serie NOVA TERRA nº 49

margin and various sectors in the proximity of the West African Craton (WAC). The original basins of the studied series from the Iberian Massif can be placed with some clarity. In this way, the geochemical and isotopic features have enabled us to establish a clear criterion of correlation between the NW and SWBasal Allochthonous Units in the Iberian Massif. The T DM observed in the Basal Units as a whole are very old (1499 - 2156 Ma), clearly greater than those from sedimentary sequences of similar age from the Central Iberian Zone (1256 - 1334 Ma). Thus, all the Basal Units of the Iberian Massif would come from a single paleogeographic domain, very close to Gondwana, from which they would receive a dominant contribution of old crustal materials. The original sedimentary basins had to be located very close to the WAC, since this is the only possible interpretation to explain the very old T DM that have been found. The sedimentary basins, where the series of the relative Autochthonous domain (Cen- tral Iberian Zone) were deposited, occupied more distant positions in relation to the WAC, probably displaced towards more oriental sectors along the paleomargin of Gondwana. Fina- lly, the positive values of [ Nd and the much younger T DM of the Upper Allochthonous Units of NW Iberia (720 - 1215 Ma), places their origin in positions very close to the active volcanic arc, but always within the back-arc basin. This volcanic arc would be the source of the abun- dant juvenile material needed to explain the T DM values, typical of these units. A fundamental conclusion of the present work is that the combined research, using geo- chemistry of major and trace elements and isotopic geochemistry (Sm-Nd), represents a cri- tical tool when studying the geodynamic context of deposition and the source areas of old siliciclastic series. The value of these methodologies is even greater for the study of azoic, altered and metamorphic series, where the application of a traditional stratigraphic or pa- leontological analysis is virtually impossible. vi