Tobacco Root Mountains, Montana

Meta-ultramafi c rocks occur as small (2 to 100 m long), podiform bodies in all three major Precambrian rock suites of the Tobacco Root Mountains of southwest Montana. Most samples consist of a randomly oriented, coarse-grained assemblage of orthopyroxene, olivine, and magnesiohornblende ± spinel, partially replaced by a fi ne-grained assemblage that may include anthophyllite, talc, cummingtonite, magnesiohornblende, chlorite, serpentine, and/ or magnetite. Blackwall reaction zones of anthophyllite, actinolite, chlorite, and/or biotite surround several of the meta-ultramafi c bodies. The absence of clinopyroxene with orthopyroxene limits the metamorphic pressure-temperature history of these rocks to temperatures below ~800 °C. The presence of anthophyllite with olivine indicates that these rocks passed through 650–700 °C at pressures below 0.6 GPa. These constraints are consistent with the detailed pressure-temperature path determined for the surrounding upper-amphibolite to granulite facies metamorphic rocks. Whole-rock chemical analyses of the meta-ultramafi c rocks show them to be rich in SiO 2 (44–54 wt%) and poor in MgO (21–34 wt%) relative to mantle peridotites and typical Alpine-type ultramafi c rocks. Rare earth element concentrations are all enriched relative to chondritic values and the light rare earth elements are especially enriched (10–30 times), inconsistent with either an upper-mantle or a ko matiitic origin. All samples have similar TiO 2 /Zr ratios, suggesting that they have a common or related origin, and that TiO 2 and Zr were conserved in the processes that led to the observed chemical variations. Together, the chemical data point to a protolith that was an ultramafi c cumulate rich in orthopyroxene, and therefore probably formed in a continental setting from a basaltic magma enriched in silica. One possible time of origin is a magmatic event during the continental rifting at 2.06 Ga that led to the intrusion of a suite of mafi c dikes.