RbSr and 40Ar39Ar laser probe dating of high-pressure phengites from the Sesia zone (Western Alps): underscoring of excess argon and new age constraints on the high-pressure metamorphism

The combined use of the RbSr and 40Ar39Ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the Sesia zone (Western Alps) despite systematic 40Ar39Ar plateau ages. 40Ar39Ar phengite ages from two different areas show integrated age spreads between 65.4 ± 0.3 Ma and 109.5 ± 0.4 Ma. The youngest 40Ar39Ar plateau age (65.9 ± 0.4 Ma) is concordant with two RbSr phengite-whole-rock isochron ages from the same outcrop which display a mean age at 64.2 ± 2.5 Ma and older than a Rb-Sr biotite-whole-rock isochron age at 53.0 ± 1.0 Ma. This age concordance, being incompatible with the concept of isotopic closure temperature, is probably fortuitous and probably results from an excess argon contamination. However, the concordance of the RbSr phengite isotopic closure temperature with the temperature reached during the eclogite-facies event suggests that the age at 64.2 ± 2.5 Ma could be a crystallization age probably contemporaneous with the eclogite-facies event in the Sesia zone. When compared to recent age data from the Piemont zone and the Dora Maira Massif, the entire data set (which needs to be confirmed by extra measurements) support diachronous high-pressure (HP) events in the Western Alps.