Sudbury igneous complex: Impact melt or endogenous magma? Implications for lunar crustal evolution

If the Sudbury Igneous Complex is a differentiated impact melt, then currently favored views of how the Moon's crust formed may be wrong. The lunar highlands crust is thought to comprise a diverse array of magmatic cumulates, but the primary record of lunar crustal evolution has been obscured by intensive impact brecciation, melting, and mixing. Based on the petrologic and geochemical characteristics of terrestrial impact deposits, criteria have been developed to help distinguish primary lunar crustal rocks from impact-produced mixtures. Application of these criteria to the Sudbury Igneous Complex (SIC) would indicate it is also a primary magmatic suite, raising the possibility that some or all of the pristine lunar highlands cumulates actually crystallized from impact melts. This would remove the direct links between these rocks and the Moon's petrologic and thermal evolution. As the basis for such a fundamental conceptual change, the case favoring an impact melt origin for the SIC should be overwhelmingly persuasive. This paper argues that geochemical evidence cited as favoring an impact origin is not compelling. In particular, the crust-rich composition of the SIC does not require impact melting, but can be produced by endogenous magmatic processes. At least two other aspects of the SIC are also better explained by endogenous magmatism rather than impact melting: (1) Contact Sublayer xenoliths with compositional affinities to the SIC, and (2) PGE concentrations and Re-Os isotopic compositions of SIC ores that indicate terrestrial, not meteoritic compositions. The spatial and temporal association of Sudbury magmatism with a basin-forming impact event may nonetheless have important implications for early planetary evolution.