A PETROCHRONOLOGICAL COMPARISON OF NEOARCHEAN-PALEOPROTEROZOIC MONAZITE FROM SW MONTANA

The Hell Roaring Creek shear zone (HCRsz) is located in the Precambrian rocks of the Northern Madison Range near the NW margin of the Wyoming Province in SW Montana. It is a dextral transpressive amphibolite facies ductile shear zone. Recent structural and lithological description has shown that the HRCsz contains overlapping structures from at least three distinct deformation events. However, the precise timing of events, as well as their broader tectonic implications, remain poorly understood.

Our research aimed to construct a timeline of deformation and high-grade metamorphism in and around the HRCsz. We employed comparative analysis of geochemical, textural, and geochronological data collected from monazite. Five sites were sampled for monazite: three within the HRCsz, and two from the Moon Lake block (MLb) ~2 km SE of the shear zone. The MLb was chosen for comparison due to the presence of low-strain folding that may be related to the HRCsz’s formation and clearer preservation of pre-HRCsz events.

U-Th-Total Pb dating revealed a complex tectonic history spanning from the Neoarchean to the Paleoproterozoic. Monazite crystallization occurred during two main intervals - 2570-2470 Ma (old) and 1770-1730 Ma (young). Samples within the HRCsz were dominated by young monazite. Conversely, the MLb contained majority old growths.

This age stratification implies localized disparities during the 1770-1730 Ma growth phase, in which the HRCsz experienced conditions more conducive to monazite crystallization than the MLb. We suspect this resulted from activation of the shear zone itself. dominant old monazite from the MLb is almost entirely absent in samples taken from Bear Basin ~2 km to the northwest of the HRCsz in previous studies. This suggests the HRCsz played a significant role in the tectonic evolution of the broader region.

Geochemical analysis of complex monazite grains found multiple phases of growth in the young population within the HRCsz and in the old population from the MLb. In the case of the HRCsz samples, this may be evidence of the Big Sky orogeny (ca. 1800-1700 Ma) and HRCsz formation occurring in close temporal proximity. For MLb samples, this could indicate similar successive fluctuations in deep crust conditions from 2570-2470 Ma.