Time-strain evolution of shear zones from petrographically constrained Rb–Sr muscovite analysis

Abstract

Some isotopic ratios in minerals like monazite, apatite, and mica can record the thermal response to mylonitization, but such systems may fail to track the time of low- to medium temperature fabric realignment (recrystallization). New analytical methods that allow spatially resolved measurement isotopic ratios in distinct microstructures offer the potential to characterize the time-strain evolution of mid-crustal shear zones over a temperature range conducive to capturing the growth timing of those structures. Here we present high spatial resolution electron backscatter diffraction (EBSD), mineral chemistry, and in situ Rb–Sr dates from two texturally distinct muscovite populations in a mid- to upper-greenschist facies granitic mylonite from Top Up Rise, Western Australia. Coarse-grained muscovite fish retain a primary magmatic composition, displaying microstructural evidence of mechanical (kink and folds) and crystal-plastic deformation. The muscovite fish yield a Rb–Sr isochron date of 1614±75 Ma that either records the crystallization of the granitic protolith or cooling from high-grade metamorphic overprinting as constrained by in situ garnet Lu–Hf dates in related metapelites (1696±43 Ma and 1670±36 Ma). Conversely, fine-grained muscovite texturally associated with shear bands is chemically distinct, display microstructural evidence of fluid-assisted dynamic recrystallization and yield a distinctly younger isochron date of 609±14 Ma. The Rb–Sr dates from the coarse-grained muscovite fish cannot be unequivocally linked to mylonitization as they are indistinguishable from regional metamorphic processes. Muscovite grains behaved as rigid porphyroclasts during later medium-temperature mylonitization, leading to fish geometries, whilst maintaining intact Rb–Sr systematics. However, recrystallized/neoblastic fine-grained muscovite records the timing of medium-temperature mylonitization, establishing a direct time–strain relationship. These results highlight the potential for significant diachronicity in mineralogical components of mylonitic fabrics, demonstrating the need to establish direct time-strain relationships in order to accurately reconstruct deformation histories.