serie NOVA TERRA nº 49

95 4.1.2. Upper sequence The metapelitic schists of this sequence show less silicic (SiO 2 =57.63 – 59.77) and more aluminic (Al 2 O 3 =14.58 – 19.82 wt.%) compositions than the detrital rocks from the lower sequence. They also have higher contents in K 2 O, Fe 2 O 3 , MgO, MnO and TiO 2 , and lower values in Na 2 O and CaO ( Table 1 ). The SiO 2 /Al 2 O 3 (2.9 – 3.1) and Al 2 O 3 /TiO 2 (21.1 – 26.2) ratios fi t those of the upper continental crust ( McLennan, 2001 ). The schists are classi fi ed as shales in the diagram of Herron (1988) . Sample B-21 is an arkose, but this is a local and very restricted composition in the upper sequence ( Fig. 3 a), and it is shown in Fig. 2 b. The total REE content ( Σ REE) ranges between 88 and 235 ppm, with moderately to highly (sample B-21) fractionated chondrite-normalized patterns ( Fig. 3 c). The schist samples B-17, B-18 and B-19 showREE pat- terns almost identical to those of the greywackes of the lower sequence, in turn similar to the average composition of the upper continental crust represented by the PAAS ( Taylor and McLennan, 1985 ). All the samples show a slight to pronounced Eu anomaly (Eu/Eu*=0.71 – 0.20). The samples with shale composition have an average Th/Sc value of 0.7, slightly lower than PAAS and probably suggesting the presence of an important volume of ma fi c rocks in the source area. This conclusion is also based on the Ti/Zr (39.3; PAAS=28.2) and Zr/Sc (6.6; PAAS=13.1) ratios, as well as on the high V contents (124 – 164 ppm). 4.2. Tectonic setting The tectonic discrimination diagrams developed by Bathia and Crook (1986) allow to distinguish clearly among the four tectonic set- tings considered to be the most common environments for greywacke deposition: (A) oceanic island arc, (B) continental island arc, (C) active continental margin, and (D) passive margin. Only siliciclastic rocks from the lower sequence showing greywacke com- positions were plotted in these diagrams ( Fig. 4 ). The schists from the upper sequence do not meet the requirements (pelitic composi- tion) to be plotted in them, and were consequently ruled out for tec- tonic discrimination studies. The diagrams presented in Fig. 4 allow to discard the type-A set- ting for the sedimentation of these greywackes, as their origin seems clearly related to a tectonic setting with the presence of an older continental crust. In the Th-Co-Zr diagram the metagreywackes generally plot in the B fi eld, which points to a basin located either in an island arc built on a mature continental crust, or in a magmatic arc built on a thinned continental margin. On the other hand, the Th-Sc- Zr and La-Th-Sc diagrams suggest some passive margin af fi nity. This af fi nity is especially clear in Fig. 4 b, but can be also deduced from the diagram 4c where it is not possible to differentiate type-C or type-D greywackes, although the incompatibility with type-C is clear after projections in Fig. 4 a and b. In summary and according to the diagrams of Bathia and Crook (1986) , the metagreywackes from the lower sequence were generated in relation to a magmatic arc built on a thinned continental margin, but probably closer to the main continental domain and to certain distance from the areas with the most important magmatic activity. This setting would prob- ably explain the observed relative af fi nity to passive margins. A back- arc setting or a retro-arc setting would explain the compositions ob- served in the metagreywackes, as well as the whole sedimentary characteristics of the lower sequence. This tectonic setting was previ- ously suggested by Díez Fernández et al. (2010) and it will be exam- ined in more detail in a next section. Fig. 5 shows PAAS-normalized plots of the most signi fi cant ele- ments for tectonic setting discrimination. The diagrams are plotted according to the criteria of Thompson (1982) . The patterns de fi ned by the metagreywackes ( Fig. 5 a) are quite similar to those typical of continental island arc or active margins ( Winchester and Max, 1989 ). The plots are characterized by depletion in most of the large ion lithophile elements (LILE: Cs, Rb, Th, Ce and K 2 O), which deviate slightly from 1, with the exception of La, U and P 2 O 5 . The high fi eld strength elements (HFSE: Zr, Hf, HREE, Sm, TiO 2 and Sc) are generally close to 1, with a slight characteristic negative TiO 2 anomaly, typical of this type of greywackes ( Winchester and Max, 1989 ). In general a fl at pattern is observed, with an average close or below 1. It can be also stressed the general absence of a pronounced Sr anomaly, typical of the most characteristic passive margins. The schists of the upper sequence showmore variable patterns, specially when the anomalous Fe-shale Shale Arkose Wacke Litharenite Sublitharenite Subarkose Fe-sand 0 0.5 1 1.5 1 0.5 0 -0.5 -1 log (Fe 2 O 3 /K 2 O) log (SiO 2 /Al 2 O 3 ) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 1 10 1000 Sample / Chondrite 100 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 1 10 1000 Sample / Chondrite 100 a b c Santiago schists (upper sequence) Ceán Schists (upper sequence) Metagreywackes (lower sequence) Albite schists (lower sequence) Fig. 3. Chemical diagrams for the metasedimentary rocks from the basal units of NW Iberian Massif. (a) Classi fi cation diagram ( Herron, 1988 ). (b, c) Chondrite- normalized rare earth elements plots for the metasedimentary rocks of the lower and upper sequences, respectively; the dotted line corresponds to the PAAS (Post Archean Australian Shale; Taylor and McLennan, 1985 ). Normalizing values are from Nakamura (1974) . 202 J.M. Fuenlabrada et al. / Lithos 148 (2012) 196 – 208