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

John P. Murphy

doi:10.7273/000006960

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 metamorphic 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, Lemhi subbasin, and Salmon River suture zone (SRSZ). In these areas garnet Lu-Hf ages can be broadly grouped into three separate age ranges: ~1.4-1.0 Ga, ~150-100 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. Previous Lu-Hf garnet geochronology in this region has identified garnet ages spanning ~1.4 to 1.0 Ga. In the present study, a new garnet analysis of a metasedimentary garnet schist yielded a Lu-Hf age of 1360 ± 12 Ma and a Sm-Nd age of 1111 ± 46 Ma; metasedimentary samples collected along Kelly Creek and the North Fork of the Clearwater River yielded garnet Lu-Hf ages of 1149 ± 11 Ma for a biotite-rich quartzitic gneiss sample and 1124 ± 18 Ma for a garnet-sillimanite-biotite gneiss. These ages provide evidence of Grenville-aged metamorphism along Laurentia’s western margin, which has not been widely recognized. The Lemhi subbasin in east-central Idaho is less understood in terms of the metamorphic history of the area, with metamorphic events documented in the Mesoproterozoic and Cretaceous that are both accompanied by cobaltite mineralization. Xenotime U-Pb ages intended to date cobaltite formation in the metasedimentary units suggest initial cobalt formation at 1.37 Ga within the metasedimentary host rock. Subsequent cobaltite formation is also documented with Cretaceous xenotime formation and is coincident with pre- existing garnet geochronology in the area (113 ± 8 Ma and 94 ± 5 Ma) but there is still uncertainty about the thermal history of the area. Three newly collected metasedimentary samples yielded a Lu-Hf age of 1353 ± 8 Ma and a Sm-Nd age of 1238 ± 8 Ma for a Lemhi subbasin migmatite, a Sm-Nd age of 1164 ± 52 Ma for a Lemh subbasin phyllite, and a Lu-Hf age of 1112 ± 8 Ma for a garnet-biotite schist. These new Mesoproterozoic ages broaden the known area of Mesoproterozoic metamorphism in eastern Idaho. Samples from the Salmon River suture zone document Cretaceous garnet Lu-Hf ages to the west. The suture zone 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 to 90 Ma. Ages older than 100 Ma are interpreted to be the result of viterrane accretion and crustal thickening, where ages younger than 100 Ma are thought to be related to the emplacement of the Idaho batholith.

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

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