Metamorphosed Precambrian ultramafic rocks in the Ruby Range, Montana

Abstract

Ultramafic rocks occur in many localities throughout the Archean Ruby Range metamorphic complex. Consistent field, petrologic, petrofabric and textural relationships in the ultramafic rocks of the Ruby Range indicate that they were emplaced early and participated in the deformation and upper amphibolite facies metamorphic event of the area. Textures (included S-surfaces), mineral assemblages (magnesio-hornblende, orthopyroxene, olivine, green spinel, ± titanian hydroxyl-clinohumite) and associated rock types (meta-rodingites) indicate the bodies were serpentinized prior to metamorphism. Typically, the ultramafic rocks are elongate, elliptical bodies which tend to parallel the regional foliation and range in size from a few meters to hundreds of meters in length. They commonly have concordant foliations with their host rocks, occur at or near the crests of isoclinal folds, and occasionally form boudinage structure. Mineral equilibria in the ultramafic rocks indicate the conditions of their metamorphism were on the order of 710°C at 5–7 kbar. These estimates are consistent with geothermometry and barometry in the host rocks. Titanian hydroxyl-clinohumite was stabilized under these conditions by an appreciable fluorine content. After the upper amphibolite facies metamorphism, reserpentinization of the ultramafic bodies occurred.