RECONSTRUCTING A MESOPROTEROZOIC TECTONIC PUZZLE BY REVISITING THE PROVENANCE OF THE BELT SUPERGROUP IN THE WESTERN US

The provenance of the Belt Supergroup (Idaho and Montana) plays an important role in the reconstruction of the Nuna supercontinent, as it received some of its detritus from another continental block. The identity of the western continent has been described in contrasting models: the SWEAT model (linking East Antarctica to the western U.S and Australia to western Canada) and the AUSWUS model (linking Australia to the western U.S and East Antarctica to the south). To test these models, we determined the U-Pb ages and trace element compositions of detrital zircon and monazite from the Prichard Formation, Ravalli Group, and Lemhi Group of the Belt.

Most detrital zircons from the Prichard Formation yield ages of ~1.60 Ga with a minor peak at ~1.70 Ga. The Ravalli Group yields zircon ages of ~1.65 Ga, including some components at 2.60-2.70 Ga. Quartzites from the Lemhi Group yield zircon ages of ~1.70-1.75 Ga with a few grains dating at 2.20-2.50 Ga. Gd/Yb and U/Yb in zircon display a slightly decreasing trend with decreasing age for all three groups, an indication that the younger zircon population came from a less evolved source. Rare detrital monazites in the Prichard Formation have ages ranging from 1.56 to 1.72 Ga whereas detrital monazites in the Ravalli Group yield age peaks at 1.50, 1.60, and 1.70 Ga. Monazites from the Lemhi Group displays one major peak at 1.67 Ga and a smaller peak at 1.77 Ga. The older zircon and monazite peaks from the Lemhi group could be derived from Yavapai/Mazatzal sources from SW US, whereas the younger ages from the Prichard Formation could be derived from Australian sources.

Future work will include U-Pb dating of detrital rutile, Hf isotope analysis of previously dated zircons, and Pb isotope analysis of detrital feldspars. These data together may allow for the more detailed pinpointing of specific off-continental sources for the Prichard and Ravalli and SW North American sources for the Lemhi Group.