AbstractsEarth & Environmental Science

The meteorite impact that formed the 32-km-wide 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. Crater-fill deposits exposed on the ring of islands can be divided into three major successive units, which are, in ascending order: 1) a red, friable impact-melt-bearing fragmental breccia 0 to 20 m thick cut by small (cm to dm) dykes of impact melt; 2) a red, massive, clast-rich impact-generated melt about 18 meters thick containing up to 25% clasts > 1 mm in size, and 3) a massive poorly jointed clast-poor impact-generated melt containing <15% clasts greater than 1 mm in size, at least 85 m thick, with a matrix that shows an ophitic to subophitic texture. The average composition of the West Clearwater Lake impact melt, which has a bulk composition similar to that of a trachyandesitic volcanic rock, can be modeled by mixing the average chemical compositions of three types of lithologies present in the vicinity of the impact structure: 1) granitic rocks, 2) granodioritic, enderbitic and tonalitic rocks, and 3) mafic and ultramafic rocks, according to the following proportions: 1) 31%, 2) 62% and 3) 6%. The order of crystallization of the melt was plagioclase + Fe-Ti oxides + clinopyroxene + sanidine + apatite + quartz. The initial clast content in the clast-poor melt, before the onset of crystallization, must have been much higher. This conclusion is supported by the widespread occurrence in the clast-poor impact melt rock of plagioclase grains with an irregular, partially resorbed relict core. In thin section, plagioclase xenocrysts now form ~5% to ~15% of the clast-poor impact melt rock, depending on the thin section. The preferential preservation of plagioclase as a clast implies that the melt quickly equilibrated to temperatures below the liquidus and within the range of plagioclase crystallization. Two-feldspar thermometry calculations suggest that the last plagioclase and the first alkali feldspar crystallized at temperatures of about 781 to 816°C (±44°C). Quartz, the last phase to crystallize in the clast-poor impact melt rock, formed at temperatures of 700 to 730°C, with an activity of Ti in quartz between 0.5 and 0.6. This study is the first to employ Ti-in-quartz geothermometry to estimate the temperature of solidification of a melt sheet created by a meteoritic impact. Because the melt crystallized anhydrous minerals, it is possible that the melt was highly undersaturated at the liquidus but had achieved H2O saturation at the solidus. Le cratère du Lac à l'Eau Claire Ouest, une structure de 32 km de diamètre, a été formé pendant le Pennsylvanien supérieur, il y a 285 ± 23 Ma; il se trouve environ 125 km à l'est de l'arc de la Baie d'Hudson. L'impact météoritique à l'origine de ce cratère a produit une nappe de magma, et les roches qui en résultent se trouvent maintenant exposées dans un anneau d'îles occupant le centre du cratère. Ces roches peuvent…