DETRITAL ZIRCON U-PB PROVENANCE OF THE UPPER PURCELL SUPERGROUP, SOUTHEASTERN BRITISH COLUMBIA, CANADA; IMPLICATIONS FOR BELT-PURCELL BASIN MODELS AND PALEOGEOGRAPHIC RECONSTRUCTIONS

This study reports >400 new detrital zircon U-Pb SHRIMP-II ages from the Mesoproterozoic (~1.4 Ga) Upper Purcell Supergroup (UPSG) of SE BC. The goal of our study is to constrain the depositional, tectonic and paleogeographic setting of the Belt-Purcell basin. Five samples were collected along the eastern extent of exposed Purcell strata; one sample was collected from the western limit of strata.

All samples are characterized by subordinate numbers of detrital zircons that yield Paleoproterozoic and Archean ages. Detrital zircon ages from the Sheppard Fm. are dominated by 1500, 1700, 1750, and 1850 Ma grains. The overlying Gateway Fm. is dominated by 1400-1450, 1700, 1850, and 1900 Ma zircon grains. The overlying Phillips, Rooseville (east), and Mt. Nelson formations are dominated by detrital zircon ages between 1375-1450 Ma and 1650-1800 Ma. Detrital zircon ages from the Rooseville Fm. (west) are dominated by 1500-1625 Ma grains.

Paleoproterozoic and Archean detrital zircon ages from the eastern UPSG samples could have been derived from source terranes within W Laurentia, including the U.S. SW. The influx of young (~1375-1450 Ma) zircon grains requires syn-depositional magmatism in a nearby source terrane. Anorogenic granites (~1430 Ma) and related ryholites of the U.S. SW are a possible source of these young ages. However the series of ~1380 Ma granitoid intrusions that make up the Salmon River Arch, and related granitic intrusions into Lower Belt-Purcell Supergroup strata constitute a potential local source for young zircons. In contrast, detrital zircons from the western extent of Purcell strata are better matched to NE Australian source terranes. ~1576 Ma basement exposed in the Priest River core complex in E Washington and W Idaho cannot be correlated with any known autochthonous Laurentian basement and is the likely source of exotic detrital zircon found in the UPSG. The Priest River basement is interpreted to be allocthonous with respect to North America, and may represent a stranded fragment of the long since departed cratonic terrane which formerly constituted the west margin of the Belt-Purcell basin. We interpret the UPSG to have been deposited in a transpressional pull-apart basin setting, adjacent to a convergent/translational plate margin bound to the west by terranes now located in NE Australia.