anorthosite

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anorthosite Plutonic rock composed almost entirely of plagioclase feldspar (90% or more) which may be oligoclase, andesine, labradorite, or bytownite. These rocks occur most frequently in Precambrian shield areas as non-stratiform intrusions or in layered complexes. Anorthosite also constitutes the lunar highlands where it is largely composed of the calcium end-member anorthite.