Within the moderately eroded Manicouagan structure a sheet of clast‐laden impact melt 230 m thick and 55 km in diameter forms an annular plateau surrounding an uplift of shocked anorthosite. The melt sheet is divided into three vertically gradational units based on decreasing clast abundance and coarsening of the melt above the base. A very fine‐grained lower unit, rich in millimeter‐ and centimeter‐sized inclusions, thickens radially outward but is overlapped and replaced toward the center by a coarser middle unit containing fewer, larger inclusions. The upper unit is medium grained, virtually clast free, and texturally the most homogeneous of the three melt units. Within the lower and middle units a variety of textures are present. Textural development is a function of the cooling rate determined by stratigraphic position and the degree of supercooling determined by initial clast content. The mineralogy of the melt rocks is similar in all units and consists of zoned plagioclase, sanidine, Ca‐poor pyroxene, augite, quartz (rare in the lower unit), magnetite‐ilmenite intergrowths, smectite, and apatite, Pseudomorphs after olivine, and pigeonite in various stages of inversion to hypersthene, are widespread only in the upper unit, while minor biotite and hornblende are confined to the lower and middle units. Replacement of olivine and much of the Ca‐poor pyroxene by smectite, and alteration of iron oxides occurred during late stage crystallization and subsolidus cooling. The melt rocks as a group are chemically homogeneous, with a bulk composition similar to that of latite. No statistically significant regional chemical variations were found as a function of vertical, lateral, or radial position in the melt sheet. A local mafic variant represented by two samples with poikilitic texture indicates that the melt is not completely chemically homogeneous. The poikilitic rocks texturally resemble some Apollo 17 impact melt rocks and are inferred to have had a similar origin and thermal history.

Abstract. The 65‐km‐diameter Manicouagan impact structure has an eroded 230‐m‐thick sheet of clast‐laden, impact melt rock with an estimated preerosional volume of >270 km3. All samples are characterized by mineral and lithic clasts or their incompletely digested remnants. Drawing upon previous theoretical studies of shock waves, we suggest that the Manicouagan melt formed in 1 or 2 s in a 5‐km‐radius hemisphere near the point of impact. The melt accelerated to a few kilometers per second, and the melt and the less shocked debris surrounding it flowed downward and outward for a few minutes until the melt formed a lining of a 5‐ to 8‐km‐deep, 15‐ to 22‐km‐radius cavity. Extremely turbulent flow thoroughly homogenized the melt and promoted the incorporation and progressive digestion of slower moving, less shocked, cooler debris surrounding the melt. This debris had been finely fragmented, but not melted, to grain sizes of less than 1 mm by the passage of the shock waves. Because of the fine grain size, the melt and fragmented debris equilibrated thermally in about 100 s. During thermal equilibration, virtually all clastic debris (i.e. alkali feldspar, biotite, hornblende, garnet, and scapolite), other than highly refractory quartz and plagioclase as well as many of the centimeter size lithic clasts other than anorthosite, were digested. The preservation of quartz and plagioclase mineral clasts implies that the clasts and melt equilibrated to temperatures near but not above the liquidus. Plagioclase nucleation was initiated by the drop in temperature and possibly by direct nucleation on undigested debris. The initiation of crystallization vastly increased the melt viscosity, preventing settling of 10‐mm clasts of basement. Flow of melt through basement fractures is evidence that readjustment of the crater floor took place during the period of clast‐melt thermal equilibration.

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