INCIPIENT MELT FORMATION AND DEVITRIFICATION AT THE WANAPITEI IMPACT STRUCTURE, ONTARIO, CANADA

The Wanapitei impact structure is similar to 8 km in diameter and lies within Wanapitei Lake, similar to 34 km northeast of the city of Sudb ury. Rocks related to the 37 Ma impact event are found only in Pleisto cene glacial deposits south of the lake. Most of the target rocks are metasedimentary rocks of the Proterozoic Huronian Supergroup. An almos t completely vitrified, inclusion-bearing sample investigated here rep resents either an impact melt or a strongly shock metamorphosed, pebbl y wacke. In the second, preferred interpretation, a number of partiall y melted and devitrified clasts are enclosed in an equally highly shoc k metamorphosed arkosic wacke matrix (i.e., the sample is a shocked pe bbly wacke), which records the onset of shock melting. This interpreta tion 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, v esiculation, and devitrification. Peak shock pressure of similar to 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 i s comparable to that reported recently from glasses in the suevite of the Ries impact structure in Germany and may indicate that the analyze d sample experienced an annealing temperature after deposition of some where between 650 degrees C and 800 degrees C.