Co/Th value of 1.27). The greywackes preserve the scatter observed in former diagrams, what together with signi fi cantly greater Eu anomalies than PAAS favor the idea of sediments eroded from an intermediate (andes- ite – diorite) and recycled sedimentary sources ( Taylor and McLennan, 1985 ). 4.2. Tectonic setting According to the results here exposed, we do not observe atypical deviations of the Ediacaran greywackes and Early Cambrian shales chemistry from reference compositions (PAAS), ruling out signi fi cant post-depositional alterations as a result of metamorphism. Likewise, the content in certain LILE (Rb, Cs) that are linked tomicas and claymin- erals is comparable to that of unaltered rocks, whereas lowNa, Ca and Sr contents might indicate slight weathering, but with no signi fi cant consequences for using provenance and tectonic setting discrimination diagrams based on immobile trace elements (REE, Y, Zr, Ti, Nb, Th, Sc, Hf and Co). a b c d Fig. 6. Trace element diagrams with tectonic setting discrimination fi elds for Lower Alcudian greywackes ( Bhatia and Crook, 1986 ). 24 J.M. Fuenlabrada et al. / Tectonophysics 681 (2016) 15 – 30