Monazite Geochronology of the Madison Mylonite Zone and Environs, Southwestern Montana: With Implications for Precambrian Thermotectonic Evolution of the Northern Wyoming Province
Loehn, Clayton William
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Neoarchean thermotectonism at in the northern Wyoming province is preserved in metamorphic zircon rims and monazite growth throughout the Snowy shear zone (SSZ) and the Madison mylonite zone (MMZ), South Madison Range, Montana. Comparison of U-Pb and U-Th-Pb ages yielded by monazite grains from both shear zones and zircon rims from SSZ, a new timing for major SE-directed thrusting and formation of the MMZ and SSZ has been identified at ~2550 Ma. The collinearity of these two shears indicates the formation of a much larger single shear zone that extends from the North Snowy block (NE), Beartooth Mountains, through the South Madison range (SW), and is paralleled to the immediate NW by the Mirror Lake and Big Brother shear zones. A detrital zircon study of two quartzites, from the westernmost North Snowy block units, yielded concordant age populations ranging in age from 3556 Â± 10 to 2752 Â± 9 Ma indicating that these sediments were derived either from older crust located in the Beartooth Mountains or from another source that was relatively close to the region prior to ~2750 Ma. The youngest magmatic zircon core found among these quartzites yielded a U-Pb age of 2690 Â± 12 Ma, setting a new maximum age for sandstone deposition, additionally 10 metamorphic zircon rims and one monazite grain provide a new minimum U-Pb age of deposition and metamorphism at 2545 Â± 2 Ma. Driving forces behind the ~2550 Ma SE-directed thrusting in the NW Wyoming craton may have been the final stages of supercontinent Kenorland assembly, whereas the ~2450 Ma reactivation, recorded by monazite rim growth, along the SSZ-MMZ may relate to the incipient supercontinent break-up, which has been suggested to have occurred at about this time by other studies.
- Masters Theses