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 which have undergone various metamorphic conditions (350 to 600 degrees C and 3 to 7 kb) were analysed for their Rb-Sr systematics. In the greenschist 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 reflect 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 replaced 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 modifications are probably related to external fluid circulation and are also 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 events are obtained for mylonites in shear zones.