Manicouagan Basal Suevites as Lunar Granulitic Breccia Analogues

Lunar granulitic breccias are high-temperature (~1000 degrees C) metamorphic rocks with homogeneous mineral chemistries (i.e., mineral zoning is absent). The are typically ferroan or magnesian anorthositic rocks derived from the lunar highlands, although their exact protoliths are not represented in the lunar rock record (i.e., they appear to have been derived from a distinct, hitherto unrecognized suite of igneous rocks). They comprise a fine-grained matrix that encloses clasts, all of which have been thermally metamorphosed. The heat source for the granulitic breccias is either burial within the selenothermal gradient (deep origin) or juxtaposition with hot impact melt sheets (shallow origin). Because of the difficulty of exhuming deep crustal rocks on the Moon (impact is the only viable mechanism), the shallow origin is generally favoured as the formation mechanism. Here we draw analogies with so-called basal suevites that have been described from some terrestrial impact structures. At Manicouagan, the basal suevites occur between the impact melt sheet and the underlying basement. They appear to represent an early ejecta facies that lined the excavation cavity surface. After emplacement and movement of the basal suevite, the overlying impact melt could have thermally metamorphosed them to yield textures similar to lunar granulitic breccias. By inference, we suggest that the granulitic breccias may be the lunar equivalent of terrestrial basal suevites. This places them in context within the impact structure formation process.