To provide a better definition of the geodynamical processes active at the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on specific mafic Mg-K magmatic rocks (the vaugnerites), intrusive into the granite-gneiss sequences of the Variscan Vosges crystalline massif. Those rocks, though subordinate in volume, are frequently associated with late collisional granites. In the Central-Southern Vosges, they appear either as (1) pluton margin of the Southern Vosges Ballons granite complex or (2) composite dykes intrusive into migmatite and metamorphic sequences classically referred to as granite-gneiss unit (Central Vosges). Both types correspond to melanocratic rocks with prominent, Mg-rich, biotite and hornblende (20- 40 % vol., 64 < mg# < 78), two-feldspar and quartz. Those Vosges vaugnerites display geochemical signatures characteristic of mafic to intermediate, metaluminous to slightly peraluminous rocks of calc-alkaline affinity.
Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircons were extracted from a sillimanite-bearing gneiss from the granite-gneiss unit hosting the Central Vosges vaugnerites. They yielded an age at 451 ± 9 Ma, indicating a pre- Variscan Upper Ordovician protolith for the host sequence. Zircon from the four vaugnerite intrusives display U-Pb ages (± 2s) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data thus give consistent results suggesting a relatively early emplacement of the vaugnerites regarding the Late Variscan collisional history, i.e. during Middle Visean times. These results are in line with previously published ages within the Southern Vosges volcano-sedimentary sequences (Oderen- Markstein) and the nearby Mg-K granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a Mg-K magmatic event of regional significance.
Recent petrological studies on vaugnerites suggest that they derive from partial melting of a metasomatized mantle contaminated to some different degrees by elements of continental crust. We propose here the major Mg-K magmatic pulse at 340-335 Ma to be the marker of a significant evolution into the dynamics of the Rhenohercynian subduction system below the Central-Southern Vosges. In the light of recent thermo-mechanical modelling experiments on mature continental collision, it could traduce the first effects of a syn-collisional lithospheric delamination mechanism involving (1) first, continental subduction evolving towards (2) the underthrusting of the Avalonian continental margin lower crust and (3) the initiation of lithospheric delamination within the supra-subduction retro-wedge (Saxothuringian- Moldanubian continental block). This process would drive the emplacement of an asthenospheric upwelling, initially localized along the Variscan suture zones, and gradually propagating towards the southern front of the belt during the Late Carboniferous, as the delamination front migrated at the base of the crust.