Fusion of Torridonian Sandstone by a Picrite Sill in Soay (Hebrides)

Up to 92 per cent of the original minerals were fused duringprogressive metamorphism of Torridonian sediment by a picritesill. The liquid precipitated microlites of tridymite, cor-dierite,hypersthene, and magnetite until that remaining quenched toa glass. Stages of fusion were determined by petrographic methods.After decomposition of sericite and reduction of haematite,liquid developed by fusion of feldspar and quartz. The compositionof liquid in a fused xenolith at various stages was calculatedfrom chemical and modal analyses. The chemical analysis indicatesthat fusion occurred in the presence of excess water-vapour.The melting and crystallization processes compare closely withthe behaviour of similar compositions in the system NaAlSi₃O₈-KAlSi₃O₈-SiO₂-H₂O.Liquidus temperatures corresponding to the calculated liquidsprovide temperature estimates. An upper pressure limit is givenby the quartz-tridymite PT curve. It is estimated that the liquidcrystallized between 1,025?C and 935?C at a water-vapour pressureof 430 kg/cm². Reaction between picrite and fused sediment indicatesthat maximum fusion occurred when crystallization of the picritewas almost completed. The estimated picrite intrusion temperatureis at least 1,175?C.