TECTONIC ACTIVITY IN THE BORING BILLION RECORDED IN GARNET LU-HF AND SM-ND GEOCHRONOLOGY ALONG THE WESTERN MARGIN OF LAURENTIA

Recent efforts to compile new and existing garnet geochronologic data from Idaho and western Montana has yielded a database of 26 samples with Mesoproterozoic Lu-Hf and Sm-Nd ages. These samples can be grouped geographically with 22 samples in northern Idaho and western Montana, and 4 samples in east-central Idaho. Northern Idaho contains most of the Mesoproterozoic garnet ages and records a long geologic history with Archean and Paleoproterozoic basement exposures of the Clearwater and Priest River complexes with extensive cover of the Mesoproterozoic Belt Supergroup. The geology in east-central Idaho is dominated by the Lemhi subbasin metasedimentary strata, thought to be correlative to the Belt Supergroup.

The garnet ages can be grouped into three separate metamorphic periods that span ~500 Ma from ~1.5 to 1.0 Ga and record a prolonged metamorphic history on the western margin of Laurentia. The oldest metamorphic period, recognized through garnet Lu-Hf geochronology in this area, are syn-Belt deposition magmatic events dated between 1470 – 1430 Ma, solely recognized in northern Idaho. The second metamorphic period recognized in this area is the East Kootenay orogeny constrained to ages between 1379 and 1325 Ma with garnet Lu-Hf and titanite Pb-Pb geochronology; garnet Lu-Hf and Sm-Nd ages coincident with this period are found throughout northern Idaho and east-central Idaho. New ages determined in this study coincident with the East Kootenay orogeny include one sample from northern Idaho with a Lu-Hf age of 1360 +/- 12 Ma and a Sm-Nd age of 1111 +/- 46 Ma. The last metamorphic period recognized in this area is a Grenville-aged tectonic event from ~1.2 – 1.0 Ga, recognized in both east-central and northern Idaho. Two new coincident ages from this study in east-central Idaho include an Sm-Nd age of 1164 +/- 52 Ma and a Lu-Hf age of 1112 +/- 8 Ma. An additional Lu-Hf age of 1149 +/- 11 Ma coincident with Grenville orogenesis was determined for a sample in northern Idaho.