Interpreting isotopic ages as deformation ages when they are acquired from moderate-temperature metamorphic environments can be a challenging task. Syros Island (Cyclades, Greece) is famous for Eocene high-pressure metamorphic rocks reworked by localized Miocene greenschist-facies deformation. In this work, we investigate phengites from coarse-grained marbles, which experienced the high-pressure event, and phengites from fine-grained localized marble shear zones attributed to the low-grade Miocene deformation. Based on structural criteria, both events can be easily discriminated because of their opposing kinematics. Laser-heating 40Ar/39Ar analysis on phengite yielded a 40 ± 1.6 Ma age for the host rock and a 37 ± 1.3 Ma age for the shear zone. Both ages are statistically indistinguishable, consistent with the regional Eocene event, and not the Miocene deformation event responsible for the formation of the shear zone. Thermodynamic modeling indicates that the observed high-variance mineral assemblage is stable without compositional change along the pressure-temperature path followed by the rocks of Syros. Although the marble within the shear zone was deformed at extremely fast strain rates (10−10 s−1), we observed no intracrystalline deformation of phengite grains and no resetting in the isotopic system, because strain was mostly accommodated by calcite. Consequently, a high strain rate does not necessarily create deformation ages in rocks with high-variance assemblages, such as marble mylonites.
- Received 11 March 2015.
- Revision received 15 May 2015.
- Accepted 16 June 2015.
- © 2015 Geological Society of America