2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 6
Presentation Time: 2:45 PM

PROVENANCE OF THE UPPER BELT SUPERGROUP AND LEMHI GROUP: CONSTRAINTS FROM HF AND ND ISOTOPES


STEWART, Eric D., Geosciences, Idaho State University, 921 S. 8th Ave. Stop 8072, Pocatello, ID 83209, LINK, Paul, Department of Geosciences, Idaho State University, 921 S. 8th Ave. Stop 8072, Pocatello, ID 83209, FANNING, C. Mark, Research School of Earth Sciences, Australian National Univ, Canberra, ACT, Australia, FROST, Carol D., Department of Geology and Geophysics, University of Wyoming, Dept. 3006, 1000 University Avenue, Laramie, WY 82071 and MCCURRY, Michael, Department of Geosciences, Idaho State University, Pocatello, ID 83209, steweric@isu.edu

U-Pb ages on detrital-zircon grain populations from 12 samples of Mesoproterozoic Missoula and Lemhi Groups of western Montana and east-central Idaho yield unimodal spectral peaks derived from a major 1660 to 1780 Ma magmatic source. Six samples are statistically identical and combined yield an age peak of 1734 Ma on 330 zircon grains. Smaller syn-Belt (1470-1400 Ma) and Archean populations are present in some samples. Hf isotopes from 1675 to 1775 Ma Missoula Group detrital zircons have a wide range in εHf(i) (+9 to -12); with a similar range for Lemhi Group strata ( εHf(i) between +8 and -7). Thus although the ages of the zircons are unimodal, the Hf isotopes show major variance. Syn-Belt (1470-1400 Ma) grains from the Bonner Formation, Missoula Group have a wide range in εHf(i) values between +5 and -9, incompatible with direct derivation from A-type magmatic sources in the Mojave and Yavapai terranes. Finally, new bulk rock Nd isotopes from Lemhi Group argillites which show unimodal detrital zircon spectra yield a range in εNd(i) between +1.1 and -5.4, also incompatible with direct derivation from the Mojave and Yavapai terranes.

The new Hf isotope data and the bulk-rock Nd data support an isotopically heterogeneous 1660-1780 Ma source crystallized from mixed Paleoproterozoic juvenile mantle and Archean crust. Several km of south-derived Missoula and Lemhi Group coarse-grained feldspathic strata suggest a proximal source to the southwest. This provenance is best explained by a now-covered southwestern part of the Selway Terrane or a rifted fragment of Laurentia, rather than the Mojave-Yavapai-Mazatzal terranes far to the south.