serie NOVA TERRA nº 49

98 attenuation of the magmatic activity in the arc, or the progressive ocean ward translation of this activity, or the drifting of the whole arc system away from the continental margin. The Nd model ages of the 23 analysed samples are Paleoproterozoic and range between 1782 Ma and 2223 Ma (average value 1919; Fig. 6 ). The Nd model ages are slightly younger in the upper sequence than in the lower sequence, but altogether they de fi ne a single population. Therefore the two metasedimentary rock sequences can be clearly re- lated. Sedimentation probably took place in the same basin located in the continental platform of Gondwana, whose main source areas did not change during the Late Neoproterozoic and Cambrian times. The Nd model ages are very old and they seem to be only compatible with Paleoproterozoic – Archean source areas. The participation of younger sources was probably limited, although both the metagreywackes and the metapelitic rocks contain Mesoproterozoic and Neoproterozoic (ac- tually the most abundant grains) detrital zircon populations ( Díez Fernández et al., 2010 ). The chronology recorded by the main detrital zircon population and by the NdT DM is not contradictory as each method supplies a different information ( Liégeois and Stern, 2010 ). Source areas dominated by Paleoproterozoic – Archean materials intruded by Pan-African granitoids, and with variable participation of detrital sedimentary rock series through time, explain the Sm – Nd isoto- pic data and the age populations of detrital zircons (see Liégeois and Stern, 2010 , and references therein). The signi fi cant difference between the ages reported by the two methods suggests that the involved Pan- African granitoids were the result of partial melting of Paleoproterozoic – Archean basement and its erosion products, and con- sequently they share a same Sm – Nd isotope signature. The important participation of Neoproterozoic zircons can be explained by the high abundance of zircons in the Cadomian – Pan-African granitoids (750 – 550 Ma), as it is known that granitoids are the most important source for detrital zircons. The North African margin of Gondwana shows suitable features to be considered the source area for these sedimentary rocks. It is dif fi cult to ascertain the precise location of the sedimentary basin, because its position can only be determined presently using provenance studies based in U – Pb geochronology of de- trital zircons. In the context of North Africa a key element is the presence/absence of a Stenian – Tonian population of detrital zircons (750 – 1150 Ma), which it is also present in the sedimentary series of the basal units. Avigad et al. (2012) suggested that Gondwanan series Uppermost Units (Órdenes Complex, NW Spain) Range of Nd-model ages: 0 0.5 1.0 1.5 2.0 2.5 Age (Ga) ε Nd -20 -8 -4 0 4 8 12 DEPLETED MANTLE -12 -16 West Avalonia (Atlantic Canada) East Avalonia (Uriconian, southern Britain) East Avalonia (southern Britain-2) East Avalonia (southern Britain-1) West Africa Armorica (Brittany, Normandy, Channel Islands) Mid German Crystalline Rise (Germany, Spessart) "Moldanubian Zone" (Germany, Schwarzwald, Vogesen) Carolina Florida Polish Sudetes Icartian (Brittany) Reservoir dominated by Grenvillian and (?) post-Grenvillian/ pre-Cadomian crust Reservoir dominated by pre-Grenvillian (>0.9-1.1 Ga) cratonic crust 3200 Ma Amazonian Craton Sunsas-Grenvillian (Amazonian Craton, eastern part) Tocantins Province (Brasil) Grenville (NE North America) T DM : 1743-2223 (n = 22) T DM : 720-1215 (n= 20) CHUR Metagreywackes (lower sequence) Albite schists (lower sequence) Santiago schists (upper sequence) Ceán Schists (upper sequence) Órdenes greywackes (uppermost units) Fig. 6. T DM model ages ( DePaolo, 1981 ) of the metasedimentary rocks from the basal units of the allochthonous complexes of NW Iberian Massif. Triangles and circles show the ε Nd values at 560 Ma and 500 Ma, reference ages for the deposition of the lower and upper sequences, respectively ( Díez Fernández et al., 2010 ). Nd data of the uppermost greywackes of the Órdenes Complex (upper units; Fuenlabrada et al., 2010 ) are included for comparison. Data source for comparative model ages from different regions taken from Linnemann and Romer (2002) . 205 J.M. Fuenlabrada et al. / Lithos 148 (2012) 196 – 208