EVIDENCE FOR ~2550 MA CONVERGENT THERMOTECTONISM IN SW MONTANA, WITH IMPLICATIONS FOR NEOARCHEAN SUPERCONTINENT ASSEMBLY

The Madison mylonite zone (MMZ) and Snowy shear zone (SSZ) are prominent, collinear, NE-trending and NW-dipping, deformational features in the NW Wyoming craton, which transect Archean metamorphic rocks in the South Madison Range (SMR) and NW Beartooth Range (North Snowy Block, NSB), respectively. Electron microprobe (EMP) and ion microprobe (SHRIMP) analyses have been conducted on monazites occurring in highly deformed rocks sampled from the MMZ and SSZ, as well as from an intervening locality in the SW Beartooths (Yankee Jim Canyon, YJC), in order to constrain the timing of regional, SE-directed thrusting. EMP analysis of metamorphic monazites in diverse rocks from the MMZ and environs yielded total-Pb ages of ~2660, ~2560, and ~2480 Ma. SHRIMP analysis of radiogenic accessories in two SSZ quartzites yielded ²⁰⁷Pb/²⁰⁶Pb ages of 2557 ± 7 Ma for metamorphic rims on ~3050-3800 Ma detrital zircons and 2547 ± 8 Ma for metamorphic monazite in which EMP analysis further revealed a ~2450 Ma metamorphic rim too thin to be analyzed by SHRIMP. Likewise, EMP analysis of metamorphic monazite in YJC felsic gneisses also yielded total-Pb ages of ~2550 Ma (grain cores) and ~2450 Ma (rims). The regional preservation of ~2550 Ma ages from MMZ to SSZ is suggestive of a single intracratonic shear zone dating back to at least this time. If true, then these ages can be further interpreted as recording Neoarchean juxtaposition of metasedimentary and magmatic terranes, as previously inferred from SSZ rocks only. Further, published occurrences of disturbed ~1850-1750 Ma dates of MMZ micas and amphiboles, once considered to record earliest MMZ thrusting, are reinterpreted here in terms of reactivation of a ~2550 Ma(?) shear zone. Older ~2660 Ma ages recorded mainly in the SMR reflect the first thermotectonism to have affected local supracrustals deposited earlier. Moreover, the ~2660-2560 Ma events documented in the SMR coincide with known times of convergence in the nearby Teton Range, whereas ~2560 Ma convergence is also known in the E Wyoming (Black Hills) and S Superior cratons. Thus, ~2560 Ma collisional ages are widespread and possibly related to final assembly of Kenorland. Likewise, the ~2480-2450 Ma ages documented here coincide with the previously inferred onset of Kenorland breakup, but the role of SW Montana in this event remains enigmatic.