Incipient melt formation and devitrification at the Wanapitei impact structure, Ontario, Canada

Abstract

Abstract-The Wanapitei impact structure is -8 km in diameter and lies within Waniipitei Lake, -34 km northeast of the city of Sudbury. Rocks related to the 37 Ma impact event are found only in Pleistocene glacial deposits south of the lake. Most of the target rocks are metasedimentary rocks of the F'roterozoic Huronian Supergroup. An almost completely vitrified, inclusion-bearing sample investigated here represents either an impact melt or a strongly shock metamorphosed, pebbly wacke. In the second, prefei~ed interpretation, a number of partially melted and devitrified clasts are enclosed in an equally highly shock metamorphosed arkosic wacke matrix (i.e., the sample is a shocked pebbly wacke), which records the onset of shock melting. This interpretation is based on the glass composition, mineral relicts in the glass, relict rock textures, and the similar degree of shock metamorphism and incipient melting of all sample components. Boulder matrix and clasts are largely vitrified and preserve various degrees of fluidization, vesiculation, and devitrification. Peak shock pressure of -50-60 GPa and stress experienced by the sample were somewhat below those required for complete melting and development of a homogeneous melt. The rapid cooling and devitrification history of the analyzed sample is comparable to that reported recently from glasses in the suevite of the Ries impact structure in Germany and may indicate that the analyzed sample experienced an annealing temperature after deposition of somewhere between 650 "C and 800 OC.

Petrographic studies of “fallout” suevite from outside the Bosumtwi impact structure, Ghana

“…6c and 6d) was found in some of the thin sections. Similarly textured ballen quartz has been found in impact glasses and crystalline impact melts of many impact craters (e.g., French et al 1970;Engelhardt 1972;Carstens 1975;Dressler et al 1997). This ballen quartz has been interpreted to represent pseudomorphs after cristobalite, which, in turn, is thought to have replaced lechatelierite, originally formed by shock-induced melting of quartz (Carstens 1975), or to represent recrystallized diaplectic quartz glass that underwent transition to cristobalite and then α-quartz (Bischoff and Stöffler 1981;1984).…”

Preservation of detrital shocked minerals derived from the 1.85 Ga Sudbury impact structure in modern alluvium and Holocene glacial deposits

“…2) is a smaller (8 km) and younger (ca. 37 Ma) impact structure located on the eastern margin of the Sudbury Basin (Dressler et al, 1997), and thus it could potentially have contributed detrital grains to the sediment samples described here. While it may be somewhat challenging to distinguish the two sources given the similarity of target rock lithologies (Fig.…”

“…2), we view the probability of a signifi cant contribution of detrital shocked minerals from the Wanapitei structure to be low. No in situ shocked rocks have been documented at Wanapitei (shocked rocks have only been found as "fl oat"), and the three localities around Lake Wanapitei where PDFs in quartz have been reported are attributed to the Sudbury impact (Dressler et al, 1997). In addition, the two sediment samples from Holocene eskers we collected on the shore and to the southwest of Lake Wanapitei (Fig.…”

“…2). The North and South Ranges are defi ned by an E-W tectonic zone (Rousell, 1984a), whereas the East Range is defi ned by deformation related to the formation of the Eocene Lake Wanapitei impact crater (Dressler et al, 1997). Bedrocks of the North and East Ranges are part of the Archean Superior Province and are therefore older than rocks in the South Range, which are dominated by Proterozoic rocks.…”

Planar microstructures and Dauphiné twins in shocked quartz from the Charlevoix impact structure, Canada

“…The discrepancy between experiment and nature is apparent in the absence of basal PDFs and Brazil twins in experimentally shocked quartz crystals. Additionally, stress localization, as indicated by the varying PDF density on the grain scale, differences in mineralogy (Kieffer, 1971;Dressler et al, 1997), crystallographic orientation (Langenhorst, 1994;Langenhorst and Deutsch, 1994) and pre-shock temperature (Langenhorst, et al, 1992;Huffman et al, 1993;Huffman and Reimold, 1996), as well as grain size effects (Walzebuck and Engelhardt, 1979;Grieve and Robertson, 1976) limit the use of PDFs as a pressure gauge. Nevertheless, the occurrence of characteristic PDF sets can yield important information on shock pressure attenuation (e.g., Robertson, 1975).…”