The investigated HP/LT metasedimentary units
of the Valaisan and adjacent European domains occupy
a key position in the Alpine belt for understanding
the transition from early subduction-related HP/LT
metamorphism to collision-related Barrovian overprint
and the evolution of mountain belts in general. The
timing of high-pressure metamorphism, subsequent
retrogression and following Barrow-type overprint was
studied by 40Ar/39Ar dating of biotite and several white
mica generations that are well characterized in terms
of mineral chemistry, texture and associated mineral
assemblages. Four distinct age populations of white
mica record peak pressure conditions (42–40 Ma) and
several stages of subsequent retrograde metamorphic
evolution (36–25 Ma). Biotite isotopic analyses yield
consistent apparent ages that cluster around 18–16 Ma for
the Barrow-type thermal overprint. The recorded isotopic
data reveal a significant time gap in the order of some
20 Ma between subduction-related HP/LT metamorphism
and collision-related Barrovian overprint, supporting the
notion of a polymetamorphic evolution associated with
a bimodal P-T path