Potassic magmas of the Vosges Mts. (NE France) delimit the areal extent and nature of long-gone Variscan orogenic mantle domains

Highly magnesian and potassic (Mg–K) magmatism is a characteristic feature of the European Variscan Belt. These magmatic rocks have an odd dual geochemical character: they have both high mg# and transition metal contents indicative of being derived from the mantle (and not having lost that character via magmatic differentiation), while simultaneously they are highly enriched in LILE and other fluid-mobile mantle-incompatible elements. There is a growing consensus that these magmas are likely produced by partial melting of metasomatized domains in the subcontinental lithospheric mantle (SCLM) that formed by influx of fluids rich in incompatible elements derived from subducted crust. The Sr and Nd isotopic signature of these fluids determined the isotopic character of the resulting mafic potassic magmas, and can be variable, depending on whether the subducted crust was continental vs. oceanic.