Assessing the geochemical and tectonic impacts of fluid-rock interaction in mid-crustal shear zones: a case study from the intracontinental Alice Springs Orogen, central Australia

Abstract

The Reynolds–Anmatjira Ranges, central Australia, form part of a high-grade basement terrane dissected by intensely metasomatized transpressional shear zones active during the Ordovician–Carboniferous Alice Springs Orogeny. Unlike typical retrograde structures associated with discrete fluid flow, the mid-crustal setting and intracontinental nature of these shear systems present significant problems for the source and ingress mechanism of the fluid involved in their rehydration. To address these issues, we describe two detailed traverses across deformed and metasomatized basement rocks in this region, and interrogate their record of fluid–rock interaction from various perspectives. Both traverses combine structural and petrological observations with Zr-in-rutile and Ti-in-quartz thermobarometry, oxygen and hydrogen stable isotope analysis, and major, trace and rare earth element mobility trends. Each technique is critically evaluated for its utility in this study and its more widespread applicability to alternative field areas, providing a strategic framework for the general investigation of fluid-affected shear zones. Ultimately, the integrated data sets specify pressure–temperature conditions of ∼530 °C and 4–5 kbar, implying average apparent thermal gradients of 29–36 °C km−1 and depths of 14–18 km. Other characteristic features to emerge include strongly variable element mobilities and pronounced isotopic depletion fronts consistent with the alteration effects of an externally derived, non-equilibrium fluid. This is confirmed by calculated fluid compositions indicative of contributions from a fluid of meteoric origin, with estimated δ18O and δD values as low as 2.3‰ and −59.8‰, respectively. We propose that these surficial fluid signatures are imposed on the mid-crust by the prograde burial and dehydration of hydrothermally altered fault panels produced during pre-orogenic basin formation. Progressive fluid release with continued subsidence then leads to the accumulation of increasing fluid volumes in the vicinity of the brittle–ductile transition, promoting extensive hydration, metasomatism and reaction softening at the locus of stress transmission from plate-boundary sources. The sustained injection of externally derived fluids into refractory crustal material may thus stimulate a critical reduction in the long-term strength of the lithosphere, providing strong impetus for the initiation and advancement of intracontinental orogenesis.