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摘要
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 Ar-40/Ar-39 method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300-315 Ma) 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 degrees C and 4.8-7 +/- 1 kbar in an H2O-satured system. In this low temperature environment, Ar-40/Ar-39 analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited Ar-40/Ar-39 isotopic resetting, all ages scattering between 296 and 315 Ma. Under conditions of intense ductile deformation and large-scale fluid circulation. muscovite grains formed during the Variscan retain their much older ages. Ar-40/Ar-39 dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34-20 Ma) for each shear zone. In detail, Ar-40 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 Ar-40 excess. These results demonstrate that the Ar-40/Ar-39 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, Ar-40/Ar-39 dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (<400 degrees C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15-21 km depth are dated between 33 and 20 Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6 +/- 0.6 Ma or in the range (2) 26.8 +/- 0.7 Ma-26.3 +/- 0.7 Ma. In the West of the massif, younger shear zone ages range between (3) 22.2 +/- 0.3 Ma and (4) 20.5 +/- 0.3 Ma.
- 出版日期2011-7
