Petrology and precursors of lithic clasts from feldspathic fragmental breccia 67975

Abstract

Fourteen lithic clasts were extracted from feldspathic fragmental breccia 67975 and analyzed by instrumental neutron activation analysis (INAA) by Lindstrom (1984), who classified the clasts into three granulite groups based upon color and grain size. The INAA data demonstrated that there are significant differences in the chemistry among these three groups, with both magnesian and ferroan rocks present in the clast suite. Evaluation of the textures and mineral chemistries of five of these lithic clasts indicates that several rock types are present, including feldspathic microporphyritic melt breccia, metamorphosed mafic‐rich melt breccia with zoned and mantled clasts, “typical” reequilibrated lunar feldspathic granulites with granoblastic texture and homogeneous mineral chemistry, hornfels, and a recrystallized mafic‐rich granulite with relict igneous texture and the composition of a ferroan anorthositic norite. These five lithic clasts were derived from both ferroan‐anorthosite suite and Mg‐suite precursors, with most of them containing more than one precursor. The ferroan anorthositic norite clast represents an unusual but important precursor that is a possible component of many of these clasts. The two light gray clasts (Lindstrom, 1984) examined in this study differ greatly; one is a plagioclase‐rich, possibly monomict rock with ferroan anorthositic precursor(s), and the other represents a mixture of feldspathic granulite and ferroan anorthositic norite lithologies. Lindstrom's dark gray group is a suite of feldspathic microporphyritic melt breccias and the light green group consists of clasts of metamorphosed polymict mafic melt breccias that have distinctive texture, mineralogy, and REE contents.