serie NOVA TERRA nº 49

97 5. Discussion Different paleogeographic models have shown that large magmat- ic arcs developed in the peri – Gondwanan realm. Their activity took place between Neoproterozoic (Cadomian – Pan ‐ African cycle) and Early Ordovician times ( Murphy and Nance, 2002; Stamp fl i and Borel, 2002 ), and they left an imprint in the sedimentological and magmatic record of many of the terranes forming part of the Variscan Belt ( Linnemann et al., 2004, 2007; Rodríguez Alonso et al., 2004; Ugidos et al., 2003 ). The geochemical features of the metagreywackes of the lower sequence suggest that they were generated in relation to an arc system built on a thinned continental margin, although they also record certain chemical transition to passive margin greywackes. A back-arc setting or a retro-arc setting as those presented in the models of Fig. 7 (a, b), would explain most of the geochemical fea- tures observed in these lithologies. The lower sequence of the basal units was probably deposited in an Ediacaran – Early Cambrian back- arc/retro-arc setting, closer to the external platform of the Gondwana Mainland, in a region where the most important sedimentary supplies would come from source areas located in the interior of the continental domain. The in fl uence of the Cadomian magmatic arc on the composition of the greywackes is clear, as indicated by their detrital zircon age populations ( Díez Fernández et al., 2010 ). However, the sedimentary basin was apparently located far away from the region with the most important igneous activity, because the oldest igneous rocks intruding the greywackic sequence are Middle Cambrian. The origin of the Cambrian sedimentary rocks of the upper se- quence is more uncertain, as they are mainly constituted by meta- pelitic schists whose compositions are not ideal for tectonic setting discrimination. The available geochemical data are not abundant, but they seem to suggest some af fi nity with passive margin sedi- ments; i.e., these sediments were probably deposited close to the continental domain and to certain distance from the most active zones in the magmatic arc. The absence of intrusive granitic rocks, which are widespread in the lower sequence, supports this interpre- tation ( Fig. 2 b). It is likely that during Late Cambrian times the previ- ously deposited series of the lower sequence were located towards the most active part of the arc system, while the sediments of the upper sequence were deposited closer to the continental platform. In addition, it is important to consider that the contact between upper and lower metasedimentary sequences is a low-angle fault (ex- tensional detachment), so these series could have been located at dif- ferent positions along the margin. On the other hand, opening of the Rheic Ocean and transition of the Gondwana margin to a typical pas- sive margin setting occurred between the Middle Cambrian and the Early Ordovician ( Arenas et al., 2007; Murphy et al., 2010; Nance et al., 2010 ). This agrees with the observed sedimentary evolution and explains the presence of N-MORB ma fi c rocks in the sedimentary se- ries ( Rodríguez Aller, 2005 ), which are particularly more abundant in the upper sequence ( Fig. 2 b). Although there are not many geochem- ical data about the metasedimentary rocks of the upper sequence, the compositional patterns (PAAS-normalized plots) of some samples ( Fig. 5 b) could indicate an evolution from a back-arc setting to a pas- sive margin. This transition might be related either with the Cs Rb U Nb Ce Nd Sm Hf TiO 2 Y Yb Cr Ba Th K 2 O La P 2 O 5 Sr Zr Eu Dy Er Sc Ni 10 0.1 0.01 Sample / PAAS 1 Cs Rb U Nb Ce Nd Sm Hf TiO 2 Y Yb Cr Ba Th K 2 O La P 2 O 5 Sr Zr Eu Dy Er Sc Ni 10 0.1 0.01 Sample / PAAS 1 a b Santiago schists (upper sequence) Ceán Schists (upper sequence) Metagreywackes (lower sequence) Albite schists (lower sequence) Fig. 5. (a, b) PAAS-normalized trace elements plots for the metasedimentary rocks from the lower and upper sequences, respectively. PAAS after Taylor and McLennan (1985) . Table 3 Whole rock Nd isotope data of metasedimentary rocks. Sample Sm Nd 147 Sm/ 144 Nd 143 Nd/ 144 Nd 2 σ ε Nd(0) ε Nd(560) a TDM (Ma) b B-1 4.00 20.67 0.1169 0.511676 3 − 18.8 − 13.1 2078 B-2 4.14 21.83 0.1146 0.511670 3 − 18.9 − 13.0 2041 B-3 4.75 23.42 0.1227 0.511950 3 − 13.4 − 8.1 1782 B-4 4.10 21.31 0.1164 0.511852 3 − 15.3 − 9.6 1817 B-5 4.49 22.80 0.1191 0.511800 4 − 16.3 − 10.8 1940 B-6 3.73 19.69 0.1146 0.511731 3 − 17.7 − 11.8 1956 B-7 3.03 14.81 0.1239 0.511809 4 − 16.2 − 11.0 2020 B-8 5.58 30.14 0.1118 0.511712 3 − 18.1 − 12.0 1934 B-9 5.62 29.26 0.1162 0.511844 4 − 15.5 − 9.7 1824 B-10 3.33 17.93 0.1123 0.511727 6 − 17.8 − 11.7 1921 B-11 6.24 35.72 0.1055 0.511663 3 − 19.0 − 12.5 1893 B-12 4.41 22.83 0.1167 0.511728 3 − 17.7 − 12.0 1999 B-13 3.61 19.94 0.1095 0.511708 3 − 18.1 − 11.9 1898 B-14 4.05 21.25 0.1152 0.511729 3 − 17.7 − 11.9 1971 B-15 3.71 19.80 0.1133 0.511753 3 − 17.3 − 11.3 1902 B-16 4.04 21.90 0.1115 0.511700 3 − 18.3 − 12.2 1946 B-22 2.46 12.91 0.1150 0.511771 3 − 16.9 − 11.1 1906 B-23 4.55 22.19 0.1238 0.511885 3 − 14.7 − 9.5 1902 B-17 4.84 26.06 0.1123 0.511846 4 − 15.4 − 10.1 1756 B-18 5.19 26.38 0.1189 0.511877 3 − 14.9 − 9.9 1824 B-19 5.33 26.66 0.1208 0.511954 4 − 13.3 − 8.5 1743 B-20 1.87 9.26 0.1223 0.511892 3 − 14.6 − 9.8 1863 B-21 3.05 12.60 0.1464 0.512007 3 − 12.3 − 9.1 2223 a ε Nd(t) calculated for 560 Ma. b Nd model ages calculated according to DePaolo (1981) . 204 J.M. Fuenlabrada et al. / Lithos 148 (2012) 196 – 208