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). Weakly deformed granite grades into progressively more deformed
gneiss and whiteschist. A garnet-talc-chloritoid-calcite-pyrite-quartz
boudin marks the center 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 Mg and H2O, with minor Si,
Fe and K. Na, Ca, Sr, Ba, Pb and locally U are depleted. 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
fluid source. We suggest that a pre-Alpine argillitic alteration leads
to a chlorite-rich metasomatic alteration of the granite around gangue
mineralization. Strain was localized during high-pressure metamorphism
and subsequent exhumation into these rocks. This formed a deformational
weak zone in the otherwise dry Monte Rosa granite. The presence of
hydrous phases prior to metamorphism led to abundant recrystallization
during high-pressure metamorphism.