The Rb87-Sr87 whole-rock isochrons of Hercynian granites in the Montagne Noire (southern Massif Central, France) show that two different episodes of granitization occurred during Hercynian time: early alkalic granites were emplaced 340 to 330 m.y. ago, and calc-alkalic granites were emplaced 290 to 280 m.y. ago. The Hercynian orogeny may have occurred between 340 and 270 m.y. ago in this area. Tectonic phases are apparently confined to the beginning of the orogenesis and are short compared to the total lapse of time of the orogeny.
The Sr87/Sr86 initial ratios of the granites and of surrounding crustal rocks allow us to put some limitations on the relative amounts of mantle-derived material in each granitic type. The early granites represent remobilized continental crust, whereas in the later ones the mantle contribution increases with time. The mantle-derived material is presumed to be a highly differentiated product, probably of a dioritic composition.
Rb and Sr concentrations are used as tracers to test models of granite genesis, equilibrium partial melting, and (or) fractional crystallization. The mineral isochrons reveal that no important thermal event occurred after the Hercynian episode. The youngest rehomogenization is dated at 280 to 290 m.y. ago. A temperature-time curve summarizes the Hercynian orogeny and represents an attempt to connect the tectonic phases with periods of granitization and metamorphism.