EMPLACEMENT OF A TABULAR TONALITE INTO A FOLD HINGE, WAKEMUP BAY TONALITE, MINNESOTA

The Archean Wakemup Bay tonalite is a small plutonic body that intrudes amphibolite-grade biotite schist on the extreme southern edge of the Quetico sub-province, Minnesota.  Immediately surrounding the pluton, the foliation within the schist defines a doubly plunging, EW-trending anticline with moderately dipping limbs (F3).  The Wakemup Bay tonalite is spatially associated with the hinge area of this fold.         A gravity study suggests that most of the pluton is less than 0.5 km thick and contains two root zones with depths of up to 4.0 km.  The deepest root zone lies beneath the schist, on the south side of the surface exposure of the pluton.  The biotite schist structurally overlies the tonalite in a small area in the center of the pluton, hence the depth recorded by the gravity inversion represents the true thickness of the pluton.  The Anisotropy of Magnetic Susceptibility technique provides magnetic fabric orientation of the igneous body.  The magnetic foliation paralleling the measured field foliation indicates a doubly plunging anticline. The magnetic lineation is predominantly oriented EW, plunges shallowly, and is parallel to both the long direction of the Wakemup Bay pluton and the F3 fold hinge.  The intrusion of the Wakemup Bay pluton into a broad anticlinal hinge during the F3 folding event is supported by the thin shape of the pluton, the consistent magnetic lineations, and the field relations.

The Wakemup Bay tonalite and the Burnside gneiss have a similar composition and structural style and are offset by ~45 km along the Vermillion fault.  We are currently conducting similar analysis on the Burntside gneiss to determine if the two plutonic bodies were originally continuous and constitute a piercing point for the Vermillion fault.