The age of the Mont-Blanc granite (Western Alps): a heterogeneous isotopic systeme dated by Rb-Sr whole rock determinations on its microgranular enclaves

Abstract

New Rb-Sr whole rock age data have been obtained for the Mont-Blanc calc-alkaline Hercynian granite. Different facies of the granite have been sampled, as well as locally abundant microgranular enclaves of magmatic origin. The analytical data for the granite and for some of the enclaves are scattered within the Sr evolution diagram, which points to isotopic disequilibrium. Other enclaves form a homogeneous system which gives an age of 316.1 ± 19.5 Ma and an initial 87Sr/86Sr ratio of 0.7058 ± 0.0005. The following evolution is plausible: - individualization of microgranular enclaves due to the dismembering of mafic synplutonic dikes within the granitic magma; - complete isotopic equilibrium between granite and enclaves during a late-magmatic stage; - remobilization ofthe granite and certain enclaves in a post-magmatic stage and during Alpine orogenesis. Certain enclaves are not affected and retain the original characteristics of the system. The 316 Ma age of these "relics" and their initial ratio of 0.7058 are therefore thought to be valid for the granite as well. These results are similar to those obtained for other calc-alkaline Hercynian plutons in Western Europe. They indicate an origin for the granitic magma in the deep crust with involvement of mantle-derived material in unknown proportions, represented by the microgranular enclaves.