U-PB GEOCHRONOLOGY OF MONAZITE AND ZIRCON IN ARCHEAN METAMORPHIC ROCKS FROM THE SOUTH MADISON AND BEARTOOTH RANGES, SW MONTANA, WITH IMPLICATIONS FOR THERMOTECTONIC EVOLUTION OF THE WYOMING PROVINCE

The Madison mylonite zone (MMZ) and the Snowy shear zone (SSZ) are prominent, NE-trending and NW-dipping, features that transect Archean metamorphic rocks in the South Madison Range (SMR) and NW Beartooth Ranges, respectively. Monazite and zircon have been dated in an effort to constrain the timing of SE-directed thrusting along these two apparently collinear mylonite zones and to test a longstanding hypothesis that they share a common history. In three unmylonitized schists and two rocks from the MMZ, electron microprobe analysis was conducted on 471 spots in 12 monazite grains. Most spot U-Th-Pb ages in the unmylonitized rocks define apparent age populations of ≥2660, ~2560 Ma, and ≤2480 Ma, indicating that the SMR experienced complex polyphase thermotectonism, whereas monazites as young as ~1700 Ma are locally preserved adjacent to fluid-mediated alteration zones. In contrast, within one mylonitized rock from the MMZ these ages are systematically lower by ~50 ± 20 Myr and within another a subordinate population of ~1700-1750 Ma monazite is also preserved. These results suggest that the older monazites were incipiently overprinted at ~1700-1750 Ma. In two quartzites from the SSZ, ion microprobe analysis (SHRIMP-RG) of zircon grains and overgrowths on ~3050-3800 Ma detrital grains, and of one monazite grain, yields ²⁰⁷Pb/²⁰⁶Pb spot ages of 2557 ± 7 and 2547 ± 8 Ma, respectively, which are interpreted as the age of metamorphism associated with SSZ deformation. The ~2660 and ~2560 Ma metamorphic ages coincide with known times of regional convergence and magmatism, whereas the ~2480 Ma ages coincide with a known time frame of global rifting. Finally, the coincidence of ~2560 Ma metamorphic ages in both the SSZ and MMZ is suggestive of a single intracratonic boundary dating back to at least this time. In this context, the MMZ may have been reactivated at ~1700-1750 Ma, consistent with its previous characterization as a foreland structure related to Paleoproterozoic collision of the Wyoming and Medicine Hat cratons.