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

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