The Composition of the Prebasin Crust in the Central Highlands of the Moon

RANDY L.KOROTEVDepartment of Earth and Planetary Sciences and McDonnell Center for the Space Sciences,Washington University, St. Louis, Missouri 63130, USAAuthor's e-mail address: rlk@levee.wustl. edu"I / _/ () /(Received 1995 June 19; accepted in revised form 1996 January 31)Abstract-Two types of texturally and compositionally similar breccias that consist largely of fragmentaldebris from meteorite impacts occur at the Apollo 16 lunar site: Feldspathic fragmental breccias (FFBs) andancient regolith breccias (ARBs). Both types of breccia are composed of a suite of mostly feldspathiccomponents derived from the early crust of the Moon and mafic impact-melt breccias produced during thetime of basin formation. The ARBs also contain components, such as agglutinates and glass spherules,indicating that the material of which they are composed occurred at the surface of the Moon as fine-grainedregolith prior to lithification of the breccias. These components are absent from the FFBs, suggesting thatthe FFBs might be the protolith of the ARBs. However, several compositional differences exist between thetwo types of breccia, making any simple genetic relationship implausible. First, clasts of mafic impact-meltbreccia occurring in the FFBs are of a different composition than those in the ARBs. Also the feldspathic"prebasin" components of the FFBs have a lower average Mg/Fe ratio than the corresponding components ofthe ARBs; the average composition of the plagioclase in the FFBs is more sodic than that of the ARBs; andthere are differences in relative abundances of rare earth elements. The two breccia types also have differentprovenances: the FFBs occur primarily in ejecta from North Ray crater and presumably derive from theDescartes Formation, while the ARBs are restricted to the Cayley plains. Together these observationssuggest that although some type of fragmental breccia may have been a precursor to the ARBs, the FFBs ofNorth Ray crater are not a significant component of the ARBs and, by inference, the Cayley plains. Theaverage compositions of the prebasin components of the two types of fragmental breccia are generallysimilar to the composition of the feldspathic lunar meteorites. With 30-31°,5 AI203, however, they areslightly richer in plagioclase than the most feldspathic lunar meteorites (-29% A1203) , implying that thecrust of the early central nearside of the Moon contained a higher abundance of highly feldspathicanorthosite than typical lunar highlands, as inferred from the lunar meteorites. The ancient regolith breccias,as well as the current surface regolith of the Cayley plains, are more mafic than (1) prebasin regoliths in theCentral Highlands and (2) regions of highlands presently distant from nearside basins because they contain ahigh abundance (-30%) of mafic impact-melt breccias produced during the time of basin formation that isabsent from other regoliths.INTRODUCTIONFragmental breccias are common products of large meteoriteimpacts on the Earth and Moon. They consist of clastic fragments ofvarious target lithologies that were lithified by the heat and shock ofimpact. Also known as ctastic matrix breccias, fragmental brecciasoccur in continuous cjecta deposits outside craters and as layers insidecraters (StOffler et aL, 1979). At the Apollo 16 landing site in thelunar Central Highlands, the most common rock typc recovered fromthe rim of North Ray crater (-I km diameter) is "feldspathic frag-mental breccia," i.e., fragmental breccia that consists largely of plagio-clase-rich fragments (St6fflcr et al., 1980; James, 1981 ). Regolith breccias are known from airless bodies, i.e., the Moonand meteorite parent bodies. Although also dominated by rock andmineral fragments, regolith breccias contain components unique tosurface regolith, namely, glass spherules, agglutinates, and gasesderived from solar and galactic irradiation. Many regolith brecciashave a glassy matrix, but those that do not can be difficult to distin-guish from fragmental brcccias. For example, Apollo 16 samples63588 and 63595 have been classified as feldspathic fragmentalbreccias by St0ffler et al. (1985) and as regolith breccias by McKayet al. (1986). A subset of regolith breccias from Apollo 16 is designated"ancient" because the breccias are composed of regolith that existedduring the time of basin formation ~3.9 Ga ago (McKay et al., 1986;Wentworth and McKay, 1988b). The ancient regolith breccias(ARBs) and feldspathic fragmental breccias (FFBs) of Apollo 16 aresimilar to each other texturally (Wentworth and McKay, 1988a) andcontain many of the same nominal clastic components, e.g., anortho-sites, granulitic breccias, and impact-melt breccias. In particular, bothFFBs and ARBs contain clasts of marie (17-22% A1203), KREEP-bearing impact-melt breccias that were produced during the time ofbasin formation. Because they are more mafic than the presenthighlands surface, these melt breccias are believed to be dominatedlargely by material from the lower crust or upper mantle of the Moonthat was excavated and mixed by large impacts (e.g., Ryder andWood, 1977). However, a significant difference between the FFBsand ARBs is that the ARBs contain glasses with a wide range ofcompositions, including KREEP-rich and ultrahigh-Mg" varieties(Wentworth and McKay, 1988b; Mg" = mol% Mg/[Mg + FeD. Suchglasses, which presumably are derived by impact mixing at thesurface, are absent from the FFBs. This observation led Takeda et aL(I 990) to suggest that the FFBs might be precursors to the ARBs.403