REVISITING THE PROVENANCE OF THE BELT SUPERGROUP WITH AN EXPANDED DETRITAL MINERAL TOOLKIT

LA-ICP-MS U-Pb dating of detrital zircon is an important tool in sedimentary provenance research, but useful additional information can be gleaned by simultaneously measuring trace element ratios in zircons and also by U-Pb dating other detrital minerals. Here, we show the benefits of this approach in a study of the Mesoproterozoic (~1.4 Ga) Missoula Group of the Belt Supergroup in Montana and Idaho.

Detrital zircons obtained from samples of the Mt. Shields, Bonner, Garnet Range, and Pilcher Formations yield similar U-Pb age systematics, with a large number of 1.7 to 1.8 Ga grains and a small number of Mesoproterozoic and Neoarchean grains. The Garnet Range Formation shows a main age peak around 1.79 Ga while the other three units share a principle age peak between 1.70 and 1.73 Ga. The Paleoproterozoic zircons of the Mt. Shields, Bonner, and Pilcher share a decline in U/Yb and Gd/Yb with decreasing age.

Large amounts of detrital monazite and rutile, both of which occur most commonly as metamorphic minerals, were obtained from the Bonner Formation. Monazite grains yield a single age peak at 1.70 Ga and rutile ages yield a peak at 1.62 Ga with an asymmetric tail towards older ages. A small number of monazites recovered from the Mt. Shields Formation yield an age comparable to the Bonner monazites, and rutile obtained from the Garnet Range Formation yield an age comparable to the Bonner rutiles.

Taken together, the data require a source or mixture of source regions that experienced extensive magmatism between 1.8 and 1.7, with perhaps less crustal melting and greater mantle input later in that time interval. Major metamorphism occurred at 1.70 Ga and cooling and unroofing proceeded slowly, reaching the rutile U-Pb closure temperature (~500-600°C) by 1.62 Ga. Terranes with the appropriate history are not present nearby. Instead, our results support the previously proposed connection between the Missoula Group of the Belt basin and the terranes of the southwest US, such as the Yavapai and Mazatzal.