GEOCHEMISTRY, ND-HF ISOTOPIC COMPOSITION AND DETRITAL ZIRCON GEOCHRONOLOGY OF THE SNOWCAP ASSEMBLAGE, YUKON: INSIGHTS INTO THE PROVENANCE AND COMPOSITION OF THE BASEMENT TO YUKON-TANANA TERRANE

The Snowcap assemblage is a pre-Devonian metasedimentary package that forms the basement to Carboniferous volcanic arcs of the Yukon-Tanana terrane throughout much of the Yukon and northern B.C. It comprises varying amounts of quartzite, pelite, psammite, marble and calc-silicate, and minor mafic metavolcanic and meta-intrusive rocks. Geochemical data for the clastic meta-sedimentary rocks illustrate that these rocks have REE and trace element signatures consistent with derivation from typical upper crust. The Nd (eNd_t = -4.3 to -23.7) and Hf (eHf_t = -7.8 to -40.9) isotopic compositions and depleted mantle model ages (T_DM(Nd) = 1.98-3.37 Ga; T_DM(Hf) = 1.15-2.51 Ga) are all consistent with derivation from North American cratonic sources. Detrital zircon geochronology from a sample of impure quartzite indicate derivation from Paleoproterozoic (ca. 1.87, 1.95, 2.08 and 2.37 Ga) and Archean (ca. 2.72 Ga) sources, supporting the Nd-Hf isotopic data and more specifically pointing to the northwestern North American craton as a source region. Mafic rocks have alkalic affinities with ocean island basalt (OIB) and enriched mid-ocean ridge basalt (E-MORB) signatures and Nd-Hf isotopic compositions similar to rift-related rocks in the Cordilleran miogeocline. Taken together these data suggest that the Snowcap assemblage represents detrital material derived from the northwestern North American craton, deposited along the Cordilleran miogeocline in Late Proterozoic-early Paleozoic, and subsequently rifted away during the mid-Paleozoic opening of the Slide Mountain ocean.