Maes, S M
Department of Geology, University of Wisconsin-Eau Claire, 115 Garfield Avenue, Eau Claire, WI 54702 United States
Czeck, D. M.
Dept. of Geosciences, University of Wisconsin - Milwaukee, 3209 N. Maryland Ave., Milwaukee, WI 53211 United States
Sturm, C. L.
Department of Geology, Oberlin College, 52 W. Lorain Street, Oberlin, OH 44074 United States
Fein, E. M.
Department of Geology, Kent State University, Lincoln and Summit Streets, Kent, OH 44242 United States
The Algoman suite of granitic plutons intruded along the boundary between the Wabigoon-Quetico subprovinces of the Archean Superior Province at about 2686 Ma. Due to the apparent undeformed nature of the plutons, their age had previously been used to constrain the cessation of late Archean regional deformation. However, the Algoman plutons are more competent than the surrounding rocks; therefore macroscopic deformation fabrics may not be observed even if they were present during deformation. An AMS study and a simple 2-D gravity survey were used to assess the relative timing of regional deformation and pluton emplacement. The subjects of the study were the Ottertail Pluton and a smaller unnamed pluton, both Algoman plutons located in the Rainy Lake region of Ontario along the Minnesota border. The vertical magnetic foliations are roughly coincident with the regional tectonic foliation observed in adjacent rocks. Magnetic lineation orientations throughout most of the pluton are highly variable, though most are moderately to steeply plunging. Macroscopic lineations are less common, and the magnetic and macroscopic lineations rarely coincide. The observed variation in lineation orientations could result from monoclinic ductile transpression with variable inclined extrusion or be a record of magma flow during emplacement. Two-dimensional gravity models indicate that both plutons are relatively shallow. The Ottertail has a two- dimensional dish-shaped geometry with a flat base that shallows to the north and south as well as to the east and west. No correlation is observed between steep lineations and deepening of the plutons, which commonly results from upward flow of magma from a root zone. The AMS fabrics and the shallow nature of the plutons are consistent with emplacement during regional dextral transpression, possibly into shallow rhombochasms created by en echelon P shears related to major shear zones.
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