Texturally targeted multi-isotopic dating of ductile deformation: the Clarke River Shear Zone, northeastern Australia

Dating deformation in shear zones is challenging, because mylonitisation can disturb isotopic systems. Here we tackle this problem by applying an integrated microstructural/geochronological approach, combining high-lateral resolution microstructural analysis with zircon U–Pb, mica step-heating 40 Ar/ 39 Ar, and in situ Rb–Sr geochronology. Our results from the Clarke River Shear Zone (northeastern Australia) reveal a complex history of shear zone reactivation (constrained by Rb–Sr and 40 Ar/ 39 Ar dating), which affected an orthogneiss protolith (476 ± 2 Ma; U–Pb zircon crystallisation age). An early reactivation phase involved dextral general shear under amphibolite-facies conditions at 416 ± 9 Ma, as constrained by in situ Rb–Sr dating of syn-kinematic mica, and biotite 40 Ar/ 39 Ar analysis. This phase was approximately coeval with the syn-tectonic emplacement of a tonalite at 419 ± 1 Ma (amphibole 40 Ar/ 39 Ar). A subsequent phase of dextral shearing at 348 ± 13 Ma, constrained by Rb–Sr analyses of recrystallised muscovite within a secondary mylonitic foliation, reflects deformation under sub- to lower greenschist facies conditions. The results highlight that texturally targeted Rb–Sr analyses can successfully record multiple deformation episodes. 40 Ar/ 39 Ar analysis of bulk grains was only able to record the older Devonian event, possibly due to limited 40 Ar susceptibility to deformation or the use of whole-grain rather than in situ 40 Ar/ 39 Ar analyses. In situ Rb–Sr analysis of muscovite grains dynamically recrystallised at temperatures below the typical closure temperature (i.e., <450 °C) can record the timing of deformation, thereby effectively capturing both overprinting events and the ages of deformed relics.