Impact melt rocks from New Quebec Crater, Quebec, Canada
作者:
Richard A. F. Grieve,
Richard B. Bottomley,
Michel A. Bouchard,
P. Blyth Robertson,
Charles J. Orth,
Moses Attrep,
期刊:
Meteoritics
(WILEY Available online 1991)
卷期:
Volume 26,
issue 1
页码: 31-39
ISSN:0026-1114
年代: 1991
DOI:10.1111/j.1945-5100.1991.tb01012.x
出版商: Blackwell Publishing Ltd
数据来源: WILEY
摘要:
Abstract—Approximately 1500 g of float samples of impact melt rocks have been recovered from gravel deposits ∼4 km north and northeast of the rim of the 3.4 km diameter New Quebec Crater (61°17′N; 73°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 ∼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 ∼2% meteoritic contamination. Stepwise40Ar‐39Ar 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 glacia
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