PETROGRAPHY AND MINERALOGY OF ULTRAMAFIC PODS IN THE RUBY RANGE WITH SPECIAL ATTENTION TO IDENTIFYING ACCESSORY MINERAL PHASES, INCLUDING ZIRCON

The Ruby Range is an uplifted exposure of Archean to Early Proterozoic rocks in southwestern Montana exposed during Laramide uplift of the Wyoming Province basement. Pod-shaped bodies of meta-ultramafic rocks are found in all the Precambrian rock groups of the Ruby Range. Pods range from 1 m to 50 m in diameter. There is controversy regarding the protolith(s) of these rocks. The main hypotheses are emplacement as igneous cumulates or serpentinized slivers of mantle. Ruby Range meta-ultramafic samples were analyzed by petrographic microscope, SEM and cathodoluminesence. All pods show a remarkably similar mineral assemblage of olivine, orthopyroxene and magnesian hornblende. This main phase is replaced mainly by serpentine, anthophyllite and magnetite and a minor suite of hydrothermal minerals is present. Preserved igneous textures are found in the form of small prismatic spinel grains in orthopyroxene. Later metamorphism is recorded in the form of poikiloblastic textures in orthopyroxene and the presence of metamorphic zircon with sector zoning. This is the first reported case of zircon found in the Ruby Range ultramafic rocks. The findings of this study are consistent with studies of ultramafic rocks in the neighboring Tobacco Root Mountains that suggest an igneous cumulate origin of these rocks prior to metamorphism and in contrast to interpretations of serpentinization prior to metamorphism. Future geochemical and geochronological studies of the rocks and zircons are needed to further characterize the protolith of the Ruby Range ultramafic rocks and constrain the age of metamorphism.