serie NOVA TERRA nº 49

Abstract T he Iberian Massif contains an excellent representation of the Lower Paleozoic strati- graphy, as well as its transition to the Upper Neoproterozoic. The geochemistry (ma- jor and trace elements) of the siliciclastic series and their isotopic sources (Sm-Nd) have been scarcely investigated until recently, even though these aspects are important to infer the geodynamic context of the sedimentary basins and their location along the paleomargin of Gondwana. This PhD Thesis presents a study based on these methodologies, whose main objective is the investigation of the stratigraphic series that deϐine the transition between Lower Cambrian and Ediacaran times. The studied series correspond to the Upper Alloch- thonous Units and the Basal Allochthonous Units of NW Iberia, the Lower Cambrian and Al- cudian series from the southern sector of the Central Iberian Zone and to the SW Basal Allo- chthonous Units of the Iberian Massif. The whole research has been synthesized in several works already published, which have been included in this PhD Thesis constituting its main content. The document has also been completed with an introductory section incorporating the description of the used methodology and a ϐinal chapter that presents a synthesis of the conclusions within the geological frame of the Iberian Massif. As a general conclusion, the obtained results indicate a signiϐicant change in the inϐluence of a Cadomian volcanic arc on the sedimentary processes that took place along the Gondwa- na margin and consequently, on the chemical composition of the Ediacaran and Cambrian siliciclastic sequences. This change meant a progressive evolution of the depositional con- text, probably linked to the opening and widening of an extensive back-arc basin. During the Lower Paleozoic, this opening caused a progressive separation of the sedimentary basins from the main focus of the magmatic activity, which apparently was also decreasing, and a very slow but continuous evolution towards a passive margin during most of the Cambrian. The transition is especially evident regarding the geochemistry (major and trace elements) of the Lower Cambrian and Alcudian (Ediacaran) sedimentary sequences of the southern sector of the Central Iberian Zone. It is also very noticeable in the composition of the detrital sequences of the Basal Allochthonous Units of NW of Iberia. The investigation of the isotopic sources (Sm-Nd) reϐlects the same geodynamic evolution of the peri-Gondwanan terranes between Ediacaran and Cambrian times. It is particularly visible in the case of the metasedi- mentary sequences of the Central Iberian Zone, where the evolution of [ Nd values indicates a lower inϐluence of the volcanic arc during the sedimentation of the Cambrian series. The older T DM recorded in the Lower Cambrian sequences (1444 - 1657 Ma), are compatible with an increase in the contributions from older Gondwana sources and a decrease in the juvenile contributions from the volcanic arc. In the light of former results regarding the known distribution of the Ediacaran terranes and the cratonic masses in North Africa, this PhD Thesis proposes a paleogeographic recons- truction model for the location of the peri-Gondwanan terranes represented in the Iberian Massif during the Ediacaran - Cambrian transition. According to this model, the different terranes, that are currently part of a stack of nappes, would originally occupy lateral posi- tions within an extensive back-arc basin. The recorded model ages allow us to locate all the terranes relative to an active volcanic arc, which would cover the outermost position in the v