Petrogenesis of melt rocks, Manicouagan Impact Structure, Quebec

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.