serie NOVA TERRA nº 49

96 sample B-21 (arkose) is considered. However, samples B-17, B-18 and B-19 show patterns close to 1 with some differences in relation to the metagreywackes from the lower sequence. They do not show deple- tion in some LILE elements (Rb, Th, Ce, K 2 O) although negative anom- alies exist for U, Sr, Hf and TiO 2 , which could suggest passive margin af fi nity ( Fig. 5 b). 4.3. Sm – Nd isotope systematics The Sm – Nd isotopic analyses were performed at the Centro de Geocronología y Geoquímica Isotópica from the Universidad Complutense de Madrid. They were carried out in whole-rock powders using a 150 Nd- 149 Sm tracer by isotope dilution-thermal ionization mass spectrometry (ID-TIMS). The samples were fi rst dissolved through oven digestion in sealed Te fl on bombs with ultra pure reagents to perform two-stage conventional cation-exchange chromatography for separa- tion of Sm and Nd ( 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 b 0.0001) and normalized to 146 Nd/ 144 Nd=0.7219 to cor- rect for mass fractionation ( Table 3 ). The LaJolla Nd international isoto- pic standard was analysed during sample measurement, and gave an average value of 143 Nd/ 144 Nd=0.5114840 for 9 replicas, with an inter- nal precision of ±0.000032 (2 σ ). These values were used to correct the measured ratios for possible sample drift. The estimated error for the 147 Sm/ 144 Nd ratio is 0.1%. In crustal evolution models based on Nd isotopic composition, the main fractionation event during the formation and evolution of conti- nental 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 ex- traction 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 in- terpretation of Nd model ages and detrital zircon ages has proven to be a powerful tool for investigating the evolution of continental crust, especially in orogenic belts (e.g., Linnemann et al., 2004 ). The investigated metasedimentary rocks show relatively uniform ε Nd(0) values, ranging between − 19.0 and − 12.3 (average − 16.4; Table 3 ). ε Nd(T) ranges − 13.1 to − 8.1 in the lower sequence (maximum depo- sitional age of 560 Ma; Díez Fernández et al., 2010 ) and − 10.1 to − 8.5 in the upper sequence (maximum depositional age of 500 Ma; Díez Fernández et al., 2010 ). The analysed metasedimentary rocks show 147 Sm/ 144 Sm ratios ranging between 0.1055 and 0.1464, with average at 0.1174. This is a ratio below the upper limit of 147 Sm/ 144 Sm=0.165 suggested by Stern (2002) appropriate to perform NdT DM calculations. The T DM model ages ( DePaolo, 1981 ) are Paleoproterozoic and range between 2223 and 1782 Ma, with an average value of 1919 Ma ( Fig. 6 ). A collection of Nd model ages from different regions ( Linnemann and Romer, 2002 ) is also included in Fig. 6 . 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). In this collection of Nd model ages, similar Paleoproterozoic ages are only reported in elements of the West African Craton, the Amazonian Craton and the Icartian gneisses of Brittany. The rest of terranes included in this collection show younger Nd model ages. Moreover, Fig. 6 also includes for com- parison Nd data of the uppermost greywackes of the Órdenes Complex (upper units; Fuenlabrada et al., 2010 ). These Cambrian greywackes (maximum age of sedimentation at 530 – 500 Ma) have much younger model ages ranging between 1215 and 720 Ma. Metagreywackes (lower sequence) Albite schists (lower sequence) B D C A Co Zr / 10 Th B D C A Sc Zr / 10 Th Th Sc La A B C , D a b c A - Oceanic island arcs B - Continental island arcs C - Active continental margins D - Passive margins Fig. 4. Trace element tectonic setting diagrams for the greywackes from the lower metasedimentary rock sequence. The analysed schists from the upper sedimentary sequence are not included in these diagrams, as they show a scattered projection due to compositions different from typical greywackes. Diagrams are after Bathia and Crook (1986) . 203 J.M. Fuenlabrada et al. / Lithos 148 (2012) 196 – 208