LATE VARISCAN GRANITES OF THE BELLEDONNE MASSIF (FRENCH WESTERN ALPS)- AN EARLY VISEAN MAGNESIAN PLUTONISM

Belledonne, one of the External Crystalline Massifs of the Alps, has w idespread Late Variscan granitoids in its northeastern domain that con stitute three strongly elongate plutons (Sept Laux, Saint Colomban, La Lauziere) intruding gneisses and migmatites. These plutons mainly con sist of variably foliated biotite granites and granodiorites, associat ed with different proportions of enclaves and stocks of amphibole-biot ite mafic rocks (e.g. vaugnerite, durbachite, monzonite, quartz syenit e). They define similar highly magnesian suites of the subalkaline typ e (i.e. intermediate between alkaline and calc-alkaline or, in other w ords, alkali-calcic), usually rich in Ba, Sr, Th, U and light REE, tha t evolved independently of each other. Their respective emplacements h ave been dated by the single-zircon lead-evaporation technique at 343 +/- 16 Ma (Saint Colomban), 341 +/- 13 Ma (La Lauziere), and 335 +/- 1 3 Ma (Sept Laux). Geochemical and chronological data show that the thr ee plutons belong to a single Early Visean magmatic episode, possibly short-lived, comparable to the Mg-K plutonism reported from other Vari scan massifs (e.g. Aar, Corsica, Southern Vosges, Bohemia). Their empl acement is related to the beginning of a first major period of extensi on and wrench tectonics that affected the entire Late Variscan belt of Western Europe. These plutons, probably composed of hybrid granitoids , may have been derived from both crustal and mantle source rocks.