Metamorphism on the Moon Recorded by the Granulite Suite

J. F. Pernet-Fisher and K. H Joy
Department of Earth and Environmental Sciences, University of Manchester, M13 9PL, UK
E-mail: [e-mail]
Metamorphic rocks on the Moon are an important yet under-studied suite of lunar lithologies that have been identified in the Apollo and lunar meteorite collections. These rocks, with granoblastic textures, are generally referred to as granulites, however, unlike their terrestrial counterparts, they are considered to represent the products of only high-temperature (more than 1000°C) thermal metamorphism that completely re-crystallizalised their protoliths.
Lunar granulites are commonly sub-divided into two main compositional groups related to their protolith lithologies: The Fe-granulites, found at most Apollo landing sites, are generally accepted to derive from metamorphosed plagioclase-rich igneous cumulates, termed the ferroan anorthosite (FAN) suite. The FAN suite are important lithologies as they represent products of the primary lunar crust. The Mg-granulites, found mostly at the Apollo 16 landing site and within lunar meteorite samples. The protolith of this group is not so well understood. Early studies have linked the protolith to secondary magmatic intrusions into the primary anorthositic crust (termed the Mg-suite), however, recent studies have tentatively connected the protoliths to a Mg-rich variation of the primary crustal plagioclase cumulates (termed the MAN suite). The occurrence of MANs is controversial, indeed, it is unclear how the MAN suite fits into canonical lunar crustal formation models. In the absence of sample return missions, by identifying the protolith lithologies of the Mg-granulite suite, we greatly enhance our understanding of the range of lithologies that make up the primary crust. In turn, this enables lunar crustal formation models to be better constrained.
We present preliminary plagioclase trace-element data, in combination with published peak metamorphic temperatures, to assess the protoliths of two Apollo granulites. Furthermore, we aim to investigate the extent to which thermal metamorphism affects the diagnostic trace-element signatures that are commonly used to classify lunar highland lithologies.
Lindstrom & Lindstrom, 1986, JGR, 91(B4), 263-276.

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