ACCRETIONARY LAPILLI FROM THE SUDBURY IMPACT EVENT: OBSERVATIONS FROM FIVE SOURCES

Accretionary lapilli have been observed at numerous outcrops of Sudbury impact ejecta. The lapilli have various amounts of lithic fragments, occur in different sizes, ranging from 0.8 cm to 2.5 cm, some are zoned, and the geochemical compositions vary based on their location. Five sites were selected to examine the macro- and microscale differences between lapilli: Connors Creek, McLure, Hwy-588, Hillcrest Park, and Pine River. These sites are 450 km to 700 km from the center of the Sudbury impact structure. The lapilli were examined by petrographic analysis to determine the amount and type of fragments included in the lapilli, and the geochemical composition of the lapilli was determined by neutron activation analysis.

The largest lapilli are most common near the impact structure. At all sites, their semi-spherical shape is preserved, although some flattening has taken place as a result of post-depositional compaction. Lapilli occur at differing zones within individual outcrops, ranging from a thick cross-bedded layer at the top of the Connors Creek section to a thin layer in the middle of the Pine River deposit.

Zonation is site dependent, with some sites showing multiply zoned lapilli, while others show almost no zonation. Serial slicing through a lapilli-bearing sample shows that zones within lapilli are often variably deformed within individual lapilli, even though the outer edge of the lapilli remains spherical, which suggests that deformation took place as layers are being accreted.

Each location has petrographically distinct lapilli. Differing types of lithic clasts make the framework of lapilli from each site. Quartz and phyllosilicates are the most common components of lapilli, though the McLure site has abundant Fe-rich particles and the Hwy-588 site includes abundant C-rich particles. A small number of lapilli include shocked quartz.

Lapilli also show significant differences in geochemical composition between sites, but are more homogeneous within sites. Enrichment in Ni and Cr varies depending on distance from the Sudbury structure. The traditionally proposed mechanism for lapilli formation as accreting from the vapor plume may need to be revised to fully explain the range of compositions observed for Sudbury lapilli.