Mineralogy, petrology, and chronology of the lunar granulitic breccias

Before the return of the Apollo and Luna samples, many basic geological questions about the Moon remained unanswered. The study of returned samples, in addition to remote sensing data and the growing collection of meteorites has revealed that the Moon is a geologically complex body with a history dominated by impact events.;Lunar meteorites provide samples of the crust far removed from the Apollo landing sites and are probably more representative of the average lunar crust. SaU 300 was previously misclassified as an anorthositic regolith breccia. Here, I reclassify it as a polymict crystalline impact-melt breccia with an anorthositic norite bulk composition. SaU 300 is a new meteorite that is unpaired with any of the currently known lunar meteorite samples.;The main objective of this study was to gain a better understanding of the early evolution of the Moon by studying some of the oldest samples available: the granulitic breccias. The following samples were studied: Apollo samples 60035, 77017, 78155, and 79215 and paired meteorites NWA 3163/4881/4483.;Granulitic breccias exhibit poikiloblastic to granoblastic matrix textures and occur as individual rocks and as fragments in impact-melt rocks, the regolith, and in lunar meteorites. These rocks contain 70 -- 80% anorthite, low concentrations of incompatible trace elements, and moderately high concentrations of siderophile elements. Their history was dominated by impact events and thermal metamorphism. Matrix pyroxenes in the granulitic breccias last equilibrated at ∼1050+/-50°C. 40Ar/39Ar data reveal that 60035, 77017, and 78155 have peak metamorphic ages of 4.1 Ga, while 79215 has a peak metamorphic age of 3.9 Ga. NWA 3163 has a peak metamorphic age of 3.3 Ga. Samples 60035, 77017, and NWA 4881 were partially reset by low temperature heating events following metamorphism.;Granulitic breccias have been described as "homogeneous on a millimetre scale" throughout the literature. Although they appear to be compositionally homogeneous under petrographic and electron microscopes EMP analysis shows that they retain relic igneous minor and trace element zoning in their larger mafic mineral clasts.;Based on the results of this study, I proposed that granulitic breccias form beneath thick, superheated melt sheets associated with impact basins and that they were heated for 104 -- 106 years.