Abstract
Dating specific pressure-temperature-time-deformation-fluid (P-T-t-d-f) events is a major petrological issue, particularly for polymetamorphic assemblages. In order to better assess such events using mica populations, this study uses a combined in situ Rb-Sr and Ar-Ar dating approach coupled with chemical mapping, with application to an exhumed subduction complex (Schistes Lustrés, Western Alps). Geochronological investigation of the most Si-rich (Tschermak substitution) mica population allows us to investigate the (near-)peak burial ages of the various tectonometamorphic units of this high-pressure/low-temperature subduction complex. In the blueschist-facies units, a wide range of ages between ∼36 (younger than previously obtained peak ages) and ~ 52 Ma suggests a diachronous slicing of units with different initial paleogeography and/or a long residence time at near-peak conditions. The combined use of Ar-Ar and Rb-Sr isochron ages reveals the existence of some excess argon in the studied metamorphic rocks, interpreted as the result of (i) heterogeneous contamination from an argon-rich fluid in the higher grade eclogite-facies units and (ii) partial removal, during pseudomorphic recrystallization, of argon inherited from earlier stages of metamorphism in the lower grade blueschist-facies units.