FLUID ADVECTION IN SHEAR ZONES - EVIDENCE FROM GEOLOGICAL AND GEOCHEMICAL RELATIONSHIPS IN THE AIGUILLES-ROUGES MASSIF (WESTERN ALPS, SWITZERLAND)

At the Southeastern margin of the Aiguilles Rouges Massif, Western Alp s, the development of a ductile shear zone has occurred during the Alp ine Tertiary events. The feldspathic gneisses of the crystalline basem ent are transformed into highly-deformed dark mylonites. This study de scribes mass-transfer and stable isotope behaviour related to the tran sformation of gneiss to mylonite. Mass balance calculations using majo r element data show that the deformation of the gneiss occurred withou t significant changes in volume and under essentially isochemical cond itions, except for calcium. The calcium mobility is probably associate d to the shift from a ductile towards a more brittle behaviour of the shear zone in the latest stage of the deformative process. This result ed from opening of the system to fluids of an external origin. DeltaO- 18 quartz values measured in mylonites and gneisses range from +10.2 t o +12.6 parts per thousand and from +10.9 to 12.4 parts per thousand r espectively, while whole rock samples have deltaO-18 values ranging fr om +8.2 to +9.9 parts per thousand from least to most deformed mylonit es and from +9.8 to +11 parts per thousand for the gneisses, which wer e only weakly deformed, with the exception of two samples showing pecu liar conditions. In agreement with the chemical data, the measured O-1 8/O-16 ratios suggest a limited participation of external fluids durin g the mylonitic deformation, while the calcium mobility may be explain ed in terms of a contamination of a localized Ca-bearing fluid which m oved through the system during a later stage of deformation.