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U-Pb ages of zircons from the Lower Belt Supergroup and proximal crystalline basement: Implications for the early evolution of the Belt Basin

By
Paul Mueller
Paul Mueller
Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA
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David Mogk
David Mogk
Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA
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Joseph Wooden
Joseph Wooden
U.S. Geological Survey, 785 Nob Ridge Drive, Marietta, Georgia 30064, USA
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Drew Spake
Drew Spake
Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA
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Published:
September 01, 2016

The Neihart Quartzite and LaHood Formation are the lowermost units exposed in the Helena embayment, which forms the eastern and southeastern margins of the Belt Basin. Ages of detrital zircons from the Neihart Quartzite (quartz arenite) and a range of lithologies in the LaHood Formation (conglomerates to arkoses to siltstones) show that these units do not share a common provenance. The dominant provenance is Paleoarchean for the LaHood Formation and Paleoproterozoic for the Neihart Quartzite. Provenance is further constrained by the geochemistry and U-Pb ages of zircons from cobbles from the classic LaHood conglomerate in Jefferson Canyon (Tobacco Root Mountains), ages of Paleoproterozoic crystalline basement in the Beaverhead-Tendoy Mountains (1.8–2.45 Ga), and elemental and Sm-Nd isotopic data for select samples of both sedimentary rocks and crystalline basement within the basin. These data show a pronounced lack of detritus from abundant, proximal Neoarchean (2.7–2.9 Ga) and Paleoproterozoic (1.9–2.5 Ga) crystalline basement exposed in Laramide uplifts and the soles of Sevier-style thrust faults within and near the basin.

Analyses of detrital mineral assemblages in the Lower Belt Supergroup units clearly indicate that the finer-grained portions of the LaHood Formation were not locally derived, based on abundant white mica in sections overlying tonalite-trondhjemite-granodiorite (TTG) basement and lack of amphibole in units overlying hornblende tonalites. Significant fractionation also exists between sand- and cobble-size components in conglomerate of the LaHood Formation in terms of elemental abundances, isotopic compositions, and the U-Pb ages of zircons. Stratigraphically, the differences in the ages of the youngest zircons in all LaHood Formation samples and the Neihart Quartzite (1.71 Ga, Neihart; 1.78 Ga, LaHood) do not refute any proposed stratigraphic correlations. Nonetheless, age spectra of detrital zircons from the Neihart Quartzite, all LaHood lithologies, and previously published data for the Newland Formation show distinctions of provenance and an apparent lack of interaction among the sediment-supply systems of these three formations. This contrast suggests that distinct, likely fault-bounded, sedimentologically restricted subbasins characterized the initial stages of development of the eastern Belt Basin along the Perry line (southeastern margin of the Helena embayment), in the manner of a modern, but partially submerged, Basin and Range topography. The time of development of this topography is not clear, but it may have been related to the collapse phase of the Great Falls orogeny at ca. 1.7 Ga for the Helena embayment. The primary, north-south–trending Belt Basin also developed subsequent to the Great Falls orogeny along the western paleomargin of the newly amalgamated Wyoming–Medicine Hat–Hearne craton.

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GSA Special Papers

Belt Basin: Window to Mesoproterozoic Earth

John S. MacLean
John S. MacLean
Department of Physical Science, Southern Utah University, 351 W. University Boulevard, Cedar City, Utah 84720, USA
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James W. Sears
James W. Sears
Department of Geosciences, University of Montana, 32 Campus Drive #1296, Missoula, Montana 59812-1296, USA
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Geological Society of America
Volume
522
ISBN print:
9780813725222
Publication date:
September 01, 2016

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