Production of the Mont Blanc granite batholith by felsic plumes

In the Alps, the current view is that the Mont Blanc granite batholith is an inherited Paleozoic lens and was not produced by the Alpine subduction. Here, I show that widespread hornblende amphibolites lenses and breccias, enclosed within plagioclase gneiss, are transformed into plagioclase rich lithologies. At Mont Tondu, a large lens of Mesozoic sedimentary schists is enclosed within plagioclase gneiss. Numerous anastomosing gneissic lenses grow within these sedimentary schists, suggesting that they are transformed into gneiss. This suggests that a metamorphic front has developed into an ophiolitic slab and into a Mesozoic sedimentary slab. The transformation of ophiolitic and sedimentary materials into felsic lithologies has been generated by aqueous fluids. These considerations show the occurrence of felsic plumes containing a significant amount of water. These felsic plumes have generated a metamorphic front into an ophiolitic slab and into a Mesozoic sedimentary slab. They have generated the Mont Blanc granite batholith between 29 and 22 Ma at a depth in the range of 20 km.