INVESTIGATING METAMORPHIC AND TEMPORAL TRENDS EAST OF THE ARC-CONTINENT BOUNDARY IN NORTH-CENTRAL IDAHO

The Mesozoic tectonics of west-central Idaho are complex and long-lived, where multiple stages of collision, accretion, igneous intrusion, and crustal deformation at shallow and deep levels have shaped the Idaho segment of the U.S. Cordillera orogeny. The accretion of arc terranes onto continental North America has been presumed to have occurred between 130 Ma and 93 Ma. The Sr “0.704-0.706” line is used to define a boundary between the accreted terrane and continental North America. While trends in metamorphic grade and age of the accreted terrane assemblage on the west side of the arc-continent boundary have been investigated by multiple workers in recent years of the arc-continent boundary, the geology on the east side remains understudied. The tectono-structural border between accreted terrane and rocks associated with western Laurentia is cut by the South Fork of the Clearwater River, revealing a cross section of intensely deformed, high-grade metamorphic rocks. This series of exposures offer unparalleled access to the high-grade metamorphic rocks on the Laurentian side of the arc-continent boundary. In the Elk City, Idaho region, there are high-grade metasedimentary rock sequences that are considered equivalent to the Lemhi subbasin of the Mesoproterozoic Belt Supergroup. Garnet-bearing pelitic schists and amphibolites in this region are conducive to thermobarometry. We introduce new thermobarometric constraints to compliment publish garnet Lu-Hf ages.

Previous studies along this transect report garnet ages east of the arc-continent boundary are younger, ~89.9 Ma, compared to samples on the arc side, which yield ages of ~117-118 Ma. Metamorphic thermobarometry indicates rocks to the east, continent side, of the arc-continent boundary have a peak metamorphic grade of 641 °C and 8.4 kilobars, which is consistent with upper amphibolite facies. Peak temperature and pressure estimates indicate a potential maximum burial depth of ~31 km. This research aims to constrain metamorphic conditions to the east of the arc-continent boundary, which aids in the understanding of the tectono-metamorphic evolution of the Elk City region of north-central Idaho.