Evolution of Mg/Fe Ratios in Late Variscan Plutonic Rocks from the External Crystalline Massifs of the Alps (France, Italy, Switzerland)

Abstract

Late Variscan plutonic bodies are widespread in the External Raumer et al., 1993; Menot et al., 1994). From south to Crystalline Massifs of the Alps (Argentera, Pelvoux, Belledonne, north, they are the Argentera, Pelvoux, Belledonne, Grandes Rousses, Mont Blanc, Aiguilles Rouges, Aar, Gotthard). Grandes Rousses, Mont Blanc and Aiguilles Rouges They can be classified on the basis of their Mg/(Fe+ Mg) ratio massifs (Western Alps), and the Aar and Gotthard massifs and mafic mineral content (expressed by the B = Fe + Mg + (Central Alps) (Fig. 1). They appear as dome-like strucTi parameter). Together with available ages of emplacement, this tures of crystalline basement surrounded by a mostly classification highlights the existence of two plutonic suites, one Mesozoic sedimentary cover. As a whole, they define an early, Visean (~330–340 Ma), and highly magnesian, the other arcuate structure, possibly inherited from the Variscan later, mainly Stephanian (~295–305 Ma), and more ferriferous. orogeny (Bogdanoff et al., 1991). This evolution from a magnesian plutonism to a more ferriferous During the Variscan orogeny, the ECM were part of one, which also occurs in other Variscan massifs (e.g. Corsica), the Helvetic–Moldanubian terrane of the internal zone might be accounted for by a combination of factors related to the of the Variscides (von Raumer & Neubauer, 1993). nature of the source of the magmas, the physical and chemical Precambrian metamorphic units and Precambrian to conditions of melting, and the Late Variscan geodynamic setting. Palaeozoic sediments, interlayered with volcanic and As a basis for these considerations a comprehensive review is ultramafic rocks, display in most areas a polymetamorphic presented of all the External Crystalline Massifs and their history and underwent a highly complex pre-Variscan Late Variscan intrusions. and Variscan evolution. Although they share a number of similarities, the ECM each have also specific characteristics (von Raumer et al., 1993). In addition to orthogneisses deriving from Early Palaeozoic plutonic (and volcanic?) protoliths (Bussy & von