Marble enclaves in the melt sheet at the West Clearwater Lake impact crater, northern Quebec.

摘要

The meteorite impact that formed the 32-km-diameter Upper Pennsylvanian (285 +/- 23 Ma) West Clearwater structure, situated about 125 km east of the Hudson Bay arc, northern Quebec, formed a sheet of impact melt now exposed on a central ring of islands. At least seven marble enclaves have been mapped in the melt sheet; this report is based on observations made on samples from two of these enclaves. These blocks represent recrystallized remnants of a cover of Middle Ordovician limestone, now completely eroded in the area. While the impact melt was still above its solidus, the locally fossiliferous dolomite-bearing limestone recrystallized to granoblastic calcite + periclase marble. The silica content of the marble led to the formation of a sanidinite-facies assemblage of unusual nesosilicates such as spurrite, Ca5(SiO4)2(CO3), merwinite, Ca 3Mg(SiO4)2, and monticellite, CaMgSiO4. Aluminian srebrodolskite, Ca2(Fe3+,Al)2O 5, the Fe3+-dominant analogue of brownmillerite, Ca 2(Al,Fe3+)2O5, is found in material with a composition similar to "Portland cement clinker". At its type locality, srebrodolskite is attributed to the calcining of ankeritic carbonate. Phase equilibrium and stratigraphic data constrain peak conditions of metamorphism to at least, T ≈ 815° +/- 15°C, P 100 bar and X(CO2) > 0.1. Retrograde mineral assemblages indicate that X(CO2) 0.1 was achieved shortly after peak metamorphism owing to influx of H2O. Portlandite, Ca(OH)2, is one of several species formed during late hydration of the assemblage upon cooling.