High Precision U-Pb zircon and monazite dating in the Aiguilles
Rouges-Mont Blanc area allowed discrimination of three short-lived
bimodal magmatic pulses: the early 332 Ma Mg-K Pormenaz monzonite and
associated 331 Ma peraluminous Montees Pelissier monzogranite; the 307
Ma cordierite-bearing peraluminous Vallorcine and Fully intrusions; and
the 303 Fe-K Mont Blanc syenogranite. All intruded syntectonically along
major-scale transcurrent faults at a time when the substratum was
experiencing tectonic exhumation, active erosion recorded in detrital
basins and isothermal decompression melting dated at 327-320 Ma. Mantle
activity and magma mixing are evidenced in all plutons by coeval mafic
enclaves, stocks and synplutonic dykes. Both crustal and mantle sources
evolve through time, pointing to an increasingly warm continental crust
and juvenile asthenospheric mantle sources. This overall
tectono-magmatic evolution is interpreted in a scenario of
post-collisional restoration to normal size of a thickened continental
lithosphere. The latter re-equilibrates through delamination and/or
erosion of its mantle root and tectonic exhumation/erosion in an overall
extensional regime. Extension is related to either gravitational
collapse or back-are extension of a distant subduction zone.