serie NOVA TERRA nº 49

62 related to the dynamics of the arc itself. Thus, it is probable that the present position of the turbiditic series partly re fl ects its primary location (although the contacts with the underlying series are tectonic), and that greywacke deposition took place in an intra-arc basin above thick metamorphic and plutonic complexes and possible pre-arc continental basement, the existence of which has not been investigated in the allochthonous complexes of NW Iberia. The T DM model ages of the greywackes range from the Middle Neoproterozoic to the Upper Mesoproterozoic (720 – 1215 Ma, Fig. 8 ). This, together with the absence of detrital zircons with these ages in the greywackes ( Fernández-Suárez et al., 2003 ), which rules out the presence of Grenvillian terranes in the source area, indicates that the T DM model ages re fl ect amixing of components of the Paleoproterozoic and Ediacaran ages. Both sources are represented in the detrital zircon population and are characteristic of the realms bordering the West African Craton. The presence of Mesoproterozoic basement (1100 – 1600 Ma) below most of the NW Iberian Massif has been proposed by Murphy et al. (2008) . The Paleozoic passive margin sedimentary sequence of the NW Iberian Massif, from the Schistose Domain of the Galicia-Trás- os-Montes Zone to the Cantabrian Zone, may have been deposited on this basement. The combined interpretation of the detrital zircon and Nd model ages from the greywackes of the uppermost levels of the Órdenes Complex, however, indicates a provenance incompatible with the participation of such a basement. If this basement exists, the sedimentary basin in which the greywackes were deposited would have to have been located on a sector of the Gondwanan margin well removed from that represented by the rest of the NW Iberian Massif, although still at the periphery of the West African Craton. A collection of Nd model ages for various terranes of Gondwanan af fi nity is included in Fig. 8 , in addition to the data from the Grenville belt of NE North America ( Linnemann and Romer, 2002 ). Although the ages overlap and the terranes included represent a rather general group, the Nd model ages of the greywackes of the Órdenes Complex com- pare most closely with those of West Avalonia, Florida and the Carolina terranes, and differ slightly from those of East Avalonia and other terranes, and the domains of the Bohemian Massif. Fig. 9 shows the probable location of the peri-Gondwanan arc immediately prior to the opening of the Rheic Ocean. The opening of this ocean caused the rifting and separation of the external part of the arc, which was transported to the north, carrying with it the sedimentary basin in which the turbiditic series was deposited. The data obtained from the greywacke provenance of the Órdenes Complexcon fi rmthe equivalenceoftheupperunitsoftheallochthonous complexes of NW Iberia with an arc-derived terrane originally located at the periphery of the West African Craton. This arc was probably generated in the Avalonian peri-Gondwanan realm, in a more westerly position than that occupied by the Lower Paleozoic autochthonous sequence of NW Iberia ( Fig. 9 ). This is consistent with previous data related to the provenance of this allochthonous terrane ( Gómez Barreiro et al., 2007 ). No domain containing the Mesoproterozoic basement was involved in the genesis of the arc. The analysed greywackes were likely deposited adjacent to a mature volcanic arc with abundant felsic magmatism. Although the speci fi c location of the turbidite basin within the volcanic arc is uncertain, the present data suggest deposition in an intra-arc basin generated during the main phase of volcanic arc activity, roughly coincident with the intrusion of the largest plutonic bodies. Acknowledgements Financial support for this research has been provided by Spanish project CGL2007-65338-CO2-01/BTE (Ministerio de Ciencia e Innova- ción). The authors thank Jean Paul Liégeois and an anonymous referee Fig. 9. Paleogeographic reconstruction for the Cambrian – Ordovician limit showing the probable location of the peri-Gondwanan arc where the greywackes were deposited. The fi gure shows the moment immediately previous to the opening of the Rheic Ocean. Modi fi ed after Arenas et al. (2007) . 349 J.M. Fuenlabrada et al. / Gondwana Research 17 (2010) 338 – 351