Contrasting geochemical features of the Central Iberian Zone shales (Iberian Massif, Spain): implications for the evolution of Neoproterozoic–Lower Cambrian sediments and their sources in other peri-Gondwanan areas

Abstract

More than 90 samples of shales have been collected for geochemical purposes from Neoproterozoic and Lower Cambrian sedimentary series in an extensive area of the Central Iberian Zone (CIZ). For similar SiO 2 and Al 2O 3 abundances, the Neoproterozoic shales (NS) are characterized by relatively higher TiO 2 and Zr abundances and by higher Cr/Th and Ti/Nb ratios. By contrast, the Lower Cambrian shales (LCS) have higher Al 2O 3/TiO 2, Rb/Zr, (La/Yb) cn and Th/Yb ratios. Diagrams such as Al 2O 3–TiO 2, Al 2O 3/TiO 2–Ti/Nb, TiO 2–Zr, TiO 2–Zr/Nb and Ti/Nb–Rb/Zr are useful to define two distinct fields that discriminate the Neoproterozoic from the Lower Cambrian compositions. These and other geochemical features suggest that these rocks were derived from different source compositions and that the Lower Cambrian shales (LCS) were chemically more fractionated than those of the Neoproterozoic. These distinctive chemical features are also recorded by Neoproterozoic and Cambrian fine-grained rocks in other areas in the Central Iberian Zone and in other zones within Iberia. Published neodymium isotopic results consistently distinguish both compositions and, together with the geochemical data, support a relatively fractionated nature for the Lower Cambrian sediments compared to the Neoproterozoic ones. It is proposed that continental deposits derived from the interior orogens in North Gondwana would have been the sources for Neoproterozoic sediments in the Iberian zones, whereas the Lower Cambrian sediments in these zones would have been derived from older crust beneath those continental deposits. Thus, the Neoproterozoic–Lower Cambrian stratigraphic succession in Iberia reflects an inverse stratigraphy of the source area and contrasts with zones of Central and Western Europe where equivalent sedimentary series record important coeval igneous contributions and show less fractionated geochemical characteristics. Finally, the possible connection between the Central Iberian Zone and the Meguma Zone (MZ) in Canada is discussed on the basis of certain sedimentological and geochemical features. It is proposed that the sedimentary series of the Central Iberian Zone and of the Meguma Zone represent, respectively, proximal and distal North Gondwana-derived sediments.