Protracted magmatism and crust-mantle interaction during continental collision: insights from the Variscan granitoids of the external western Alps

Variscan granitoids and associated mafic rocks exposed in the External Crystalline Massifs (ECM) of the Western Alps document the Variscan stages from the early Carboniferous collision to the early Permian post-collisional setting. Our study focuses on the Central part of the ECM, synthesizing newly acquired and existing geochronological, whole-rock geochemical and isotopic data. We identified two distinctive magmatic series: (i) high-K calc-alkaline granitoids, which range from magnesian (MgG) to ferro-magnesian (FeMgG) rocks; (ii) ultra-high-K metaluminous (UHKM) rocks ("durbachites"). These series were emplaced roughly simultaneously between ca. 350 and 300 Ma, with two main episodes during the Visean (ca. 348-335 Ma) and the late Carboniferous (305-299 Ma), with a more limited activity in between. A younger Permian event at ca. 280-275 Ma has also been identified in one granitoid pluton. Contemporaneous emplacement of these two series reflects concomitant crustal anatexis and melting of LILE-LREE-rich metasomatized lithospheric mantle. Trace elements and Nd-Sr isotopes reveal significant hybridization between these two magmatic end members, by magma mixing, or assimilation of crystallized mafic ultrapotassic enclaves in the high-K calc-alkaline granitoids. Granitoid composition evolves over time, especially SiO2, Mg#, Sr/Y, La/Yb and Nb/Ta, possibly explained by increasing differentiation of magmas over time, changes in the crust versus mantle sources mass-balance, and decrease in melting pressure due to the orogenic collapse. The epsilon Ndi values of both high-K calc-alkaline granitoids and durbachites decreases from [- 3.8; - 2.9] to [- 6.4; - 5.2] between 345 and 320 Ma, possibly indicating an increasing influence of subducted/relaminated crustal material contaminating the lithospheric mantle source. epsilon Ndi values then rise to [- 3.7; - 0.5] during the late Carboniferous, possibly due to progressive exhaustion of the enriched mantle source, or advection of the asthenosphere during the post-collisional stage.Graphic abstract Possible geodynamic scenario along the central-eastern segment of the Variscan Belt, which may account for the temporal evolution of Variscan magmatism in the External Western Alps.