Proterozoic Metamorphism of the Tobacco Root Mountains, Montana
Textures and mineral assemblages of metamorphic rocks of the Tobacco Root Mountains are consistent with metamorphism of all rocks during the Big Sky orogeny (1.77 Ga) at relatively high pressure (P >1.0 GPa) followed by differential reequilibration on a clockwise P-T path at lower pressures (0.6–0.8 GPa). The highest pressures are documented by coarse-grained kyanite and orthopyroxene in aluminous orthoamphibolites, which require P ≥ 1.0 GPa. Other higher-pressure mineral assemblages of note include kyanite + orthoamphibole and kyanite + K-feldspar. Abundant textural evidence for partial melting in pelitic and basaltic rocks includes leucosomes, very large (several cm across) porphyroblasts of garnet, and an absence of primary (foliation-defining) muscovite. Partial to complete overprinting of the coarse-textured, high-pressure assemblages by lower-pressure assemblages and textures occurred across the Tobacco Root Mountains, especially where assisted by deformation and the availability of water. In aluminous rocks, sillimanite bundles typically replace kyanite, and garnet may be rimmed by cordierite + orthopyroxene symplectite or, in quartz-absent rocks, sapphirine + spinel + cordierite symplectite. Orthoamphibolites with partial pseudomorphs of garnet by cordierite are common. Garnet necklaces surround orthopyroxene in orthopyroxene-plagioclase gneisses, whereas orthopyroxene + plagioclase pseudomorphs of garnet occur in nearby hornblende amphibolites. These features appear to require nearly isobaric cooling at pressures near 0.8 GPa, followed by nearly isothermal decompression at temperatures near 700 °C. The resulting P-T path is believed to be the result of tectonic denudation late in the orogenic cycle. Quartz-plagioclase-garnet-hornblende amphibolites occur throughout the Tobacco Root Mountains. Near-rim mineral compositions from these rocks have been used to calculate Ts of 650–750 °C at Ps of 0.7–0.9 GPa across the terrane. There is no systematic variation in calculated P and T between units nor geographically within units; differences appear to reflect variations in thermometer closure possibly due to the availability of water during cooling. Field relations involving metamorphosed mafic dikes, as well as geochronological data from monazite and zircon, demonstrate that some rocks were first metamorphosed at high temperatures and pressures at 2.45 Ga. However, we have not identified mineral assemblages that can be assigned unequivocally to this earlier event.
Document Object Identifier (DOI)
Cheney, J.T., Brady, J.B., Tierney, K.A., DeGraaff, K.A., Mohlman, H.K., Frisch, J.D., Hatch, C.E., Steiner, M.L., Carmichael, S.K., Fisher, R.G.M., Tuit, C.B., Steffen, K.J., Cady, P., Lowell, J., Archuleta, L.L., Hirst, J. Wegmann, K.W., Monteleone, B. (2004). Proterozoic Metamorphism of the Tobacco Root Mountains, Montana. Geological Society of America Special Papers, 377, 105-129. doi: 10.1130/0-8137-2377-9.105