Phase Relations and Crystallization Sequence in a Contact-Metamorphosed Rock from the Gunflint Iron Formation, Minnesota

Abstract

Portions of the Gunflint Iron Formation, originally a ferruginous sediment, were metamorphosed by the intrusion of the Duluth Complex to assemblages containing: pigeonite (Wo10En24Fs66)+olivine (Fo13Fa37)+Fe-Ti oxide (Mt62Usp34Hc4)+plagioclase (An94Ab6)+ vapor+augite (Wo40En20Fs40) or cummingtonite Fe/(Fe+Mg) ˜ 0.69; quartz was present but probably was not in equilibrium with olivine. Comparison with synthetic phase-equilibrium studies indicate conditions of initial recrystallization of T≥ 800 °C, Ptotal ≥ 2kb, fo2 slightly below that of the pure fayalite-magnetite-quartz assemblage, and PH2O < Ptotal. During the slow cooling process following initial recrystallization, the phases present underwent a complex series of exsolution, inversion, oxidation, and hydration reactions. Pigeonite initially exsolved augite along (001), then inverted to orthopyroxene, which then exsolved augite along (100). The augite exsolved only pigeonite on (001) during its cooling history. The Fe-Ti oxide for the most part oxidized to an intergrowth of magnetite and ilmenite, although unoxidized portions later exsolved ulvöspinel. Cummingtonite exsolved actinolite, forming irregular patches of the latter. Olivine, orthopyroxene, and augite reacted with plagioclase to form retrograde amphiboles. Orthopyroxene had difficulty nucleating during this slow cooling process, forming only at widely spaced points in mosaics of primary pigeonite grains, and never nucleating within primary augite grains. The resulting orthopyroxene grains are much larger than the original pigeonite grains.