serie NOVA TERRA nº 49

117 Like trace elements analysis, Sm – Nd systematics and U – Pb dating of detrital zircon have become suitable approaches to infer the evolution and the relative position of the paleo-basins, as well as the potential provenance of metasedimentary rocks ( Linnemann and Romer, 2002; Linnemann et al., 2004 ). The Sm – Nd system has been widely used as a powerful tool to represent crustal and mantle evolution. Both elements are compatible and have low coef fi cients of partition in most petroge- netic processes, being rarely affected by intracrustal processes (anatexis, fractionation or metamorphism) and processes following sedimentation (diagenesis, weathering or hydrothermal processes). These processes cause minimal effects on the Sm/Nd ratio of a given sedimentary rock, being thus completely inherited from the depleted mantle ( DePaolo, 1981; DePaolo and Wasserburg, 1976 ). Since Nd model ages (TDM) represent the average age of extraction from the de- pleted mantle of the constituents in a clastic rock, these ages have proved to be a reliable tool to constrain the provenance of sediments, as well as to reveal some of their pre-and post-depositional history ( Allègre and Rousseau, 1984 ). Similarly, Nd model ages, along with de- trital zircon ages have become crucial to infer the evolution of the con- tinental crust, especially in orogenic belts (e.g., Linnemann and Romer, 2002; Linnemann et al., 2004 ). 4. Whole rock geochemistry 4.1. Composition and classi fi cation 4.1.1. Greywackes Our greywacke samples show no signi fi cant chemical variations in major elements ( Table 1 ), with SiO 2 contents (70.1 wt.%, on average) being higher than the shale samples, and lower contents of Al 2 O 3 , Fe 2 O 3 and K 2 O (13.59 wt.%, 5.14 wt.% and 2.18 wt.%, respectively). They show very low CaO content (0.61 wt.%, on average) compared to typical upper crust (4.2 wt.%). Following Crook (1974) , all the greywacke samples are quartz-intermediate (K 2 O/Na 2 O: 0.54 – 1.15), and on the diagram proposed by Herron (1988) they plot into the greywacke fi eld ( Fig. 4 a). SiO 2 /Al 2 O 3 (4.1 to 6.0), and low K 2 O/Na 2 O (0.5 to 1.1) ratios re fl ect immature sediments, with a predominance of plagioclase over K-feldspar, and values close to those of the upper crust (4.3 and 0.9, re- spectively). Immaturity is also con fi rmed by the Al 2 O 3 /Na 2 O (3.9 – 5.6) and Al 2 O 3 /TiO 2 (16.0 – 23.5) ratios, which lay within the typical range of the upper continental crust (3.9 and 30.4, respectively; Taylor and McLennan, 1985 ). The Rb/Sr ratio varies with weathering and dia- genetic processes, being increased by the loss of Sr from plagioclase. McLennan et al. (1993) interpreted Rb/Sr values over 0.5 as related to post-depositional processes of alteration. Our greywacke samples display values between 0.4 and 0.8, very close to PAAS. A low degree of post-depositional alteration is also con fi rmed by relatively low CIA values (Chemical Index of Alteration, Nesbitt and Young, 1982 ), which range from 56 to 66 ( Table 1 ). The greywacke samples are characterized by (La/Yb) N (7.50, on av- erage) and (La/Sm) N (3.22, on average) slightly lower than PAAS (9.08 and 4.02, respectively) ( Table 3 ). Chondrite normalized ( Nakamura, 1974 ) fractionation patterns for these samples exhibit enrichment in LREE relative to HREE, which display nearly fl at patterns, with values of (Gd/Yb) N close to the unity (1.40, on average), and similar to those of the PAAS ((Gd/Yb) N : 1.34). The greywacke samples, especially those collected in the Agudo – Valdemanco Anticline (CIA-21 to CIZ- 24), show similar patterns to PAAS ( Fig. 5 a), with lower LREE values and higher dispersion of HREE abundances. All the samples show a negative Eu anomaly, whose values range from 0.60 to 0.79 (Eu/Eu*, calculated according to Taylor and McLennan, 1985 ). Th/Nb ratios (between 0.8 and 1.0) obtained for the greywackes are similar to those of PAAS (0.77), while Ti/Zr (14.0 – 21.6) and Zr/Sc (16.0 – 30.7) ratios have lower and higher values than PAAS, respective- ly. These results point to a mixed source dominated by felsic material, which is con fi rmed by lower Cr content (83 ppm, avg. value) and Ni content (37 ppm, avg. value); and by La/Th values (3.0 – 3.6) ( Fig. 4 b) higher than PAAS (2.6). 4.1.2. Shales The most relevant geochemical feature of our shale samples is their compositional homogeneity ( Table 2 ). They show SiO 2 values ranging between 58.59 and 61.36 wt.%, high values of Al 2 O 3 (18.4 – 19.39 wt.%.) and low CaO (from 0.24 to 0.93 wt.%), Na 2 O (1.43 – 2.03 wt.%.) and K 2 O contents (3.34 – 3.66 wt.%). Fe 2 O 3 (6.66 – 7.63 wt.%), MgO (2.51 – 3.00 et.%) and TiO 2 (0.72 to 0.79 wt.%) are similar to PAAS. SiO 2 and K 2 O/ Na 2 O values (1.68 – 2.56) indicate that all samples are quartz-rich ( Crook, 1974 ). According to the diagram by Herron (1988) to evaluate maturity in sedimentary rocks, all samples can be classi fi ed as shales, grouping close to the wacke fi eld ( Fig. 4 a). Al 2 O 3 , CaO, Na 2 O and K 2 O contents similar to those of PAAS make strong chemical alteration processes after sedimentation unlikely. Low values of Ti/Zr ( Table 2 ) con fi rm this assertion. Alteration of plagioclase following deposition would explain the strong Sr anomaly compared to PAAS and hence the higher Rb/Sr ratio observed in these samples (average: 1.8), well above 0.5. The CIA values for shale samples vary from 69 to 74, falling within the typical values for average shales (70 – 75) and similar to those of PAAS (70), which is considered to represent a low to moderate degree of alteration. The Sr negative anomaly could be explained after weathering and decomposition of a b Fig. 5. Chondrite normalized rare earth element plots. (a) Late Ediacaran greywackes. (b) Early Cambrian shales. The dotted line corresponds to the PAAS (Post Archean Australian Shale; Taylor and McLennan, 1985 ). Normalizing values are from Nakamura (1974) . 23 J.M. Fuenlabrada et al. / Tectonophysics 681 (2016) 15 – 30