Abstract

At Heaney’s Find, Meier’s Find, and Queen Victoria Rocks in the greenstone belt terrain (~2.6–2.7 Gyr) of the Yilgarn Block of Western Australia, banded iron-formations have been regionally metamorphosed. The primary metamorphic assemblages in the iron-formations contain quartz, grunerite, clinopyroxene, orthopyroxene, magnetite, and minor hornblende and pyrrhotite. Fayalite is also present at Heaney’s Find and Queen Victoria Rocks. Feshales interbanded with the iron-formations contain hornblende, almandine, and minor biotite, plagioclase, ilmenite, magnetite, and pyrrhotite. Smooth, curved grain boundaries between various combinations of the above minerals suggest that they are equilibrium assemblages. Actinolite and some grunerite with ragged boundaries against other minerals are interpreted as having formed after the primary assemblages. Asbestiform grunerite, ragged magnetite, and greenalite are retrograde. Based on (a) two-pyroxene geothermometry, (b) olivine–orthopyroxene geothermometry, and (c) the absence of evidence for the crystallization of pigeonite, the peak metamorphic temperature is estimated to have been 670 ± 50°C. Using Smith’s (1971) quartz–olivine–orthopyroxene geobarometer, the composition of orthopyroxene coexisting with fayalite + quartz in the iron-formation assemblages indicates that the pressure during metamorphism was 3–5 kbar. A somewhat lower pressure, as indicated by experimental data for the quartz + olivine → orthopyroxene reaction (Bohlen et al. 1978), is also consistent with the geology of these three areas.

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