Mg-Suite Volcanics. A Magmatic or Impact Origin?

Mg-suite rocks are generally thought to represent one of the first periods of magmatism associated with crust-building. This event occurred soon after the formation of the ferroan anorthositic crust during the primordial differentiation of the Moon. Numerous models have been proposed for the origin of the Mg-suite that include melting during a large basin forming impact event or derivation from a variety of mantle sources (e.g., hybrid mantle). Recent chronology studies distinguish between the timing of FAN and Mg-suite events and imply that the Mg-suite may be petrogenetically related to the generation of alkali suite and evolved magmas, and that these magmatic events occurred over a limited period of time (e.g., 20-30 Ma). The protracted duration to produce these post-FAN magmas has profound implications for their origin. Barboni et al. suggested that this limited time span of melt production may be related to biased sampling, a major mantle event (e.g., LMO cumulate over-turn), or a large basin forming event.
Volcanic lithologies associated with Mg-suite magmatism are rare. Prissel et al. explored the potential for Mg-suite volcanics and identified possible volcanic deposit locations. A fine-grained magnesian clast in an Apollo 16 impact melt breccia was proposed to be an extrusive product of Mg-suite magmatism. Yen et al. identified a potential Mg-suite extrusive lithology within Apollo 17 core 73001/2 (73002,1017C) and a related glass bead. Pb-Pb chronological results (15-point isochron) reported by Yen et al. yield a crystallization age of approximately 4246 ± 4 Ma for 73002,1017C. Further examination of the 73001/2 core revealed other similar (but not identical) lithic fragments [14,15]. Here, we examine one of these lithic fragments in the <1 mm size fraction. Using a variety of analytical approaches, we explore relationships among these distinct lithologies, evaluate the origin of this particular lithology, and deduce the petrologic implications for Mg-suite magmatism.