Petrology of the Western Highland Province: Ancient crust formation at theApollo 14 site

Plutonic rocks found at the Apollo 14 site comprise four lithologic suites: the magnesian suite, the alkali suite, evolved lithologies, and the ferroa n anorthosite suite (FAN). Rocks of the magnesian suite include troctolite, anorthosite, norite, dunite, and harzburgite; they are charaterized by pla gioclase approximate to An(95) and mafic minerals with mg#s 82-92. Alkali s uite rocks and evolved rocks generally have plagioclase approximate to An(9 0) to approximate to An(40), and mafic minerals with mg#s 82-40. Lithologie s include anorthosite, norite, quartz monzodiorite, granite, and felsite. F erroan anorthosites have plagioclase approximate to An(96) and mafic minera ls with mg#s 45-70. Whole rock geochemical data show that most magnesian su ite samples and all alkali anorthosites are cumulates with little or no tra pped liquid component. Norites may contain significant trapped liquid compo nent, and some alkali norites may represent cumulate-enriched, near-liquid compositions, similar to KREEP basalt 15386. Evolved lithologies include ev olved partial cumulates related to alkali suite fractionation (quartz monzo diorite), immiscible melts derived from these evolved magmas (granites), an d impact melts of preexisting granite (felsite). Plots of whole rock mg# ve rsus whole rock Ca/(Ca+Na+K) show a distinct gap between rocks of the magne sian suite and rocks of the alkali suite, suggesting either distinct parent magmas or distinct physical processes of formation. Chondrite-normalized r are earth element (REE) patterns show that rocks of both the magnesian suit e and alkali suite have similar ranges, despite the large difference in maj or element chemistry. Current models for the origin of the magnesian suite call for a komatiitic parent magma derived from early magma ocean cumulates ; these melts must assimilate plagiophile elements to form troctolites at l ow pressures and must assimilate a highly enriched KREEP component so that the resulting mixture has REE concentrations similar to high-K KREEP. There are as yet no plausible scenarios that can explain these unusual requireme nts. We propose that partial melting of a primitive lunar interior and buff ering of these melts by ultramagnesian early magma ocean cumulates provides a more reasonable pathway to form magnesian troctolites. Alkali anorthosit es and norites formed by crystallization of a parent magma with major eleme nt compositions similar to KREEP basalt 15386; If the parent magma of the a lkali suite and evolved rocks is related to the magnesian suite, then that magma must have evolved through combined assimilation-fractional crystalliz ation processes to form the alkali suite cumulates.