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 ore deposit models. Accurate interpretations of geological, magnetic, and petrographic evidence of magma flow provides essential and independent insights into the physics of sill emplacement but are rarely considered in combination with one-another. We integrate multiscale observations of the Whin Sill, located in the north of England, to discern between syn- and post-emplacement processes. The Whin Sill is a mafic sill that intruded into Carboniferous-aged sediment 295 ± 6 Ma, which is outcrops across northern England with coastal exposures and ridges, such as below Hadrian’s Wall.

Field observations of sill finger orientation and ropy structures provide the best indication of primary magma flow directions, whereas plagioclase feldspar crystals do not as they show weak shape and crystallographic preferred alignments (SPO and CPO). Early Ti-poor titanomagnetite with a low-inclination K_max anisotropy of magnetic susceptibility (AMS) tensor records magma finger inflation and variable flow during sill growth. A second Ti-poor titanomagnetite population records the migration of melt upwards and the post-solidification influx of hydrothermal fluids via cooling joints. This is captured through a steep K_max anhysteretic anisotropy of remanent magnetisation (AARM) and inverse AMS fabrics. By integrating our results from multiple techniques, we have proposed a multi-stage emplacement mechanism for the Whin Sill in this area: 1) initial propagation as magma fingers based on field observations, 2) magma finger inflation and variations in magma flow direction, based on weak CPO and low-inclination AMS K_max tensors, and 3) influx of fluids and upward melt migration syn-emplacement, based on high-inclination AARM and inverse AMS fabrics. These novel results highlight the complex dynamics of sill emplacement that can be reconstructed only through multiscale and multimethod analysis. Conducting similar analysis on samples taken from multiple locations across an intrusion would allow for further detail to be obtained, and for a greater understanding of the complex processes occurring. Not using an integrated approach risks over simplistic models with incorrect magma flow trajectories and inaccurate source locations.