Abstract

40Ar/39Ar laser-probe dating of phengitic micas has been carried out by step-heating and spot-fusion procedures. These micas represent successive deformation stages in the structural evolution of the internal Dora Maira nappe, western Alps. Single phengites from a gneiss affected by a single ductile strain under retrogressive conditions (sample 99.1) display complete isotopic resetting with nearly homogeneous intracrystalline Ar distribution and yield plateau ages of about 40 Ma. Small clusters of phengites from an earlier foliation were selected from a polydeformed mica schist (sample PTX3). They show a partial isotopic resetting in response to overprinting during the retrogressive deformation stage with a "concentric" age zoning from 68 Ma on the rim to 87 Ma in the core on one cleavage plane. This zonation is fully consistent with the laser-derived discordant age spectrum, which ranges from 40 to 90 Ma from low to high temperatures. According to the deformation history of both samples, these preliminary data suggest a deformation control on Ar migration during recrystallization processes, and they are consistent with the timing of the collisional evolution previously reported for southern Dora Maira units. This study shows that the 40Ar/39 Ar continuous laser-probe dating technique produces data accurate enough to discriminate several tectonometamorphic episodes recorded in single hand samples.

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