Miocene Magmatism in Northern Tunisia: Characterization and Geodynamic Implications

The Nefza mining district in northern Tunisia comprises a wide variety of magmatic rocks related to the Cenozoic Mediterranean geodynamic history. Magmatic rocks consist mainly of granodiorite and rhyodacite emplaced during the Serravallian–Tortonian (14–8 Ma) and basalts emplaced during the Messinian (8–6 Ma). This magmatism belongs to the magmatic belt that extends along the Mediterranean coast of the Maghreb from eastern Tunisia to northern Morocco. The differences in geochemical composition were attributed to changes in mantle dynamics but the implications are still unclear in northern Tunisia due to the paucity of data. In this contribution, we present new bulk-rock geochemical data of the granodiorite and rhyodacite of Nefza recovered from OB45 drill core. Samples from the OB45 drill core give a unique opportunity to follow the geochemical evolution of the magmatic rocks with depth. Geochemical (major and trace elements) compositions of OB45 drill core rocks indicate a peraluminous and potassic nature. In the total alkali versus silica (TAS) classification diagram, samples plot in the rhyolite, dacite and trachydacite fields. Trace element compositions of Nefza magmatic rocks show typical subduction-related signatures, with relative depletion of high field strength elements (HFSE: Nb, Zr, Ta, Hf), large ion lithophile elements (LILE: Rb, Ba, Pb) and light rare earth elements (LREE: La, Ce, Nd, Gd, Sm), compared to the Continental Arc Andesite (CAA) geochemical composition. However, most of the OB45 drill core rocks show negative Eu anomalies. The wide geochemical composition of the igneous rocks emplaced during this tectonic evolution confirms a strong heterogeneity of the Mediterranean upper mantle composition. The comparison of the major and trace element as well as isotope characteristics of the magmatism in the Nefza and nearby regions (such as Kabylies and Sardinia) provides insights into the petrogenesis of the magmas and the nature of the main mantle source and its geodynamics.