Manicouagan Impact Melt, Quebec, 1, Stratigraphy, petrology, and chemistry

Abstract

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.