Geochemical and isotopic (SmNd) provenance of Ediacaran-Cambrian metasedimentary series from the Iberian Massif. Paleoreconstruction of the North Gondwana margin

Abstract

The geochemical and isotopic (SmNd) features of the Ediacaran-Cambrian siliciclastic series from the Central Iberian Zone and the Iberian Allochthonous Domains indicate a significant paleogeographic change at the northern margin of Gondwana, associated with the development of the Avalonian-Cadomian arc. During the Neoproterozoic, the opening of a peri-Gondwanan back-arc basin and its subsequent widening during the Cambrian led to a progressive separation of the sedimentary basins from the main focus of the probably waning Cadomian magmatic arc activity. This work presents a paleogeographic reconstruction of the distribution of the Iberian terranes along the North Gondwanan margin during Ediacaran and Cambrian times, based on the geochemical and Nd-isotope data from the Iberian Allochthonous and Autochthonous Domains. These results suggest a location close to the West Africa Craton for both the allochthonous and autochthonous terranes, but they would be distributed laterally along the northern margin of Gondwana, occupying arc- or continental-ward positions within a wide back-arc basin. In this context, the peri-Gondwanan volcanic arc acted as the main supplier of the abundant juvenile material, as reflected in positive values of εNd and young TDM model ages (720–1215 Ma) of the Middle Cambrian siliciclastic series from NW Iberia Upper Allochthonous Units. On the other hand, the rather equivalent geochemical and isotopic features of the sedimentary series of the NW and SW Iberia Basal Allochthonous Units suggest a common paleolocation for those series, yet within the back-arc basin but closer to the mainland and older isotope sources. The sedimentary series of these terranes present the oldest TDM ages (1499–2156 Ma), clearly greater than the TDM ages that characterize coeval sedimentary series of the Iberian Autochthonous Domain (1256–1334 Ma). This finding allows us to place Iberian Basal Allochthonous Units very close to the West Africa Craton, from where they received a dominant contribution of old crustal materials, whereas the Iberian Autochthonous Domain occupied a more eastern paleoposition closer to the Sahara Metacraton.