Mineral fractionation and pre- and post-emplacement processes in the uppermost part of the Shiant Isles Main Sill

Reinvestigation of the top 25 m of the 165 m thick Shiant Isles Main Sill r equires reinterpretation of its internal structure, especially of the last unit emplaced. - the granular olivine picro-dolerite. New mineral analyses allow re-assessment of the magmatic processes involved in the intrusion and differentiation of this classic multiple sill. C-shaped vertical mineral c omposition profiles, which occur in olivine-rich, alkaline basaltic sills, reflect reaction between phenocrysts and variable composition liquids. A ca se is made for vertical migration of interstitial liquids within and across the boundaries of intrusive units, for the mixing of such liquids from adj acent units, and for their influence on mineral chemistry on both sides of the interface.