Emplacement of an Archean gneiss dome, northern Ontario, Canada: Inflation inferred from magnetic fabrics

Previous field structural studies of the Ash Bay gneiss dome revealed an upward convex foliation with an integral east‐west lineation defined by a quartzo‐felspathic matrix. Unfortunately, the amphibolite facies gneisses lack traditional indicators of microfabric evolution, such as kinematic indicators, so that it is difficult to relate the foliation's orientation distribution to an emplacement mechanism for the dome. Fortunately, anisotropy of magnetic susceptibility isolates a distinct microfabric because of the preferred orientation of high‐susceptibility, accessory magnetite. The magnetite subfabric developed late in the metamorphic crystallization history, and its magnetic foliation and magnetic lineation are similarly oriented to the S‐L fabric of the quartz‐feldspar matrix. However, the magnetite foliation dips less steeply and forms a less convex domal surface than the field schistosity. The metamorphic schistosity and the magnetite foliation thus isolate the domal deflection at different stages, confirming the inflation and upward motion of the Ash Bay diapir during the closing stages of amphibolite facies metamorphism.