serie NOVA TERRA nº 49

138 There is neither a large variation in major element contents, nor sig- ni fi cant differences relative to PAAS (Post Archean Australian Shale; Taylor and McLennan, 1985 ). Sample average values fi t accurately those displayed by the average shale, with homogenous SiO 2 , Al 2 O 3 , Fe 2 O 5 , CaO, K 2 O, Ti 2 O and P 2 O 5 contents (61, 17.5, 6.2, 0.9, 3.5, 0.7 and 0.2 avg. wt%, respectively). Higher Na 2 O contents (3.7 wt%) diverge from those of PAAS (1.2 wt%), as expected for albite-rich rocks such as the ones analyzed here. Chemical Index of Alteration (CIA; Nesbitt and Young, 1982 ) show values that correspond to low weathered rocks, ranging from 56 to 65, and with an average value of 61, far from those typical for average shale (70 – 75; McLennan et al., 1993 ) ( Table 1 ). No important alteration at the source area is deduced from PIA values (Plagioclase Index of Alteration; Fedo et al., 1995 ), which vary between 57 and 75, with an average value of 65, the latter revealing to be lower than PAAS (79), thus indicating no signi fi cant transformation from plagioclase/K-feldspar to clay minerals. This is also suggested by Na 2 O and K 2 O contents (average values of 3.7 and 3.5 wt%, respectively), very close to those of the upper continental crust (UCC: 3.35 and 3.10 wt%, respectively; Condie, 1993 ). Negative correlation between CIA values and Ba/Sr ratios ( − 0.22) con fi rms their low weathering character. A general immature character can be inferred from the SiO 2 /Al 2 O 3 ratio (3.5 on average), which is lower than that of the UCC (4.5 on average) and indicative of a low fractionation of Al 2 O 3 from SiO 2 . In ad- dition, Al 2 O 3 /Na 2 O (5.1 on average), Al 2 O 3 /TiO 2 (25 on average) and Ti/ Zr (22.8 on average) ratios are similar to those of the UCC (4.5, 24.0 and 21.4, respectively), thus con fi rming the general immature character for these clastic sediments. Some of the samples (EV-2 to EV-7), however, show differentiated Zr/Sc and Th/Sc ratios, which indicate that those particular sedimentary beds could have undergone slight recycling processes ( McLennan et al., 1993 ). The REE-abundances of the Ediacaran metasedimentary rocks from the Évora Massif ( Table 2 ) show slight variability (bulk content ranging from 156 to 280 ppm). Their chondrite-normalized patterns are very similar to those of PAAS ( Fig. 5 b), characteristic of felsic igneous sources and upper crustal provenance. REE distribution patterns display a signif- icant LREE enrichment featuring a REE fractionation with high (La/Yb) N (6.9 – 11.4), almost fl at HREE with (Gd/Yb) N close to unity (1.3 – 1.6), and negative Eu anomalies (Eu/Eu* = 0.44 – 0.76, average of 0.61). The analyzed Ediacaranmetagreywackes vary in the Al 2 O 3 /TiO 2 ratio from 20.5 to 41.7, which fi t with an intermediate-felsic source (e.g., Roser and Korsch, 1986 ), with some of the values well above the limit of a felsic provenance ( Table 1 ). This dominant felsic character is con fi rmed by Cr/Th ratios, with an average value of 8.2 (1.6 – 17.5); and the Eu/Eu* ratio, with an average value of 0.58 (0.44 – 0.61). These numbers meet the values proposed by Cullers (1994) for sedimentary rocks with a dominant felsic provenance (Cr/Th = 2.5 – 17.5 and Fig. 4. (a) Banded appearance (sedimentary layering) of the paragneisses of the Lower Sequence. Lighter bands are richer in albite. Note the lens of felsic orthogneiss in the central part. (b) Main (external) foliation (S e , short-dashed line) of the albite-bearing paragneisses of the Lower Sequence (EV-6; quartz + biotite + white mica + plagioclase ± opaque minerals ± tourmaline).Note the obliquecharacter of the internal foliation(S i ,long-dashed line) inthealbite porphyroblasts.(c) Paragneiss oftheLower Sequence (EV-1) with albite porphyroblasts oriented to the main foliation and including an internal straight fabric that ranges from parallel to normal to the main foliation, or a folded internal fabric. (d) Internal foliation (quartz + white mica + chlorite + opaque minerals ± garnet) of an albite porphyroblast in the paragneisses of the Lower Sequence. Note the growth of biotite over this fabric. (e) Continuity between the internal and external foliation in an albite porphyroblast (EV-3) of the paragneisses of the Lower Sequence. Note the polygonal arcs (pointed line) that occurinthequartz-richbandsoftheparagneiss.(f)Albite-bearingparagneissesoftheEdiacaranseriesincludedinthebasalallochthonousunitsofNWIberia(from DíezFernández,2011 ). 292 R.D. Fernández et al. / Lithos 268 – 271 (2017) 285 – 301