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.

Mineralogy and petrology of melt rocks from the Popigai impact structure, Siberia

“…The fine-grained texture of the melt rocks, which can reach total thicknesses approaching 800 m (780 m in core 4674), is anomalous when compared with impact melt rocks from other impact structures. For example, the grain size is significantly smaller than that developed in substantially thinner melt rock units from the >80 m thick Mistastin melt sheet (Grieve, 1975) and the >230 m thick Manicouagan melt sheet (Grieve and Floran, 1978). The presence of finely shattered target rock clasts, which mayor may not be reflected in the present clast content, could have facilitated rapid, widespread nucleation and consequent limited growth ofcrystals within the Popigai impact melt rocks.…”

“…Overgrowths that have developed on potassic feldspar are significantly less potassic and more calcic than the clast cores. Overgrowth rims on plagioclase clasts have been noted in melt rocks from other impact structures, where they are typically calcic (e.g., Grieve, 1975;Bischoff and Stoffler, 1984), although they can be sodic or potassic (Grieve, 1978;Floran et al, 1978). The overgrowth rims at Popigai bridge and "seal" the melt channels at the margins ofthe checkerboard plagioclase clasts (Fig.…”

Incipient melt formation and devitrification at the Wanapitei impact structure, Ontario, Canada

“…The sample, therefore, is not derived from a thick melt sheet from within the crater cavity. As such it would have developed a geochemically probably homogeneous, crystalline matrix possibly with typical reaction coronas of pyroxene and/or biotite around quartz grains as described from melts of other impact structures such as Manicouagan (Floran et al, 1978). However, minor element transport in the supercooling glass of our sample had occurred as suggested by Fig.…”

Calcul de l’érosion à long terme en région de socle autour de grands astroblèmes du Québec et de France

“…La morphostructure est caractérisée par la présence d'une couche de brèches d'impact et de roches fondues d'environ 55 km de diamètre (Floran et al, 1978). Ces matériaux forment un plateau annulaire, dont les bordures externes pré-sentent généralement des altitudes plus élevées qu'à l'inté-rieur.…”

“…8) (Floran et al, 1978 ;Pohl et al, 1978). Dans le bassin intérieur de ces deux cas, les contacts géologiques entre ces brèches et le substrat sous-jacent montrent généralement des variations altimétriques assez faibles.…”