Abstract

Precambrian metamorphic iron-formations from the Tobacco Root and Ruby Mountains and the Gravelly Range of southwestern Montana were studied by optical, electron microprobe, and X-ray diffraction techniques. Associated pelitic rocks in the Tobacco Root Mountains are of sillimanite-orthoclase grade, and those of the Carter Creek area of the Ruby Mountains, though still of sillimanite grade, are inferred to have been metamorphosed at a slightly lower temperature or higher РН2O. Pelitic rocks associated with the iron-formation in the Ruby Creek area of the Gravelly Range are of chlorite grade.

The high-grade iron-formation assemblages are mainly of the type quartz + magnetite ± hematite + one or two pyroxenes + two amphiboles ± garnet. Tobacco Root Mountains assemblages contain optically homogeneous ferrohypersthene which may coexist with a ferrosalite of similar Fe/Mg ratio showing exsolved ferrohypersthene lamellae. Associated amphiboles are grunerite and hastingsitic or tschermakitic hornblende, both with visible or submicroscopic exsolution lamellae. Both amphiboles and pyroxenes are subhedral and unaltered, though the amphiboles are finer in grain size than the pyroxenes. Some assemblages are aluminous and may contain abundant almandine, as well as subordinate biotite or feldspar. Similar assemblages from the Carter Creek area of the Ruby Mountains are relatively amphibole-rich, with minor ferrohypersthene and andradite garnet. Amphiboles in the Carter Creek area iron-formations contain more exsolution lamellae than do Tobacco Root Mountains amphiboles of the same composition. A common assemblage in the Carter Creek area iron-formation contains aegirine-rich clinopyroxene + sodic tremolite (rimmed by mag-nesioriebeckite) + quartz + iron oxides. The Gravelly Range iron-formation contains a very different assemblage, quartz + magnetite + hematite + hydrobiotite.

Calculations based on the compositions of coexisting silicates in the iron-formations from the Tobacco Root Mountains confirm conditions of metamorphism as deduced from pelitic assemblages: 650–750°C and 4–6 kbar. The prevalence of two-pyroxene assemblages in the Tobacco Root Mountains may imply slightly higher temperatures or lower PH2O than in the amphibole-rich, one- or no-pyroxene assemblages in the Carter Creek area of the Ruby Mountains. The assemblage of the oxide iron-formation in the Ruby Creek area of the Gravelly Range is consistent with chlorite grade metamorphic conditions: <400°C and 2–4 kbar.

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