Nova Terra 52

475 International Journal of Earth Sciences (2021) 110:467–485 13 Tentudía formations, in comparison with the UCC, appears in lower values of CaO content (1.93, 0.73 and 3.36 wt%, respectively). The higher amount of CaO in the Montemolín Formation than in the Tentudía Formation is likely linked with the occurrence within the Montemolín Formation of abundant mafic material (Montemolín amphibolites; Eguíluz et al. 1990). The average compositional ranges of TiO 2 , MnO, MgO and P 2 O 5 are 0.75 and 0.73 wt%; 0.07 and 0.06 wt%; 2.38 and 2.23 wt%; 0.18 and 0.20 wt%, for Monte- molín and Tentudía rock samples, respectively. Whole-rock anhydrous chemical compositions of the Montemolín and Tentudía samples have been plotted in the classification diagram published by Herron (1988; Fig. 4a). The rocks sampled in both formations appear at the out- crop as little deformed common greywackes. However, they exhibit a relative compositional heterogeneity when plot- ted in this classification diagram. A part of the Tentudía and the whole Montemolín samples appear represented into the shale field. This depiction, in the case of Montemolín Formation, seems motivated for their relatively low SiO 2 contents. While for Tentudía Formation, such plotting can be explained by its low K 2 O values, which may reflect a sig- nificant post-depositional weathering characteristic of this element. It is appropriate to use other indexes and immobile ratios for accurate measuring of the maturity and weathering of these rocks. The average values of the SiO 2 /Al 2 O 3 , K 2 O/Na 2 O, Al 2 O 3 / Na 2 O, Al 2 O 3 /TiO 2 ratios for the Montemolín Formation (4.14, 0.91, 4.99, and 21.18, respectively; Tables 1 and 2) show no significant deviation from those in the Tentudía Formation (4.34, 0.67, 4.73, and 21.27, respectively), falling within the range estimated by Condie (1993) for the UCC (4.46, 0.93, 4.47 and 24.18, respectively), which suggests an immature nature. The very low values of the K 2 O/Na 2 O and Al 2 O 3 /Na 2 O ratios, with respect to the PAAS (Post Archean Australian Shale, Taylor and McLennan 1985), confirm this immaturity as well as a certain predominance of plagioclase over K-feldspar (except for the TE-03 and TE-10 samples) and white mica in the sedimentary protoliths. The limited effect of weathering processes and the immature character of the sedimentary protoliths from both formations are also supported by a negative correlation between the SiO 2 and the rest of major elements, which is reflected in the homogene- ous and restricted major element variation of all the samples (Fig. 4b). The CIA (Chemical Index of Alteration; Nesbitt and Young 1982) and PIA (Plagioclase Index of Altera- tion; Fedo et al. 1995) values for the Tentudía Formation (avg. 63.71 and 76.30, respectively) are slightly higher than those of the Montemolín Formation (avg. 60.24 and 71.51, respectively), and lower than those of the PAAS (avg. 73.70 and 79.02, respectively). These values are consistent with a limited degree of post-depositional alteration and a scarce influence of the sedimentary transport over plagioclase and K-feldspar. The REE chondrite-normalized fractionation patterns (Nakamura 1974) in Montemolín and Tentudía formations exhibit an enrichment of the LREE (avg. La N /Sm N : 3.90 and 3.79, respectively; Tables 1 and 2), and relatively flat pat- terns for the HREE, with Gd N /Yb N values close to unity (avg. 1.64 and 1.51, respectively). The metagreywacke samples from the Tentudía Formation display REE patterns (avg. La/Yb N 9.51) closer to those of the PAAS (9.06), than samples of the Montemolín Formation (avg. La/Yb N 10.68) (Fig. 4c). All the samples show slightly negative Eu anoma- lies, whose average values for the Montemolín and Tentudía formations are 0.83 and 0.82, respectively (Eu/Eu* calcu- lated according to Taylor and McLennan 1985). Trace elements such as La, Zr, Th, Nb, Sc, Y, Ti, and Co, as well as their ratios, are useful tectonic setting and provenance discriminators for (meta)sedimentary rocks Table 2 (continued) Sample TE-01 TE-02 TE-03 TE-04 TE-05 TE-06 TE-07 TE-08 TE-09 TE-10 TE-11 TE-12 Er 2.73 2.52 2.15 2.48 1.49 2.22 2.01 2.17 2.28 2.32 2.39 2.4 Tm 0.40 0.39 0.34 0.37 0.22 0.33 0.30 0.33 0.33 0.34 0.34 0.36 Yb 2.68 2.56 2.25 2.51 1.44 2.17 1.98 2.17 2.17 2.39 2.42 2.39 Lu 0.41 0.37 0.34 0.38 0.23 0.33 0.31 0.32 0.33 0.38 0.33 0.36 ΣREE 121.08 116.99 132.93 125.64 124.50 136.21 136.26 173.75 178.36 182.12 187.27 168.31 Eu/Eu* 0.82 0.70 0.92 0.83 0.81 0.82 0.78 0.82 0.82 0.88 0.85 0.75 La N /Yb N 5.94 5.93 7.37 6.85 12.58 8.32 9.73 11.62 11.99 11.70 11.63 10.44 Gd N /Yb N 1.25 1.19 1.31 1.19 1.88 1.40 1.58 1.74 1.79 1.45 1.66 1.63 sumREE 121.08 116.99 132.93 125.64 124.50 136.21 136.26 173.75 178.36 182.12 187.27 168.31 LREE 230.98 221.29 253.14 244.35 251.37 262.74 269.84 345.80 356.03 366.11 376.89 336.18 HREE 91.31 84.95 76.63 82.41 56.15 76.98 73.08 83.53 83.03 81.67 89.00 88.26 LREE/HREE 2.53 2.60 3.30 2.97 4.48 3.41 3.69 4.14 4.29 4.48 4.23 3.81 La N /Sm N 3.14 3.42 3.39 3.60 3.68 3.68 3.71 4.04 4.02 4.86 4.14 3.83 Iberian–Bohemian correlations