Introduction
The Variscan orogeny (~ 380–300 Ma) is the geological event most largely represented in the basement of the European continent. It was assembled between Ordovician and Carboniferous from the larger collision of Gondwana with the northern plate of Laurentia–Baltica, which involved the microplates of Avalonia and Armorica (Matte, 2001). Variscan units (Fig. 1) extend from the Ibero-Armorican block of southern Spain to the Bohemian Massif of Poland. Large remnants of Variscan basement are preserved in the southern Variscides, within the Alpine chain, where they are located in external positions (Fig. 1).
In the Western Alps, such remnants are identified as External Crystalline Massifs (Mont Blanc, Aiguilles Rouges, Grandes Rousses, Belledonne, Pelvoux, and Argentera), which record the general evolution common to all Pangean Europe (von Raumer et al., 2009). They are generally composed of a complex metamorphic basement intruded by Permo-Carboniferous granitoids. In these Massifs, the Alpine metamorphic overprint is weak and commonly limited to shear zones. The exhumation of the External Crystalline Massifs from below their sedimentary cover units and the Alpine nappes initiated in the Miocene (Argentera: Bigot-Cormier et al., 2006), i.e. at the end of the Alpine orogeny.
Figure 1. European Variscides
Distribution of the European Variscan massifs and units (modified from O'Brien, 2000; Stampfli et al., 2002; von Raumer and Bussy, 2004). A: Argentera; BA: Mont Blanc – Aiguilles Rouges; C: Corsica; MT: Maures-Tanneron; RH: Rheno-Hercynian; S: Sardinia; SW: Schwarzwald; V: Vosges.