IMPACT MELT ROCKS FROM NEW QUEBEC CRATER, QUEBEC, CANADA

Approximately 1500 g of float samples of impact melt rocks have been recovered from gravel deposits approximately 4 km north and northeast of the rim of the 3.4 km diameter New Quebec Crater (61-degrees 17'N; 73-degrees 40'W) in northern Quebec, Canada. Previously, only two small samples of impact melt rocks were known. The newly recovered samples have cryptocrystalline to microcrystalline matrices with microlites of andesine and pigeonite. Mineral clasts of quartz and feldspar occur and, in some cases, show shock metamorphic features. The melt rocks have a normative mineralogy corresponding to approximately 70% quartz, orthoclase and albite and are compositionally similar. Their major element composition can be modeled as a mix of granitic gneisses that make up the target rocks. The melt rocks show enrichments, however, in Cr (21 ppm), Co (9 ppm), Ni (12 ppm) and Ir (1.5 ppb) over the target rocks. Interelement ratios suggest a chondritic impacting body, although they do not define a specific type. Assuming a C-1 chondrite, the impact melt rocks average approximately 2% meteoritic contamination. Stepwise Ar-40-Ar-39 dating using a laser on three chips from three samples give integrated ages of 0.6-2.5 Ma. From the best plateau ages, the age of the New Quebec impact is taken to be 1.4 +/- 0.1 Ma, which places it before the first major northern hemisphere continental glaciation of the Pleistocene. A number of considerations suggest that the impact melt rocks were originally deposited in fractures in the crater wall and later transported to their discovery site by glacial ice and melt water.