The Traigh Bhan na Sgurra Sill, Isle of Mull: Flow Localization in a Major Magma Conduit

Field evidence points to localization of magma flowing in a Tertiary doleritic sill on the Isle of Mull, Inner Hebrides, Scotland. Regions of the sill in which flow was short-lived have chilled margins, a narrow or absent metamorphic aureole, and alignment of plagioclase crystals resulting in pronounced, flow-parallel, lineations on fracture surfaces (Type I regions). Prolonged flow resulted in blocky, coarse-grained dolerite with no chilled margins, and an extensive metamorphic aureole (Type II regions). The distribution of Type I and Type II regions shows no spatial pattern, with stagnant solidifying sections immediately adjacent to sections with contemporaneous active and sustained flow. The transition between Type I and Type II regions occurs as the sill thickness exceeds 3[middot]5 m. Our observation that sustained flow was only possible in regions of the sill thicker than 3.5 m is consistent with previously published theoretical models which predict a critical sill thickness in the range 2[middot]2-5 m. Regions of the sill narrower than this experienced a single injection of magma. Simple models to determine flow duration from the width of the metamorphic aureole demonstrate that progressive focusing of flow into wider parts of the conduit created discrete channels active for up to 5 months.