Geochemistry of a talc-kyanite-chloritoid shear zone within the Monte Rosagranite, Val d'Ayas, Italy

Kyanite-talc-chloritoid-bearing rocks are exposed in a shear zone and other places within the Monte Rosa granite in the upper Val d'Ayas (Italy). Weak ly deformed granite grades into progressively more deformed gneiss and whit eschist. A garnet-talc-chloritoid-calcite-pyrite-quartz boudin marks the ce nter of this shear zone. The occurrence of weakly deformed whiteschists and discordant whiteschist-granite contacts indicates a pre-deformation origin of the whiteschist chemistry. Mass balance calculations were done in order to assess the nature and amount of mass transfer necessary to produce the whiteschist from a granitic protolith. The results indicate enrichment of M g and H2O, with minor Si, Fe and K. Na, Ca, Sr, Ba, Pb and locally U are de pleted. Stable isotope investigations on calcite from the carbonate zone in the center of the shear zone were performed to constrain the fluid source. delta C-13 values of calcite range from -9.60 to -9.81 parts per thousand. (PDB), while their delta O-18 values range from +9.61 to +10.14 parts per thousand. (VSMOW). These light isotopic values point towards a magmatic flu id source. We suggest that a pre-Alpine argillitic alteration leads to a ch lorite-rich metasomatic alteration of the granite around gangue mineralizat ion. Strain was localized during high-pressure metamorphism and subsequent exhumation into these rocks. This formed a deformational weak zone in the o therwise dry Monte Rosa granite. The presence of hydrous phases prior to me tamorphism led to abundant recrystallization during high-pressure metamorph ism.