CONSTRAINTS ON MESOPROTEROZOIC AND CRETACEOUS METAMORPHISM THROUGH GARNET GEOCHRONOLOGY IN IDAHO AND WESTERN MONTANA

The region of Idaho and western Montana has a long, complex geologic history due to its proximity to the ancient continental margin of Laurentia. To better understand the complexity of thermal events in this region, we present a compilation of garnet geochronology dates with the addition of new garnet Lu-Hf dates determined in this study. The studied areas include the Clearwater metamorphic complex, Blackbird mining district, and Salmon River suture zone (SRSZ). In these areas garnet Lu-Hf ages can be broadly classified into three separate age ranges: ~1.4-1.0 Ga, ~120-100 Ma, and 100-90 Ma.

The Clearwater complex is located in north-central Idaho near the Idaho-Montana border and was exhumed during the early Eocene within the Lewis and Clark fault zone. The Clearwater complex contains Archean basement rocks covered by the Mesoproterozoic Belt Supergroup. Here, Lu-Hf garnet geochronology has identified garnet ages spanning ~1.4-1.0 Ga. While more work is required to assign these ages into distinct metamorphic events, new garnet Lu-Hf analyses returned ages of 1123.8±1.7 Ma and 1149.6±1.8 Ma from garnets collected near Kelly Creek and the North Fork of the Clearwater River. This is most notably evidence of Grenville-aged metamorphism along Laurentia’s western margin, which is largely unstudied.

The Blackbird district in east-central Idaho contains cobalt deposits hosted within Belt Supergroup rocks in the Lemhi subbasin. Xenotime and zircon U-Pb ages from these metasedimentary units show cobalt formation at 1.37 Ga. In addition, cobaltite formation is also documented within Cretaceous metamorphism and garnet growth. However, since xenotime matrix effects may impact the reliability of their U-Pb analyses, three new garnet age are currently being collected to further the understanding of this area together with the two existing Lu-Hf garnet ages in the Lemhi subbasin, 93.7±5 Ma and 112.8±7.7 Ma. The SRSZ documents more Cretaceous garnet Lu-Hf ages to the west. The SRSZ is widely accepted to be the boundary between oceanic terranes accreted in the Cretaceous and the North American craton. Here, Lu-Hf garnet ages document metamorphism from ~120-90 Ma. Ages older than 100 Ma are interpreted to be the result of terrane accretion and ages younger than 100 Ma are thought to have more pluton influence, namely the Idaho batholith.