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

Abstract

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.