Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)

Abstract

In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300–315Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375±30°C and 4.8–7±1kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34–20Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (<400°C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15–21km depth are dated between 33 and 20Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6±0.6Ma or in the range (2) 26.8±0.7Ma–26.3±0.7Ma. In the West of the massif, younger shear zone ages range between (3) 22.2±0.3Ma and (4) 20.5±0.3Ma.