REVISED INTERPRETATIONS OF DETRITAL ZIRCON POPULATIONS IN THE MESOPROTEROZOIC BELT AND PURCELL SUPERGROUPS OF MONTANA, IDAHO AND BRITISH COLUMBIA

Detrital zircon grain populations in the internally drained, intracratonic Mesoproterozoic Belt-Purcell Basin differ in the eastern, southern and western parts of the basin. The eastern population in the lower Belt, Lemhi, and Piegan groups with ~1900 to 1750 Ma and scattered Archean grain clusters are attributed to Laurentian sources to the east. The southern population, with mostly ~1800 - 1600 Ma grains are centered in the Lemhi sub-basin. They are mostly correlated with Missoula Group rocks and are attributed to Yavapai, Mazatzal, and Mojave sources far to the south. The western population in the lower Belt, Ravalli, and Piegan groups has a cluster of 1610-1490 Ma (North American Magmatic Gap) zircons attributed to western non-Laurentian sources, mixed with a group of ~1800-1600 Ma zircons from the south. With these are small, scattered populations of sun-Belt (~1480 - 1440 Ma) grains. The dominance of ~1800 – 1600 Ma grains in the Lemhi sub-basin that spread northward high in the Belt are interpreted to indicate that the western source detached high in the Belt, and that the basin reorganized tectonically. These interpretations are based on the premise that Belt formations and groups can be correlated on the basis of their zircon populations.

Multidimensional scaling of catalogued detrital zircon ages reveals significant heterogeneity of zircon populations. Komogorov-Smirnov tests show a low internal consistency of ~30% within the Belt-Purcell Basin, indicating high spatial and temporal grain heterogeneity. As locations become more separated, they become more dissimilar. A “good fit” is surpassed beyond 1000 km of geographic separation, and grains become completely dissimilar beyond ~2000 km. We attribute this geographic variance in ages to predominantly proximal rather than mixed distal sources.

These local source areas question the premise that Belt units can be lithically and temporally correlated across the basin on the basis of their zircon populations. We hypothesize that the southern sources were less than ~500 km away throughout Belt deposition, and contributed sediment that interfingered with the Ravalli and Piegan groups. As erosion to the south accelerated high in the Belt, southern sediments overwhelmed eastern and western sources, with minimal evidence for rifting or tectonic reorganization.