serie NOVA TERRA nº 49

58 pressure upper units) were collected in order to study their geo- chemistry, provenance and tectonic setting. Samples were collected along three sections on the coastline surrounding Redes and Ares ( Fig. 4 ). Sample preparation was carried out at Universidad Complu- tense de Madrid, and whole rock major and trace elements analyses were performed at Activation Laboratories Ltd. (Actlabs) in Canada. The method used for sample digestion was fusion with lithium metaborate/tetraborate, and the analytical techniques for major and trace element determination were ICP-OES and ICP-MS, respectively. The chemical analyses of the greywackes are shown in Tables 1 and 2 . 4.1. Composition and classi fi cation The analysed greywackes are characterized by variable SiO 2 contents (59.8 – 75.7 wt.%), with an average of 65.7 wt.%. Only three samples have SiO 2 contents higher than 70 wt.% (SO-13 to SO-15). The grewackes have relatively high, homogeneous Na 2 O contents (2.5 – 3.9 wt.%), with an average of 3.1 wt.%, and relatively low and variable contents in CaO (0.1 – 3.1 wt.%) and K 2 O (1.5 – 3.4 wt.%), with averages of 1.1 and 2.5 wt.%, respectively. The compositional ranges of the remaining major elements are Al 2 O 3 (11.4 – 18.1 wt.%), Fe 2 O 3 (3.6 – 8.0 wt.%), MnO (0.04 – 0.15 wt.%), MgO (1.0 – 2.9 wt.%), TiO 2 (0.54 – 0.89 wt.%) and P 2 O 5 (0.07 – 0.18 wt.%). Based on the SiO 2 contents and the K 2 O/Na 2 O ratio (0.4 – 1.2), most of the greywackes are quartz-intermediate ( Crook, 1974 ). A negative correlation exists between SiO 2 and Fe 2 O 3 , MgO, MnO, CaO and TiO 2 , whereas a positive correlation exists between TiO 2 and Al 2 O 3 . There is also signi fi cant scatter in the SiO 2 /K 2 O and SiO 2 /Na 2 O ratios due to the high mobility of Na and K during alteration processes. The chemical classi fi cation of sedimentary rocks differentiates between mature and immature sediments. One of the most widely used parameters is the Fe 2 O 3 /K 2 O ratio ( Herron, 1988 ), which is especially applicable to arkoses. This ratio is better than the Na 2 O/K 2 O ratio used by Pettijohn et al. (1972) , and can be applied to uncon- solidated fi ne- to coarse-grained sediments. Based on the chemical classi fi cation diagram by Herron (1988) , most of the analysed samples cluster in the greywacke fi eld ( Fig. 5 a). Only samples SO-14 and SO-15 fall in the fi eld of litharenites. The SiO 2 /Al 2 O 3 ratio of the samples is low (3.3 – 6.7), which is indicative of immaturity. The variable K 2 O/Na 2 O ratio (0.4 – 1.2) and average value of 0.8, can be interpreted in the same way ( Asiedu et al., 2004; Spaletti et al., 2008 ). The Rb/Sr ratio, which also re fl ects recycling processes, varies between 0.15 and 0.88, with an average value of 0.5. Weathering processes and, in some cases, diagenesis can lead to an important increase of the Rb/Sr ratio, due largely to Sr loss during plagioclase alteration. Values higher than 0.5 are interpreted by McLennan et al. (1993) as a weathering and sedimentary recycling index. An average value of 0.5 in the greywackes indicates a certain increase relative to the average value of the Rb/Sr ratio in the upper crust (0.32; Taylor and McLennan, 1985 ). This, in turn, suggests that alteration processes during the sedimentary history of the greywackes were minor. The effects of homogenization in sedimentary processes result in a relatively uniform distribution of REE in detrital rocks, the pattern re fl ecting the abundance of REE in the upper crust. REE are generally considered to be immobile, with only slight changes during sedi- mentation processes. The results of the REE analysis can be seen in Table 2 . The analysed greywackes show little variability in Σ REE, with values ranging between 79 (sample SO-16, with a marked depletion in LREE) and 169 (sample SO-5, with a pronounced positive Ce anomaly). The samples also show similar chondrite-normalized ( Nakamura, 1974 ) fractionation patterns, with slight enrichment in LREE (La – Sm) relative to HREE ( Fig. 5 b). Likewise, the samples show a weak negative Eu anomaly, which varies from 0.76 to 0.96 (calculated according to Taylor and McLennan, 1985 ). Eu anomalies are usually interpreted in sedimentary rocks as being inherited from the igneous source rocks. The samples also display an unfractionated HREE pattern. As can be seen in Fig. 5 b and c, despite some intersample differences in abundance, the REE patterns of the greywackes are similar to those of PAAS (Post Archean Australian Shale; Taylor and McLennan, 1985 ), which are considered representative of upper continental crust. The depletion of HREE relative to LREE is usually related to low concentrations of heavy minerals, especially zircon. Here, the low concentration of Zr in the greywackes (average=187 ppm) is consistent with that interpretation. Almost all the samples have Gd N /Yb N values between 1.0 and 2.0 (0.73 – 1.40) and low Eu/Eu ⁎ ratios (0.76 – 0.96), indicating a provenance from upper continental Fig. 5. Chemical diagrams for the metagreywackes from the uppermost levels of the Órdenes Complex. (a) Classi fi cation diagram ( Herron, 1988 ). (b) Chondrite-normalized rare earth elements plots; the dotted line corresponds to the PAAS (Post Archean Australian Shale; Taylor and McLennan, 1985 ). (c) Compositional range of the chondrite- normalized rare earth elements plots. Normalizing values are from Nakamura (1974) . 345 J.M. Fuenlabrada et al. / Gondwana Research 17 (2010) 338 – 351