Lunar ferroan anorthosites: mineralogy, compositional variations, and petrogenesis

Detailed petrologic study of a suite of lunar ferroan anorthosites indicates that their mineral compositional variations are generally consistent with derivation of these anorthosites from a common parent magma. There are continuous compositional variations in major elements and smooth trends in minor elements (Mg, Fe in plagioclase; Al, Ti, Cr in pyroxene). Relict textures, zoning, and possible primary trends of mineral compositional variations are preserved in many of the anorthosites. The presence of heterogeneous, bimodal, and/or trimodal pyroxene compositions suggests that some of the anorthosites are polymict rocks; however, they are composed entirely of ferroan-anorthosite-suite lithologies. Some aspects of the mineral composition data suggest that complex processes operated during formation of the ferroan anorthosites. The lack of a well-defined, positive or negative trend of pyroxene Mg/(Mg + Fe) versus plagioclase anorthite content, the homogenization of mineral compositions in some anorthosites, and the varied and apparently enriched Mn contents of pyroxenes in some anorthosites suggest that original igneous compositional characteristics have been altered during and/or after crystallization. Processes operative during anorthosite formation may have included some mixing of different melts, trapping of variable amounts of intercumulus liquid, postcrystallization redistribution of elements, or perturbations both during adcumulus growth and subsequent to crystallization by impact events. FeO and MgO contents of the highly calcic plagioclase typical of these anorthosites suggest that the plagioclase-melt distribution coefficients for these elements need to be reassessed.