Journal of Petrology, Volume 40, Issue 7: July 1999.

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

FRANÇOIS DEBON AND MARIE LEMMET

LABORATOIRE DE GÉODYNAMIQUE DES CHARNES ALPINES, UPRES-A CNRS 5025, INSTITUT DOLOMIEU, 15 RUE MAURICE-GIGNOUX, 38031 GRENOBLE CEDEX, FRANCE

Late Variscan plutonic bodies are widespread in the External Crystalline Massifs of the Alps (Argentera, Pelvoux, Belledonne, Grandes Rousses, Mont Blanc, Aiguilles Rouges, Aar, Gotthard). They can be classified on the basis of their Mg/(Fe + Mg) ratio and mafic mineral content (expressed by the B = Fe + Mg + Ti parameter). Together with available ages of emplacement, this classification highlights the existence of two plutonic suites, one early, Viséan (~330-340 Ma), and highly magnesian, the other later, mainly Stephanian (~295-305 Ma), and more ferriferous. This evolution from a magnesian plutonism to a more ferriferous one, which also occurs in other Variscan massifs (e.g. Corsica), might be accounted for by a combination of factors related to the nature of the source of the magmas, the physical and chemical conditions of melting, and the Late Variscan geodynamic setting. As a basis for these considerations a comprehensive review is presented of all the External Crystalline Massifs and theirLate Variscan intrusions.

Keywords: Alps;geochronology;Late Variscan plutonism; major and trace element geochemistry

Pages 1151-1185