Migmatization of Archean aluminous metasediments from the eastern Beartooth Mountains, Montana, U.S.A.

Geothermobarometry, mineral compositions and textures, and thermodynamic models suggest biotite dehydration melting occurred in the peraluminous rocks of the eastern Beartooth Mountains. These Archean metapelitic migmatites are metatexites and diatexites, and have typical metamorphic assemblages of Qtz + Pl + Kfs + Bt + Sil +/- Grt +/- Crd. The subsequent in situ crystallization of the magma derived from the biotite dehydration created migmatitic leucosomes in the rocks. These leucosomes are primarily composed of quartz, plagioclase and potassium feldspar. Water derived from the dehydration of biotite was dissolved in the melted phase. Crystallization of the magma reintroduced this water to the system, allowing reversal of the biotite dehydration melting reactions on the retrograde pressure/temperature path. This rehydration event produced melanosomes, primarily composed of biotite, sillimanite and garnet. The volumetric and conceptual significance of theses melanosomes suggest that retrograde processes are of major importance to the formation of textures in pelitic migmatites. Geothermobarometry suggests that these rocks attained peak conditions of 795° +/- 42° C and 7.0 +/- 0.9 kbar. The interpreted pressure/temperature path is isobaric heating above four kilobars to 795°. At peak temperature, a nearly isothermal compression occurred, raising the pressure to seven kilobars. This roughly counterclockwise (in P/T space) trajectory is consistent with a thermal event due to local emplacement of granitic plutons, subsidence due to magmatic thickening, and later uplift and unroofing. Trace element heterogeneities in sillimanite, revealed by Scanning Electron Microscopy-Cathodoluminescence Imaging (SEM-CL), suggest multiple stages of growth and dissolution of this mineral throughout the metamorphic cycle. The interpretation of these heterogeneities involves initial prograde production of sillimanite, and subsequent dissolution during the biotite dehydration melting reactions. Further sillimanite and biotite is formed during retrograde metamorphism, and enriched in chromium by a late-stage hydration event