The Hebridean Igneous Province plumbing system: A phase equilibria perspective

The architecture of magmatic plumbing systems and the behaviour of the crystal mush underlying active volcanoes can be assessed by examining the crystal chemistry and microstructure preserved in solidified magma bodies, i.e. dykes, sills, and layered intrusions. Combining field and petrographic observations of the Little Minch Sill Complex in Scotland, with analyses of its mineral chemistry, demonstrates that these shallow tabular intrusions formed by two distinct magma pulses, a cargo-rich magma and a cargo-free magma, in a relatively closed system. The co-genetic nature of the crystal cargo and its carrier liquid permits the application of phase-equilibria modelling to estimate the pressures and temperatures of crystallisation and sill emplacement. Our calculations using thermodynamic modelling software (Perple_X) suggest the presence of a of magma chamber at ~25 km depth and the formation of sill and dyke network at ~3–16 km below the surface. Our new estimates of the pressures of magma storage and crystallisation are similar to those found for the Iceland and the rest of the British Tertiary Igneous Province, suggesting the behaviour of magmas in the crust and the architecture of the plumbing system are comparable. This apparent temporal consistency supports the use of preserved intrusive bodies as a means for understanding and constraining processes occurring in the upper crust beneath active volcanoes.