Table Mountain Quartzite and Moose Formation (new names) and associated rocks of the middle Proterozoic Belt Supergroup, Highland Mountains, southwestern Montana

New sensitive high resolution ion microprobe (SHRIMP) UPb zircon analyses from two tuffs and a felsic flow in the middle and upper Belt Supergroup of northwestern Montana significantly refine the age of sedimentation for this very thick (15-20 km) Middle Proterozoic stratigraphic sequence. In ascending stratigraphic order, the results are (1) 1454 ± 9 Ma for a tuff in the upper part of the Helena Formation at Logan Pass, Glacier National Park; (2) 1443 ± 7 Ma for a regionally restricted porphyritic rhyolite to quartz latite flow of the Purcell Lava in the Yaak River region; and (3) 1401 ± 6 Ma for a tuff in the very thin transition zone between the Bonner Quartzite and Libby Formation, west of the town of Libby. Combining these ages with those previously published by other workers for ca. 1470-Ma sills in the lower Belt in Montana and Canada indicates that all but the uppermost Belt strata (about 1700 m) were deposited over a period of about 70 million years, considerably reducing the time span from longstanding estimates ranging from 250 to 600 million years. Calculated sediment accumulation rates between dated samples indicates rapid, but not unreasonable, values for early Belt strata, with decreasing rates through time. These ages also suggest the inadequacy of previously published paleomagnetic data to resolve Belt Supergroup chronology at an appropriate level of accuracy.

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Proterozoic evolution of the western margin of the Wyoming craton: implications for the tectonic and magmatic evolution of the northern Rocky Mountains

Canadian Journal of Earth Sciences ◽

10.1139/e06-052 ◽

2006 ◽

Vol 43 (10) ◽

pp. 1601-1619 ◽

Cited By ~ 106

Author(s):

David A Foster ◽

Paul A Mueller ◽

David W Mogk ◽

Joseph L Wooden ◽

James J Vogl

Keyword(s):

Sedimentary Cover ◽

Magmatic Evolution ◽

Geochronological Data ◽

Western Margin ◽

Northern Rocky Mountains ◽

Southwestern Montana ◽

Wyoming Craton ◽

Belt Supergroup ◽

Western Side ◽

Rifted Margin

Defining the extent and age of basement provinces west of the exposed western margin of the Archean Wyoming craton has been elusive because of thick sedimentary cover and voluminous Cretaceous–Tertiary magmatism. U–Pb zircon geochronological data from small exposures of pre-Belt supergroup basement along the western side of the Wyoming craton, in southwestern Montana, reveal crystallization ages ranging from ~2.4 to ~1.8 Ga. Rock-forming events in the area as young as ~1.6 Ga are also indicated by isotopic (Nd, Pb, Sr) signatures and xenocrystic zircon populations in Cretaceous–Eocene granitoids. Most of this lithosphere is primitive, gives ages ~1.7–1.86 Ga, and occurs in a zone that extends west to the Neoproterozoic rifted margin of Laurentia. These data suggest that the basement west of the exposed Archean Wyoming craton contains accreted juvenile Paleoproterozoic arc-like terranes, along with a possible mafic underplate of similar age. This area is largely under the Mesoproterozoic Belt basin and intruded by the Idaho batholith. We refer to this Paleoproterozoic crust herein as the Selway terrane. The Selway terrane has been more easily reactivated and much more fertile for magma production and mineralization than the thick lithosphere of the Wyoming craton, and is of prime importance for evaluating Neoproterozoic continental reconstructions.

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