Using monazite geochronology to constrain timing of deformation within the Hell Roaring Creek Shear Zone, SW Montana

Abstract:

A wide range of tectonic events from the middle Archean to the Paleoproterozoic have shaped Wyoming Craton, including its collision with the Medicine Hat block, resulting in the Big Sky Orogeny (~1.8-1.7 Ga) in southwest Montana. Here, we examine the ductile deformation preserved within the Hell Roaring Creek shear zone (HRCsz), which is a ~2 km wide dextral transpressive amphibolite facies high strain zone within the Northern Madison Range. The structure divides two separate domains: the Bear Basin block and the Moon Lake block. From previous work, it was concluded that the main deformation fabrics in the Bear Basin block and that immediately preceded the shear zone effects developed around 1750-1740 Ma. Our goal was to specifically constrain the time in which the HRCsz formed, using samples directly from the shear zone and the Moon Lake block.

High resolution geochemical and geochronological techniques were used to analyze the monazite grains because they are compositionally zoned and this zoning is linked to the timing in which the domain grew. Backscatter electron images and x-ray maps for relevant elements were collected by electron microprobe to identify monazite domains and areas of interest. We used the Th-U-total Pb method to date the domains.

In the shear zone samples, most of the monazite grains range in age from 1740-1760 Ma, suggesting that the shear zone occurred during the Big Sky Orogeny and developed just after the deformation in the Bear Basin block.