Dating microstructures by the Ar-40/Ar-39 step-heating technique: deformation-pressure-temperature-time history of the Penninic Units of the Western Alps

Abstract

White micas from the Penninic units of the Urtier Valley, in the Western Alps, have been dated by the 40Ar/39Ar step-heating method. A strong correlation is observed between measured ages, mica composition and deformation fabrics. Samples characterized by a single mineral reservoir yielded relatively flat age spectra, allowing to estimate an age of 44.4 ± 0.4 Ma for the formation of phengite-bearing fabrics in eclogite facies conditions (SHP) and an age of 42.0 ± 0.4 Ma for the microstructurally younger muscovite-bearing shear fabrics, which formed in greenschist facies conditions (SZgr1). Several micaschist samples are characterized by the overprint of the phengite-bearing SHP fabric by a younger muscovite-bearing fabric (FU) that is axial planar to tight upright folds that post-date the SZgr1 shearing event. BSE images reveal that the different generations of syn-kinematic micas are not exclusively tied to their respective fabric, but muscovite is commonly found also in the SHP microlithons, while relict phengite is often transposed into the new FU axial planar fabric. All the samples containing these two generations of white micas yielded convex-upward spectra, which, following considerations based on the 36Ar/40Ar Vs 40Ar/39Ar plots, have been interpreted to indicate mixing of two argon reservoirs of different ages, related to the older phengite and the younger muscovite visible with optical and electron microscopy. This method allowed to estimate a maximum age of ca. 36.5 ± 0.5 Ma for the folding of the shear-related structures. The ages estimated for SHP and SZgr1 in the study area are consistent with those obtained in other parts of the Penninic units with other geochronological techniques. Therefore, the post-crystallization loss of argon from the phengite associated with SHP and the muscovite defining SZgr1 is negligible, despite evidence indicating that conditions of T = 480-510 °C were reached during the later upright folding event.