The petrology, geochemistry, and emplacement of the Foy Offset Dike, Sudbury impact structure (Ontario)

The Foy Offset Dike is a non-traditional igneous body located in the North Range of the Sudbury impact structure. Its origin is intimately related to the impact event that occurred 1.85 Ga ago. It consists of a polymict impact melt breccia with variable inclusion and melt contents. This research defined the petrology and geochemistry of six distinct groundmass variants, defined the petrology and geochemistry of inclusions, determined the relative distribution of inclusions to host groundmass at various outcrops, determined the aspect ratio of aligned inclusions at one locality, and compiled published and unpublished geological data. From this information it is proposed tensional radial fracturing upon catastrophic rebound of the Sudbury transient cavity during the modification stage of cratering facilitated the downward injection of an early bulk melt composition. Lithological variations along strike are interpreted to be related to the subsequent radial increase of deuteric waters. This is believed to be the consequence of a large geothermal gradient imposed by a large structural uplift comprising the Levack Gneiss Complex. The embayment, the segment of the Foy Offset Dike that connects to the Sudbury Igneous Complex (SIC), is interpreted to be influenced by fractional crystallization juxtaposed with a thermal aureole from the overlying SIC. Mass balance calculations between selected target rocks indicate the melt is strongly influenced by an East Bull Lake anorthositic component. A subtle geochemical shift occurs between 22 and 24 km from the SIC associated with down-faulted supracrustal host rocks from the distal Foy. If the length of the Foy represents the extent of a central uplift, the Sudbury Structure may be the largest presently known impact structure on Earth according to current crater scaling equations.