Eo/Oligocene (35 Ma) high-pressure metamorphism in the Gornergrat Zone (Monte Rosa, Western Alps): implications for paleogeography

Zircons from a phengite-rich metaquartzite of the Gornergrat Zone (Monte Ro sa nappe) have rims with irregular shape and weak cloudy zoning. Their appa rent ages scatter from Late Permian to Eo/Oligocene. These rims are thought to have formed during a metamorphic event, which is dated by the four youn gest rims that have very low Th/U ratios and yield an age of 34.9 +/- 1.4 M a. The rock paragenesis (quartz, phengite, epidote, albite and titanite) in dicates equilibration at HP conditions (P < 14-15 kbar and T < 500-550 degr ees C) and is not compatible with greenschist-facies retrogression. Based o n the rock assemblage and the geological evolution of the area, this age is interpreted as dating the high-pressure/medium-temperature metamorphism in the Gornergrat Zone. The new age confirms the hypothesis of Tertiary subdu ction in the Western Alps. The metamorphic assemblage in the sample dated i s very similar to the one found in high-pressure rocks of the Monte Rosa ba sement, suggesting that these two units shared the same Alpine evolution. I n our view, the new data are best explained with a paleogeographic location of the Monte Rosa nappe on the European margin of the Tethys.