Within-plate calc-alkaline rocks: Insights from alkaline mafic magma–peraluminous crustal melt hybrid appinites of the Central Iberian Variscan continental collision

Central Iberian Variscan S-type granitoid plutons are cut by small-scale coeval appinitic stocks. Despite their whole-rock calc-alkaline character a subduction-related origin is not indicated for the appinites: field, petrographic, mineral chemistry, geochronological data and geodynamic information all point towards an extension-related genesis and primary alkaline composition. Appinite intrusion coincides temporally and spatially with the principal phase of extensional collapse of the Central Iberian Variscan orogen and is, we suggest, the result of adiabatic melting of the lithospheric mantle related to associated uplift. Once emplaced, controls on the petrogenetic evolution of the appinitic stocks changed from closed system fractional crystallization in a central mafic facies to a combination of fractional crystallization and open system mingling with the host granodiorite in an intermediate facies to mingling dominated in a border rim facies. We propose that alkaline magmas, produced by small-scale melting of heterogeneous metasomatized lithospheric mantle, act as markers of a change in tectono-magmatic conditions during orogenesis.