Unraveling the tectonomagmatic history of north-central Idaho through accessory mineral geochronology of Proterozoic quartzites in the Clearwater metamorphic complex

Session: Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records (Posters)

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Abstract:

The Clearwater metamorphic complex of north-central Idaho is one of a series of larger core complexes (e.g., Priest River complex) that extends into southern British Columbia, and which were unroofed during Eocene extension (56-46 Ma). The Clearwater complex contains extensive exposures of 2.6 Ga and 1.86 Ga orthogneiss and 1.79 Ga anorthosite enclosed by quartzite and schist of uncertain age/affinity. Hietanen (1984) called these metasedimentary rocks the Boehls Butte Formation and considered them basement to the Mesoproterozoic Belt-Purcell Supergroup. In British Columbia and Montana, similar rocks are interpreted as remnants of a fluvial sand sheet that covered Paleoproterozoic North America. However, robust correlation of pre-Belt rocks throughout this region suggests a more complex tectonomagmatic history than previously thought. We present new zircon, monazite, and rutile U-Pb ages for Boehls Butte quartzites in the interior footwall of the Clearwater complex, as well as nearby Prichard Formation and Ravalli Group quartzites. Boehls Butte quartzites have major zircon age modes at 2.8-2.5 Ga and 1.8-1.7 Ga, whereas ages in Belt samples range from 1.7 to 1.5 Ga with very few Archean grains. This supports correlation of Boehls Butte quartzites with the Neihart Formation and its equivalents, but unlike the Neihart, fine-grained intervals (now schist) are also thought to be part of the pre-Belt strata. In lower Belt and pre-Belt samples, monazite grew in situ from 85 to 60 Ma, but pre-Belt samples also have detrital populations at 1.80 Ga and 1.64 Ga, with additional in situ growth at 1.36 Ga and two less recognized events at 740 Ma and 705 Ma. Rutiles give a weighted mean age of 48 Ma, consistent with core complex exhumation. Collectively, these data suggest that north-central Idaho received detritus from the Great Falls and Farmington tectonic zones (1.86-1.67 Ma), was affected by the East Kootenay orogeny (1.4-1.3 Ga), and experienced two enigmatic Neoproterozoic events (740 Ma and 705 Ma) that may represent early phases of Rodinian rifting. In contrast, lower Belt units in the Clearwater complex record a different history related to both Grenville (1.2-1.0 Ga) and Late Cretaceous events.