Fluid or melt? distinguishing syn-deformational interaction pathways

Understanding whether deformation occurred in the presence of aqueous fluid or silicate melt is crucial for interpreting ductile shear zones, impacting their thermal and geochemical evolution, and having rheological consequences. To identify the syn-deformational fluid type, we investigate contrasting shear zones active during the Alice Springs Orogeny in central Australia, focusing on their effects on dry granulite facies gneisses transformed into greenschist–amphibolite facies schists. Shear zones in the north-western part of the orogen (Reynolds–Anmatjira Ranges) exhibit greenschist–lower amphibolite facies muscovite–chlorite assemblages, quartz veins and microstructures indicative of solid-state deformation. These features collectively suggest deformation in the presence of aqueous fluid. In contrast, shear zones in the south-eastern part (Strangways Range) display upper amphibolite facies garnet–biotite–sillimanite assemblages, along with granitic dykes and lenses retaining igneous textures. Microstructures, such as ‘string of bead’ textures and felsic minerals forming films along grain boundaries or exhibiting low apparent dihedral angles, indicate the former presence of melt in high strain rocks. This suggests that hydration in the south-eastern shear zones was driven by externally sourced silicate melt and melt–rock reactions. Differentiating between the two types of shear zones using whole rock major and trace element data is challenging. However, rare earth element (REE) analyses show potential. Limited REE metasomatism is observed where aqueous fluids are inferred, with three samples in a transect displaying consistent patterns. In contrast, where silicate melt is interpreted as the metasomatic agent, REE metasomatism is more variable, exhibiting atypical REE patterns relative to common rock types and considerable variability between samples in a transect. This contrast is attributed to greater mobility of REEs in silicate melt compared to aqueous fluid.