Unravelling the complex record of magma flow and solidification in sills

Sill emplacement is fundamental to the development of volcanic plumbing systems and their impact on volcanic hazard assessments, geothermal heat potential estimates, and critical hydrothermal and magmatic ore deposit models. Accurate interpretations of geological, magnetic and petrographic evidence of magma flow provide essential and independent insights into the physics of sill emplacement, but these datasets are rarely considered in combination. We integrate multiscale observations of the Whin Sill, UK, to discern between syn- and post-emplacement processes. Magma finger orientation and ropy structures provide the best indication of primary magma flow directions, whereas the plagioclase feldspar crystals do not as they show a weak crystallographic preferred orientation. Euhedral Ti-poor titanomagnetite records magma finger inflation and variable flow during sill growth through a low-inclination K max anisotropy of magnetic susceptibility (AMS) tensor. A needle-like Ti-poor titanomagnetite population shows upward melt migration and post-solidification influx of hydrothermal fluids via cooling joints through a steep K max anisotropy of anhysteretic remanent magnetization tensor and inverse AMS fabrics. These novel results highlight the complex dynamics of sill emplacement that can be reconstructed only through multiscale and multimethod analysis. Not using this approach risks over-simplistic models with incorrect magma flow trajectories and inaccurate source locations.