EVIDENCE THAT KYANITE REPLACED M1 ANDALUSITE DURING M2 METAMORPHISM IN THE SNOW PEAK AREA, NORTHERN IDAHO

Kyanite clusters that grew during the second metamorphic episode, M2, are common in the highest grade metapelitic rocks in the Snow Peak area of northern Idaho. Kyanite in the clusters is polycrystalline and is commonly associated with coarse-grained muscovite in rectangular prismatic shapes that are clearly pseudomorphous after some earlier porphyroblastic mineral formed in the first metamorphic episode, M1. At lower grade, some of the coarse grained muscovite pseudomorphs have staurolite cores and seem to have formed from staurolite, others are more ambiguous. Kyanite in some pseudomorphs above the kyanite isograd, especially where coarse grained muscovite is rare or absent, is comprised of sheaf-like mats of slightly mis-aligned kyanite laths that mimic rectangular prisms or herringbone-textured branching crystals. These textures suggest direct replacement of an earlier Al₂SiO₅ polymorph by kyanite. We have recently found one or two kyanite pseudomorphs that show a strong resemblance to the chiastolite cross that only forms in andalusite. The most chiastolite-like pseudomorph consists predominantly of a kyanite crystal that is shaped like a cross with stubby arms. Careful examination of this pseudomorph reveals that its arms are comprised of many slightly mis-aligned, strained kyanite crystals. The re-entrants between the arms of the kyanite cross are rich in graphite, as is the center of the kyanite cross, which is characteristic of chiastolite. This kyanite must have formed by replacement of chiastolite; therefore, andalusite must have been formed during the first metamorphic episode in the Snow Peak area. Although coarse-grained muscovite pseudomorphs that replaced staurolite commonly contain, or are rimmed by Fe-bearing minerals like staurolite or biotite, many of the kyanite pseudomorphs do not have associated Fe-rich minerals, which is consistent with their formation from an Fe-free aluminosilicate like andalusite.

Formation of andalusite during M1 metamorphism and kyanite during M2 metamorphism in northern Idaho puts important constraints on the tectonometamorphic history of this area, which is now known to have experienced Proterozoic metamorphism (Vervoort, et al., 2007, GSA Abs with Programs, v. 39, p. 245).