U-Th-Pb dating of collision in the external Alpine domains (Urseren zone, Switzerland) using low temperature allanite and monazite

Dating metamorphism in external domains of orogenic belts is a challenging task due to the few chronometers available and common inheritance or disturbance of the isotopic systems at low-temperature metamorphic conditions. In the external domains of the Central Alps (Urseren zone), the occurrence of distinct populations of monazite and allanite in one single outcrop offers a unique chance to evaluate the potential of these two REE-minerals to date successive metamorphic stages within a collisional cycle. The studied outcrop (locality of Garvera) exposes Mesozoic metasediments that were metamorphosed under greenschist facies conditions (T<450°C). Under these conditions, REE-mineral grains (or domains) are typically small (~5-50μm), and in this case also occur as porphyroblast in pelites at the contact with carbonate layers or veins (allanite grains >200μm). Based on the texture and mineral assemblages, REE-mineral growth conditions were attributed to be detrital (Mnz1), syn-kinematic (Mnz2 and Aln1), post-kinematic at the thermal peak (Aln2) and retrograde during late-stage deformation (Mnz3). To constrain temporally the successive crystallisation conditions, REE-minerals have been dated using SHRIMP ion microprobe. U-Th-Pb analyses show that the Alpine REE-minerals have a high Th/U and while most of the ²⁰⁶Pb is non-radiogenic (f₂₀₆ that can reach up to 99% for allanite), Th-Pb ages can be obtained using ²⁰⁶Pb_c-corrected Th-Pb isochron. The age of different generations of allanite and monazite constrains the timing of successive collisional stages: for the prograde accretion at 22.5±1.5Ma (Mnz1 and Aln1), thermal peak at 19.3±2.0Ma (Aln2) and late stage hydrothermal veining at 13.6±1.4Ma (Mnz3). These novel geochronological data confirm the regional diachroneity through the Central Alps with younger ages (burial, thermal peak and exhumation) toward the external domains. The similarity between metamorphic ages of samples taken in the Urseren zone and the nearby crystalline massifs suggests that the metasedimentary cover of the Urseren zone remained closely juxtaposed to the external crystalline basement during the Alpine collision cycle. The monazite age of the late stage vein testifies for active tectonic deformation at 13.6±1.5Ma, which likely played a crucial control on the exhumation of the external massifs in the Middle Miocene.