Bailey Drover MSc thesis abstract

Thesis Title: 
A record of transpression-related brittle-ductile deformation and associated alteration within granitoid plutons of the Wabigoon subprovince, Superior Province

The Wabigoon subprovince is a 900 km long by 150 km wide Archean-aged granite greenstone belt. Large, synvolcanic batholiths with smaller late to post-tectonic stocks cut the numerous greenstone belts of the subprovince. One of the post-tectonic stocks is the Taylor Lake Stock, located within the western Wabigoon subprovince with a late Archean crystallization age that has been interpreted to infer the cessation of regional tectonics in the area. However, granitoids are highly competent and dry rocks that are difficult to deform, leading them to appear undeformed/unmetamorphosed in the field even though they may have undergone ductile deformation, brittle deformation and associated hydrothermal/ metasomatic alteration. In this study, field mapping and sampling, petrographic analysis, mineralogical compositional analysis, stable isotope geochemistry and cathodoluminescence imaging of twelve granitoid plutons (including the Taylor Lake Stock) across the subprovince are used to constrain the relationship between the brittle deformation, ductile deformation and alteration of the plutons to provide insight into the tectonic history of the Wabigoon subprovince.

The twelve plutons record chlorite and/or epidote infilled steeply dipping shear fractures with sub-horizontal lineations that indicate an oblique strike-slip displacement, characteristic of transpression. The strike of the infilled shear fractures varies across the individual plutons, possibly as a result of non-coaxial strain in which the rigid and competent granitoid bodies have undergone a component of rigid body rotation, resulting in a shifting trend of the maximum elongation direction. Evidence for this stems from the shape of the Ottertail pluton in the western Wabigoon subprovince that resembles a porphyroclast entrained within a dextral shear zone. Adjacent to the chlorite and/or epidote infilled shear fractures, wall-rock alteration common to all plutons includes white mica (mostly phengite) ± epidote alteration of the feldspar and chloritization of biotite. Epidote, chlorite and sphene are also commonly seen along microfractures within the host rock. Alteration of the plutons is noted to be lesser in the low strain areas of the plutons where brittle deformation is not as pervasive. Cathodoluminescence imaging of the feldspars from the various strain zones supports lower degrees of alteration in lower strain zones, as feldspars are more consistent with their color hues in the lowest strain samples.

Each of the twelve plutons also record solid-state deformation microstructures within quartz and feldspar, demonstrating dislocation creep was active in both mineral phases. As dislocation creep does not become an effective process within feldspar until temperatures reach ⁓450°C, these solid-state deformation microstructures provide evidence for amphibolite facies metamorphism of the plutons. Ductilely overprinted quartz veins and the presence of chlorite and/or epidote shear fractures within shear zones demonstrates brittle deformation during regional ductile deformation and associated alteration, which is seen in six of the plutons studied (Sabaskong batholith, Dryberry batholith, Revell batholith, Ottertail pluton, Irene-Eltrut batholithic complex and the Croll Lake Stock), providing evidence that regional-scale transpression consisted of coeval brittle-ductile deformation and associated alteration within the granitoid plutons. The δDfluid and δ18Ofluid values of the hydrothermal fluid calculated from measured δD and δ18O values of chlorite infilled shear fractures from six samples in two plutons range from -30 to -45‰ and 5.6 to 7.1‰, respectively, recording a metamorphic water signature that likely stems from the devolatilization of the surrounding host greenstone during regional Archean metamorphism.

XRD qualitative mineral phase analysis of two chlorite infilled shear fractures also shows the presence of the illite 2M1 polytype associated with the vein, suggesting that some of the hydrothermal fluid circulation occurred at temperatures within the realm of the illite 2M1 stability field (lower than roughly 300°C). This coupled with the presence of a chlorite infilled cataclasite, which represents a lower temperature brittle feature, suggests that at least a component of the brittle deformation and associated hydrothermal fluid flow occurred post peak metamorphism, likely into exhumation of the granitoid plutons.

Furthermore, the alteration and deformation of the twelve plutons studied demonstrates evidence for coeval brittle-ductile deformation and associated hydrothermal fluid flow in the amphibolite facies of metamorphism, with brittle fracturing and alteration continuing into exhumation. The late Archean crystallization age date from the Taylor Lake Stock should not be used to mark the cessation of regional tectonics within the western Wabigoon subprovince.