Metamorphic petrology of the Negaunee Iron Formation, Marquette District, northern Michigan; mineralogy, metamorphic reactions, and phase equilibria

Progressive regional metamorphism of the Negaunee Iron Formation in the Marquette district from chlorite to sillimanite grade has resulted in the development of three metamorphic zones and two subzones. The diagnostic zone assemblages are, in order of increasing grade: 1, siderite + quartz + or - ankerite + or - minnesotaite + or - stilpnomelane + or - magnetite; 2, grunerite + or - quartz + or - ankerite + or - magnetite and grunerite + or - siderite + or - ankerite + or - magnetite; 3a, grunerite + or - garnet + or - quartz + or - magnetite and grunerite + or - Ca amphibole + or - quartz + or - calcite + or - magnetite; 3b, grunerite + or - olivine + or - clinopyroxene + or - quartz + or - calcite + or - magnetite and grunerite + or - Ca amphibole + or - olivine + or - magnetite. The grunerite, garnet, and Ca amphibole, and olivine and clinopyroxene isograds define the boundaries between the respective zones and subzones. Temperature estimates are approximately 300 degrees C for zone 1 and approximately 600 degrees C for subzone 3b. Pressure is estimated at between 200 to 300 MPa. Microprobe analyses of coexisting minerals indicate general attainment of exchange equilibrium and a strong localized influence of bulk composition on mineral chemistry.Theoretical analysis of phase relations on isobaric T-X (sub CO ₂ ) diagrams leads to the following conclusions:1. Throughout zone 1, minnesotaitc-bearing assemblages are diagnostic of low X (sub CO ₂ ) values, stilpnomelane-bearing assemblages represent low to medium values, and chlorite-bearing assemblages represent medium to high X (sub CO ₂ ) values. Juxtaposition of these three assemblages at one locality indicates that fluid phase compositional gradients most likely existed.2. Throughout zone 3, actinolite-, hornblende-, and garnet-bearing assemblages reflect similar low to high X (sub CO ₂ ) values. The continued juxtaposition of these various assemblages suggests the continued presence of fluid-phase compositional gradients into the highest grade metamorphic zones.3. Metamorphic reactions caused substantial, localized CO ₂ enrichment in the coexisting fluid phase. A diversity of T-X (sub CO ₂ ) paths was followed, typically differing on the scale of individual mesobands.