ALBERT

Description: The magmatic evolution of the Alboran region (westernmost Mediterranean) contains important clues for improving our understanding of the origin of Mediterranean-style back-arc basins and the desiccation of the Mediterranean Sea in the Messinian. We use new laser 40Ar/39Ar age and geochemical (major and trace element and O–Sr–Nd–Pb isotope) data from igneous rocks from southern Spain, the Alboran Sea and northern Morocco to reconstruct the magmatic evolution of the westernmost Mediterranean since the Eocene. Lower Oligocene dikes near Malaga (33.6±0.6 Ma) and Middle to Upper Miocene volcanic rocks from the Alboran Sea area (6.57±0.04 to 11.8±0.4 Ma) can be subdivided into two groups: (1) LREE-depleted (relative to N-MORB), primarily tholeiitic series, and (2) LREE-enriched, primarily calc-alkaline series volcanic rocks. Both groups are generally enriched in fluid-mobile elements (e.g. Rb, Th, U, K and Pb) relative to fluid-immobile elements (e.g. Nb, Ta, LREE). The LREE-depleted group has 143Nd/144Nd (0.5128–0.5130) isotope ratios similar to Atlantic MORB but higher 87Sr/86Sr (0.7046–0.7100). In contrast, the LREE-enriched group has less radiogenic Nd (0.5121–0.5126) and tend to more radiogenic Sr (0.7066–0.7205) isotopic composition. Pb isotope ratios are surprisingly uniform and have compositions similar to marine sediments. Analyses of mineral separates show that mafic melts with relatively low δ18O (5.6–7.2‰) had high 87Sr/86Sr (0.7048–0.7088), Δ7/4 (10.6–14.1) and Δ8/4 (40.0–49.3). Modeling of the trace elements and Sr–Nd–Pb–O isotopic compositions provides compelling evidence for the contamination of the mantle source with hydrous fluids/melts, which can be explained through subduction of oceanic lithosphere beneath the Alboran Basin but not through detachment/delamination of lithospheric mantle. We present a geodynamic model that reconstructs the Late Eocene to Quaternary evolution of the western Mediterranean through westward roll-back of subducted Tethys oceanic lithosphere. Slab roll-back resulted in a large-scale reorganization of the western Mediterranean paleogeography, causing the closure of marine gateways linking the Atlantic Ocean to the Mediterranean Sea. Isolation of the Mediterranean Sea led to its desiccation, causing the Messinian Salinity Crisis.