Shock metamorphic features in mafic and ultramafic inclusions in the Sudbury Igneous Complex: Implications for their origin and impact excavation

The lowermost, discontinuous parts of the impact-generated Sudbury Igneous Complex (Canada), comprising the Sublayer and Offset Dikes, are distinguished from overlying Main Mass norite rocks by the presence of abundant inclusions and Ni-Cu-PGE (PGE—platinum group element) sulfide mineralization. The majority of the felsic to mafic inclusions appear to be derived from the exposed country rocks, but the volumetrically important olivine-bearing mafic and ultramafic inclusions have only very rare equivalents in the surrounding country rocks. We record the discovery of abundant shock metamorphic features (e.g., mosaicism in olivine; strong fracturing and partial isotropization of plagioclase) in the olivine-bearing mafic and ultramafic inclusions consistent with a shock pressure of 20–30 GPa. Olivine compositional data are inconsistent with a local country rock or mantle origin for these inclusions. Abundant plagioclase, the absence of garnet or Mg-spinel, and calculated low pressures (<500 MPa) provide evidence for derivation of the inclusions from unexposed mafic-ultramafic intrusions in the upper to middle crust that were disrupted during formation of the transient crater, incorporated into the impact melt sheet, and preserved because of their relatively refractory compositions. These observations support models involving intermediate, rather than very deep or very shallow, excavation for the Sudbury impact event.