VISCOUS EMULSION DIFFERENTIATION OF SUPERHEATED IMPACT MELT SHEETS

The Sudbury Igneous Complex (SIC), Ontario, Canada, is a compositionally stratified magma body that formed by impact melting of a heterogeneous section of continental crust in the middle Proterozoic. The division of the SIC into an upper granophyre layer and a lower norite layer reflects the essentially bimodal nature of the target zone. When these rocks were melted and disrupted during the impact event, they formed a magmatic emulsion. Felsic magma formed a continuous phase, in which mafic droplets and blobs of all sizes were dispersed. Although chemically miscible, these two liquids remained discrete because of their contrasting viscosities. After cratering motions ceased, the emulsion quickly separated. The dense mafic blobs and droplets sank to the bottom and underwent compaction, leaving a relatively pure felsic layer at the top.

Initially, the melt sheet was superheated and underwent vigorous thermal convection that homogenized both layers and cooled the magma down to its liquidus. Additional cooling was conductive, with the norite cooling from the base up and the granophyre cooling from the top down. The interface between the norite and granophyre, which is geochemically and mineralogically distinct, was the last to cool below the solidus. During the final stages of solidification, different types of foreign debris were "frozen" into the melt sheet. The debris includes blocks of granite that floated into the norite from the crater floor as well as remobilized fallback breccia that foundered and sank into the granophyre.

The emulsion model explains the geochemistry of the SIC, the presence of exotic rock types in the center of the complex, and also the textural variations through the norite. Impact melting is a process that was common during early Earth history and is capable of maximizing igneous diversity by concentrating preexisting heterogeneity.