Impact resetting of the U-Pb, Pb-Pb, and Rb-Sr chromometers in the 4.3 Ga lunar granitoids

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Abstract
Understanding when lunar granitoids formed remains a long-standing challenge hampering efforts to place these rock types into the wider context of the geological evolution of the Moon. In particular, it is poorly understood what their relationship is to KREEP-rich mafic lithologies such as alkali gabbronorites that have been linked to the generation of felsic melts through processes such as crustal underplating. Here we report in situ Rb-Sr, U-Pb, and Pb isotope systematics for lunar granitoid and alkali gabbronorite clasts from a range of Apollo landing sites. Our data indicate that the dates derived from the Rb-Sr, U-Pb, and Pb isotope systematics recorded by plagioclase, K-feldspar, Ca-phosphates, and glass most likely reflect impact resetting events rather than initial crystallisation ages. Granitoid and alkali gabbronorite samples 67975,9011, 14303,330, 72275,520, and 73255,9083 record dates consistent with the Imbrium impact event at 3.92 Ga, whereas samples 14305,111, 73235,73, and 14305,102 record modification by younger impacts at ∼3.5 to ∼3.6 Ga. Currently, zircon U-Pb isotope dates provide the most robust constraints on the formation ages of Apollo sampled granitoids. The majority of zircon dates are significantly older than those calculated from crater counting for lunar silicic domes (mostly between ca. 4 to 3.6 Ga). Consequently, it is not evident that mechanisms for generating large volumes of evolved melt by processes such as crustal underplating of mafic lithologies are needed. Formation within granophyre layers in magmatic layered intrusions or differentiated impact melt sheets cannot be ruled out.

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