Strain localization in the Spanish Creek mylonite, Northern Madison Range, southwest Montana, U.S.A.

The Spanish Creek mylonite zone formed within multiply deformed granitic orthogneisses in the Northern Madison Range in southwestern Montana. An integrated approach incorporating fieldwork, microstructural analysis, electron backscatter diffraction (EBSD), and U-Pb zircon geochronology was utilized to characterize this mylonite zone and place its evolution into a broader regional context. The mylonitic rocks are characterized by a well-developed L-S fabric and a variably porphyroclastic character. U-Pb zircon geochronology yielded a magmatic crystallization age of 2824 ± 47 Ma (2σ) for the granitic protolith of the porphyroclastic Spanish Creek mylonite, indicating that the mylonitic deformation could have occurred during either a late Archean tectonic event or the late Paleoproterozoic Big Sky Orogeny, potentially activated and then reactivated with each successive event. Three distinct structural realms defined within the study area on the basis of structural observations and EBSD data reveal differences in the degree of mylonitic deformation. Two models are herein proposed to explain these significant differences, which are manifest in deformation microstructures and CPO fabrics between structural realms. In the first model, the intrusion and mylonitization of the granitic protolith for the rocks of the third structural realm were broadly coeval during a late Archean tectonic event, allowing for high-temperature mylonitic deformation. In an alternate model, the mylonitic deformation significantly postdated the ∼2.8 Ga intrusion of the granitic protolith, with factors aside from temperature influencing the nature of the deformation. Strain localization provides a viable explanation for the observed differences in the intensity of deformation between structural realms.