serie NOVA TERRA nº 49

140 If compared to PAAS in a multivariation diagram ( Fig. 5 e), the patterns obtained are consistent with those exhibited by sediments deposited on an active margin setting ( Winchester and Max, 1989 ). REE patterns show a relatively positive slope from a slight depletion displayed by most LILE elements (Cs, Th, Rb, Sr and K) to HSFE elements contents (Ti, Zr, Hf, Sc, Sm and HREE), very close to those of PAAS, with a characteristic negative Ti anomaly ( Fig. 5 e). Unlike Winchester andMax conclusions, no signi fi cant P 2 O 5 depletion is observed, probably because of the presence of some apatite and monazite, which exert a high in fl u- ence on P 2 O 5 concentration. 3.3.3. Sm – Nd isotope geochemistry The Nd model ages of the high-P metasedimentary rocks of the Edi- acaran series of the Évora Massif are shown in a ε Nd vs Age diagram ( Fig. 6 ). 147 Sm/ 144 Nd values range from 0.1117 to 0.1244 (average of 0.1168), and are similar to those of the continental crust (ca. 0.12). All the ratios are under 0.165, which is considered the upper limit applica- ble for T DM calculations ( Stern, 2002 ). The ratios are also coherent with those proposed as normal values for clastic sediments (0.1 – 0.13; Zhao et al., 1992 ). The samples have an ε Nd (0) average value of − 13.1, while ε Nd (560) values vary from − 10.2 to − 4 (maximum depositional age of ca. 560 – 550 Ma; Pereira et al., 2008 ). Nd T DM model ages ( DePaolo, 1981 ) are early Mesoproterozoic – Paleoproterozoic (1499 – 1853 Ma), with an average value of 1692 Ma. 4. Correlation of major tectonic units from NW Iberia and the Évora massif A reinterpretation of lithostratigraphy and structure in the Évora Massif leads to the conclusion that it can be correlated with equivalent units in NW Iberia ( Díez Fernández and Arenas, 2015 ). In both areas, high-grade domains lacking an initial high-P overprint occur under a strongly deformed sequence of Ediacaran and Cambrian schists, paragneisses and orthogneisses that experienced high-P metamor- phism in Late Devonian times. These gneisses are overlain by a sequence lacking high-P metamorphism and dominated by metabasites with MORB-af fi nity that may alternate with some layers of metasedimentary rocks. Such piling of tectonometamorphic units seems to mimic the nappe stack de fi ned by the basal and ophiolitic allochthonous units in NW Iberia, and is, consequently, considered here yet another klippe of the allochthonous complexes of the Iberian Massif ( Fig. 1 ). Correlatives of the upper allochthonous units that are located on top of the ophiolitic units do not occur in the Carvalhal-Cabrela synform. The erosional na- ture of the basal contact of the overlying Cabrela Unit (Carboniferous syn-orogenics) may well explain such absence. Following this interpretation for the Évora Massif, the Escoural and Monfurado units together would be a correlative to the lower sequence of the basal allochthonous units of NW Iberia. The upper sequence of the basal units seems to be absent in the Cabrela-Carvalhal synform, since a series dominated by mica schists, containing minor metabasites, and af- fected by conspicuous high-P metamorphism, is yet to be found in this area. A plausible reason for its lack in this particular section of SW Iberia is that the basal contact of the Carvalhal Unit is an extensional detach- ment. Note how its basal contact cuts the underlying gneisses in the Cabrela-Carvalhal synform, thus removing the underlying crust located along its trace ( Fig. 3 a and b). The Carvalhal Unit, dominated by metabasites with MORB-af fi nity, represents a good candidate for an ophiolitic sequence in the Évora Massif as it has almost the same lithological constitution, structural po- sition, and probable age than the Vila de Cruces ophiolite, one of the lower ophiolitic units of NW Iberia. Ma fi c units like these represent tec- tonic slices of transitional oceanic crust (oceanic supracrust) that usual- ly account for a broad transition from a continental-oceanic crust (e.g., upper sequence of the basal allochthonous units; Díez Fernández et al., 2010, 2013 ) to an oceanic-continental crust (e.g., Vila de Cruces and Carvalhal ophiolites; Arenas et al., 2007b ). In this regard, a group of Cambrian-Ordovician dismembered ophiolitic sequences has been described southeast of the Évora Massif, the so-called Internal Ossa- Morena Zone Ophiolite Sequences ( Fonseca et al., 1999; Pedro et al., 2010 ). Those ophiolitic sequences have been interpreted as a complete (but dismembered) section of oceanic lithosphere, including dunites Age (Ga) -20 -16 -12 -8 -4 12 2.5 2.0 1.5 1.0 0.5 0 4 8 0 ε Nd CHUR DEPLETED MANTLE Depositional ages Iberian Autochthon T DM : 1256-1334 Ma (n=12) Central Iberia metagreywackes (Iberian Autochthon) Malpica-Tui metagreywackes (NW Basal Units) Escoural metagreywackes (SW Basal Units) Younging of Sm-Nd model ages Central Iberian Zone Autochthon (Lower Alcudian; Fuenlabrada et al., 2016). TDM: 1256-1334 Ma (n=12) SW Basal Allochthonous Units (Évora Massif; present work). TDM: 1499-1853 Ma (n=11) SW Basal Allochthonous Units (Central Unit; López-Guijarro et al., 2008). TDM: 1640-1900 Ma (n=3) NW Basal Allochthonous Units (Malpica Tui Unit; Fuenlabrada et al., 2012). TDM: 1829-2156 Ma (n=18) BASAL ALLOCHTHONOUS UNITS Fig.6. TDMmodelages( DePaolo,1981 )ofthelate Ediacaranmetagreywackesofthebasalallochthonousunitsof SWIberia(IberianMassif).Diamondsshowthe ε Ndvaluesatca.560Ma. Model agesfrom late Ediacaran metasedimentary rocks ofthe basal allochthonousunits ofNW Iberia(Malpica-TuiUnit; Fuenlabradaetal., 2012 ),thebasal allochthonousunits ofCentral Iberia (Central Unit; López-Guijarro et al., 2008 ), and the Autochthon of the Central Iberian Zone ( Fuenlabrada et al., 2016 ), are included for correlation and comparison. 294 R.D. Fernández et al. / Lithos 268 – 271 (2017) 285 – 301