REINTERPRETATION OF THE “POST-DEFORMATIONAL” NATURE OF THE OTTERTAIL PLUTON USING ANISOTROPY OF MAGNETIC SUSCEPTIBILITY AND MICROSTRUCTURES

The Ottertail Pluton, located within the Superior Province of Northwestern Ontario, is part of the Algoman suite of quartz monzonites and granodiorites. This suite intruded surrounding rocks along the Wabigoon-Quetico subprovince boundary approximately 2686 Ma based on previous U-Pb zircon dating. Deformation most likely occurred along this boundary during a collision between the Wabigoon and Wawa arcs, with the Quetico metasediments getting caught between them. The Algoman plutons are the youngest rocks in the region, showing no obvious structural features. The apparent macroscopic undeformed nature of the Algoman plutons has led previous researchers to interpret them as post-deformational and use their age as a constraint for the cessation of regional deformation. However, the contrasting competency between the relatively rigid granites and the surrounding metavolcanic and metasedimentary rocks suggests that the plutons would not necessarily show obvious deformation fabrics even if they were present during deformation. Is it possible that the Algoman plutons were actually present during some part of the regional tectonic deformation? To answer this question, we have conducted a detailed Anisotropy of Magnetic Susceptibility (AMS) and microstructural study to determine the emplacement history of the Ottertail pluton relative to the regional deformation.

The magnetic mineralogy is dominated by multidomain magnetite in many cases, or by paramagnetic contributions of biotite and hornblende. The AMS signal is very strong and roughly coincident with the regional EW, steep tectonic foliations measured in surrounding rocks. Therefore, it seems unlikely that the AMS signal is unrelated to the regional deformation. In addition, petrographic analysis shows microstructures such as undulose extinction and subgrain formation in quartz, indicating that there was some solid-state deformation in the granite accommodated by dislocation creep mechanisms. Therefore, the pluton was present during at least some of the tectonic deformation, and was most likely syn-tectonic. Thus, the age of the Ottertail Pluton (~2686 Ma) and other similar Algoman plutons do not provide a conclusive constraint to regional deformation as previously thought.