serie NOVA TERRA nº 49

57 development of the basal Ponte Carreira detachment (PCD; Fig. 3 ). This basal extensional detachment prevents the identi fi cation of original relationships with the underlying part of the intermediate-pressure upper units. From a lithological point of view the uppermost terrigenous series can be divided into two members: (1) a lower member with a maximum thickness of c. 1 km consisting of black metapelites with intercalations of grey to black quartzites and lydites of variable thickness; and (2) an upper member, c. 2 km thick, that has a fl yschoid appearance and consists of alternations of metagreywackes and grey to black metapelites with conglomeratic intervals and minor green phyllites and calcsilicate layers. The entire succession appears to represent an upward shoaling megasequence. Two detailed partial stratigraphic columns were previously measured and studied by Gutiérrez Alonso et al. (2000) , who identi fi ed facies indicating various settings within a deep submarine fan model (mainly lower-middle fan and upper fan, with a few facies representing the slope). The identi fi ed facies' association suggests a type II turbiditic system ( Mutti, 1985 ), mostly formed by channel and sand lobe complexes. Conglomeratic levels consist of pebbles of granitic rocks, quartz and greywacke intraclasts. Most of the sandstones can be classi fi ed as feldespathic greywackes with a framework of quartz and weakly altered plagioclase. Rock fragments of vitric and microgranular texture are common in polymictic conglomerates and coarse-grained greywackes, together with slates, cherts and bipyramidal volcanic quartz fragments. Although recrystallization under greenschist facies conditions in the chlorite and biotite zones and the presence of two cleavages hinder detailed textural analysis, the sandstones are typical immature, fi rst-cycle sandstones with angular to subangular, poorly sorted grains in a muddy matrix. Heavy minerals are dominated by unabraded zircons and less abundant epidote and rutile. Detrital zircons of two metagreywacke samples collected near Ares and Redes ( Fig. 4 ) were studied by Fernández-Suárez et al. (2003) in order to constrain the provenance of the greywacke series and its maximum depositional age. The age groups of the zircons (480 – 610 Ma, 1900 – 2100 Ma and 2400-2500 Ma) and the absence of Mesoproterozoic zircons suggest an origin in a Neoproterozoic – Early Palaeozoic peri-Gondwanan realm along the periphery of the West African Craton. The maximum depositional age of the greywackes, initially thought to be c. 480 Ma (Early Ordovician), was reassessed in this study. Using only analyses with<10% discordance (shown in Fig. 4 for the youngest group of detrital zircons), the maximum depositional age can be estimated at c. 530 Ma, based on the average age of the largest group of youngest zircons ( Elliot and Fanning, 2008 ). However, it is possible that the age of the youngest zircon in this group is signi fi cant, in which case the maximum age of deposition could be as young as 510 – 520 Ma. 4. Whole rock geochemistry The chemical composition of sedimentary rocks depends on numerous factors, including the nature of the source areas, and the subsequent processes, such as weathering, diagenesis or metamor- phism. Likewise, the tectonic setting in which the sedimentary basin developed also exerts a signi fi cant control over the fi nal composition of the resulting rocks ( Bathia, 1983; Bathia and Crook, 1986; Ranjan and Banerjee, 2009; Hegde and Chavadi, 2009 ). The abundance of some elements, such as rare earth elements (REE), Hf, Ti, Cr, Co, Zr, Nb, Y, Th and Sc, is preserved in sedimentary rocks through the weathering processes. These elements have very low residence times in oceanic waters, being transferred almost quantitatively to sedi- mentary rocks. Thus, they provide excellent discriminating factors for determining the provenance and tectonic setting of sedimentary rocks in both ancient successions and far travelled terranes. Twenty metagreywacke samples from the sedimentary series that constitutes the upper levels of the Órdenes Complex (intermediate- Table 2 Whole rock rare earth element data of the top metagreywackes from the Órdenes Complex. Sample SO-1 SO-2 SO-3 SO-4 SO-5 SO-6 SO-7 SO-8 SO-9 SO-10 SO-11 SO-12 SO-13 SO-14 SO-15 SO-16 SO-17 SO-18 SO-19 SO-20 La 24.4 18.6 25.7 21.8 26.0 15.7 23.2 26.9 24.3 29.0 26.3 31.3 21.1 21.9 17.9 12.9 33.1 22.8 34.6 23.6 Ce 50.1 35.2 56.6 41.5 79.6 36.3 49.0 54.9 50.3 61.7 57.2 62.4 43.4 40.2 35.1 29.0 50.6 34.1 49.9 43.6 Pr 6.05 50.70 6.51 5.51 6.69 4.44 5.80 6.05 6.12 7.06 6.62 7.40 4.96 5.26 4.35 3.50 8.51 6.30 8.20 5.67 Nd 24.5 20.1 25.2 22.1 27.0 17.2 22.6 23.2 24.1 28.2 25.9 29.7 19.1 20.6 17.3 14.1 33.0 25.2 31.4 22.1 Sm 5.09 4.18 5.32 4.55 5.84 3.67 4.62 4.53 5.08 5.82 5.45 6.12 3.97 4.00 3.62 3.26 6.88 5.72 6.49 4.53 Eu 1.38 1.19 1.29 1.28 1.48 0.99 1.22 1.20 1.21 1.41 1.32 1.59 1.05 1.04 0.94 0.87 1.74 1.50 1.75 1.30 Gd 4.72 3.49 4.52 3.95 5.52 3.23 3.84 3.92 4.77 5.20 4.87 5.84 3.51 3.59 3.30 3.05 6.43 5.81 6.31 4.04 Tb 0.82 0.62 0.80 0.67 0.98 0.61 0.66 0.67 0.85 0.93 0.90 1.06 0.65 0.62 0.59 0.62 1.11 1.08 1.12 0.71 Dy 4.96 3.88 4.85 4.07 5.99 4.04 4.30 3.80 5.25 5.48 5.39 6.22 4.00 3.75 3.51 4.13 6.74 6.86 6.92 4.32 Ho 0.98 0.77 0.91 0.80 1.19 0.87 0.90 0.72 1.03 1.07 1.05 1.22 0.78 0.72 0.69 0.85 1.26 1.39 1.36 0.85 Er 2.90 2.37 2.79 2.40 3.61 2.74 2.83 2.08 3.11 3.21 3.18 3.64 2.37 2.20 2.13 2.76 3.78 4.22 4.02 2.56 Tm 0.446 0.386 0.455 0.380 0.569 0.449 0.471 0.333 0.482 0.513 0.502 0.558 0.370 0.339 0.345 0.484 0.597 0.686 0.602 0.395 Yb 2.92 2.67 3.11 2.59 3.71 3.05 3.18 2.23 3.17 3.40 3.32 3.70 2.44 2.30 2.30 3.33 3.91 4.54 3.89 2.63 Lu 0.45 0.42 0.5 0.4 0.57 0.48 0.49 0.34 0.51 0.52 0.516 0.589 0.374 0.358 0.348 0.523 0.850 0.670 0.596 0.380 Σ REE 130 99 139 112 169 94 123 131 130 154 143 161 108 107 92 79 158 121 157 117 Eu/Eu ⁎ 0.87 0.96 0.81 0.93 0.80 0.88 0.89 0.88 0.76 0.79 0.79 0.82 0.86 0.84 0.84 0.85 0.80 0.80 0.84 0.93 (La/Sm) N 2.96 2.75 2.98 2.96 2.75 2.64 3.10 3.66 2.95 3.07 2.98 3.16 3.28 3.38 3.05 2.44 2.97 2.46 3.29 3.21 (Gd/Yb) N 1.29 1.04 1.16 1.22 1.19 0.84 0.96 1.40 1.20 1.22 1.17 1.26 1.15 1.24 1.14 0.73 1.31 1.02 1.29 1.22 (La/Yb) N 5.59 4.66 5.53 5.63 4.69 3.44 4.88 8.07 5.13 5.7 5.30 5.66 5.78 6.37 5.20 2.59 5.66 3.36 5.95 6.00 Rare earth element data in parts per million (ppm). 344 J.M. Fuenlabrada et al. / Gondwana Research 17 (2010) 338 – 351