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

Abstract

Up to 92 per cent of the original minerals were fused during progressive metamorphism of Torridonian sediment by a picrite sill. The liquid precipitated microlites of tridymite, cordierite, hypersthene, and magnetite until that remaining quenched to a 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 composition of liquid in a fused xenolith at various stages was calculated from chemical and modal analyses. The chemical analysis indicates that fusion occurred in the presence of excess water-vapour. The melting and crystallization processes compare closely with the behaviour of similar compositions in the system NaAlSi_3O_8-KalSi_3O_8-SiO_2-H_2O. Liquidus temperatures corresponding to the calculated liquids provide temperature estimates. An upper pressure limit is given by the quartz-tridymite PT curve. It is estimated that the liquid crystallized between 1,025°C and 935°C at a water-vapour pressure of 430 kg/cm^2. Reaction between picrite and fused sediment indicates that maximum fusion occurred when crystallization of the picrite was almost completed. The estimated picrite intrusion temperature is at least 1,175°C.

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