U-PB GEOCHRONOLOGY AND HF ISOTOPE SIGNATURES OF DETRITAL ZIRCON GRAINS FROM THE NEOPROTEROZOIC AMUNDSEN BASIN OF NORTHWESTERN CANADA

One of the first U-Pb detrital zircon studies was published 20 years ago using sandstones from the Tonian-Cryogenian Shaler Supergroup. At that time, 25 zircon grains from 3 samples were analyzed by ID-TIMS. The GSC’s Geo-mapping for Energy and Minerals program has allowed us to re-sample and re-analyze the Shaler Supergroup via modern techniques to: 1) test the conclusions of our original study; 2) understand observed stratigraphic changes in provenance in more detail, and 3) establish maximum depositional ages (MDA) for key stratigraphic horizons. In this study, we analyzed >400 zircon grains from 10 samples using SHRIMP. Quartz-arenites were selected from throughout the ~4000m-thick section, including (in ascending stratigraphic order) the Escape Rapids, Nelson Head, Fort Collinson, Wynniatt, Kilian and Kuujjua formations. An arkose from the unconformably underlying Husky Creek Formation, in the Coppermine Homocline, has a MDA of about 1230 Ma and exhibits an age profile consistent with local reworking of underlying strata. The Escape Rapids Fm. displays a wholesale change to mainly 1600-1400Ma sources. Rocks of this age are unknown in the region, suggesting non-Laurentian provenance. A fluvial sandstone from the overlying Nelson Head Fm. preserves northwesterly paleocurrents and well-rounded grains of Grenvillian age (1400-1000Ma). The lower Wynniatt Fm. shows almost no Grenvillian signature and a dominant mode at 1870Ma, suggesting uplift with associated sedimentary by-pass of Grenvillian detritus. The upper Wynniatt and overlying fluvial Kuujjua formations signal a rejuvenation of the Grenvillian source (strong 1080 peak; MDA~780Ma) and/or recycling of underlying units. A tuffaceous sandstone from the Kilian Fm. has unusual modes of 950, 900 and 800 Ma (MDA~800 Ma). Provenance from an adjacent craton such Siberia and/or South China is supported by recent Rodinia reconstructions. Approximately 150 zircon grains from 5 samples were also analyzed for their Hf isotope composition via LA-ICPMS. Initial analysis indicates that Mesoproterozoic grains have mainly juvenile εHf values, consistent with provenance from arc terranes of the Grenville Province of southeastern Laurentia. Paleoproterozoic grains have mixed juvenile and evolved signatures and a few Archean grains have juvenile signatures.