THE SPANISH CREEK MYLONITE: A NEWLY RECOGNIZED ZONE OF HIGH STRAIN IN THE NORTHERN MADISON RANGE, SOUTHWESTERN MONTANA

Recent fieldwork in Archean rocks of the Northern Madison Range in southwestern Montana has revealed a zone of high strain approximately 2 km long and at least 800 meters wide, herein referred to as the Spanish Creek mylonite. A multipronged structural approach incorporating fieldwork, microstructural analysis, and geochronology is applied to study of this mylonite. The dominant rock type in this area is biotite-hornblende granitic to granodioritic orthogneiss with variable porphyroclastic character. The mylonitic foliation within the zone is dominantly NE-striking and NW-dipping (~225°/45°) and is often accompanied by a subhorizontal mineral lineation that plunges shallowly to either the northeast or southwest. Along the margins of the zone, the mylonitic fabric is superimposed upon an earlier fabric (~300°/65°). Preliminary examination of thin sections reveals the presence of high-temperature microstructures such as quartz ribbons and polygonal, recrystallized grains of quartz and feldspar. Asymmetric fabrics are rare in both thin section and outcrop, suggesting a large component of pure shear. Microstructural differences between samples confirm field observations of strong strain gradients associated with this zone.

Microstructures in the Spanish Creek mylonite will be further examined using electron backscatter diffraction (EBSD). This approach will provide information on the deformation mechanisms and temperatures associated with this episode of mylonitization. It is expected that the EBSD results will confirm that high-temperature slip systems – prism <c> slip in quartz as well as (010)[001] and (001)<110> slip in plagioclase feldspar – were active during the mylonitic deformation. Additionally, in situ electron microprobe U-Pb monazite geochronology will be used to constrain the timing of mylonitization. Dating of deformation overgrowths in monazite will likely link the development of the Spanish Creek mylonite to one of two major orogenic events in the northwestern Wyoming Province – a Late Archean event at ~2.7-2.5 Ga or the Early Proterozoic Big Sky Orogeny at 1.78-1.71 Ga. If this mylonite is indeed related to the Big Sky Orogeny, it would provide a measure of validation towards recent studies that cite a broader extent for this orogenic event in the Wyoming Province.