The Occurrence and Chemistry of Epidote-Clinozoisites in Mafic Gneisses from the Ruby Range, Southwestern Montana

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Contributions to Geology

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Geochemistry | Geology


Optically and chemically zoned epidote-clinozoisite occurs as a retrograde mineral in Precambrian mafic gneisses from the Ruby Range, southwestern Montana. Computer-corrected electron microprobe analyses place the chemical boundary between epidote and clinozoisite at 15.45 ± 1.55 mole percent pistacite (Fe+3/(Fe+3+AlVI)). Zoning profiles indicate that epidote-clinozoisite crystals formed during a single episode in which: (1) temperatures declined gradually (in the range from 745°C to 550°C); and/or (2) oxygen fugacity increased. Superimposed on these changes were local and relatively minor T-fO2 fluctuations, as evidenced by oscillatory Al-Fe+3 zonation in single crystals. These physicochemical variations probably characterize conditions that prevailed in mafic gneisses during the post-2700 m.y. uplift event inferred by Dahl (1979a, 1979b) for the Ruby Range.