The geochemistry of shocked and country rocks from the Lake Wanapitei impact structure, Ontario.

Abstract

Lake Wanapitei is located in central Ontario, 40 km northeast of Sudbury. The geophysical and morphological evidence suggest that the crater originally measured 8500 m in diameter. The original ground surface has been lowered by some 300 m; material scoured from the crater floor has been deposited as glacial float on and south of the southern shoreline. The shock metamorphosed rock fragments in these deposits consist of glassy melt rocks and suevitic breccias with lithic clasts of mainly quartzite, arkose, wacke, siltstone, and diabase. This work concentrates on establishing a compositional relationship between the Wanapitei crater lithologies and the unshocked country rocks at the area. Due to the relatively young age of the structure, the impact melt glasses are well preserved, with a low content of quench plagioclase and other alterations resulting from devitrification. Presented here are the results of bulk rock XRF and microprobe analyses of the Wanapitei crater rocks together with analyses of country rocks, that may have been exposed to the impact at the time of event. The suspected target rocks are represented by Proterozoic quartzitic sediments of Mississagi, Bruce, and Gowganda Formations, and diabase dikes of Nipissing Intrusions. A least-squares mixing model has been applied to determine which of the country rocks, were mixed to produce the observed impact glass lithologies. The results indicate 56% of Mississagi and 44% of Gowganda Formations; however, secondary evidence suggests a limited contribution of the Nipissing rocks. Based on these results, the meteoritic content of the impact melts has been established, and by comparison of siderophile geochemical data with the Wanapitei samples, a chondrite has been confirmed as the most probable projectile. The grade of shock metamorphism recorded in the Wanapitei shocked rocks suggests at least 65-70 GPa for maximum pressures and up to 2500$\sp\circ$C for maximum temperatures.