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

¹ Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410, USA
² Department of Geosciences, State University of West Georgia, Carrollton, Georgia 30118, USA

The Stillwater gabbroic complex was emplaced at about 2700 Ma in a metasedimentary section consisting of pelitic schist, iron formation, and quartzite. Contact metamorphism resulted in the development of a 500–1000 m hypersthene hornfels zone at the contact and a 500–1500 m cordierite + cummingtonite hornfels zone farther from the contact. Weakly foliated biotite schist occurs beyond the hornfels. Pelitic rocks are poor in potassium, generally lack muscovite, and typically contain the assemblage garnet + staurolite + gedrite + chlorite + cordierite + biotite + plagioclase + ilmenite + quartz. The diagnostic hornfels assemblage is cordierite + cummingtonite + biotite + plagioclase + ilmenite + quartz. Cummingtonite is replaced by hypersthene near the contact. The assemblages in the biotite schist formed during a low-pressure regional metamorphism. Temperature ranged from 450–625 °C in biotite schist to 650–800 °C in hypersthene hornfels. Pressure estimates range from 100–250 MPa in biotite schist to 50–300 MPa in hypersthene hornfels. The _H2O in the hypersthene zone is estimated to have been 0.27. Orthoamphibole in the biotite schist has compositions that fall on a well-defined linear trend from Fe_3.5Mg_3.5Si₈O₂₂(OH)₂ to Na_0.7Fe_3.5Mg_1.4Al_2.0(Al_2.5Si_5.5)O₂₂(OH)₂. There is no evidence for any immiscibility between anthophyllite and gedrite at the pressure of metamorphism. Orthoamphibole was replaced by clinoamphibole with increasing grade. The petrogenetic grid, calculated from estimates of thermodynamic data for the observed compositions of minerals, indicates that most reactions in biotite schist occur in the temperature range 525–550 °C at 200 MPa. The formation of the assemblage cordierite + cummingtonite occurs at 600 °C, and hypersthene is stable above 700 °C. The net reaction during formation of cordierite + cummingtonite hornfels from biotite schist indicates little loss of H₂O and a small increase in solid volume. The pressure and temperature of metamorphism are consistent with subvolcanic emplacement of the complex at a depth of 6–7 km in metasedimentary country rock following a period of regional metamorphism.

Key Words: Archean • contact metamorphism • cordierite • cummingtonite • Stillwater Complex

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