Geothermobarometry of metamorphosed mafic dikes and sills, Tobacco Root Mountains, southwestern Montana

Sarah Carmichael
Department of Geology
Smith College

At least two generations of metamorphosed mafic dikes and sills (MMDS) of various thicknesses (2 cm to 30 m) and orientations are scattered throughout the Archean rocks of the Tobacco Root Mountains. The MMDS cross-cut gneissic banding in amphibolite and quartzofeldspathic gniess of the Indian Creek Metamorphic Suite (ICMS) and the Pony-Middle Mountain Metamorphic Suite (PMMMS) (Burger et al., 1994). However, MMDS are not present in the adjacent Spuhler Peak Metamorphic Suite (SPMS), a metamorphosed mafic oceanic (?) sequence. We have interpreted these field relations to indicate that the MMDS were intruded into the ICMS and PMMMS prior to their juxtaposition with the SPMS, but after the first metamorphism of the ICMS and PMMMS. All of the rocks (MMDS, ICMS, PMMMS, and SPMS) were heated and deformed together during a subsequent metamorphism as indicated by parallel axes for ductile folds in all units (Harms et al., 1996).

The primary mineral assemblages in MMDS are Hbl+Plag(An25-65)+Cpx+Gar ±Ilm±Bio, indicating upper-amphibolite, lower-granulite phase metamorphism. More hornblende but less augite is found near the margins of the MMDS, suggesting that availability of water influenced the modal composition. Crystal size also varies from the margins (e.g. garnets 0.1 mm) to the centers of the thicker MMDS (e.g. garnets 1 mm). Data using Powell's (1984) gar+hbl thermobarometer and Kohn and Spear's (1990) gar+hbl+plag+qtz barometer suggest temperatures ranging from 600 to 730°C, and pressures of 7 to 11 kb. Gar-cpx geothermobarometers on the same samples give similar results. Using the same garnet-hornblende geothermobarometer calibrations on coarse grained SPMS samples reveals temperatures and pressures of 650°C and ~6 kb for near-rim compositions. Although the temperatures and pressures found in the SPMS reflect different conditions than those found in the MMDS, and thus those in the ICMS and PMMMS, the data are consistent with initial prograde metamorphic pressures >10 kb and re-equilibration either during decompression to ~5.5 kb on a clockwise P-T path, or with reheating at a lower pressure (Cheney et. al, 1994).