PHASE EQUILIBRIA MODELING, PSEUDOMORPHS, AND CRUSTAL LOADING ALONG THE NORTHEAST MARGIN OF THE MOUNT STUART BATHOLITH, NORTH CASCADES, WA

Pseudomorphs of sillimanite and/or kyanite after andalusite around the margins of the 93-96 Ma Mount Stuart Batholith in the North Cascades, WA, are well known and often cited examples of polymetamorphic textures. They are interpreted to locally record a post Mount Stuart emplacement crustal loading event that occurred at or before 88-86 Ma based on garnet Sm-Nd ages from the nearby Heather Lake area. New phase diagram section calculations in the MnNCKFMASH system for metapelite samples near the northeast margin of the Mount Stuart Batholith near Jove Peak and Labyrinth Mountain place constraints on the P-T conditions of metamorphism and magnitude of crustal loading.

The phase diagram section for 03NC19A, a garnet-biotite-staurolite-kyanite-muscovite-plagioclase-quartz schist from Labyrinth Mountain predicts the observed assemblage to be stable from 6.6 to 8.8 kilobars & 630 to 650 °C, values which approximate the conditions of assemblage development. Garnet-biotite and garnet-plagioclase-quartz-kyanite thermobarometry indicates 6.8 kbar @ 632 °C to 7.9 kilobars @ 665 °C, broadly consistent with the assemblage stability field. These pressures are interpreted to represent conditions during or after crustal loading.

Pseudomorph-bearing 03NC152 from within the Mount Stuart contact metamorphic aureole near Jove Peak contains andalusite replaced by kyanite, staurolite, muscovite, biotite, and quartz. Garnet and fibrolitic sillimanite occur in the matrix. Andalusite stability is predicted at pressures < 3-4 kilobars, and the andalusite-replacement assemblage at > 6.0-6.5 kilobars & 620 to 655 °C, indicating a minimum pressure increase of 2 kilobars after andalusite growth. Calculations addressing the stability of this assemblage as a function of bulk-rock Al₂O₃ content may refine the pressure estimate of pseudomorph development. A pressure increase of 2 kilobars is compatible with inferred pressure increases in the Heather Lake region where inferred pseudomorphs of sillimanite after andalusite are observed, but phase equilibria modeling results from Heather Lake were unable to directly calculate the magnitude of pressure increase during crustal loading. The Jove Peak area could be an important locality for detailed geochronology studies aimed at directly dating the crustal loading event.