The Barrovian Metapelitic Rocks In Northern Idaho: An Outstanding Suite For Teaching Undergraduate Metamorphic Petrology

The Barrovian metapelitic rocks in the St. Joe-Clearwater region in northern Idaho provide an outstanding suite for the study of many aspects of regional metamorphism in an undergraduate petrology class. Metamorphosed Belt Supergroup and associate rocks, located along the northwest border zone of the Idaho batholith, record the effects of three phases of recrystallization. The first phase of metamorphism (M1) is recognized by the presence of (M2) muscovite pseudomorphs after staurolite. The second phase of metamorphism, M2, was Barrovian and ranges in grade from biotite zone through K-feldspar + sillimanite zone. M2 porphyroblasts are commonly quite large and easily recognizable in hand sample as well as petrographically. Four and five AFM phase assemblages present in these rocks can be used to discuss the effects of extra components or variable fluid compositions on stable mineral assemblages. The presence of epidote-bearing plutonic rocks, emplaced during M2 metamorphism, provides additional data on the tectonometamorphic evolution of the region. In the central part of the area are unusually Al- and Mg-rich metapelitic rocks associated with several anorthosite bodies. These rocks contain kyanite, andalusite, and sillimanite in a single thin section but the polymorphs are not in textural equilibrium. These rocks also contain cordierite coronas around kyanite and andalusite. The inversion of kyanite to andalusite and the late growth of cordierite occurred during M3 metamorphism. M3 recrystallization was generated by a phase of rapid, nearly isothermal decompression. The texturally late, lower pressure overgrowths of cordierite are explained to undergraduates through the use of petrogenetic grids. Published mineral analyses for rocks in this area are available, as are limited geochronologic data. Ongoing research on the structural evolution of the region provides a context for the integration of tectonics and petrology.