Joshua J. Selagi HBSc thesis abstract

The Coldwell Complex is a large Proterozoic alkaline igneous complex on the north shore of Lake Superior west of Marathon, Ontario.  From east to west across the complex, three distinct magmatic centres form an overlapping sequence; Center 2 is characterized by biotite-bearing alkaline gabbro and nepheline syenite (Mitchell and Platt, 1982).  Within Center 2, a fine-grained porcelaineous variety of nepheline syenite is recognized as being chemically eqivalent to the more typical coarse-grained nepheline syenite.  This porcelaineous nepheline syenite outcrops in a narrow arcuate zone along the western shore of Redsucker Cove.

Microstructural evidence for dislocation creep indicates that the porcelaineous syenite is a mylonite formed by localized solid-state deformation.  As the Coldwell Complex has not been subjected to regional metamorphism, the high temperature required for ductile solid-state deformation in the procelaineous syenite must be related to the magmatic activity.  Localized high-temperature ductile strain within this arcuate zone could have led to catastrophic brittle failure along ring faults culminating in caldera collapse.  This discovery may have implications for understanding the processes associated with caldera collapse in large alkaline igneous complexes.