NEOCRYSTALLIZATION OR COOLING? 40AR39AR AGES OF WHITE MICAS FROM LOW-GRADE MYLONITES

Abstract

40Ar39Ar data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or, alternatively, ages of deformation-induced neocrystallization. To address these interpretations three types of information are needed: (1) the effective diffusion length scale controlling volume diffusion of argon in white micas; (2) an estimate of the diffusion parameters for volume diffusion of argon; and (3) the thermal history. Completely recrystallized quartzites exhibit smooth grain size distributions for white micas, with good exponential or power law fits, suggesting that the grains grew under similar conditions. The microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history. Measured grain size distributions provide a maximum limit for the diffusion length scale. Thermal histories are estimated by 40Ar39Ar diffusion experiments on K-feldspars using the multi-diffusion-domain method of Lovera [Lovera, O.M., Richter, F.M., Harrison, T.M., 1989. The 40Ar39Ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes. J. Geophys. Res. 94, 17917-17935.]. These results are consistent with the expected temperature-time path for the thrusting environment, and predict maximum temperatures of 250-350°C during the time span recorded by the white micas. Experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystallization ages. Forward modeling of the passage of the white micas through the closure window for argon diffusion, using the thermal histories derived from the K-feldspars, indicates that the white micas record neocrystallization rather than cooling, and supports the idea that the white micas record the timing of when ductile deformation ceased.