RB-SR SYSTEMATICS OF RECRYSTALLIZED SHEAR ZONES AT THE GREENSCHIST-AMPHIBOLITE TRANSITION - EXAMPLES FROM GRANITES IN THE SWISS CENTRAL ALPS

In the Swiss Central Alps, several metagranite ductile shear zones whi ch have undergone various metamorphic conditions (350 to 600 degrees C and 3 to 7 kb) were analysed for their Rb-Sr systematics. In the gree nschist facies, the rubidium content increases in mylonites while the strontium content decreases. Opposite chemical variations are observed in amphibolite facies shear zones. These chemical mass transfers refl ect the different behaviour of the initial magmatic paragenesis during deformation. In amphibolite facies, the initial paragenesis is stable , K-feldspar and oligoclase recrystallize to a fine grained mylonite. In the greenschist facies, the feldspathic phase is progressively repl aced by a new metamorphic assemblage (quartz-phengite-albite). In both metamorphic cases, Sr isotope changes of the whole rocks are strongly suspected to occur in the mylonites. These chemical and isotopic modi fications are probably related to external fluid circulation and are a lso affected by progressive mineral changes. In each studied area, the whole rock Rb-Sr isotopic dating method records magmatic ages of the undeformed granite, while spurious ages with respect to tectonic event s are obtained for mylonites in shear zones.