RB-SR MICA AGES IN THE ALPINE SHEAR ZONES OF THE TRUZZO GRANITE - TIMING OF THE TERTIARY ALPINE P-T-DEFORMATIONS IN THE TAMBO NAPPE (CENTRAL ALPS, SWITZERLAND)

The Truzzo granite, located in the southern part of the Tambo nappe, h as recorded the Tertiary history of Alpine deformation. Rb-Sr whole ro ck dating method of this late Variscan complex leads to an age of intr usion around 280 Ma. During the Tertiary deformation, the Truzzo grani te has undergone a finite heterogeneous strain at all scales. The stud y of Rb-Sr ages on micas from shear zones of the granite yields ages b etween 25 Ma-32 Ma for white micas and between 18 Ma-22 Ma for biotite s. In this part of the Alps, these Rb-Sr ages are interpreted as cooli ng ages. These new results, associated with a compilation of previousl y published studies in the Tambo and Suretta nappes, allow to propose a tectono- metamorphic evolution for these Brianconnais units: (i) a h igh pressure event combined with a top to the NNW shearing during the stacking of these Pennine nappes is proposed between 55-35 Ma. (Valais subduction) (ii) a strong decompression, associated with a slight dec rease of temperature in the upper Pennine units between 35-30 Ma occur red during the progressive activity of the Frontal Pennine Thrust (con tinental collision). (iii) This onset of the collision is followed by a typical Barrovian metamorphism, contemporaneous with the uplift of t his part of the Alps and of the activity of the External Crystalline M assifs Thrust between 30-20 Ma.