High-strain deformation and fluid infiltration diachronism of the middle crust: New Devonian–Permian Alice Springs ages (365–290 Ma) of shear zones in the Strangways Metamorphic Complex, Central Australia

Abstract

The Proterozoic granulite-facies Strangways Metamorphic Complex (SMC) in the Arunta Region of Central Australia was partially hydrated during the Devonian–Carboniferous (380–320Ma) Alice Springs Orogeny by amphibolite-greenschist-facies high-strain deformation. Stable and radiogenic isotope analyses on coeval muscovite and biotite from shear-zone samples of the SMC were performed to analyse the composition and date the timing of fluid circulation and cooling.Stable isotope data (δ18O and δD) indicate that in the south of the SMC, the source of fluids appear to be the thick successions of Neoproterozoic Heavitree Quartzite, while another source in the north and central part of the SMC may be related to meteoric fluids. The heterogeneous δ18OFluid and homogenous δDFluid values for muscovite reflect changes in water/rock ratios with initial high values in the south and lower in the north, possibly suggesting a migration of fluids from south to north (as indicated by the evolving δ18OFluid and homogenous δDFluid values). Rb/Sr muscovite ages of 365–340Ma approximate the time of crystallization and fluid infiltration. They also indicate that the shear zones were active at different times during the Alice Springs Orogeny and that some were active prior to the peak Carboniferous amphibolite-facies metamorphism. 40Ar/39Ar muscovite ages of 350–325Ma are mostly interpreted as cooling ages. Rb/Sr biotite ages of 335–290Ma are interpreted as cooling ages at ~300°C (Sr closure temperature for biotite) and extend the activity of the Alice Springs Orogeny until the Permian.Cooling histories of the Wallaby Knob and Harry Creek shear zones, which serve as the tectonic boundaries of the SMC to the north and south respectively, indicate slow cooling (<3.5°CMa−1) for c. 30Ma (360–330Ma) at both boundaries continuing until 290Ma in the western portion of the Harry Creek shear zone. This slow cooling is associated with passive exhumation (i.e. erosion). Rapid cooling (20–10°CMa−1) then occurred over c. 10Ma (330–320Ma) in the Wallaby Knob shear zone and in the east portion of the Harry Creek shear zone. This change in cooling rate is associated with the combined initiation in the Carboniferous of dextral and thrusting deformation by the Pinnacles Bore and Erontonga shear zones.