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 northwesternAfrican continental margins contains important clues for theunderstanding of a possible causal connection between movementsin the Earth's upper mantle, the uplift of continental lithosphereand the origin of circum-Mediterranean igneous activity. Systematicgeochemical and geochronological studies (major and trace element,Sr–Nd–Pb-isotope analysis and laser ⁴⁰Ar/³⁹Ar-agedating) on igneous rocks provide constraints for understandingthe post-collisional history of the southern Iberian and northwesternAfrican continental margins. Two groups of magmatic rocks canbe distinguished: (1) an Upper Miocene to Lower Pliocene (8·2–4·8Ma), Si–K-rich group including high-K (calc-alkaline)and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene(6·3–0·65 Ma), Si-poor, Na-rich group includingbasanites and alkali basalts to hawaiites and tephrites. Maficsamples from the Si–K-rich group generally show geochemicalaffinities with volcanic rocks from active subduction zones(e.g. Izu–Bonin and Aeolian island arcs), whereas maficsamples from the Si-poor, Na-rich group are geochemically similarto lavas found in intraplate volcanic settings derived fromsub-lithospheric mantle sources (e.g. Canary Islands). The transitionfrom Si-rich (subduction-related) to Si-poor (intraplate-type)magmatism between 6·3 Ma (first alkali basalt) and 4·8Ma (latest shoshonite) can be observed both on a regional scaleand in individual volcanic systems. Si–K-rich and Si-poorigneous rocks from the continental margins of southern Iberiaand northwestern Africa are, respectively, proposed to havebeen derived from metasomatized subcontinental lithosphere andsub-lithospheric mantle that was contaminated with plume material.A three-dimensional geodynamic model for the westernmost Mediterraneanis presented in which subduction of oceanic lithosphere is inferredto have caused continental-edge delamination of subcontinentallithosphere associated with upwelling of plume-contaminatedsub-lithospheric mantle and lithospheric uplift. This processmay operate worldwide in areas where subduction-related andintraplate-type magmatism are spatially and temporally associated. KEY WORDS: post-collisional magmatism; Mediterranean-style back-arc basins; subduction; delamination; uplift of marine gateways     

Petrogenesis of Basanitic to Tholeiitic Volcanic Rocks from the Miocene Vogelsberg, Central Germany

The Miocene Vogelsberg volcano in Central Germany produced maficmagmas ranging in composition from basanite to quartz tholeiiteand limited amounts of evolved magmas. Trace element and Nd,Sr and Pb isotopic compositions reveal the presence of threedistinct mantle sources: (1) a trace element enriched, asthenosphericplume-type source, similar to the European Asthenospheric Reservoircomposition inferred for many other Tertiary volcanic provincesin Central Europe; (2) a depleted mantle source, located inthe lithospheric mantle or uppermost asthenosphere; (3) a veinedlithospheric mantle source. The oldest basanites of the Vogelsbergvolcano have distinctly higher Ti, Al, Sc and V contents thanyounger basanites. These high-Ti basanites may have been producedby partial melting of a veined lithospheric mantle source, formedduring the earliest stages of uplift of the Rhenish Shield,