Post-Collisional Transition from Subduction- to Intraplate-type Magmatism in the Westernmost Mediterranean: Evidence for Continental-Edge Delamination of Subcontinental Lithosphere

Post-collisional magmatism in the southern Iberian and northwestern African continental margins contains important clues for the understanding of a possible causal connection between movements in the Earth’s upper mantle, the uplift of continental lithosphere and the origin of circum-Mediterranean igneous activity.Systematic geochemical and geochronological studies (major and trace element, Sr-Nd-Pb-isotope analysis and laser 40 Ar/ 39 Ar-age dating) on igneous rocks provide constraints for understanding the postcollisional history of the southern Iberian and northwestern African continental margins.Two groups of magmatic rocks can be distinguished: (1) an Upper Miocene to Lower Pliocene (8Á2-4Á8 Ma), Si-K-rich group including high-K (calc-alkaline) and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene (6Á3-0Á65 Ma), Si-poor, Na-rich group including basanites and alkali basalts to hawaiites and tephrites.Mafic samples from the Si-K-rich group generally show geochemical affinities with volcanic rocks from active subduction zones (e.g.Izu-Bonin and Aeolian island arcs), whereas mafic samples from the Si-poor, Na-rich group are geochemically similar to lavas found in intraplate volcanic settings derived from sub-lithospheric mantle sources (e.g.Canary Islands).The transition from Si-rich (subduction-related) to Si-poor (intraplate-type) magmatism between 6Á3 Ma (first alkali basalt) and 4Á8 Ma (latest shoshonite) can be observed both on a regional scale and in individual volcanic systems.Si-K-rich and Si-poor igneous rocks from the continental margins of southern Iberia and northwestern Africa are, respectively, proposed to have been derived from metasomatized subcontinental lithosphere and sub-lithospheric mantle that was contaminated with plume material.A three-dimensional geodynamic model for the westernmost Mediterranean is presented in which subduction of oceanic lithosphere is inferred to have caused continental-edge delamination of subcontinental lithosphere associated with upwelling of plume-contaminated sub-lithospheric mantle and lithospheric uplift.This process may operate worldwide in areas where subduction-related and intraplate-type magmatism are spatially and temporally associated.