Fluid-driven HTLP metamorphism at Aberfoyle, NSW: geological and geochemical insights

Hot fluids in the crust are widely recognised, yet the sources and prevalence of fluid-driven metamorphism (metasomatism and advective heat transport) remain debated. Here, we document a previously unrecognised high-temperature − low-pressure (HTLP) metamorphic block at Aberfoyle, ∼13 km west of the well-studied Wongwibinda Metamorphic Complex (WMC) in the southern New England Orogen, eastern Australia. This fault-bounded block comprises Carboniferous accretionary complex metasedimentary rocks and notably lacks associated igneous intrusions, providing a natural setting to isolate the effects of fluid flux. A steep metamorphic gradient from greenschist to amphibolite facies over ∼1 km indicates a highly localised and anomalous heat source. The rock types and metamorphic assemblages closely resemble those of the WMC, suggesting the Aberfoyle block was laterally displaced westwards along a major fault. In contrast to the WMC, the absence of pegmatite dykes allows investigation of early-stage HTLP complex development during fluid-driven metamorphism. Structural observations reveal deformation-assisted fluid channelisation within ductile shear zones is overprinted by complex veining. Integrated geological and whole-rock geochemical data reveal localised fluid infiltration and channelised migration, with fluid–rock interaction intensifying systematically with metamorphic grade. We propose that during crustal extension, aqueous fluids derived from metamorphism of fertile lower crust were channelled up the Wongwibinda Shear Zone, advecting heat and initiating HTLP metamorphism in the upper crust. Subsequent channelised migration of S-type granitic melts in the Wongwibinda Shear Zone likely sustained HTLP conditions at Aberfoyle and along the shear zone margins.