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Shocked lithologies at the Wanapitei impact structure, Ontario, Canada
Author(s) -
Grieve R. A. F.,
Ber T. J.
Publication year - 1994
Publication title -
meteoritics
Language(s) - English
Resource type - Journals
eISSN - 1945-5100
pISSN - 0026-1114
DOI - 10.1111/j.1945-5100.1994.tb00775.x
Subject(s) - impact structure , lithology , geology , mineralogy , geochemistry , matrix (chemical analysis) , homogeneous , augite , breccia , quartz , plagioclase , impact crater , materials science , astrobiology , composite material , paleontology , physics , thermodynamics
The ∼7.5 km diameter Wanapitei impact structure (46°45′N; 80°45′W) lies entirely within Lake Wanapitei in central Ontario, Canada. Impact lithologies are known only from glacial float at the southern end of the lake. Over 50% of the impact lithologies recovered from this float can be classified as suevite, <20% as highly shocked and partially melted arkosic metasediments of the target rock Mississagi Formation or, possibly, the Serpent Formation and <20% as glassy impact melt rocks. An additional <5% of the samples have similarities to the suevite but have up to 50% glass clasts and are tentatively interpreted as fall‐back material. The glassy impact melt rocks fall into two textural and mineralogical types: a perlitically fractured, colorless glass matrix variant, with microlites of hypersthene with up to 11.5% Al 2 O 3 and a “felted” matrix variant, with evidence of flow prior to the crystallization of tabular orthopyroxene. These melt glasses show chemical inhomogeneities on a microscopic scale, with areas of essentially SiO 2 , even when appearing optically homogeneous. They are similar in bulk composition for major elements, but the felted matrix variant is ∼5×more enriched in Ni, Co and Cr, the interelement ratios of which are indicative of an admixture of a chondritic projectile. Mixing models suggest that the glassy impact melt rocks can be made from the target rocks in the proportions: ∼55% Gowganda wacke, ∼42% Serpent arkose and ∼3% Nipissing intrusives. Geologic reconstructions suggest that this is a reasonable mixture of potential target rocks at the time of impact.