NEW PETROLOGIC AND GEOCHRONOLOGICAL CONSTRAINTS ON PALEOPROTEROZOIC TECTONOMETAMORPHISM ALONG THE NW MARGIN OF THE WYOMING CRATON, N. MADISON RANGE, SW MONTANA

Late Paleoproterozoic thermotectonism along the NW margin of the Wyoming Craton has been recognized for some time, however the spatial extent and details of this event remain enigmatic. This high-grade Paleoproterozoic metamorphism and deformation, referred to as the Big Sky orogen, have been identified in the nearby Tobacco Root Mountains. The northern Madison range in southwestern Montana is a ~50 km long NW-SE trending Laramide basement cored uplift of multiply deformed and metamorphosed Archean rocks. Although the most recent ductile deformation was previously interpreted as Archean, newly combined geochronology, thermobarometry, petrologic forward modeling, and structural analysis indicate ~1.75 Ga deformation and metamorphism across much of range. Metamorphosed mafic dikes in the NW record equilibrium P-T conditions of 1.2 GPa, 800 °C linked to 1753 ± 9 Ma metamorphic zircon. Preliminary work on the Hell Roaring Creek shear zone (HRCsz) near the east end of the range indicates that it is a 1-2 km wide NE-striking, moderate to steeply NW-dipping structure with a near down-dip stretching lineation and NW-side-up and dextral kinematic indicators. On the NW side of this structure, pelitic and aluminous quartzite samples yield equilibrium P-T conditions of ~0.85 GPa, 700-750 ºC linked to U-Th-total Pb monazite dates of 1745 ± 5 Ma. On the E side of the HRCsz, a folded mafic dike and a nearby orthogneiss yield undated P-T conditions of 1.0 GPa 700-750 °C. Possible explanations for the higher pressures found in the footwall of the HRCsz may be 1) tectonic loading or 2) equilibrium conditions from an earlier high grade possibly Archean metamorphic event. Farther to the SE across strike in Gallatin Canyon, structural evidence for late flexural slip folding and fabrics defined by greenschist-facies minerals that have reactivated older gneissic layering that are kinematically compatible with the HRCsz. The Madison Mylonite zone in the nearby southern Madison range also records Paleoproterozoic greenschist-facies deformation. Ongoing investigations are focused on more precisely identifying the location, timing, and grade of Paleoproterozoic structures related to the Big Sky orogen, as well as the thermochronological signature and length-scale of the hinterland-foreland transition.