Tourmaline (dravite) from the Boehls Butte anorthosite, Idaho, U.S.A.

Role of replacement in the genesis of anorthosite in the Boehls Butte area, Idaho

In the Boehls Butte area, Idaho, three large and numerous small lenses of layered to massive anorthosite consisting of two, and locally three, types of plagioclase and minor hornblende and micas occur in aluminum silicate-rich garnet mica schist. In most of this anorthosite, andesine megacrysts with bytownite inclusions are embedded in a fine-grained groundmass of bytownite or anorthite. In places labradorite occurs instead of 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 that were changed to two-plagioclase rocks by partial replacement of bytownite by andesine.