serie NOVA TERRA nº 49

60 are generally close to one, with a slight characteristic negative TiO 2 anomaly. In general, a slightly increasing pattern can be observed, culminating in a fl at pattern close to one. However, three important differences exist between the diagram for continental island arcs and active margins proposed by Winchester and Max (1989) and that obtained for the analysed greywackes: (1) there is essentially no negative P 2 O 5 anomaly, perhaps because of low contents of apatite and monazite; (2) there is a less marked positive Sr anomaly, perhaps as a result of alteration processes; and (3) the Cr and Ni abundances are signi fi cantly lower, suggesting an essentially felsic provenance for the greywackes, although the values lie close to the analytical detection limits. The felsic provenance appears to be corroborated by Hf values and La/Th ratio, and is a likely indication of the association of the greywackes with an evolved (mature) volcanic arc. 4.3. Sm – Nd isotope systematics Isotopic analysis of the greywackes was performed at the Centro de Geocronología y Geoquímica Isotópica at the Universidad Com- plutense de Madrid. For whole rock Nd isotopic analysis by isotope dilution-thermal ionization mass spectrometry (ID-TIMS), the sam- ples were fi rst dissolved in ultra-pure reagents in order to perform isotope separation by exchange chromatography ( Strelow, 1960; Winchester, 1963 ), and subsequently analysed using a Sector 54 VG- Micromass multicollector spectrometer. The measured 143 Nd/ 144 Nd isotopic ratios were corrected for possible isobaric interferences from 142 Ce and 144 Sm (only for samples with 147 Sm/ 144 Sm<0.0001) and normalized to 146 Nd/ 144 Nd=0.7219 in order to correct for mass fractionation ( Table 3 ). The La Jolla Nd international isotopic standard was analysed during sample measurement, and gave an average value of 143 Nd/ 144 Nd=0.511859 for 7 replicas, with an internal precision of ±0.000015 (2 σ ). These values were used to correct the measured ratios for possible sample drift. The error estimated for the 147 Sm/ 144 Nd ratio is 0.1%. In crustal evolution models based on Nd isotopic composition, the main source of fractionation during the formation and evolution of continental crust takes place during partial melting of lithospheric mantle to generate crustal rocks ( McLennan and Hemming, 1992 ). The ε Nd model age of a sedimentary rock represents the average age of the extraction of its components from the mantle. In the case of detrital rocks, model ages usually re fl ect complex mixing based on the different age and provenance of their components. The combined interpretation of model ages and detrital zircon ages has proved to be a powerful tool for investigating the evolution of continental crust, especially in orogenic domains (e.g., Linneman et al., 2004 ). The Nd model ages calculated for the metagreywackes are included in the ε Nd vs. the time diagram in Fig. 8 . The analysed metagreywackes show 147 Sm/ 144 Nd ratios <0.145, which is an appropriate ratio for NdT DM calculations. Stern (2002) suggests that a 147 Sm/ 144 Nd ratio of 0.165 is the upper limit for performing NdT DM calculations. The T DM model ages ( DePaolo, 1981 ) range between 720 and 1215 Ma ( Table 3 ), with an average value of 995 Ma ( Fig. 8 ). ε Nd (0) values vary from − 4.1 to 0, while ε Nd (500), that is, the ε Nd value at the time of greywackes sedimentation, ranges between 0.3 and 4.8 ( Table 3 ). A collection of Nd model ages from different regions ( Linnemann and Romer, 2002 ) is also included in Fig. 8 . These ages have been divided into two groups according to the age of the dominant source (Grenvillian and post- Grenvillian/pre-Cadomian crust, or pre-Grenvillian, >0.9 – 1.1 Ga, cratonic crust). Fig. 7. (a) PAAS-normalized trace elements plots for the metagreywackes. (b) Plot showing the compositional range of the PAAS-normalized trace elements composition of the metagreywackes. PAAS after Taylor and McLennan, 1985 . Table 3 Whole rock Nd isotope data of the top metagreywackes from the Órdenes Complex. Muestra Nd Sm 147 Sm/ 144 Nd 143 Nd/ 144 Nd 2 σ ε Nd (0) ε Nd (500) ⁎ T DM (Ma) a SO-1 22.60 4.93 0.1318 0.512531 3 − 2.1 2.0 986 SO-2 18.44 4.22 0.1383 0.512555 4 − 1.6 2.1 1017 SO-3 24.49 5.77 0.1425 0.512480 3 − 3.1 0.4 1215 SO-4 20.51 4.62 0.1363 0.512633 4 − 0.1 3.8 855 SO-5 26.83 5.85 0.1318 0.512456 3 − 3.5 0.6 1111 SO-6 16.39 3.78 0.1393 0.512518 3 − 2.3 1.3 1098 SO-7 20.64 4.69 0.1373 0.512529 3 − 2.1 1.7 1052 SO-8 19.82 4.07 0.1241 0.512589 3 − 1.0 3.7 818 SO-9 25.97 4.82 0.1122 0.512482 3 − 3.1 2.3 878 SO-10 29.29 6.07 0.1253 0.512497 3 − 2.8 1.8 973 SO-11 26.13 5.56 0.1287 0.512518 3 − 2.4 2.0 974 SO-12 31.11 6.56 0.1276 0.512537 3 − 2.0 2.4 932 SO-13 19.56 4.06 0.1255 0.512587 4 − 1.0 3.6 833 SO-14 21.11 4.22 0.1207 0.512636 3 0.0 4.8 720 SO-15 17.66 3.57 0.1224 0.512506 3 − 2.6 2.2 930 SO-16 15.76 3.66 0.1402 0.512481 3 − 3.1 0.5 1180 SO-17 33.57 7.11 0.1281 0.512427 3 − 4.1 0.3 1115 SO-18 26.85 6.20 0.1396 0.512468 3 − 3.3 0.3 1194 SO-19 32.82 6.81 0.1254 0.512467 3 − 3.3 1.2 1021 SO-20 23.21 4.80 0.1252 0.512476 3 − 3.2 1.4 1003 a Nd model ages calculated according to DePaolo (1981) . ⁎ ε Nd(t) calculated for 500 Ma. 347 J.M. Fuenlabrada et al. / Gondwana Research 17 (2010) 338 – 351