29-33 Using anisotropy of magnetic susceptibility and geochronology from the Bush Lake pluton, Florence County, WI to understand the final stages of the Penokean orogeny

Session: Undergraduate Research, Part II (Posters)

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Abstract:

The Bush Lake granite in Florence County, Wisconsin, is interpreted to have intruded at ~1835 Ma as a late-stage intrusion of the Paleoproterozoic Penokean orogeny. The Bush Lake intrusion occurs just south of the Niagara fault zone, a major southward-dipping structure that marks the Penokean-age suture between the Pembine-Wausau terrane and the Superior craton. The Niagara fault zone may have been reactivated by the ca. 1.7 Ga Yavapai orogeny, the ca. 1.6 Ga Mazatzal orogeny, and/or the ca. 1.4 Ga Baraboo orogeny. The Bush Lake intrusion is therefore well located to constrain the timing of granite magmatism relative to Proterozoic deformation events. Although geochronologic, geochemical, and structural data exists for nearby intrusions that are inferred to have been coeval and cogenetic with the Bush Lake granite, there has been no direct geochronology or modern structural analysis of the Bush Lake granite to date. Here, we present microstructural analysis, anisotropy of magnetic susceptibility (AMS) fabric data, and SIMS U-Pb zircon geochronology for the Bush Lake pluton to constrain its tectonomagmatic setting.

            The Bush Lake pluton has an exposed area of ~40 km² and may extend as deep as 4.5 km below the surface based on gravity studies. The intrusion contains the major minerals quartz, plagioclase, alkali feldspar, and biotite, as well as accessory allanite, zircon, titanite, and apatite. Cathodoluminescence (CL) imaging shows that most Bush Lake zircons preserve oscillatory zoning of likely magmatic origin, though many zircon crystals also have irregular, disturbed zoning and low-CL regions consistent with alteration. The granite exhibits solid-state deformation microstructures in thin section. Both the foliation measured in the field–defined by the alignment of biotite and quartz aggregates–and AMS foliation measurements on the west side of the pluton have a similar orientation, striking NW-SE and subvertical. The NW-SE striking foliation is subparallel to the nearby Niagara fault zone and metamorphic wall rock foliations that align with the dominant regional Proterozoic fold-and-thrust belt orientations. Based on the presence of solid-state fabric that parallels regional deformation fabrics, as well as mineralogic, geochemical, and preliminary geochronological similarities between the Bush Lake granite and other regional 1835 Ma granitoids, we interpret the Bush Lake granite to record the regional stress field and convergence in the final stage of the Penokean orogeny.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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