Geochemistry of the impact‐generated melt sheet at Manicouagan: Evidence for fractional crystallization

摘要

Recent exploration drilling of the Manicouagan impact structure has revealed local developments of impact melt that are substantially thicker than the previously accepted average of ～300 m and a more complex melt sheet–basement interface showing considerable “topography.” The thickest section (1045 m of clast‐free to clast‐poor impact melt overlying 425 m of clast‐laden impact melt) is associated with a centrally located, fault‐bounded graben that was in place prior to melt crystallization. Here we report the results of a geochemical investigation of 115 samples obtained from nine drill holes and a set of field samples. The results reveal that most of the drill core comprises an undifferentiated unit, showing minimal geochemical variation. This is typically ～300 m thick and equates with previous studies performed on samples from exposed rock. In contrast, the deeper section exhibits differentiation and is divided into three layers based on chemical, mineralogical, and textural variations: a quartz monzonite to quartz monzodiorite upper zone (276 m), a quartz monzodioritic middle zone (244 m), and a mainly monzodioritic lower zone (525 m). The mineralogy is defined by plagioclase > orthoclase > clinopyroxene > orthopyroxene, with ubiquitous amphibole and biotite and olivine locally developed in the lower zone and below. Our results reveal that the thicker section of the impact melt sheet has undergone fractional crystallization. Differentiation of parts of the Manicouagan impact melt indicates that lunar impact melt samples possessing different chemistries, previously considered to be derived from distinct impact basins, may be related via the fractionation of common, much smaller diameter impact structures.