serie NOVA TERRA nº 49

139 Eu/Eu* = 0.48 – 0.78). Cr and Ni, common elements in sediments derived from ultrama fi c rocks, show abundances in some cases below the determination limit, and Cr/Ni ratios are very low (2.4 on average) and similar to PAAS (2.0), what along with a Th/Cr ratio higher than PAAS are good indicators of a felsic provenance. La/Th (3.7), Th/Sc (1) and Th/Co (1) average values are in agreement with the felsic character of the sources too ( Cullers, 2002 ). The samples have La/Th, Th/Sc and Th/Co ratios similar or even higher than PAAS (2.6, 0.9 and 0.6 respectively; Taylor and McLennan, 1985 ) and UCC (2.7, 0.7 and 0.7, respectively; Condie, 1993 ), what means higher content in La and Th, elements preferentially accumulated in felsic rocks. 3.3.2. Tectonic setting No signi fi cant deviations in the geochemical composition relative to PAAS have been observed in the Ediacaran metasedimentary rocks of the Évora Massif. Moreover, the lower CIA values of the samples, compared to those of PAAS, rule out a strong weathering and alteration during post-deposition processes. Consequently, the samples are suitable for the use of provenance and tectonic setting discrimination diagrams based on immobile trace elements. Following Bhatia and Crook (1986) , the chemical discrimination of the Ediacaran greywackes shows a clear af fi nity to active continental margin settings ( Fig. 5 c and d). Most of our samples plot into or very close to fi eld B, suggesting deposition on the realm of a continen- tal arc. This result is also supported by average values of La/Th, Ti/Zr, Zr/Th and Zr/Hf ratios (3.7, 22.8, 16.1 and 39.0, respectively), which are close to the values proposed by Bhatia and Crook (1986) for greywackes deposited in a continental arc setting, in contrast with the typical values displayed by sediments deposited over a thick con- tinental margin. -1.0 -0.5 0.0 0.5 1.0 1.5 1.0 0.5 0.0 log(SiO 2 /Al 2 O 3 ) log(Fe 2 O 3 /K 2 O) Fe-shale Fe-sandstone shale litharenite arkose wacke Present work Pereira et al. (2006) 1 10 100 1000 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sample/Chondrite Escoural Unit (Basal allochthon of SW Iberia) PAAS Th Sc La C, D B A Sc Zr/10 Th A B C D Present work Pereira et al. (2006) A - Oceanic island arcs B - Continental island arcs C - Active continental margins D - Passive margins 0.01 0.10 1.00 10.00 Cs Ba Rb Th U K Nb La Ce Sr Nd P SmZr Hf Eu Ti Dy Y Er Yb Sc Cr Ni Sample/PAAS Escoural Unit (Basal allochthon of SW Iberia) PAAS (a) (b) (c) (d) (e) Fig. 5. Chemical diagrams for the Ediacaran siliciclastic metasedimentary rocks from the Escoural Unit (basal allochthonous units of SW Iberian Massif). Previous data obtained by M.F. Pereira et al. (2006) are indicated. (a) Chemical classi fi cation (after Herron, 1988 ). (b) Chondrite normalized REE plot. The dotted line corresponds to the PAAS (Post Archean Australian Shale; Taylor and McLennan, 1985 ). Normalizing values are from Nakamura (1974) . (c) and (d) are trace element diagrams with tectonic setting discrimination fi elds (after Bhatia and Crook, 1986 ). (e) PAAS-normalized trace elements diagrams (PAAS after Taylor and McLennan, 1985 ). 293 R.D. Fernández et al. / Lithos 268 – 271 (2017) 285 – 301