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

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.(c) 2022 Elsevier B.V. All rights reserved.