serie NOVA TERRA nº 49

61 5. Discussion According to the age groups recorded by the detrital zircons, the greywackes of the upper levels of the Órdenes Complex have a maximum depositional age of 510 – 530 Ma (Middle Cambrian). The complete absence of detrital zircons of Mesoproterozoic age (1000 – 1600 Ma) indicates that the sedimentary basin in which they were deposited was probably located in the peri-Gondwanan realm along the periphery of the West African Craton ( Fernández-Suárez et al., 2003 ). The geochemistry of the greywackes clearly shows that their deposition took place on a convergent margin in a volcanic arc, and speci fi cally (according to the criteria of Bathia and Crook, 1986 ), in a continental island arc. Within this setting, Bathia and Crook (1986) include sedimentary basins located in the inter-arc, back-arc and fore-arc environments of volcanic arcs developed over thinned continental crust. These arcs have a crystalline continental basement, but lack the thick continental basement of typical arcs generated at active continental margins. The analysed greywackes show very different chemical compositions to those deposited in passive margins or oceanic island arcs. Instead, the geochemical signature of the greywackes indicates deposition in a mature, highly evolved volcanic arc with abundant felsic magmatism. Pinpointing the location of this sedimentary basin and volcanic arc system turns is more problematic. The similarity in the timing of the greywacke sedimentation (c. 510 – 530 Ma) and magmatism in the upper units (c. 500-520 Ma; Abati et al., 1999; Fernández Suárez et al., 2007 ), requires deposition of the turbidites to have taken place during the main phase of volcanic arc activity. Synchronicity of sedimen- tation with active magmatism is supported by the fact that the greywackes are cross-cut by a coeval diabasic swarm ( Díaz-Garcia et al., 2010-this issue ). The chemistry of the greywackes also points to the possible presence of thinned continental crust in the root of the arc. Furthermore, the turbiditic series presently forms the upper levels of the arc-derived terrane, and lies structurally above a complex series of high-grade metamorphic rocks (both orthogneisses and paragneisses), the tectonothermal record of which is at least partly Fig. 8. T DM model ages ( DePaolo, 1981 ) of the top metagreywackes from the Órdenes Complex. Triangles show the ε Nd values at 500 Ma, a reference age for the deposition of the turbiditic series. Data source for comparative model ages from different regions taken from Linnemann and Romer (2002) . 348 J.M. Fuenlabrada et al. / Gondwana Research 17 (2010) 338 – 351