Rb-Sr ages from phengite inclusions in garnets from high pressure rocks of the Swiss Western Alps

- We apply Rb/Sr geochronology to phengite extracted from garnets.
- The garnets grew prograde in eclogites-facies metasediments.
- We show that garnet acted as a shield for the included phengites.
- Rb and Sr isotope exchange with the bulk rock was limited after inclusion.
- The phengites included in garnets are about 4 Ma older than the matrix phengites.

The Zermatt-Saas Fee Zone (ZSZ) was subducted to eclogite-facies conditions, reaching peak pressures and temperatures of 20-28 kbar and 500-630 °C. The rocks were partially overprinted under greenschist-facies conditions during exhumation. Previous Rb-Sr isochron ages obtained on matrix phengites in metasediments of the ZSZ have been interpreted to date early exhumation of the ZSZ. Here we present new Rb-Sr geochronology on phengite inclusions in garnets to date prograde growth of garnets. We show that garnet acted as a shield for the included phengites, limiting Rb and Sr isotope exchange with the bulk rock, upon complete enclosure of the mica, during garnet growth, even if peak metamorphism exceeded the Rb-Sr blocking temperature. Similarly, garnet isolated the micas from the matrix during subsequent recrystallization due to fluid infiltration or deformation during exhumation. Phengite inclusion ages for two metapelitic samples from the same locality (Triftji) are 44.86±0.49Ma and 43.6±1.8Ma, and are about 4 m.y. older than the corresponding matrix mica ages of 40.01±0.51Ma and 39.5±1.1Ma, respectively. The results confirm previous Sm-Nd and Lu-Hf geochronology on the ZSZ that indicated protracted garnet growth during prograde metamorphism, and confirm that at least parts of the ZSZ underwent peak metamorphic HP conditions less than 43 m.y. ago, followed by rapid exhumation to upper greenschist-facies conditions around 40 Ma ago.