Dating deformation using Rb-Sr in white mica: Greenschist facies deformation ages from the Entrelor shear zone, Italian Alps

Ages of deformation have been obtained by Rb-Sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation. Since white micas commonly define deformation fabrics in medium-grade metamorphic rocks, these ages can be directly related to structural geometries with regional context. This direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust. It does require that the dated minerals attained isotopic equilibrium with the dominant Sr reservoir at temperatures lower than the closure temperature. This resetting was apparently achieved during dynamic recrystallization of white micas in greenschist-facies metasediments and metagranitoid units in the western Alps. The results suggest that the Sr isotopic composition of the new mica is buffered by the coexisting high-Sr phases (calcite, feldspar or epidote) via the grain boundary network. High-strain rocks from the Entrelor shear zone system of the western Alps have yielded indistinguishable white mica Rb-Sr ages along 30 km of individual and kinematically linked shear zones. The age of the back-thrusting event is constrained at 34+/-1 Ma, the age yielded by the younger generation of synkinematically crystallized white micas. This event was short-lived, involving at least 20 km of shortening in similar to 1 m.y. or less. An earlier, variably overprinted component, dating from 38 to 37 Ma, has been identified in the mica fabric, but its kinematic significance is uncertain. This method of dating strain fabrics offers a powerful tool for tectonic studies, since isotopic resetting can be directly linked to structural geometries, microstructural textures, and PT conditions. It allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust.