In this area in N Idaho, three large and numerous small lenses of layered to massive anorthosite consisting of two, and locally three, types of plagioclase with minor hornblende and mica occur in aluminium silicate-rich garnet mica schist. In most of this anorthosite, megacrysts of andesine with bytownite inclusions are embedded in a fine-grained groundmass of bytownite or anorthite; locally, labradorite occurs rather than andesine. Some labradorite laths show Carlsbad twinning and rims of andesine around anorthite inclusions. Along the contacts, lenses of fine-grained bytownite anorthosite with some hornblende or garnet and quartz are common. These lenses could represent calcic parent rocks converted to two-plagioclase rocks by partial replacement of bytownite by andesine. -R.A.H.
Additional publication details
Role of replacement in the genesis of anorthosite in the Boehls Butte area, Idaho.