DETRITAL ZIRCON RESULTS FROM PROTEROZOIC AND PALEOZOIC ROCKS OF WESTERN MONTANA AND IMPLICATIONS FOR GEOLOGIC EVOLUTION OF THE WESTERN NORTH AMERICAN MARGIN

We report new detrital zircon geochronologic results from twelve sandstone samples collected from western Montana. Samples include Middle Proterozoic McNamara, Bonner, Garnet Range and Pilcher Formations of the upper Missoula Group (Belt Supergroup), as well as Cambrian Flathead and Devonian Maywood Formations. Sampled localities include the Flint Creek Valley at Porters Corner, the headwaters of Bear Creek in the Garnet Range, Mulkee Gulch, and the lower Clark Fork River Valley near Alberton, Montana. Based on LA-ICP analysis of 1432 detrital zircons, we observe the following: 1) All samples are characterized by a lack of ages corresponding to the North American Magmatic Gap (~1.50 – 1.65 Ga). 2) All but two Belt samples have a strong unimodal zircon age peak at ~1.716 Ga. 3) The two anomalous Belt samples are Garnet Range Formation collected near Alberton and are characterized by a slightly older but distinctly different age mode at ~1.769 Ga. 4) All Belt sample contain a subordinate age peak at ~1.4 Ga. 5) Cambrian samples contain no zircons of ~1.7 Ga and are dominated by peaks at ~1.85 and ~1.95. 6) Devonian samples contain several zircons of ~1.7 Ga age and are dominated by zircons ranging from ~1.85 to ~2.0 Ga. 7) One sample of Garnet Range from Alberton and one sample of Devonian Maywood contain a single zircon with age of 1.0 Ga. Collectively, these results are interpreted to reflect the following: 1) All Missoula Group samples were effectively cut off from source terranes that supplied zircons of ages corresponding to the North American Magmatic Gap to the lower portion of the Belt Supergroup. 2) The dominant sources of sediment for the McNamara, Bonner, Garnet Range and Pilcher Formations included detritus from the Yavapai and Mazatzal tectonic elements to the south but also included locally-derived syn-tectonic detritus. 3) The existence of a ~1.0 Ga zircon – dated twice – in one Garnet Range Formation suggests suggests that this formation and the overlying Pilcher may significantly post-date the rest of the Belt Supergroup. 4) Cambrian and Devonian samples reflect integration of multiple sources that include the Trans-Hudson, the Great Falls tectonic zone, and the Antler Orogen.