Abstract

The Algoman suite of granites and granodiorites, located within the Rainy Lake region of the Superior Province, intruded the Wabigoon–Quetico subprovince boundary about 2686 Ma. Deformation occurred during the late Archean collision between the Wabigoon and Wawa volcanic arcs. The Algoman plutons are the youngest rocks in the region. Their apparent lack of macroscopic deformation fabrics has led previous researchers to use their age as a constraint for the cessation of regional deformation. However, contrasting competence between the relatively rigid granites and surrounding volcanic and metasedimentary rocks suggests that obvious deformation fabrics may not be observed in the plutons even if they were present during deformation. A field, anisotropy of magnetic susceptibility (AMS), gravity inversion model, and microstructural study were undertaken on two neighboring Algoman plutons to determine their emplacement history relative to the regional deformation. There is a steep macroscopic foliation in the adjacent rocks and in parts of the plutons. The magnetic foliation is very strong and roughly coincident with the field foliations. The gravity inversion models indicate that the plutons are relatively shallow (1 km and 4.5 km at their deepest points), and their shapes are consistent with emplacement during dextral transpression. Quartz microstructures, such as undulose extinction and subgrain formation, indicate that there was some solid-state deformation accommodated by dislocation creep mechanisms. The field, AMS, gravity, and microstructural evidence indicate that the plutons were most likely emplaced syntectonically. Thus, the pluton age does not provide a conclusive constraint to the cessation of regional deformation.

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