INTEGRATING PETROLOGY, STRUCTURE, AND GEOCHRONOLOGY TO EVALUATE A POSSIBLE CRUSTAL CROSS-SECTION ON THE NW MARGIN OF THE WYOMING PROVINCE, SW MONTANA

The deep crust plays a critical role in deformation of the lithosphere and crustal cross-sections are particularly valuable because they provide the opportunity to study processes across variable crustal levels. One example is potentially preserved in the northern Madison Range, SW Montana, along the northwestern margin of the Wyoming Craton. Here, Archean rocks were multiply deformed and metamorphosed to granulite facies, the most recent episode correlating with Paleoproterozoic thermotectonism recognized in nearby ranges as the Big Sky orogeny. We integrated field mapping, structural analysis, metamorphic petrology, and geochronology to understand Proterozoic paleodepths, timing, and exhumation mechanisms. Metamorphosed mafic dikes in the western part of the range record conversion of Opx-bearing protoliths to the high-pressure and temperature assemblage Grt+Cpx+Pl+Qtz with equilibrium P-T conditions of 1.2 GPa, 800°C. Metamorphic zircon dated by U-Pb ion probe indicate that these rocks were at paleodepths of 40-45 km at 1753 ± 18 Ma (2 sigma, MSWD = 1.5). Farther east in the middle of the range paleodepths of ~30 km are derived from Grt+St+Ky+Sil+Bt+Pl+Qtz schists (0.85 GPa, 700ºC). Monazite in key metamorphic phases give uniform U-Th-total Pb dates of 1745 ± 5 Ma (2 sigma), indistinguishable from the timing of high-pressure metamorphism to the west. At the eastern end of the range, Archean K-Ar biotite dates suggest paleodepths no deeper than ~13 km since that time. Ongoing research is aimed at evaluating whether this range-scale pattern represents a continuous lower to upper crustal section, or juxtaposition of multiple blocks with discrete bounding structures. Several 100’s of meters- to km-scale ductile shear zones occur throughout the range, but none have been studied in detail. Preliminary work on the Hell Roaring Creek shear zone, near the east end of the range, indicates that it is a 1-2 km wide NE-striking and moderate to steeply NW-dipping structure with a down-dip stretching lineation and NW-side-up kinematic indicators. Chronologic information will help distinguish between Archean and Proterozoic structures and evaluate the role of Proterozoic tectonometamorphism in the structural make-up and exhumation of basement rocks in this range.