Lunar granulites and their precursor anorthositic norites of the early lunar crust

Authors

  • Marilyn M. Lindstrom,

  • David J. Lindstrom


Abstract

Lunar granulites are breccias with metamorphic textures. They are not pristine, having been contaminated with meteoritic siderophiles. Nonetheless, they constitute an abundant class of highlands rocks that are important because they are ancient and KREEP-free. They may represent our best samples of the early lunar crust. The granulites can be divided into ferroan and magnesian groups, and each group can be subdivided on the basis of mineral composition and REE concentrations. Textural and compositional data show that some of the granulites (especially 67215 and 67415) may be derived from distinct anorthositic norite precursors, while some other granulites (79215, 78155) are clearly polymict. Even the polymict granulites probably had anorthositic norites as their dominant precursors, and not anorthosites, norites, or troctolites. The granulites have compositions similar to those of the two lunar meteorites, one of which (Yamato-791197) is ferroan, the other (ALHA81005) magnesian. These meteorites are soil breccias from an unknown location distant from the Apollo landing sites and contain anorthositic norites as abundant clasts. Compositions of granulites and lunar meteorites more closely resemble the average composition of lunar highlands, as determined by remote sensing, than do those of any other returned lunar samples. The predominance of plutonic anorthositic norite precursors in material having the composition typical of highlands suggests that plutonic anorthositic norites were more abundant in the early lunar crust than is implied by their scarcity among pristine rocks from Apollo missions.

Ancillary