Petrogenesis of the contact-metamorphic rocks beneath the Stillwater Complex, Montana

The Stillwater gabbroic complex was emplaced at about 2700 Ma in a metasedi mentary section consisting of pelitic schist, iron formation, and quartzite . Contact metamorphism resulted in the development of a 500-1000 in hyperst hene hornfels. zone at, the contact and a 500-1500 in cordierite + cummingt onite hornfels zone farther from the contact. Weakly foliated biotite schis t occurs beyond the hornfels. Pelitic rocks are poor in potassium, generall y lack muscovite, and typically contain the assemblage garnet + staurolite + gedrite + chlorite + cordierite + biotite + plagioclase + ilmenite + quar tz. The diagnostic hornfels assemblage is cordierite + cummingtonite + biot ite + plagioclase. + ilmenite + quartz. Cummingtonite is replaced by hypers thene near the contact. The assemblages in the biotite schist formed during a low-pressure regional metamorphism. Temperature ranged from 450-625 degr eesC in biotite schist to 650-800 degreesC in hypersthene hornfels. Pressur e estimates range from 100-250 Awa in biotite schist to 50-300 MPa in hyper sthene hornfels. The alpha (H2O) in the hypersthene zone is estimated to ha ve been similar to0.27. Orthoamphibole in the biotite schist has compositio ns that fall on a well-defined linear trend from Fe3.5Mg3.5Si8O22(OH)(2) to Na0.7Fe3.5Mg1.4Al2.0(Al-2.5,Si-5.5)O-22(OH)(2). There is no evidence for a ny immiscibility between anthophyllite and gedrite at the pressure of metam orphism. Orthoamphibole was replaced by clinoamphibole with increasing grad e. The petrogenetic grid, calculated from estimates of thermodynamic data f or the observed compositions of minerals, indicates that most reactions in biotite schist occur in the temperature range 525-550 degreesC at 200 MPa. The formation of the assemblage cordierite + cummingtonite occurs at 600 de greesC, and hypersthene is stable above 700 degreesC. The net reaction duri ng formation of cordierite + cummingtonite hornfels from biotite schist ind icates little loss of H2O and a small increase in solid volume. The pressur e and temperature of metamorphism are consistent with subvolcanic emplaceme nt of the complex at a depth of similar to6-7 km in metasedimentary country rock following a period of regional metamorphism.