The recognition of former melt flux through high-strain zones

High-strain zones are potential pathways of melt migration through the crust. However, the identification of melt-present high-strain deformation is commonly limited to cases where the interpreted volume of melt "frozen" within the high-strain zone is high (>10%). In this contribution, we examine high-strain zones in the Pembroke Granulite, an otherwise low-strain outcrop of volcanic arc lower crust exposed in Fiordland, New Zealand. These high-strain zones display compositional layering, flaser-shaped mineral grains, and closely spaced foliation planes indicative of high-strain deformation. Asymmetric leucosome surrounding peritectic garnet grains suggest deformation was synchronous with minor amounts of in situ partial melting. High-strain zones lack typical mylonite microstructures and instead display typical equilibrium microstructures, such as straight grain boundaries, 120° triple junctions, and subhedral grain shapes. We identify five key microstructures indicative of the former presence of melt within the high-strain zones: (a) small dihedral angles of interstitial phases; (b) elongate interstitial grains; (c) small aggregates of quartz grains with xenomorphic plagioclase grains connected in three dimensions; (d) fine-grained, K-feldspar bearing, multiphase aggregates with or without augite rims; and (e) mm- to cm-scale felsic dykelets. Preservation of key microstructures indicates that deformation ceased as conditions crossed the solidus, breaking the positive feedback loop between deformation and the presence of melt. We propose that microstructures indicative of the former presence of melt, such as the five identified above, may be used as a tool for recognising rocks formed during melt-present high-strain deformation where low (<5%) volumes of leucosome are "frozen" within the high-strain zone.