THE COMPLEX STRATIGRAPHY OF THE HIGHLAND CRUST IN THE SERENITATIS REGION OF THE MOON INFERRED FROM MINERAL FRAGMENT CHEMISTRY

Large impact basins are natural drill holes into the Moon, and their e jecta carries unique information about the rock types and stratigraphy of the lunar crust. We have conducted an electron microprobe study of mineral fragments in the poikilitic melt breccias collected from the Taurus Mountains at the Apollo 17 landing site. These breccias are vir tually unanimously agreed to be impact melt produced in the Serenitati s impact event. They contain lithic fragments and much more abundant m ineral fragments of crustal origin. We have made precise microprobe an alyses of minor element abundances in fragments of olivine, pyroxene, and plagioclase to provide new information on the possible source rock s and the crustal stratigraphy in the Serenitatis region. These data w ere also intended to elucidate the nature of the cryptic geochemical c omponent in breccias such as these with low-K Fra Mauro basalt composi tions. We chose the finest-grained (i.e., most rapidly quenched) brecc ias for study, to avoid reacted and partly assimilated fragments as mu ch as possible. Most of the mineral fragments appear to have been deri ved from rocks that would fall into the pristine igneous Mg-suite as r epresented by lithic fragments in the Apollo collection, or reasonable extensions of it. Gabbroic rocks were more abundant in the target str atigraphy than is apparent from the Apollo sample collection. Some pyr oxene and plagioclase, but probably not much olivine, could be derived from feldspathic granulites, which are metamorphosed polymict breccia s. Some mineral fragments are from previously unknown rocks, These inc lude highly magnesian olivines (up to Fo(94)), possibly volcanic in or igin, that exacerbate the difficulty in explaining highly magnesian ro cks in the lunar crust. It appears that some part of the lunar interio r has an mg[= 100 x Mg/(Mg/Fe) atomic] greater than the conventional bulk Moon value of 80-84. Other volcanic rocks, including mare basalts , and rapidly-cooled impact melt rocks do not contribute significantly to the fragment population. Nor do ferroan anorthosites contribute mo re than a tiny part of even the plagioclase fragment population. A few mineral fragments that are consistent with the cryptic low-K Fra Maur o chemical component were found, and these appear to be from gabbroic sources. The mineral fragment populations cannot be mixed in their obs erved proportions to produce the whole rock composition, because the f ragments are more refractory and deficient in Ti, P, and alkalis. A pr eferential contribution to the melt from a rock similar to sodic ferro gabbro can partly resolve the discrepancy, The population of mineral f ragments requires a very diverse population of igenous rocks that are not all related to each other, demonstrating the existence of a comple x crust built of numerous separate igneous plutons. Many of these plut ons may have crystallized at shallow depths. The chemical composition of the melt breccias, in combination with the mineral fragment data an d an understanding of the cratering process, suggests that the deepest crust sampled by the Serenitatis impact (not necessarily the deepest crust) was basaltic in composition, including KREEP and gabbroic rocks like sodic ferrogabbro, and lacking abundant olivine-rich material. T hese were overlain by Mg-suite rocks of varied types, including norite s and troctolites that supplied most of the olivine mineral fragments. Granulites, which are metamorphosed and more feldspathic breccias, we re abundant near the surface. Remote sensing indicates that the entire Serenitatis region lacks ferroan anorthosite, consistent with the res ults of our study. Copyright (C) 1997 Elsevier Science Ltd.