Surfing the Fra Mauro shoreline: Highlands crust at the Apollo 14 site

Plutonic rocks at the Apollo 14 site may be grouped into four lithologic su ites. In order of relative abundance these are-magnesian suite, alkali suit e, evolved lithologies, and ferroan anorthosites (FAN). Most of the samples described to date occur as clasts in lunar polymict breccias or in regolit h breccias; these clasts rarely are more than a few cm in diameter. The mag nesian suite (63 clasts) includes troctolite, anorthosite, norite, dunite, and harzburgite, all characterized by plagioclase approximate to An(95) and mafic minerals with Mg#s from 82 to 92. Alkali-suite rocks (21 clasts) and evolved rocks (10 clasts) generally have plagioclase approximate to An(90) to approximate to An(40), and mafic minerals with Mg#s from 82 to 40 Litho logies include anorthosite, norite, quartz monzodiorite, granite, and felsi te. Ferroan anorthosites (5 clasts) have plagioclase approximate to An(96) and mafic minerals with Mg#s from 45 to 70. Plots of whole-rock [Mg/(Mg+Fe)] versus whole-rock [Ca/(Ca+Na+K)] show a di stinct gap between rocks of the magnesian suite and rocks of the alkali sui te, suggesting distinct parent magmas or distinct physical processes of for mation. Chondrite-normalized REE patterns show that rocks of both the magne sian suite and alkali suite have similar ranges, despite the large differen ce in major-element chemistry. Most magnesian-suite samples and all alkali anorthosites are cumulates with little or no trapped liquid component. Nori tes may contain significant trapped liquids, and some alkali norites may re present cumulate-enriched, near-liquid compositions, similar to KREEP basal t 15386. Evolved lithologies include evolved partial cumulates related to a lkali-suite fractionation (quartz monzodiorite), immiscible melts derived f rom these evolved magmas (granites), and impact melts of pre-existing grani te (felsite). Recent models for the origin of the magnesian suite envision a komatiitic p arent magma derived from early magma-ocean cumulates; these melts must assi milate plagiophile elements to form troctolites at low pressures, and must assimilate a highly enriched KREEP component so that the resulting mixture has REE concentrations similar to high-K KREEP. As yet there are no plausib le scenarios that can explain these unusual requirements. In contrast, alka li anorthosites and norites can be shown to form from crystallization of a KREEP-basalt parent magma similar to 15386, along with some magnesian norit es and the evolved quartz monzodiorites. If the parent magma of the alkali suite and evolved rocks is related to the magnesian suite, then that magma must have evolved through combined assimilation-fractional crystallization processes to form the alkali-suite cumulates.