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A systematic investigation of the exsolution textures in titanomagnetite grains of massive magnetite layers and their associated host rocks from the entire upper zone sequence has revealed noticeable differences that can be related to changes in the oxidation state of the magma during crystallization and cooling. The observed oxidation exsolution intergrowths indicate a higher fo2 during formation of the magnetite layers than during crystallization of the disseminated titanomagnetite, as well as a decrease in the fo2 required to precipitate successive titanomagnetite-rich layers upward in the sequence.Peculiar composite lameliar intergrowths of magnetite and ilmenite in the uppermost magnetite layers and in some magnetite plugs are ascribed to a subsolvus oxidation of ulviispinel in ulviSspinel-rich magnetite. The configuration of the magnetite-ulviSspinel solvus dictates that ulviispinel in Ti-rich magnetite-ulv•ispinel solid solution high in the succession exsolves at considerably higher temperature than in Ti-poor solid solution lower in the sequence. Subsolvus oxidation of ulviispinel to ilmenite at higher temperatures near the top of the intrusion facilitated diffusion of ilmenite to produce the variety of different composite exsolution textures. The textures are not developed where abundant oxidation exsolution of ilmenite has taken place at temperatures above the solvus.

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